I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry

"Martin Perry" wrote in message
om...
I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry

I'd guess it was a function of how the two bars were chucked.
The external cut was on a tailstock-supported bar (yes?)
The internal cut - I'm guessing here - wasn't supported by the tailstock? ;-) The
tailstock itself could be *marginally* offset. This can be best checked by turning
something between centres (i.e., no chuck).

Any which way, your errors are trivial (IMHO) and could be attributed easily to just
the way you held your mouth when you tightened up the chuck.

Make sense?

--
Jeff R.

I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry



Sounds like workpiece deflection. At 2", the piece may not be bending, but it
may be moving in the chuck jaws.

John Martin

In article , Martin Perry
says...

I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

The errors you see are in the correct direction for workpiece
spring. For the OD cut, the part moves away from the tool so it
will have the largest diameter far away from the headstock. Here
you can imagine the part moving away from the tool, springing
towards the back of the machine.

For the ID cut, the part would be springing towards the front
of the machine, because that is how the force is being applied.
So it will bore small at the very end of the part.

You could re-try this test by cutting some aluminum, using
a very sharp tool, and taking only a thou off on the last
cut.

Also note (as others have mentioned) the chuck may be
flexing and allowing the workpiece to move, or the spindle
bearings may have enough slack in them to cause a similar
effect. You can check that with a good tenths reading dial
indicator.

Jim

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

The correct way to check accuracy is to use a test bar that matches your
spindle bore. Mount the bar and test for taper with a tenth indicator, first
on the bar top then on the side. This checks the headstock alignment with
the ways. A precision level (.0005" per foot per scale division) is needed
to check for bed twist and wear. This should be done without carraige,
headstock or tailstock.
Steve
"Martin Perry" wrote in message
om...
I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry

Martin Perry wrote:
I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

This is most likely due to workpiece deflection. Even a 2" bar,
especially mounted in a small chuck, will deflect some. A better
way is to use a bar of known and constant diameter, and mount it without
wobble, then read it with a dial test indicator.

..001" over 8 inches is no big surprise. If you need more accurate
diameter, you may need to turn between centers, and may also need to
grind to final diameter.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

This is also no surprise. Again, workpiece deflection, but you are pushing
in the opposite direction.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.
No, they are not opposite. Both are most likely caused by the cutting
forces deflecting the work.

As you have already figured out, it is not a misalignment between
the spindle and bed, as this would have caused the opposite pattern
in the ID boring job.

Jon

It would appear that your head is out of alignment with the ways.
R. Wink

On 13 May 2004 06:34:16 -0700, (Martin Perry) wrote:

I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry

On 13 May 2004 06:34:16 -0700, (Martin Perry)
wrote:

I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry


the bed is worn most at about 8 inches from the chuck?

(Martin Perry) wrote in message . com...
I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.

These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.

Regards, Martin Perry

Was the bar supported at the outboard end? If not, you'll get
deflection when cutting stuff with that much overhang. If you did,
you might want to check your tailstock centering with a couple of dead
centers.

On your boring project, a lot depends on how rigid your setup is, if
you're using a boring bar a lot smaller than your 2" hole diameter,
you'll get deflection that way, too. I usually make a number of
finish passes at the last cross slide setting to compensate for bar
deflection. I also try to make the boring bar a fairly close match to
the hole size if I'm trying for a good, parallel hole. A lot depends
on how deep you're boring your hole and the amount of boring bar that
sticks out past the holder.

You might also want to check to see that your lathe bed isn't twisted.
If your Boxford is the copy of the 9-10" Southbend that I think it
is, they aren't that rigid. They're nice small lathes, I've done a
lot of work on a 9" Southbend, but they do flex and if your shims have
slipped and the bed has a little wind in it, you'll end up with some
untrue results.

Stan

Wear on a lathe bed usually occurs towards the headstock end of the
bed, but not right at the headstock. It occurs where the carriage
moves the most over time. You end up with a low spot in the ways. It
is easily detected by putting a dial indicator on the carriage and
reading the bed where the tailstock slides, which usually doesn't wear
much. As the carriage goes lower sliding in the worn area, the
tailstock surface will appear to rise.

Such wear caused the tool to move vertically. Vertical movement of the
cutting tool is treacherous. It will affect small diameters more than
large diameters. You cannot correct for the effect by offsetting the
tailstock. Cutting on the outside of a bar, the cutting tool cuts
lower away from the headstock, in the worn area. This has the effect
of making the cut larger in diameter away from the headstock. If the
tool is set to bore a hole, and is on center line at the end of the
workpiece which corresponds to the low spot in the bed, it will rise
as it goes toward the headstock, causing it to cut a larger hole near
the headstock.

This is a common wear pattern on old lathes. The only really good way
to fix it is to straighten the bed.

Brownnsharp

Thanks for your help and advice. A few follow up notes:

I did not use a tailstock centre when turning the two inch bar. The
last couple of cuts were 0.001 so I thought that this would keep
deflection to a mimimum.

