make a lathe test bar
 

I'm needing a one inch lathe test bar. Finding ground and polished one inch
material is a piece of cake. The bar needs centers on each end. If I just
chuck the bar in a one inch collet and bore, I'll pass on the 0.0005 inch
runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl

Karl,
I'd start with a bar over desired size and put centers in both ends.
Then machine/ grind it (adjusting the tailstock to get your diameter true
end to end). Once done, you've set your tailstock true AND made your own
alignment bar for use the next time it is needed.

"Karl Townsend" remove .NOT to reply wrote
in message nk.net...
I'm needing a one inch lathe test bar. Finding ground and polished one
inch material is a piece of cake. The bar needs centers on each end. If I
just chuck the bar in a one inch collet and bore, I'll pass on the 0.0005
inch runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl

"Karl Townsend" remove .NOT to reply wrote
in message nk.net...
I'm needing a one inch lathe test bar. Finding ground and polished one
inch
material is a piece of cake. The bar needs centers on each end. If I just
chuck the bar in a one inch collet and bore, I'll pass on the 0.0005 inch
runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl




Best chance of success is to grind the test bar after the centers are
drilled. Precision work is generally accomplished in that fashion unless
there's a good reason to pursue other options.

Assuming you buy ground and polished material, be advised that it isn't
always what it's cracked up to be. It's not uncommon for long pieces of
material to be run through centerless grinders or sanders. Unless they are
dead straight, they often are not perfectly round, nor true to size. If
you're worried about a few tenths, and you should be, I really don't think
you're going to be happy with a test bar that isn't ground as I suggested.

Harold

If you find a good good true round bar and center it and
it ends up with your 0.0005 runout you could extend it out of your chuck
and use your compound to bore in the
center on the angle after you have tapped the bar true.
Jim
"Karl Townsend" remove .NOT to reply wrote
in message nk.net...
I'm needing a one inch lathe test bar. Finding ground and polished one
inch material is a piece of cake. The bar needs centers on each end. If I
just chuck the bar in a one inch collet and bore, I'll pass on the 0.0005
inch runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl

If you have a good chuck and soft jaws you could bore
it size and try that. Or you could try to bore a soft 5c collet. If you try
that bore it about .001 undersize don't go oversize or it will not be true.
Jim
"Karl Townsend" remove .NOT to reply wrote
in message nk.net...
I'm needing a one inch lathe test bar. Finding ground and polished one
inch material is a piece of cake. The bar needs centers on each end. If I
just chuck the bar in a one inch collet and bore, I'll pass on the 0.0005
inch runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl

In article et, Karl Townsend
says...

I'm needing a one inch lathe test bar. Finding ground and polished one inch
material is a piece of cake. The bar needs centers on each end. If I just
chuck the bar in a one inch collet and bore, I'll pass on the 0.0005 inch
runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

I guess my first question is, 'what are you going to be testing
with the bar?'

Is it going to have a taper in the end, for checking the spindle of
the machine? Or is it to be mounted between centers, for dialing
in the tailstock?

The trouble with manufacturing a test fixture like that, on the
machine it will be used on, is that the machine probably cannot
easily impart the degree of accuracy needed (I would like to see
nearly a whole extra decimal point - ie the test fixture should
be measurabley accurate to 0.0001 if you are using it to dial the
machine in to 0.001) and it sounds like that would be tough for
you to achieve, given the machine.

Jim


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If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but with
a correctly shaped 60 deg. cone. In other words, the point of the center
drill would drill larger by the amount of runout and then guide the cone in
correctly. I know, intuitively, this is not right - but help me out here.

Bob Swinney

jim rozen" wrote in message
...
In article et, Karl
Townsend
says...

I'm needing a one inch lathe test bar. Finding ground and polished one
inch
material is a piece of cake. The bar needs centers on each end. If I just
chuck the bar in a one inch collet and bore, I'll pass on the 0.0005 inch
runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

I guess my first question is, 'what are you going to be testing
with the bar?'

Is it going to have a taper in the end, for checking the spindle of
the machine? Or is it to be mounted between centers, for dialing
in the tailstock?

The trouble with manufacturing a test fixture like that, on the
machine it will be used on, is that the machine probably cannot
easily impart the degree of accuracy needed (I would like to see
nearly a whole extra decimal point - ie the test fixture should
be measurabley accurate to 0.0001 if you are using it to dial the
machine in to 0.001) and it sounds like that would be tough for
you to achieve, given the machine.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

In article , Robert Swinney says...

If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but with
a correctly shaped 60 deg. cone. In other words, the point of the center
drill would drill larger by the amount of runout and then guide the cone in
correctly. I know, intuitively, this is not right - but help me out here.

Bob Swinney

Uh oh. You just pushed Harold's hot button!

I'm pretty sure I know what he's gonna say here....

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"Robert Swinney" wrote in message
...
If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but
with
a correctly shaped 60 deg. cone. In other words, the point of the center
drill would drill larger by the amount of runout and then guide the cone
in
correctly. I know, intuitively, this is not right - but help me out here.


