Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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"Michael Koblic" wrote in message
...

1) What is the expected stiffness of a drill press spindle? Is bending arc
of 0.014" to be expected? What would you expect in a mill? Or a lathe?
2) What are the expected tolerances of drill bits? Is 0.010" out of round
usual?
3) What are the expected tolerances of Jacobs 5/8" chucks? Is 0.010-0.015"
reasonable? I understand that 3-jaw chucks on lathes are not as good at
centering as manually adjusted 4-jaw chucks. Is this about the order of
things?
4) Given that the runout was of the same order throughout the testing
would you say that it is predominantly due to the chuck rather than the
drill bits being out of round?


This seems to be a part of the answer:

http://www.jacobschuck.com/images/pr...%20(ENG)-1.pdf

The question is if the runout would be cured by buying a "name" chuck to
replace the existing People's Liberation Army one...At a cost which almost
equals the drill press as a whole.

BTW has anyone tried the hitting-it-with-the hammer adjustment (as
recommended on a couple of forums)?


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On Sep 30, 12:42*am, "Michael Koblic" wrote:
"Michael Koblic" wrote in message

...

1) What is the expected stiffness of a drill press spindle? Is bending arc
of 0.014" to be expected? What would you expect in a mill? Or a lathe?
2) What are the expected tolerances of drill bits? Is 0.010" out of round
usual?
3) What are the expected tolerances of Jacobs 5/8" chucks? Is 0.010-0.015"
reasonable? I understand that 3-jaw chucks on lathes are not as good at
centering as manually adjusted 4-jaw chucks. Is this about the order of
things?
4) Given that the runout was of the same order throughout the testing
would you say that it is predominantly due to the chuck rather than the
drill bits being out of round?


This seems to be a part of the answer:

http://www.jacobschuck.com/images/pr...20Updated%20(E...

The question is if the runout would be cured by buying a "name" chuck to
replace the existing People's Liberation Army one...At a cost which almost
equals the drill press as a whole.

BTW has anyone tried the hitting-it-with-the hammer adjustment (as
recommended on a couple of forums)?


Drill bits are probably good to a thousandth. Dowel pins are cheap and
very accurate.

The better Jacobs chucks are worth it on a milling machine, probably
not on a cheap drill press. Twist drills are flexible anyway and
center themselves on the punch mark, so runout in the chuck doesn't
matter all that much.

The one Enco keyless chuck I have is pretty good, too, although it's
on the tailstock of my lathe where I can't quickly measure the runout.
For small holes they are handier than keyed chucks, for larger ones
they have a problem with slipping.

A good chuck isn't really wasted on the drill press, you can move it
to a better machine later.
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On 2008-09-30, Jim Wilkins wrote:

Drill bits are probably good to a thousandth. Dowel pins are cheap and
very accurate.


Agreed.

The better Jacobs chucks are worth it on a milling machine, probably
not on a cheap drill press. Twist drills are flexible anyway and
center themselves on the punch mark, so runout in the chuck doesn't
matter all that much.


Well ... that depends. Yes, a typical half-inch Jacobs chuck
can't *directly* hold a drill bit much below 1/16th of an inch, and
0.007" runout is probably tolerable with a 1/16" drill bit. However, if
you are using one of the sensitive drill adaptors, and a #70 or #80
drill bit, your runout is going to be on the order of the drill
diameter, and that is likely to break the drill bit once it gets deep
enough into the workpiece. Even more so if you are using solid carbide
bits for drilling printed circuit boards. They are very brittle when
exposed to side loads.

The one Enco keyless chuck I have is pretty good, too, although it's
on the tailstock of my lathe where I can't quickly measure the runout.
For small holes they are handier than keyed chucks, for larger ones
they have a problem with slipping.


Hmm ... a real Albrecht is good at self tightening, so it tends
not so slip.

A good chuck isn't really wasted on the drill press, you can move it
to a better machine later.


