Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

On Dec 17, 7:02 am, wrote:
Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

The center holes will be no more accurate than the 3 jaw chuck. I'd
turn down the tailstock end until the cut goes all around, flip the
bar and cut the other end, then repeat to turn a smooth finish cut on
both ends of the bar without moving the crossfeed. This test bar is
easier to use with a dial indicator if you turn the space between the
ends slightly smaller.

The idea is that the work runs true in a dead center even if the
tailstock is offset sideways or the chuck or headstock center wobbles.
When you cut the bar at the tailstock, flip the bar and cut the other
end at the tailstock you've made both ends as close to the same
diameter as possible on that lathe with that tool. You can still have
errors from deflection of the tailstock or the tool, especially if it
isn't sharp.

Is that clear?

Jim Wilkins

wrote:

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel Â*20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

And what if the 3 jaw chuck is excentric? It certainly is! Put a dial
indicator to the round when it is in the chuck and rotate it. You'll
see ...

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe.

No, see above.

Will the centre drills be in the CENTRE of the bar,

No, see above

so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage,

The dial will only show how much off-center you drilled.

Its probably simple, its not a search for world peace, but I cant werk
it out....

You need to make a dead cylindrical bar between centers OR ascertain that
your center drilling is dead on drumroll center. You can do the later by
clamping the round in the chuck, center drill it AND make a small recess.
Flip it over and do the same on the other end. But then, measuring in
between is useless.

BUT, you can calculate out the excentricity by measuring the min and max
when you rotate the bar and use the average.

There's a **MUCH** better way to align your tailstock dead to the spindle
(what doesn't mean that your spindle is dead parallel to the bed):
Some bar in the chuck, a dial indicator at the end of the bar. The
indicator's tip touches a dead center in the tailstock. Rotate the spindle
and watch the indicator. Now you see how the tailstock is misaligned back
and forth and up and down. You'll be astonished!

This method can be used to align the spindle to the bed too. I'm assigning
that to you as homework. :-))


Nick
--
The lowcost-DRO:
http://www.yadro.de

On Mon, 17 Dec 2007 04:02:29 -0800 (PST), wrote:

Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

Andrew, the easy way to do this is take a piece of, say 20-25mm, steel
about a foot long. Center drill both ends and take it home. Set it up
between centers and take a cut with your home machine. Measure both
ends. If there is a difference in size offset the tailstock to
compensate. Continue until both ends of the bar are the same size. You
now have (1) centered your tail stock and (2) made a test bar that can
be used, next time, with a dial indicator to re-set the tail stock.

Setting up a machine usually starts with "leveling it". i.e., getting
the basic machine aligned (the easy way) using a precision level. This
assures that the ways are not twisted. You can then check for wear on
the ways using a straight edge and feeler gauge. If all is well then
check headstock/tail stock alignment with the test bar and Bob's your
uncle.


Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

wrote:

Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

No hope.

Your 3 jaw chuck is not nearly accurate enough.

But it's a cheap one to try and see for yourself!

If you were to center drill it, then turn the OD parallel, between
centers, different story. More accurate!



Cheers
Trevor Jones

On Mon, 17 Dec 2007 04:02:29 -0800, vk3bfa wrote:

Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment. Took a
piece of silver steel 20mm in diameter. (at least, thats what its called
here - its shiny steel rod, ground (I think) so its straight and parallel.
Set it up in a 3 jaw chuck, centre drilled each end.
snip

Could I suggest:

http://www.neme-s.org/Rollie's_Dad's_Method.pdf

Thank you Rollie's Dad.

Mike in BC

On Mon, 17 Dec 2007 17:29:55 +0000, Michael Gray wrote:

http://www.neme-s.org/Rollie's_Dad's_Method.pdf


Sorry, don't know what happened to the URL - just copy and paste the whole
thing into your browser.

Mike

On Dec 17, 12:29 pm, Michael Gray wrote:
Could I suggest:

http://www.neme-s.org/Rollie's_Dad's_Method.pdf

Thank you Rollie's Dad.

Mike in BC

That's a good quick trick but it depends on measurement rather than
comparison of equal readings, which is potentially more accurate. It
won't work at all with an indicator like this:
http://contractorstools.com/graphics...ind5series.jpg
because the reading isn't linear.

Jim Wilkins

On Dec 17, 12:02 pm, wrote:


OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

To make a test bar, you need to centre drill it as accurately as
possible. So yes do use the 4 jaw chuck and spend some time getting
it centered. before drilling each end. Using a cylindrical grinder
may make it more accurate, but that depends on how well the grinder
is set up. If the school has a bench center, use it to check the
accurary.


Dan

wrote:
So yes do use the 4 jaw chuck and spend some time getting
it centered. before drilling each end.
Dan


In addition, I'd cut the 60 degrees with a small boring bar rather than
relying on a center drill for accuracy.
Randy

Jim Wilkins wrote:

It
won't work at all with an indicator like this:
http://contractorstools.com/graphics...ind5series.jpg
because the reading isn't linear.

a) You have to know how to use them
b) They *are* accurate. Or the ones with 0.001mm resolution would be
useless.


Nick
--
The lowcost-DRO:
http://www.yadro.de

"Randy Replogle" wrote in message
...
wrote:
So yes do use the 4 jaw chuck and spend some time getting
it centered. before drilling each end. Dan


In addition, I'd cut the 60 degrees with a small boring bar rather than
relying on a center drill for accuracy.
Randy

I'm not sure I've ever read so much misinformation in all my years of
reading RCM. It is obvious, at least to me, that precious few of the
readers have an understanding of the subject at hand.