Think about something here. If you were to take and chuck a piece of stock
in a four jaw chuck and deliberately offset it by an 1/8 of an inch, then
drilled a hole in the end, how close to center do you suppose the hole would
be?

If you answered 1/8 of an inch, now consider why it is not closer to center
than that.

Once you can answer that question you should see the error of your logic.

(How did I do Harold?)

--

__
Roger Shoaf

Important factors in selecting a mate:
1] Depth of gene pool
2] Position on the food chain.

Run it in a steady rest and shave a thin chip off the 60 degree center
hole with the compound. Even if it isn't exactly centered it should
rotate around its central axis.

jw

"jim rozen" wrote in message
...
In article , Robert Swinney says...

If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but
with
a correctly shaped 60 deg. cone. In other words, the point of the center
drill would drill larger by the amount of runout and then guide the cone
in
correctly. I know, intuitively, this is not right - but help me out
here.

Bob Swinney

Uh oh. You just pushed Harold's hot button!

I'm pretty sure I know what he's gonna say here....

Jim

:-)

Yep!
When you're looking for that level of precision, the center becomes all
important. That's one of the reasons center laps were made, aside from
re-locating centers when a thou made the difference between a good part and
scrap.

What Bob suggested works, but only to a degree. The biggest problem is it's
not predictable.

What Jim said is on the money. You can't make determinations to tenths when
a test bar has more error than the one you're looking for. Standard
practice in the tool room where I was groomed was to have a tolerance of
only 10% of the product tolerance for tooling. It wasn't uncommon to find
dimensions with only a tenth tolerance. It makes sense if you want proper
results.

Harold

"Roger Shoaf" wrote in message
...

"Robert Swinney" wrote in message
...
If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but
with
a correctly shaped 60 deg. cone. In other words, the point of the
center
drill would drill larger by the amount of runout and then guide the cone
in
correctly. I know, intuitively, this is not right - but help me out
here.


Think about something here. If you were to take and chuck a piece of
stock
in a four jaw chuck and deliberately offset it by an 1/8 of an inch, then
drilled a hole in the end, how close to center do you suppose the hole
would
be?

If you answered 1/8 of an inch, now consider why it is not closer to
center
than that.

Once you can answer that question you should see the error of your logic.

(How did I do Harold?)


Frankly, quite well. Nothing magical comes from machining----you get out
what you put in----rarely achieving a silk purse from a sow's ear. In
order to work with precision results, you *must* use precision methods,
otherwise you're at the mercy of the luck of the draw. A blind squirrel
finds the occasional nut, but one with keen eyesight eats heartily.

Harold

wrote in message
oups.com...
Run it in a steady rest and shave a thin chip off the 60 degree center
hole with the compound. Even if it isn't exactly centered it should
rotate around its central axis.

jw


Rarely would that yield the desired results, although it would generally
improve the overall condition of the typical center. Grinding the part
using lapped centers assures proper results. Anything less is a crap shoot.

Harold

Harold sez;

" What Bob suggested works, but only to a degree. The biggest problem is
it's
not predictable."

Story of my life.

Bob Swinney

In article , Roger Shoaf says...

Think about something here. If you were to take and chuck a piece of stock
in a four jaw chuck and deliberately offset it by an 1/8 of an inch, then
drilled a hole in the end, how close to center do you suppose the hole would
be?

It would be almost exactly at the center!

Of the lathe spindle's axis, that is. (the only reason it would
not be is because the centerdrill would wander as it started out.

The trouble happens when the test bar is then *used* between
centers. First off it would have to be trued up so the
external part of the bar "agreed with" the centers that didn't
line up. This would mean putting it in the same lathe between
centers and taking a cut on it. The degree of tailstock offset
is still unknown so it becomes a bit of trial and error to
get the outer surface truly cylindrical. If the tailstock is
low or high that makes it even tougher.

And in the end he would wind up with a the centers on the
test bar not bearing fully on the fixture it is mounted in,
so in spite of the outer surface of the test bar being
in *principle* truly cylindrical, the end result will be
far from useable.

A thought experiment like this where you deliberately
do something far out of alignment to see 'what happens then'
is a nice way to understand how those parameters actually
affect the final accuracy of teh finished item.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

Karl Townsend wrote:
I'm needing a one inch lathe test bar. Finding ground and polished one
inch material is a piece of cake. The bar needs centers on each end.
If I just chuck the bar in a one inch collet and bore, I'll pass on
the 0.0005 inch runnout of my collet to the test bar.

How would you do this? Or do I just buy the test bar?

Karl

How long is the bar?

I've used a mandrel as a test bar before finding good cylinder squares.
I bought two cheap at auctions because none of the machinists present
knew what they were.

The mandrel is tapered but you can reverse it to indicate the same
diameter at both ends. Plus they are useful to turn wheels and pulleys.

jw