turning between centers
 

Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA

Dan Miller wrote:

Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA

I used to turn stuff held in a 3-jaw and a tailstock center a lot, then I
got in the habit of just turning between centers. It's the end-for-end swap
and the remove/replaceability I like. Also, you know the chuck isn't bending
your bar any. To get to your issue, I like to check lathe ways with a precision
level both ways. If it shows as level as you can get it then yes, just set over
your tailstock and be done with it. If your bed is misaligned you will get
similar error, yes. Make this part (by setting the tailstock to correct the
taper) and then recheck your lathe alignment.

The old way is still real good. Make a test bar to turn between centers (you
do have a drive plate and drive dogs, right?) with two collars, one at each
end. Take a very light finishing cut across both collars, striving for the
best finish, and take both cuts without touching the apron or cross-slide
wheels or adjusting your turning tool in any way, then measure the diameters
with a micrometer. If they show the same, you're home free. If not, adjust your
tailstock and repeat. This is the procedure from "How To Run A Lathe" and it
has worked for a long time for many people.

Grant

A modification of Grant's idea would be to use a piece of ground stock, do
your center drilling with your three jaw on both ends, then set it up with a
dog and face plate as though to cut but instead of cutting use your dial
indicator to see what is happening. Then make your fixes.

Wayne

"Dan Miller" wrote in message
...
Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA

Dan Miller wrote:
Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA
Chuck an indicator in the headstock and indicate the tailstock true. If
you still cut a taper after that then you need to level the ways.

On Sat, 12 Feb 2005 18:27:12 -0500, tomcas
wrote:

Dan Miller wrote:
Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA
Chuck an indicator in the headstock and indicate the tailstock true. If
you still cut a taper after that then you need to level the ways.
The above method can lead to seeing an error that isn't there. This is
because your indicator setup may droop. If this happens it will appear
to show that the tailstock ram is high. So be sure to watch out for
this.
ERS

Wayne Lundberg wrote:
A modification of Grant's idea would be to use a piece of ground stock, do
your center drilling with your three jaw on both ends, then set it up with a
dog and face plate as though to cut but instead of cutting use your dial
indicator to see what is happening. Then make your fixes.


A 3-jaw is bound to give some error, you should use
a 4-jaw and center with a test indicator.

Ken Grunke



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1. Maybe
2. Maybe

Tony
"Dan Miller" wrote in message
...
Hey all,

I'm currently turning a piece of bar stock approx. 1.25 inch in diameter
and about 7 inches long between centers in my small Myford ML7 lathe. I
have it in a 3 jaw chuck and rotating center at the tail stock. Given
I've been taking relatively heavy cuts (for me that means 20 thou) but
I'm noticing that the diameter is roughly 5 thousands larger at the
tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

Dan Miller
Seattle WA

In article ,
Wayne Lundberg wrote:
A modification of Grant's idea would be to use a piece of ground stock, do
your center drilling with your three jaw on both ends, then set it up with a
dog and face plate as though to cut but instead of cutting use your dial
indicator to see what is happening. Then make your fixes.

However, instead of using the 3-jaw to drill the centers (I
presume close up to the chuck to minimize flex), I would suggest using a
4-jaw and taking the time with an indicator to make sure that those
centers are *on* center. Otherwise, you could get false indications
depending on the orientation of the bar during the measurements.

Obviously, if your 3-jaw is a set-tru type, and you take the
time to set it true for each operation, this is as good as the 4-jaw for
the purposes.

Best, of course, would be if the ground stock was ground between
centers, so it already has the center holes. :-)

Good Luck,
DoN.

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In article , Dan Miller says...

tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks

To find out if the bed has twist, turn the same part *without*
the tailstock center in place. Then measure. If it is still
tapered, adjust the bed until it is straight. Then you
can go on and use Grant's approach of turning between
centers.

Jim


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Hey thanks for the input! I'll do some tests and get back to the group.
Grant I do have a drive plate and dogs but I only have one center...I'll
look around for another and give the true between centers method a try.
Just seemed like that driveplate dog wouldn't hold very well on a heavy
cut but perhaps that's not true. Do you mill a flat in the piece for the
dog screw?

Dan

jim rozen wrote:

In article , Dan Miller says...



tailstock end than at the chuck end. What's this mean to you guys? Do I
just need to move my tail stock over 2.5 thousands or could it also be a
twist in the bed or something? Thanks



To find out if the bed has twist, turn the same part *without*
the tailstock center in place. Then measure. If it is still
tapered, adjust the bed until it is straight. Then you
can go on and use Grant's approach of turning between
centers.

Jim

Dan Miller wrote:
Hey thanks for the input! I'll do some tests and get back to the group.
Grant I do have a drive plate and dogs but I only have one center...I'll
look around for another and give the true between centers method a try.
Just seemed like that driveplate dog wouldn't hold very well on a heavy
cut but perhaps that's not true. Do you mill a flat in the piece for the
dog screw?


No, never have yet. I try to plan it so the dog clamps onto what will eventually
be waste which gets cut away or onto a surface which will later be turned,
removing any marks, but if I have to clamp to a machined surface I use shims
cut from copper sheet.

GWE

"Dan Miller" wrote in message
...

Do you mill a flat in the piece for the dog screw?

Dan

No.
- Carl