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

Reply to
Dan Miller
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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

Reply to
Grant Erwin

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

Reply to
Wayne Lundberg

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.

Reply to
tomcas

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

Reply to
Eric R Snow

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

Ken Grunke

Reply to
Ken Grunke

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.

Reply to
DoN. Nichols

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

Reply to
jim rozen

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:

Reply to
Dan Miller

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

Reply to
Grant Erwin

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

No.

- Carl

Reply to
Carl Hoffmeyer

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