Agreed -- and if the drill press and the lathe tailstock have
the same Morse taper (or the lathe tailstock is larger so you can use
an adaptor sleeve), you can move the same chuck back and forth at need.
(Granted, it is nicer to have them each have its own good chuck. My
lathe currently has a Polish clone of the Albrecht in the 5/8" size, my
drill press has a Jacobs (close of the Albrecht, though looks a little
different), and my little Compact-5/CNC lathe has three chucks -- a 1/8"
Albrecht, a 1/4" Albrecht, and a 3/8" Rohm, which is a good clone of the
Albrecht.

A worn Albrecht with a large bit can self-tighten to the point
where you need a strap wrench to loosen it.

Enjoy,
DoN.

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"Jim Wilkins" wrote in message
...

"Drill bits are probably good to a thousandth. Dowel pins are cheap and
very accurate.

The better Jacobs chucks are worth it on a milling machine, probably
not on a cheap drill press. Twist drills are flexible anyway and
center themselves on the punch mark, so runout in the chuck doesn't
matter all that much.

The one Enco keyless chuck I have is pretty good, too, although it's
on the tailstock of my lathe where I can't quickly measure the runout.
For small holes they are handier than keyed chucks, for larger ones
they have a problem with slipping.

A good chuck isn't really wasted on the drill press, you can move it
to a better machine later."

***I was thinking along the same lines. I have actually found a real Jacobs
chuck in my possession. I think it is this one:

http://www.jacobschuck.com/images/pr...No%20Crops.pdf

Cost less than $20, but the specs are about the same I am getting now. And
it is, of course, smaller. It was just an interesting exercise.

BTW what TIR would you expect on a, say, 3-inch 3-jaw chuck in a lathe? What
sort of improvement do you expect going to 4-jaw chuck? When you are
mounting a chuck, are most of the mounts self-centering (The Advanced
Machine Work says they just screw on to the spindle but that is surely not
the case in every instance)? Can you center the chuck by using the indicator
on the periphery of the chuck or do you need to chuck something demonstrably
round and center on that?

--
Michael Koblic,
Campbell River, BC


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"Jim Wilkins" wrote in message
...
On Sep 30, 12:42 am, "Michael Koblic" wrote:
"Michael Koblic" wrote in message

...

1) What is the expected stiffness of a drill press spindle? Is bending
arc
of 0.014" to be expected? What would you expect in a mill? Or a lathe?
2) What are the expected tolerances of drill bits? Is 0.010" out of
round
usual?
3) What are the expected tolerances of Jacobs 5/8" chucks? Is
0.010-0.015"
reasonable? I understand that 3-jaw chucks on lathes are not as good at
centering as manually adjusted 4-jaw chucks. Is this about the order of
things?
4) Given that the runout was of the same order throughout the testing
would you say that it is predominantly due to the chuck rather than the
drill bits being out of round?


This seems to be a part of the answer:

http://www.jacobschuck.com/images/pr...20Updated%20(E...

The question is if the runout would be cured by buying a "name" chuck to
replace the existing People's Liberation Army one...At a cost which almost
equals the drill press as a whole.

BTW has anyone tried the hitting-it-with-the hammer adjustment (as
recommended on a couple of forums)?


Drill bits are probably good to a thousandth. Dowel pins are cheap and
very accurate.


Don't count on drill bits as gages, Jim. They're tapered, as much as 0.003"
toward the butt end. That's true of even the highest quality ones.

Put a mike on a few bits to see.

--
Ed Huntress




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On 2008-09-30, Michael Koblic wrote:

[ ... ]

***I was thinking along the same lines. I have actually found a real Jacobs
chuck in my possession. I think it is this one:

http://www.jacobschuck.com/images/pr...No%20Crops.pdf

Cost less than $20, but the specs are about the same I am getting now. And
it is, of course, smaller. It was just an interesting exercise.


I note that those are chucks with threaded mounts, not Jacobs
Taper mounts. I don't expect as good a runout on the threaded back
chucks.

BTW what TIR would you expect on a, say, 3-inch 3-jaw chuck in a lathe?


That depends on who made the chuck, how old it is, and how it
has been cared for.