No, this is not directed at you, Randy. Your comments are spot on, assuming
the material is dialed perfectly, but even then it must be dead on center at
both ends.

Harold

Harold and Susan Vordos wrote:
....... but even then it must be dead on center at
both ends.

Harold

Right. It's one of those things we've all *heard* how to do but probably
most of us never have done it, including me.
Randy

Michael Gray wrote:

On Mon, 17 Dec 2007 17:29:55 +0000, Michael Gray wrote:


http://www.neme-s.org/Rollie's_Dad's_Method.pdf



Sorry, don't know what happened to the URL - just copy and paste the whole
thing into your browser.

Mike



Apostrophes are not normally allowed in URL links.

wrote in message
...
Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.

The 4-jaw won't help if you really want the result to be dead nuts. Tell us
first, do you have a faceplate or a between-centers drive plate? What are
you using for a drive dog? You should have a drive plate and a dog with a
well-polished tail to do this the conventional way, but there is a way to do
the job with a 3- or 4-jaw.

It would be better to find out first what you have to drive the bar from the
headstock end, rather than to speculate and elaborate.

--
Ed Huntress

On Mon, 17 Dec 2007 04:02:29 -0800 (PST), wrote:

Cant get my head around this one, so, to the group........

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage, with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

Andrew VK3BFA.
Hey Andrew,

Yeah, don't make it more complicated than it is. First, as you have
the centers still in place already (don't you?), check the TIR on the
center that is in the headstock. ( If that's out, it will have to be
corrected before you do anything else. ) If the headstock run-out is
OK, then remove the centre and put on a 4-jaw chuck. Rough chuck the
DTI in the 4-jaw ( doesn't need to be center-chucked ) and bring the
tailstock up to the headstock and run it on the tailstock center. If
that's OK, then you can chuck the test-bar you made in the four-jaw to
a zero TIR run-out on the OD (20 mm), and then check the run-out on
the center hole you drilled. Do that for both ends of the test-bar.
If they are correct, then you have other problems. If they are "out",
then go ahead with what you suggested, IF you have a grinder that
will do the job.

Harold and Susan Vordos wrote:

"Randy Replogle" wrote in message
...

wrote:
So yes do use the 4 jaw chuck and spend some time getting

it centered. before drilling each end. Dan


In addition, I'd cut the 60 degrees with a small boring bar rather than
relying on a center drill for accuracy.
Randy


I'm not sure I've ever read so much misinformation in all my years of
reading RCM. It is obvious, at least to me, that precious few of the
readers have an understanding of the subject at hand.

No, this is not directed at you, Randy. Your comments are spot on, assuming
the material is dialed perfectly, but even then it must be dead on center at
both ends.

Harold


Be specific Harold.

If you are going to swing the brush, you might as well be specific
about what you beleive to be bad information.

The test bar I made, is hardened, and ground on a tool and cutter
grinder of no real great trustwortiness.

I ran out of time to get it finished, and it has been languishing in
my toolbox since 2005, according to the dates I stamped on it's end.

It is parrallel for about half it's length, within a tenth, and then
suffers a slow taper down the remainder, to the tune of about a half
thou total. The requirement was to get within a thou, for the course, so
it was a passable grade, but I never did get it back on a machine to get
it back in line.

I claim no expertise, just the experience I had.

FWIW, for the class of work most guys need tools in their shops for,
they would be better spending their money on HSS cutting bits and
grining wheels, than a commercial test bar. If one is willing to put the
time into it, one can be made, but the usefulness is limited, compared
to other tools that could be made.

Cheers
Trevor Jones

On Mon, 17 Dec 2007 17:08:48 -0500, Randy Replogle
wrote:

Harold and Susan Vordos wrote:
...... but even then it must be dead on center at
both ends.

Harold

Right. It's one of those things we've all *heard* how to do but probably
most of us never have done it, including me.
Randy

The easy way to do this is take a piece of, say 20-25mm, steel about a
foot long. Center drill both ends and take it home. Set it up between
centers and take a cut with your home machine. Measure both ends. If
there is a difference in size offset the tailstock to compensate.
Continue until both ends of the bar are the same size. You now have
(1) centered your tail stock and (2) made a test bar that can be used,
next time, with a dial indicator to re-set the tail stock.

Setting up a machine usually starts with "leveling it". i.e., getting
the basic machine aligned (the easy way) using a precision level. This
assures that the ways are not twisted. You can then check for wear on
the ways using a straight edge and feeler gauge. If all is well then
check headstock/tail stock alignment with the test bar and Bob's your
uncle.
Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

On Dec 17, 1:47 pm, Nick Mueller wrote:
Jim Wilkins wrote:
It
won't work at all with an indicator like this:
http://contractorstools.com/graphics...ind5series.jpg
because the reading isn't linear.

a) You have to know how to use them

Indeed!

b) They *are* accurate. Or the ones with 0.001mm resolution would be
useless.

You need to read the article. The process requires accurate LINEAR
measurement NOT comparison measurement.

Regards,

Robin

On Dec 17, 4:45 pm, "Harold and Susan Vordos" wrote:


I'm not sure I've ever read so much misinformation in all my years of
reading RCM. It is obvious, at least to me, that precious few of the
readers have an understanding of the subject at hand.

No, this is not directed at you, Randy. Your comments are spot on, assuming
the material is dialed perfectly, but even then it must be dead on center at
both ends.

Harold,

Couldn't agree more. BUT I think it's because there are many factors
that affect this process - an excellent example of why the call it a
"skilled trade".