The bed is worn in the area close to the chuck, 4 or 5 inches along
the bed from the chuck.

The bed is mounted on jacking screws so that I can twist the bed and
compensate for some of the wear. But it's a bit of a compromise.

I was happy with the results I got when turning the outside diameter
but I was very supprised with the results from the internal turning.

I used a 0.75 diameter boring bar for the internal cut and the last
couple of cuts were 0.001 to try and keep deflection to a minimum.

The headstock should be in perfect alignment because it mounts on the
bed ways, there are no adjustments. The taper bearing are in good
condition and correctly adjusted.

The lathe has seen a lot of heavy work over the years and I am
probably expecting too much from this "small lathe" (similar to a
Southbend).

Regards, Martin Perry

"brownnsharp" wrote in message
om...
Such wear caused the tool to move vertically. Vertical movement of the
cutting tool is treacherous. It will affect small diameters more than
large diameters. You cannot correct for the effect by offsetting the
tailstock. ...

Trolling for info here...

If your bed isn't really hell-ass worn, like, visibly (1/32" below
head/tailstock levels), which if it were I can't imagine things would be
sliding all to well anymore, then on say a 1" dia. work piece, you'll have
damn near nil change in diameter. Even by Harold's standards. G

Let's see. Say it's down 10 thou. A right triangle is drawn, one leg from
the center of the work, out straight (where the tool should be) and the
other down from here (to where the cutter *is*). The straight leg is the
radius (.5"). The 10 thou drop represents the other leg, with the
hypotenuse representing the new work diameter. So we have D' =
sqrt(D^2+h^2), which here comes to.... nearly a tenth. For a small bit of
work, say 1/8" diameter and 20 thou wear (ouch), this comes to 3.1 thou,
about 2.5%. Depending on what you're doing this could be significant.

But to show any sign of this difference (if you're measuring as you go, that
is) you need a workpiece longer than the "step" of wear, and 1/8" anything
is notoriously flexible, so you'll need a follow rest, which combined with
the flexibility of the work will utterly negate any way variations...LOL!

Tim

--
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- - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting
and Games: http://webpages.charter.net/dawill/tmoranwms

jim rozen wrote:

Martin Perry says...

I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.

The errors you see are in the correct direction for workpiece
spring. For the OD cut, the part moves away from the tool so it
will have the largest diameter far away from the headstock. Here
you can imagine the part moving away from the tool, springing
towards the back of the machine.

Just to put some numbers on it, one thou in diameter is half a thou in
radius or, in this case, possible deflection. Your 8" long, 2" diameter
steel bar is a cantilever beam.

BeamOut''(30E6,I_tube 2 1)Beam 0 (8 70)
pos'n pt. couples shear bend. slope def'l
force stress stress

0 -70 -560 -29.71 713 0 0
8 70 0 0 0 0.00009507 0.000507

Only 70 pounds force is needed to deflect the end half a thou. This is
pretty small as cutting forces go.

Aluminum has 1/3 the elastic modulus of steel so, as Jim said, the last
couple of finishing cuts woulld have to be very light.

Ted

Martin Perry wrote:

Thanks for your help and advice. A few follow up notes:

I did not use a tailstock centre when turning the two inch bar. The
last couple of cuts were 0.001 so I thought that this would keep
deflection to a mimimum.

The bed is worn in the area close to the chuck, 4 or 5 inches along
the bed from the chuck.

The bed is mounted on jacking screws so that I can twist the bed and
compensate for some of the wear. But it's a bit of a compromise.

I was happy with the results I got when turning the outside diameter
but I was very supprised with the results from the internal turning.

I used a 0.75 diameter boring bar for the internal cut and the last
couple of cuts were 0.001 to try and keep deflection to a minimum.

The headstock should be in perfect alignment because it mounts on the
bed ways, there are no adjustments. The taper bearing are in good
condition and correctly adjusted.

The lathe has seen a lot of heavy work over the years and I am
probably expecting too much from this "small lathe" (similar to a
Southbend).


Turning unsupported work 8" long on a light lathe will give you these
sorts of results. Mostly, you have probably demonstrated spindle flex.
A .001" cut on a nice aluminum alloy with a brutally sharp tool will not
need
much infeed force. Depending on the workpiece material and the sharpness
and profile of the cutting tool, you may or may not get highly accurate
cutting on very fine cuts like this. You can also have false edge buildup
on the cutting tool and thermal expansion of the workpiece to confound
you. Whenever you rough a piece, you need to allow the temperature to
return to ambient before making that last finish pass. If the part is
still cooling off from the roughing passes when you make the long, slow
finish cut, you will also get taper. It is pretty amazing how much a
1" diameter part expands when hot, when you are using a tenth-reading
mike.

Jon

Tim is right. I pictured what might happen in my head and was too
lazy to run a calculator. And I do appreciate the correction.

A bed that is worn in the middle is not likely to have worn straight
down. But I will not look at the "down" vector as particularly
important anymore. It is still an easy measurement to determine if you
have wear though.

Brownnsharp