For the 3-jaw for my Emco-Maier Compact-5/CNC (which I bought
new) I would expect less than 0.001" runout at the present stage in its
life -- assuming the jaws were clean before clamping on the workpiece.
This was made in Austria, and mounts to a flat spindle nose with through
bolts clamping it to the spindle. The mounting surface is a surface
ground plate inside the back of the chuck.

For a cheap Chinese one, still new, perhaps 0.003" (more for a
larger chuck).

What
sort of improvement do you expect going to 4-jaw chuck?


What kind of patience do you have? How much resolution on the
indicator being used to center the workpiece? How good a finish on the
workpiece? Assuming a ground finish on a dowel pin, and a tenths
reading indicator I would expect perhaps to be able to tune to 0.0001"
-- if I had the patience at that particular time.

With a rougher finish on the workpiece, it would get in the way
of measuring, and result in poorer concentricity.

When you are
mounting a chuck, are most of the mounts self-centering (The Advanced
Machine Work says they just screw on to the spindle but that is surely not
the case in every instance)?


The lathes which I have which currently have a threaded spindle
nose are the Unimat SL-1000, the Taig, and the Atlas-Clausing 6x18"
which I no longer use.

I've described the Emco-Maier above -- with the exception of
forgetting to mention that the OD of the spindle nose is cylindrical,
and slides into a pocket on the back of the chuck. This gives quite
repeatable concentricity.

The 12x24" Clausing came with a 2-1/4x8 threaded spindle nose,
but I fairly soon replaced it with a L-00 (long taper) spindle nose
which is quite self centering.

Can you center the chuck by using the indicator
on the periphery of the chuck


You can't depend on the chuck's body to be concentric with its
grip. On a good one, it will be fairly close, but what really matters
is the faces of the jaws, not the OD of the chuck. There is not even a
certainty that the register turning inside the back of the chuck is
truly concentric with the OD of the chuck -- but it *should* be
concentric with the grip of the jaws.

or do you need to chuck something demonstrably
round and center on that?


Yes -- and if you are checking the chuck, you need to check
multiple diameters of workpiece, because the centering of the jaws will
be controlled by imperfections in the thread of the scroll plate.

In a chuck with three sockets for the chuck key, one of them
*should* be (but is not always) be marked with a stamped "0" or
something similar to indicate the one which gives the best centering.
(Yes, it does make a difference which key socket you use to tighten it.
Those others are convenient for releasing the workpiece when it is
finished, but should not be used for the final stages of tightening.

Note that there are chucks available with a feature called
"set-tru" by one manufacturer, and lots of other (trademarked) names by
others. The chuck body is mounted to the backplate by screws which are
accessible from the front, and there are four radial screws near the
back which bear on a projection from the backplate. These screws can be
used like a 4-jaw chuck to adjust so a single diameter of workpiece runs
true in the jaws. Other diameters are a different thing, and if you
care about them you'll have to re-adjust at the different diameter.

Also -- you can use two-piece jaws in your larger chucks, and
bolt soft jaws onto the master jaws and turn them to perfectly fit your
workpiece for precise centering of that one diameter -- and while about
it to provide a step to set how deep into the chuck the workpiece sits
for predictable facing to length. (Actually, the little Taig comes with
two-piece jaws, with the soft jaws being made of aluminum.) I have soft
jaws for many (but not all) of my lathe chucks.

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 ---
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"DoN. Nichols" wrote in message
...
What sort of improvement do you expect going to 4-jaw chuck?

What kind of patience do you have? How much resolution on the
indicator being used to center the workpiece? How good a finish on the
workpiece? Assuming a ground finish on a dowel pin, and a tenths
reading indicator I would expect perhaps to be able to tune to 0.0001"
-- if I had the patience at that particular time.

With a rougher finish on the workpiece, it would get in the way
of measuring, and result in poorer concentricity.

When you are
mounting a chuck, are most of the mounts self-centering (The Advanced
Machine Work says they just screw on to the spindle but that is surely
not
the case in every instance)?