I go with the soft bar between centers, test cuts at each end (don't
waste time making a long pass!). Process should take about 10min or
less (depending on cutters and lathe centers being setup already or
not).

Test bars are nice, but I wouldn't trust one unless it was either
purchased AS A TEST BAR or had been hardened and ground by a competent
tool maker or grinding hand. Additionally, any nick or burr on the
internal 60º taper, or the centers will RUIN the results of the
hardened test bar.

Cutting a test bar each time is INTRINSICALLY ACCURATE (and makes for
a lighter toolbox/heavier wallet). Indeed, they don't use a test bar
when they're making test bars!

Regards,

Robin

On 2007-12-17, wrote:
Cant get my head around this one, so, to the group........

Beware that our various locales disagree on the spelling of
"center/centre", so ignore that as you read.

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Probably centerless ground -- which may have lobes which won't
show up with a standard micrometer, but which will often show up when
measured with a V-anvil micrometer.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage,

Given the typical 3-jaw chuck, the center holes which you
drilled will *not* be in the true centers of the end of the stock.

Either put them in a collet (and verify that the collet is a
good one by indicating on the 10mm or so which are projecting out the
end of the collet), or chuck up close in a 4-jaw and take the time to
center that with a tenths-reading dial gauge.

with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Yep -- typical 3-jaw chuck. You really *can't* depend on those
being true -- they get work close to center quickly, but not on center,
even brand new -- and if they've been in use some time things are likely
to be pretty bad.

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

That depends on just how accurate you want it to be. 100% is
not achievable -- unless you define it to be "errors not detectable with
the equipment which *I* have". :-)

If you have a 0.001" reading dial indicator at home, and use a
0.0001" reading dial indicator to center it in the lathe at school, it
is probably good enough. (If you're working in metric, make that a
0.001mm indicator at school, and a 0.01mm indicator at home.

And if the tailstock ram at school has any play, you'll probably
need to face off the previous center holes totally before starting over,
or the center drill will follow the previous center as much as the slop
in the ram allows it to do. And snug the lock on the ram as much as you
can while still being able to feed the ram out for drilling so the ram
does not wobble around while you're drilling.

Drilling the centers and then grinding it *between centers* will
probably give you the best results -- on a between centers grinder, not
on the lathe, because you don't know how much you can trust the
tailstock of the school lathe.

Good luck,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"DoN. Nichols" wrote in message
...
On 2007-12-17, wrote:
Cant get my head around this one, so, to the group........

Beware that our various locales disagree on the spelling of
"center/centre", so ignore that as you read.

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Probably centerless ground -- which may have lobes which won't
show up with a standard micrometer, but which will often show up when
measured with a V-anvil micrometer.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage,

Given the typical 3-jaw chuck, the center holes which you
drilled will *not* be in the true centers of the end of the stock.

Either put them in a collet (and verify that the collet is a
good one by indicating on the 10mm or so which are projecting out the
end of the collet), or chuck up close in a 4-jaw and take the time to
center that with a tenths-reading dial gauge.

I'm going to disagree with that, Don. I wouldn't use a collet or a chuck to
hold the work. You can get much more accurate results than that by setting
the job up between centers. That's the standard way of doing this test and
it has been for over 100 years.

Robin started explaining it, and this probably is where Howard was going:
First, you don't use hardened steel to make lathe test bars. Use something
soft; leaded steel, if you have it.

Turn your live center (the one in the headstock; a ball-bearing tailstock
center is not a "live" center) in place. If you have a proper taper socket
adapter for the headstock taper, and a drive plate or a face plate, set up
your *soft* live center and then machine it to a 60 deg. taper. A very light
skim cut is all you need for this.

If you don't have a socket adapter and a drive plate, take a short bar of
soft steel and chuck it, with an inch to two inches protruding out of the
chuck. The amount will depend on how close you can get your cutting tool,
because, after clamping that bar down firmly, you're going to turn it to a
sharp 60 deg. taper with the compound, machining it into a live center, in
place. Don't adjust the chuck jaws or move the bar after turning it to
shape.

The test bar can have center holes drilled in a drill press or whatever.
They don't have to be particularly accurate but do the best you can to save
time machining. You aren't going to use a ball-bearing tailstock center for
this so there's no reason to use a boring tool to cut the centers. Even a
messy hole will run true on a plain dead center unless it's really bad.

If you're doing an expert job, you use a straight-tailed dog to drive the
work. If all you have is a bent-tailed dog, make sure you've polished the
tail with abrasive paper. You should do this with any bent-tailed dog,
anyway.

If you're using the made-in-place center in a 3-jaw or 4-jaw chuck you'll
have to figure out how to make the dog jam against one of the jaws. Regular
dogs usually are too long, unless you have a big chuck. If that's the case
make a shop-made drive dog by drilling two bolt holes each in two pieces of
steel strap or thin rectangular bar. Cut, twist, and bend one end over in a
vise so it jams solidly against a chuck jaw. Clamp it on the work with two
bolts, set up like a regular dog. If this isn't clear we can explain it.
You'll need this dog to do any light between-centers work, anyway, until you
get a proper socket adapter and drive plate.

Robin suggested taking two short cuts, one at each end of the bar. That's
all you need for setting the tailstock. Scratch a witness mark on the
tailstock's cross adjustment when you get it right.

If you're checking the straightness of the lathe bed you can turn the full
length. But take it very easy and make the final cut a spring cut with a
*very* sharp tool. You can't use a follow rest or steady rest for this so
you can crash really good if the bar whips on you at the middle of the cut.