The lathes which I have which currently have a threaded spindle
nose are the Unimat SL-1000, the Taig, and the Atlas-Clausing 6x18"
which I no longer use.

I've described the Emco-Maier above -- with the exception of
forgetting to mention that the OD of the spindle nose is cylindrical,
and slides into a pocket on the back of the chuck. This gives quite
repeatable concentricity.

The 12x24" Clausing came with a 2-1/4x8 threaded spindle nose,
but I fairly soon replaced it with a L-00 (long taper) spindle nose
which is quite self centering.

Can you center the chuck by using the
indicator
on the periphery of the chuck


You can't depend on the chuck's body to be concentric with its
grip. On a good one, it will be fairly close, but what really matters
is the faces of the jaws, not the OD of the chuck. There is not even a
certainty that the register turning inside the back of the chuck is
truly concentric with the OD of the chuck -- but it *should* be
concentric with the grip of the jaws.

or do you need to chuck something
demonstrably
round and center on that?


Yes -- and if you are checking the chuck, you need to check
multiple diameters of workpiece, because the centering of the jaws will
be controlled by imperfections in the thread of the scroll plate.

In a chuck with three sockets for the chuck key, one of them
*should* be (but is not always) be marked with a stamped "0" or
something similar to indicate the one which gives the best centering.
(Yes, it does make a difference which key socket you use to tighten it.
Those others are convenient for releasing the workpiece when it is
finished, but should not be used for the final stages of tightening.

Note that there are chucks available with a feature called
"set-tru" by one manufacturer, and lots of other (trademarked) names by
others. The chuck body is mounted to the backplate by screws which are
accessible from the front, and there are four radial screws near the
back which bear on a projection from the backplate. These screws can be
used like a 4-jaw chuck to adjust so a single diameter of workpiece runs
true in the jaws. Other diameters are a different thing, and if you
care about them you'll have to re-adjust at the different diameter.

Also -- you can use two-piece jaws in your larger chucks, and
bolt soft jaws onto the master jaws and turn them to perfectly fit your
workpiece for precise centering of that one diameter -- and while about
it to provide a step to set how deep into the chuck the workpiece sits
for predictable facing to length. (Actually, the little Taig comes with
two-piece jaws, with the soft jaws being made of aluminum.) I have soft
jaws for many (but not all) of my lathe chucks.


Thank you. I just knew it was going to be simple...:-)

So the bottom line is you need a set of perfect cylinders of varying
diameters to get your chuck right on. This made perfect common sense to me
but oddly enough I could not find any reference to this in any literature I
perused. Are such sets being sold for this very purpose?

--
Michael Koblic,
Campbell River, BC


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On Oct 1, 1:28 am, "Michael Koblic" wrote:
"DoN. Nichols" wrote in message
What sort of improvement do you expect going to 4-jaw chuck?
...I would expect perhaps to be able to tune to 0.0001"
When you are
mounting a chuck, are most of the mounts self-centering ?
Can you center the chuck by using the indicator
on the periphery of the chuck?

You can't depend on the chuck's body to be concentric with its
grip. ...
or do you need to chuck something demonstrably
round and center on that?...

Note that there are chucks available with a feature called
"set-tru" by one manufacturer,...
Also -- you can use two-piece jaws in your larger chucks, and
bolt soft jaws onto the master jaws and turn them to perfectly fit your
workpiece


Thank you. I just knew it was going to be simple...:-)

So the bottom line is you need a set of perfect cylinders of varying
diameters to get your chuck right on. This made perfect common sense to me
but oddly enough I could not find any reference to this in any literature I
perused. Are such sets being sold for this very purpose?
Michael Koblic,


You are asking the wrong questions. The blank doesn't have to be
accurately centered in the chuck. As long as it is large enough to
clean up all the way around the cutting tool will generate a true
concentric cylinder. When you plan the job, assume that you will lose
some concentricity if you remove the work from the chuck no matter
what type of chuck the lathe has, even collets. If you need to remove
and replace it, turn it between centers.

Otherwise turn all surfaces that -must- be concentric at one chucking.
Plan how you will chuck it when reversed to turn the other end, and if
necessary leave extra material with enough room to hold it in a 4-jaw
and indicate it true enough.

You can rough the part close to size all over in a 3-jaw chuck, hone
the tool and finish the non-critical surfaces, then change to another
more accurate setup and dial it in carefully to finish the critical
dimensions without unclamping it.

Here's a good example. I just made a 17mm mandrel with center holes to
fit the 6203Z bearings in the HF 1 Ton I-beam trolley so I could turn
down the rollers to fit into 3 [ 5 channel iron.
http://www.harborfreight.com/cpi/cta...emnumber=97392

I chucked the rollers by the outer rim in a 3-jaw to rough them close
to size, then put them on the mandrel between centers and drove them
with a hose clamp to finish the rolling surfaces true with the bores
of the bearings. They were out about 0.020" - 0.035" from the first
chucking, mostly because the outer rims weren't machined true with the
bearing recess. I suspect the castings were machined on the two sides
in two different operations.

The 3-jaw is a new Bison Set-Tru good within about 0.001". I don't
know how they hold up in production but for hobby use it's a very good
chuck.

The roughing setup was solid and I could cut 0.025" deep without
chatter from that particular round-nosed HSS bit. The finish setup was
rather fragile, driven by a long 1/4" bolt pushing the hose clamp
screw, and only allowed 0.005" per pass. This is typical of ad hoc
faceplate driving schemes, so plan ahead.

When I spin the rollers only the rim that was about 0.035" off wobbles
visibly. The ~ 0.020" ones look OK. That should give you an idea of
how much accuracy you really need for art work.

As for the perfect setting cylinders, just use the part itself. Turn
the end slightly oversize with a good finish, reverse it and adjust it
to run true. Once you have a lathe the paradigm shifts from buying to
making the parts you need.

Jim Wilkins
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On Sep 30, 7:54*pm, "Ed Huntress" wrote:
"Jim Wilkins" wrote in message

Don't count on drill bits as gages, Jim. They're tapered, as much as 0.003"
toward the butt end. That's true of even the highest quality ones.

Put a mike on a few bits to see.
Ed Huntress


I should have written -round- to a thousandth, for indicating runout.
The dowel pins are -accurate- to perhaps 0.0002".
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On 2008-10-01, Michael Koblic wrote:

[ ... ]

Thank you. I just knew it was going to be simple...:-)

So the bottom line is you need a set of perfect cylinders of varying
diameters to get your chuck right on.


Not exactly -- you need them to see how much the off-center
behavior *varies* with diameter. You aren't going to find a 3-jaw (or a
universal 4-jaw or 6-jaw) chuck which is truly on center at all
diameters -- though if it has the adjust-tru feature, you can make it
very close at any given diameter, while it is off a bit more at other
diameters until you re-adjust it. Think of zeroing an adjust-tru for a
given diameter as similar to zeroing a 4-jaw chuck. That can be pretty
good for more workpieces of the same diameter, but worse than useless
for other diameters until you re-adjust it.

This made perfect common sense to me
but oddly enough I could not find any reference to this in any literature I
perused.


That is because it is not a permanent fix -- just a way of
checking how bad things are to decide whether it is time to purchase a
new chuck.

Note that if you check at a number of diameters (perhaps those
which cover a range representing one full turn of the scroll plate in
the back of the chuck) and find out that you have close to the same
runout through the range of testing, and much more runout than the range
which you measure from min to max, you can perhaps improve things by
grinding the tips of the hard jaws while the chuck is closed on some
spacers between the faces of the jaws) using a toolpost grinder. (Be
sure to protect the bed and ways from the grinding grit while you do
this.) This won't be perfect, but it will deal with worn jaw tips and
make things significantly better.

Are such sets being sold for this very purpose?


Not as such -- for the reasons suggested above. But if you want
a set which could be used for such testing -- consider buying an index
of hardened and ground drill blanks. (They are also called "reamer
blanks". Here is an example at MSC's web site.