If Andrew isn't familiar with these between-centers setups, let me point out
that this is the only good way to do most shaft work. Chucking a bar as
thick as this test bar, even putting it in a collet should you have one that
big, is a very bad idea if you're going to run the other end on a center.
You'll load the hell out of the center and the tailstock socket (this is one
reason they get bell-mouthed) and your work is likely not to be very
accurate. Repeating a job that way is almost impossible.

Yes, people do it all the time. But it's not the right way to do it. And
don't use ball-bearing centers for precision work like this.

If you repeat this at a later time, turn the live center again. Don't rely
on witness marks on the spindle and socket adapter, even with a proper
socket adapter, for precision work.

--
Ed Huntress

"Trevor Jones" wrote in message
news:0ME9j.16422$UZ4.14560@edtnps89...
snip--


Be specific Harold.

If you are going to swing the brush, you might as well be specific about
what you beleive to be bad information.

The test bar I made, is hardened, and ground on a tool and cutter grinder
of no real great trustwortiness.

Nor of much practical use. It would not serve well for aligning a
tailstock, and for several reasons. Nice project for the student, though.

"The idea is that the work runs true in a dead center even if the
tailstock is offset sideways or the chuck or headstock center wobbles.
When you cut the bar at the tailstock, flip the bar and cut the other
end at the tailstock you've made both ends as close to the same
diameter as possible on that lathe with that tool. You can still have
errors from deflection of the tailstock or the tool, especially if it
isn't sharp

Is that clear?

Jim Wilkins"

Yes, it's clear, but it proves nothing. If you flip the piece end for
end, it proves you've flipped the bar end for end, nothing more. The
objective in aligning the tailstock is to determine if the centeline if the
tailstock and headstock are common to one another with the ways. Flipping
the bar won't do that. Turning it at each end will, although simply
measuring the taper on either end will define if there is taper. Needless
to say, the test is far more sensitive over a longer distance, but the
results will be reliable only for that distance, with the tailstock quill
extended the identical amount.

Therefore, the bar must not be hardened, and must be machined with the cross
slide undisturbed for both the cuts on each end.

"Andrew, the easy way to do this is take a piece of, say 20-25mm, steel
about a foot long. Center drill both ends and take it home. Set it up
between centers and take a cut with your home machine. Measure both
ends. If there is a difference in size offset the tailstock to
compensate. Continue until both ends of the bar are the same size. You
now have (1) centered your tail stock and (2) made a test bar that can
be used, next time, with a dial indicator to re-set the tail stock".

Yes, you'll have a test bar that functions to align the tailstock with the
headstock---for that given length----with the tailstock quill extended the
same amount each time it is applied. Unless you have a perfect machine
(no one does have), it, otherwise is pretty useless. On a positive note,
the tailstock will be very close, regardless where it's placed -----assuming
the ways are leveled and not worn badly.

"To make a test bar, you need to centre drill it as accurately as
possible. So yes do use the 4 jaw chuck and spend some time getting
it centered. before drilling each end. Using a cylindrical grinder
may make it more accurate, but that depends on how well the grinder
is set up. If the school has a bench center, use it to check the
accurary".

A 3 jaw is more than adequate for establishing the centers. The important
issue is getting the material parallel with the centerline of the spindle
before drilling the centers on each end. An operation such as this is best
performed in soft jaws that have been machined in place. They are far
superior to hard jaws that come with chucks, and will generally grip the
material perfectly parallel, at least as good as the material is straight
from manufacturing. Hardened jaws don't often equal that condition.

As far as the grinder is concerned, most grinders run on dead centers at
both ends (not all, but they generally have ultra precision centers that do
not introduce eccentricity). The only condition you get when grinding
between centers, assuming the centers are proper, is taper. Considering a
test bar is ordinarily used by machining each end, there is no need for
grinding. A hardened test bar, for this purpose, is useless.

"In addition, I'd cut the 60 degrees with a small boring bar rather than
relying on a center drill for accuracy.
Randy"

What that does is guarantee that the center is round, and conforms to the
mating center. Center drill can leave irregular holes, although in the
hands of a skilled machinist, that isn't often the case.

"Setting up a machine usually starts with "leveling it". i.e., getting
the basic machine aligned (the easy way) using a precision level. This
assures that the ways are not twisted".

That is the essence of a proper setup. Aligning the tailstock with the
headstock by the use of a bar serves no real purpose. What a machine does
by working from the headstock alone should be the priority. If a machine
will bore a straight hole (the result of a properly leveled lathe, assuming
the headstock is parallel with the ways), only then should a test bar be
used to align the tailstock. What purpose does a lathe serve if it will
turn a straight shaft the length of any given test bar, between centers, if
it turns tapers under any other condition?

"Probably centerless ground -- which may have lobes which won't
show up with a standard micrometer, but which will often show up when
measured with a V-anvil micrometer".

The cloverleaf configuration mentioned isn't common, but certainly not
impossible to achieve. If a centerless is run on center, it is almost
assured. As the part moves away from center, either above or below, the
condition is virtually impossible to achieve. The sole exception is when
long pieces are ground, and they are not straight. A centerless, given
enough passes through the machine with the proper setup and enough material
on the part, will straighten slight deviations. When it can't, there are
places on the piece that will not be round, and may display less than round
conditions, although not necessarily the thee lobed cloverleaf that is
guaranteed by running on center. Either of these conditions will become
evident with an indicator when run on centers, regardless of the nature of
the center, be it concentric, or not.