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=2044515&PMT4NO=51073658

It is on page 64 of their current "Big Book" catalog, if you have that.
Be warned that they are expensive.

O.K. I've found a set:

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1680286&PMT4NO=51074150

for only $29.52. That is metric sizes, nine of them from 1mm to 6mm by
0.5mm steps. That makes the biggest just a little under 1/4" which
might be too small to cover the full range of a scroll plate rotation.
Pull a jaw, and measure from the start of one tooth to the start of the
next tooth. This is the range of sizes which you will need. You'll
probably need a fractional inch set, which is noticeably more expensive.

Try: http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1676488&PMT4NO=51074150

$135.49 import quality MSC part number 01105295

1/16" to 1/2" by 1/64" steps.

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.

Enjoy,
DoN.

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


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"DoN. Nichols" wrote in message
...
On 2008-10-01, Michael Koblic wrote:


big snip

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.



Thank you Don and Jim.

In my case it is more about understanding the concepts than about immediate
practical application. I am getting a better grasp.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.

--
Michael Koblic,
Campbell River, BC


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"Michael Koblic" wrote in message
...

"DoN. Nichols" wrote in message
...
On 2008-10-01, Michael Koblic wrote:


big snip

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.



Thank you Don and Jim.

In my case it is more about understanding the concepts than about
immediate practical application. I am getting a better grasp.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was
almost inevitable for the grinding dust to get inside the chuck and
causing mischief there. I am talking about grinding things clamped in the
chuck rather than doing the chuck itself as Don suggested.


Aluminum foil, newspaper and masking tape, vaseline and motor oil are your
friends if you grind in a lathe. The grit can get anywhere, including the
chuck.

I make temporary vacuum devices out of gallon milk bottles cut with a
pocketknife to fit over the wheel and the work as much as possible. Then I
tape the opening to a vacuum cleaner hose. It helps a lot. But I still use
aluminum foil, vaseline, and newspaper soaked in motor oil.

--
Ed Huntress


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On 2008-10-01, Michael Koblic wrote:

"DoN. Nichols" wrote in message
...
On 2008-10-01, Michael Koblic wrote:


big snip


Good! :-)

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.



Thank you Don and Jim.

In my case it is more about understanding the concepts than about immediate
practical application. I am getting a better grasp.


O.K.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.


Yes -- the grit can damage the chuck too. If you can, wrap
newspaper (or plastic sheeting) around it and tape to the workpiece and
the chuck body behind the jaws (and ideally covering the sockets for the
chuck key as well. Better, if you can, is to turn between centers for
the grinding, as it allows greater precision of removing and replacing
the workpiece. Since you would normally be grinding something which has
been hardened after turning, you will need to remove and replace the
workpiece. (And turn it a little oversized, as it can grow slightly in
the hardening -- maybe 0.001" per inch of diameter.)

If you have been truing the hardened (not soft) chuck jaws by
grinding, plan to strip the chuck down completely, wash it out with
kerosene or some similar solvent, blow out with compressed air (if you
have that available), and then relube as you reassemble. If you leave
the grit in there, it will both grind the teeth (interrupted threads) in
the jaws, the threads in the scroll plate, and the fit of the jaws to
the body, resulting in looser jaws which will introduce greater error
when chucking.

Probably the more significant reason to grind true the chuck
jaws is for jaws which have developed "bellmouth" -- that is they grip
firmly at the back, but are worn near the front allowing the workpiece
to tilt in the chuck under turning forces. This is typically only found
in very well used chucks.

As has already been mentioned, usually a little runout is no
problem, because you are turning off the skin of the original stock
anyway so everything turned in a single setup should be concentric. If
you expect to reverse the workpiece in the chuck after this stage,
perhaps you want soft jaws turned to precisely receive the reversed
piece, which should give you better concentricity.

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 ---
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On Oct 1, 7:29*pm, "Michael Koblic" wrote:
"DoN. Nichols" wrote in message
On 2008-10-01, Michael Koblic wrote:

big snip

But you probably don't really need to do this. *Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.


Thank you Don and Jim.

In my case it is more about understanding the concepts than about immediate
practical application. I am getting a better grasp.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. *I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.
Michael Koblic,


Opinions vary on lathe grinding. The dealer I bought mine from warned
never to grind on it because it has hardened ways. I haven't need to
anyway because I bought new chucks for it and have another small lathe
that is easily disassembled for thorough cleaning, plus a surface
grinder with a tool grinding swivel table. The small lathe and the
surface grinder each cost less than a new chuck.

Ground shafting is easy to find cheap or surplus if you can adapt to
whatever diameter you find. Old dot-matrix printers have some and you
can salvage it from hydraulic cylinders or car shock absorbers. In my
experience hydraulic cylinder rods are good steel that turns and
threads well once the chrome has been removed with a carbide bit.

If you need to improve on a lathe-turned finish you can lap it with
fine loose abrasive like Clover compound in a home-made cast lead lap.
This doesn't spread abrasive grit all over like dry grinding with a
high speed wheel.

I've been able to finish bearing surfaces to a few ten-thousandths and
a near mirror polish with fine single-cut files and black SiC
sandpaper, using the tool marks as a guide to file evenly.

Jim Wilkins
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"Jim Wilkins" wrote in message
...
On Oct 1, 7:29 pm, "Michael Koblic" wrote:
"DoN. Nichols" wrote in message
On 2008-10-01, Michael Koblic wrote:

big snip

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.


Thank you Don and Jim.

In my case it is more about understanding the concepts than about
immediate
practical application. I am getting a better grasp.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was
almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.
Michael Koblic,


Opinions vary on lathe grinding. The dealer I bought mine from warned
never to grind on it because it has hardened ways. I haven't need to
anyway because I bought new chucks for it and have another small lathe
that is easily disassembled for thorough cleaning, plus a surface
grinder with a tool grinding swivel table. The small lathe and the
surface grinder each cost less than a new chuck.


Hard or soft, grinding grit is hell on the ways of a lathe, and anything
else it gets into. Old timers avoided grinding on a good lathe if at all
possible, because even the best precautions sometimes let you down. That
grit wants to get everywhere.

If you need to improve on a lathe-turned finish you can lap it with
fine loose abrasive like Clover compound in a home-made cast lead lap.
This doesn't spread abrasive grit all over like dry grinding with a
high speed wheel.


Hand lapping is becoming a lost art. There are lots of lapping tools and
tips available in the really old books and the MAP booklets from the UK.
They really ought to be re-promoted for hobby machinists, because lapping is
easy and versatile, and it can save a lathe if the alternative is grinding.

However, it's important to keep in mind what lapping will and will not do.
It can produce better roundness than grinding. But it does not improve
concentricity. In other words, a lapped shaft may measure excellent
roundness but the concentricity at one spot may not agree with that 1/2 inch
further along the shaft. Another way to look at it is that the center of the
lapped shaft at any point may not be in line with the center at another
point. Lapping tends to follow the mean center of the shaft, at any given
point. If you start with a badly turned shaft you'll wind up with a badly
lapped shaft, even if you have a mirror finish and the roundness at any
point is near perfect.

In practice, this hardly matters. The differences are on the order of a few
millionths of an inch, if you use good technique throughout. But it could
matter in extreme circumstances.

--
Ed Huntress




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On Oct 2, 7:25*am, "Ed Huntress" wrote:
"Jim Wilkins" wrote in message
...
However, it's important to keep in mind what lapping will and will not do..
It can produce better roundness than grinding. But it does not improve
concentricity.
--
Ed Huntress


Filing and sanding can also degrade roundness and add a taper, but
they are quick and easy, all you need is a good "hand" or "pillar"
file. That's why I mentioned using the remaining tool marks as a
guide. You don't need to try to leave them, cut as close to size as
you can, filing shouldn't take off more than a few ten-thousandths and
sanding is barely measurable. At the hobbyist level a few remaining
fine marks don't harm a bearing surface. They may even help by
carrying oil.
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