"I'm going to disagree with that, Don. I wouldn't use a collet or a chuck
to
hold the work"

That is correct. For starters, when's the last time you used a collet or
chuck that had no error? Given the fact that you may have a perfectly
straight shaft of a given diameter, unless you can grip it with no error,
and have the opposite end run with no error, it serves little purpose that
most any shaft can serve. The most important function such a shaft can
serve is to determine how parallel the headstock is with the ways, and it
should not be supported by the tailstock for that function. Otherwise, the
tailstock alters the true reading. It relates poorly to tailstock
alignment, due in part to the tailstock and headstock rarely being in the
same plane. Yes, they're supposed to be, but rarely are. Most machines
have error.

". You can get much more accurate results than that by setting
the job up between centers. That's the standard way of doing this test and
it has been for over 100 years."

Yes, for aligning the tailstock for a shaft of a given length, with the
tailstock quill extended a given amount. Move either the tailstock or the
quill, and all bets are off. There is no better way to align a tailstock
that with the work piece in question. Conditions will be set properly for
the length, so with patience it can be adjusted to virtual perfection. Move
any of the components and it's likely to change, right down to installing
the next part, which may have shallower or deeper center drilled ends. That
is an ongoing problem in precision grinding.

"Robin started explaining it, and this probably is where Howard was going:
First, you don't use hardened steel to make lathe test bars. Use something
soft; leaded steel, if you have it."

Right, but make that Harold, not Howard. Leaded steel is best because it
cuts with no tearing and rarely welds to the tool, which alters finish size.
The results are far more predictable and reliable. Hard aluminum serves
well, too. 7075-T6, for example.

"If you're doing an expert job, you use a straight-tailed dog to drive the
work. If all you have is a bent-tailed dog, make sure you've polished the
tail with abrasive paper. You should do this with any bent-tailed dog,
anyway."

Does one have need to do this on a moonless night as well? I'm at a loss to
understand why a bent tail would make a difference, but I'm really tripping
over polishing the tail. Unless I've missed something, that makes no sense.
Please elaborate.

"Robin suggested taking two short cuts, one at each end of the bar. That's
all you need for setting the tailstock. Scratch a witness mark on the
tailstock's cross adjustment when you get it right."

That serves no real purpose. The alignment will likely be proper only at
that given location, under identical conditions. The moment the tailstock
is moved, it won't necessarily repeat the alignment to the same degree.
Further, the human eye can not discern less than .003", so even if you align
to the mark, you can't expect your parts to come out straight. If they do,
it's the luck of the draw. An offset of .002" will yield a taper of
..004", far more than most folks find acceptable.


"Yes, people do it all the time. But it's not the right way to do it. And
don't use ball-bearing centers for precision work like this."

That's correct. There is no guarantee that the center of a live center
runs true (it can be eccentric with the bearings, believe it or not), and it
can also be eccentric to the shank. Installing it randomly alters how
straight your works turns out. I own such a live center. Dead centers
are used in grinding because they don't introduce any errors.

I have machined since '57, and have never used an alignment shaft. They are
normally not the tool of the operator, but one used in making original
alignments of machine tools, most of which are fixed once the machine is
assembled. Why folks screw with the tailstock alignment as they do is a
total mystery to me. Those of us that have worked between centers
generally align to the condition at hand, for the specific project. Other
times, as long as the tailstock will start a small center drill without
breaking the tip, center isn't ultra critical, if for no other reason, it is
constantly changing because the lathe spindle isn't perfectly parallel with
the ways, nor is the tailstock quill perfectly parallel with the ways. You
simply have to live with a given amount of error. That's what sorts
wannabe's from machinists. Level the ways within prescribed limits, extend
the tailstock quill half way, with a dead center installed, and sweep the
point with an indicator from the spindle, held short to eliminate sag. The
alternative is to sweep the quill internal taper. That is a good average
for every day work. Make further adjustments on an individual basis, as
required, with the actual work piece. Get on with life.

Harold

Robin S. wrote:

The process requires accurate LINEAR
measurement NOT comparison measurement.

Any dial indicator can only compare.


Nick
--
The lowcost-DRO:
http://www.yadro.de

"Harold and Susan Vordos" wrote in
. net:

That's correct. There is no guarantee that the center of a live
center runs true (it can be eccentric with the bearings, believe it or
not), and it can also be eccentric to the shank. Installing it
randomly alters how straight your works turns out. I own such a live
center. Dead centers are used in grinding because they don't
introduce any errors.


Harold,
That depends on the type of live center you are using. Ours are
concentric to the taper to less than 5 microns. But these aren't $150
live centers either.
One other thing everyone has failed to take into account is quill bore
alignment to the body. You really should check tailstock alignment at two
considerably different quill extension lengths. As you stated, having the
tailstock body aligned at a particular quill setting provides you with
absolutely zero indication of how the tailstock is truely aligned.
We align tailstocks to less than 5 microns TIR at two separate points
over ~10" of quill travel. This proves that the quill is concentric to
the spindle centerline over it's entire travel. All bets are off however,
if you move the tailstock body.

--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

On Dec 17, 1:47 pm, Nick Mueller wrote:
Jim Wilkins wrote:
It
won't work at all with an indicator like this:
http://contractorstools.com/graphics...ind5series.jpg
because the reading isn't linear.

a) You have to know how to use them
b) They *are* accurate. Or the ones with 0.001mm resolution would be
useless.

Nick

Mine has a range of 0.2mm and a lever 15mm long, so the angular travel
is less than a degree and the cosine error considerably less than a
division. The more common cheaper test indicators with enough range to
read a shock absorber rod in a 3 jaw chuck rotate far enough that
cosine error is significant.

I found it at a place in Maine that sells second-hand woodworking
tools. They had no idea what it was or how much it was worth and
wanted $20.

Jim Wilkins

Harold and Susan Vordos wrote:
"Trevor Jones" wrote in message
news:0ME9j.16422$UZ4.14560@edtnps89...
snip--



Be specific Harold.

If you are going to swing the brush, you might as well be specific about
what you beleive to be bad information.

The test bar I made, is hardened, and ground on a tool and cutter grinder
of no real great trustwortiness.


Nor of much practical use. It would not serve well for aligning a
tailstock, and for several reasons. Nice project for the student, though.

I agree. Not much practical use at all. It was a grinding exercise.

Before I leave my present job, though, it is my intent to grind this
one to be parallel over it's full length. Just to get it done.

Or it is well on it's way to becoming a very handsome between centres
boring bar!

A trustworthy test bar with a taper integral to it, and a clean,
undamaged taper in the lathe spindle, seems to me a better choice if one
were to buy a hardened test bar. At least one could check that the
spindle bore was reasonably parallel to the ways, provided that the the
mating surfaces were clear of blemishes, if one wished to after a crash,
or on reinstallation, during a rebuild. Unless the ones I have seen were
anomolies, they usually have centers in them that they could be used
same as a parallel test bar, too.

For the most part, I see test bars as part of the "smoke-n-mirrors"
that get waved about to "prove" how accurately one must work to be a
machinist. That has certainly been my experience, anyway.


--much snippage--


"Setting up a machine usually starts with "leveling it". i.e., getting

the basic machine aligned (the easy way) using a precision level. This
assures that the ways are not twisted".

That is the essence of a proper setup. Aligning the tailstock with the
headstock by the use of a bar serves no real purpose. What a machine does
by working from the headstock alone should be the priority. If a machine
will bore a straight hole (the result of a properly leveled lathe, assuming
the headstock is parallel with the ways), only then should a test bar be
used to align the tailstock. What purpose does a lathe serve if it will
turn a straight shaft the length of any given test bar, between centers, if
it turns tapers under any other condition?

That second step, after the level is put to use! The bit about
actually checking what the lathe does when used.
We had a lathe delivered into our shop, that was leveled oh so
carefully, which then proceeded to cut a tidy taper until the feet were
adjusted a tiny bit. The guys that were involved with the levelling,
were quite offended, when I had at the feet of the lathe with a wrench
and tweaked it until it turned parrallel. As far as they were concerned,
levelling it was the end of the set-up, rather than the beginning. The
lathe was a Chi-Wan cast, Colchester labeled machine. I have my doubts
that it was terribly straight to begin with, but it cuts a straight and
parallel parts as it sits.


--much snippage--



"If you're doing an expert job, you use a straight-tailed dog to drive the

work. If all you have is a bent-tailed dog, make sure you've polished the
tail with abrasive paper. You should do this with any bent-tailed dog,
anyway."

Does one have need to do this on a moonless night as well? I'm at a loss to
understand why a bent tail would make a difference, but I'm really tripping
over polishing the tail. Unless I've missed something, that makes no sense.
Please elaborate.


I think he is trying to remove the influences of the friction between
the driver pin and the dog itself. Could be wrong.

Since we will have taken pains to align the two centers to each other,
there should be little or no relative movement unless we were doing a
taper between ceters, no?


"Robin suggested taking two short cuts, one at each end of the bar. That's

all you need for setting the tailstock. Scratch a witness mark on the
tailstock's cross adjustment when you get it right."

That serves no real purpose. The alignment will likely be proper only at
that given location, under identical conditions. The moment the tailstock
is moved, it won't necessarily repeat the alignment to the same degree.
Further, the human eye can not discern less than .003", so even if you align
to the mark, you can't expect your parts to come out straight. If they do,
it's the luck of the draw. An offset of .002" will yield a taper of
.004", far more than most folks find acceptable.



"Yes, people do it all the time. But it's not the right way to do it. And

don't use ball-bearing centers for precision work like this."

That's correct. There is no guarantee that the center of a live center
runs true (it can be eccentric with the bearings, believe it or not), and it
can also be eccentric to the shank. Installing it randomly alters how
straight your works turns out. I own such a live center. Dead centers
are used in grinding because they don't introduce any errors.

I have machined since '57, and have never used an alignment shaft. They are
normally not the tool of the operator, but one used in making original
alignments of machine tools, most of which are fixed once the machine is
assembled. Why folks screw with the tailstock alignment as they do is a
total mystery to me. Those of us that have worked between centers
generally align to the condition at hand, for the specific project. Other
times, as long as the tailstock will start a small center drill without
breaking the tip, center isn't ultra critical, if for no other reason, it is
constantly changing because the lathe spindle isn't perfectly parallel with
the ways, nor is the tailstock quill perfectly parallel with the ways. You
simply have to live with a given amount of error. That's what sorts
wannabe's from machinists. Level the ways within prescribed limits, extend
the tailstock quill half way, with a dead center installed, and sweep the
point with an indicator from the spindle, held short to eliminate sag. The
alternative is to sweep the quill internal taper. That is a good average
for every day work. Make further adjustments on an individual basis, as
required, with the actual work piece. Get on with life.

Harold



Thanks for the write-up Harold!


Cheers
Trevor Jones

"Anthony" wrote in message
...
"Harold and Susan Vordos" wrote in
. net:

That's correct. There is no guarantee that the center of a live
center runs true (it can be eccentric with the bearings, believe it or
not), and it can also be eccentric to the shank. Installing it
randomly alters how straight your works turns out. I own such a live
center. Dead centers are used in grinding because they don't
introduce any errors.


Harold,
That depends on the type of live center you are using. Ours are
concentric to the taper to less than 5 microns. But these aren't $150
live centers either.

Chuckle!

Yeah, I should have made mention that my comments apply to the typical home
shop equipment. There's always the exception to the rule. Yours, with
specs that are staggering, appear to be one of them. Sounds like wonderful
gear, although likely beyond the need for the home shop type.

One other thing everyone has failed to take into account is quill bore
alignment to the body. You really should check tailstock alignment at two
considerably different quill extension lengths. As you stated, having the
tailstock body aligned at a particular quill setting provides you with
absolutely zero indication of how the tailstock is truely aligned.
We align tailstocks to less than 5 microns TIR at two separate points
over ~10" of quill travel. This proves that the quill is concentric to
the spindle centerline over it's entire travel. All bets are off however,
if you move the tailstock body.

Sort of confirms my comments about moving the tailstock.

Thanks, Anthony.

Harold

On Dec 18, 8:46 am, Trevor Jones wrote:
...
A trustworthy test bar with a taper integral to it, and a clean,
undamaged taper in the lathe spindle, seems to me a better choice if one
were to buy a hardened test bar.
...
Trevor Jones

One might consider buying a cylinder square instead:

http://67.59.156.7/merchant2/merchan...uct_Code=18056

They can be home-made on an imperfect lathe the same way a test bar
can and they are useful on a surface plate whenever you aren't
realigning your tailstock.

"Trevor Jones" wrote in message
news:AaQ9j.23209$UZ4.1996@edtnps89...
Harold and Susan Vordos wrote:
"Trevor Jones" wrote in message
news:0ME9j.16422$UZ4.14560@edtnps89...
snip---


Nor of much practical use. It would not serve well for aligning a
tailstock, and for several reasons. Nice project for the student,
though.

I agree. Not much practical use at all. It was a grinding exercise.

Before I leave my present job, though, it is my intent to grind this one
to be parallel over it's full length. Just to get it done.

I can't think of one process in the shop that is more rewarding than to
accomplish fine work and have it turn out "perfect". Grinding is such
work, when in the hands of a skilled craftsman.

I fully understand your desire to correct the minor flaws. To ease your
conscience, unless you ground the shaft on a precision cylindrical grinder,
the results you achieved are in keeping with what I'd expect. I was
cornered on one job I bid when I didn't have a cylindrical grinder at my
disposal, although the shop from which I subcontracted had offered me their
universal grinder, which became the basis of quoting the job.

It was a tool & cutter type, not a cylindrical, although cylindrical work
could be performed. The parts in question were components for a
helicopter, with stringent tolerances. On a cylindrical grinder, they
would have been routine. On the cutter grinder, I fought taper and
diameter from beginning to end. Mind you, I am no stranger to precision
grinding. Using the wrong machine for such a task lowers you to the
level of using a toolpost grinder on a lathe, which I do not endorse.

For the most part, I see test bars as part of the "smoke-n-mirrors" that
get waved about to "prove" how accurately one must work to be a machinist.
That has certainly been my experience, anyway.

My observations, since visiting sites such as this, are that those that many
that don't understand the real issues of machining are caught up with
numbers, and are endlessly trying to prove that their less than adequate
machine is capable of performing at a level of serious industrial equipment.
It is a tall order, for the precision isn't there, not even when the machine
is new. That's not to say that the machine isn't worthy of being owned,
just that expectations should be lowered, and energy devoted to mastering
the concepts of machining. A fine machine tool in the hands of a fool is
just that. He will still be a fool. With talent and skill, good work can
come from lesser equipment, even with reliability. The smoke-n-mirrors
may fool those that are less informed, but it will rarely dazzle anyone with
experience.

To be a machinist is more than making chips. If a person can't start with
a print, choose the right machine, make the setup, produce the necessary
parts, reliably, to print, in reasonable time, without supervision, they
should not claim the title "machinist". No more than a hack that burns
rod should claim the title of weldor, or the common man that bandages a cut
finger should claim the title doctor.

Harold

"Nick Mueller" wrote in message
...
Robin S. wrote:

The process requires accurate LINEAR
measurement NOT comparison measurement.

Any dial indicator can only compare.


So you're suggesting that a long travel indicator isn't capable of
measurements?

Harold

Harold and Susan Vordos wrote:

So you're suggesting that a long travel indicator isn't capable of
measurements?

G Every measurement is done by comparison!
But I don't know what a long travel indicator can measure better than one
with a short travel. Except ... longer travel.


Nick
--
The lowcost-DRO:
http://www.yadro.de

"Nick Mueller" wrote in message
...
Harold and Susan Vordos wrote:

So you're suggesting that a long travel indicator isn't capable of
measurements?

G Every measurement is done by comparison!
But I don't know what a long travel indicator can measure better than one
with a short travel. Except ... longer travel.


You've missed the point. Long travel indicators are properly calibrated
for measurement--and are generally used in that capacity. Your statement
"Any dial indicator can only compare" isn't true.
I concede, you can always conclude that they are comparing against the
calibration standard------but that's not the point---which is, a long travel
indicator is able to measure. A DTI is distinctly different from a long
travel indicator in that it is specifically made for comparison readings,
due in part to the adjustable tip, which includes parallax error, a function
of the angle of the tip. None of that is an issue with a long travel
indicator, assuming it is set up in the proper attitude in relationship to
the work in question.

Harold

On 2007-12-18, Ed Huntress wrote:

"DoN. Nichols" wrote in message
...
On 2007-12-17, wrote:

[ ... ]

Given the typical 3-jaw chuck, the center holes which you
drilled will *not* be in the true centers of the end of the stock.

Either put them in a collet (and verify that the collet is a
good one by indicating on the 10mm or so which are projecting out the
end of the collet), or chuck up close in a 4-jaw and take the time to
center that with a tenths-reading dial gauge.

I'm going to disagree with that, Don. I wouldn't use a collet or a chuck to
hold the work. You can get much more accurate results than that by setting
the job up between centers. That's the standard way of doing this test and
it has been for over 100 years.

I was trying to tell him how to get the center holes drilled
very close to on center in what he had, doing the best that he can do --
on the assumption that he was *not* going to be turning the OD. Your
advice below is how to start from scratch to make the bar from mild
steel (which he does not say that he has) to do it right.

I then went on explaining how he could make the whole thing more
accurate by grinding between centers -- with the assumption that he did
not have the equipment to do this at home, and perhaps not even at the
school shop.

Robin started explaining it, and this probably is where Howard was going:
First, you don't use hardened steel to make lathe test bars. Use something
soft; leaded steel, if you have it.

[ ... ]

The test bar can have center holes drilled in a drill press or whatever.
They don't have to be particularly accurate but do the best you can to save
time machining. You aren't going to use a ball-bearing tailstock center for
this so there's no reason to use a boring tool to cut the centers. Even a
messy hole will run true on a plain dead center unless it's really bad.

Yes -- if you are going to turn the OD afterwards, the center
holes don't have to be that accurate. But if you want to make do with
the already ground drill rod, they do have to be accurate. And I also
warned that the roundness of the ground drill rod was likely to be
questionable if it was centerless ground.

If Andrew isn't familiar with these between-centers setups, let me point out
that this is the only good way to do most shaft work. Chucking a bar as
thick as this test bar, even putting it in a collet should you have one that
big, is a very bad idea if you're going to run the other end on a center.

I was having him mount it in a collet up close to the end which
he was center-drilling -- with the rest within the spindle -- assuming
that he was not going to be turning the OD. In other words, I was
trying to help him do precisely what he was trying to do more
accurately.

Feel free to tell him how to do it differently from the start
(which you did quite well).

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"Harold and Susan Vordos" wrote in
. net:

Chuckle!

Yeah, I should have made mention that my comments apply to the typical
home shop equipment. There's always the exception to the rule.
Yours, with specs that are staggering, appear to be one of them.
Sounds like wonderful gear, although likely beyond the need for the
home shop type.

Well, I have to hold +/- 4 microns on diameters on thousands of parts a
day, you can't do that with 'normal' equipment.



Sort of confirms my comments about moving the tailstock.


In our case, moving the tailstock really won't affect it all that much,
as the whole machine is one big precision instrument, but it is also a
rare occurance that you have to move it. Just reset your sensors/switches
for a different length.


--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

"DoN. Nichols" wrote in message
...

snip


I was having him mount it in a collet up close to the end which
he was center-drilling -- with the rest within the spindle -- assuming
that he was not going to be turning the OD. In other words, I was
trying to help him do precisely what he was trying to do more
accurately.

Feel free to tell him how to do it differently from the start
(which you did quite well).

I see he's getting so many different instructions that we probably have him
thoroughly confused by now. I'll have to leave it to him to sort them out.
d8-)

--
Ed Huntress

"Anthony" wrote in message
...
"Harold and Susan Vordos" wrote in
. net:

Chuckle!

Yeah, I should have made mention that my comments apply to the typical
home shop equipment. There's always the exception to the rule.
Yours, with specs that are staggering, appear to be one of them.
Sounds like wonderful gear, although likely beyond the need for the
home shop type.

Well, I have to hold +/- 4 microns on diameters on thousands of parts a
day, you can't do that with 'normal' equipment.

Interesting!

Large parts? Much easier accomplished in the miniature.

Harold

"Harold and Susan Vordos" wrote in news:NF5aj.41023
:


Interesting!

Large parts?

3" to 8" diameter parts, 2.5" to about 12" in length. Both aluminum and
steel.


Much easier accomplished in the miniature.


Nothing is easy when you are trying to hold that tight of a tolerance,
especially when you are making that many. *Everything* about the operation
matters. The list of variables that have an effect on the finished quality
magnifies expotentially.

--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

Harold and Susan Vordos wrote:

which is, a long travel
indicator is able to measure. Â* Â*A DTI is distinctly different from a long
travel indicator in that it is specifically made for comparison readings,
due in part to the adjustable tip, which includes parallax error, a
function of the angle of the tip.

Math often helps: :-)
Length of feeler: 19mm
Travel: 0.2mm
Resolution: 0.002mm

Swing in degrees: 0.6°
cos(0.6°) = 0.999945
The error is in the 5th place, two magnitudes less than the resolution

An other one:
Length: 19mm
travel: 1mm
resol.: 0,01mm

- swing 3° - error 0.9986, almost two magnitudes less than the resolution

None of that is an issue with a long
travel indicator, assuming it is set up in the proper attitude in
relationship to the work in question.

Certainly, for long travels you are absolutely right. But that's the reason
why the other type has only such short travels.


Nick
--
The lowcost-DRO:
http://www.yadro.de

Ed Huntress wrote:

I see he's getting so many different instructions that we probably have
him thoroughly confused by now. I'll have to leave it to him to sort them
out. d8-)

So maybe he needs the book "Machine Tool Reconditioning".
A friend lent it to me two days ago. I just wanted to have a look at it.
Now I need to buy it. But I knew that before. :-))


Nick
--
The lowcost-DRO:
http://www.yadro.de