What the hazelnuts is going on here! Work piece and tramming results not matching

[Maplehead]

Hi All
So I've been working all weekend on a new ball turning attachment.
When I was facing the top of one part's blank to bring the thickness down from 3/4" to a 1/2" I noticed that as I "mowed the lawn" with my roughing end mill that I could feel and see noticable ridges between each pass.
(I mow the lawn all the time to face down parts with great success.)
One part is about 1.5" wide and I am getting a 10 thou difference in thickness between one side and the other, running along its x axis. Why?
I stuck a magnetic plunge dial indicator onto my quill and then ran it across the top of my 5" vice. I got a .5 thou difference.
I the ran the indicator across a 6" paralell in the vice and I got 1 thou difference. So the work piece is laying on a pretty accurate surface.
I checked all the parts I made so far and they all have about a 10 thou difference in thickness from side-to-side. Why?
Any and all help is greatly appreciated.

 

[projectnut]

Sounds to me like the work piece is lifting when the vise is tightened. Take a piece of stock with a known thickness, set it on some parallels in the vise and measure the side to side height. Then close the jaws and measure again. I'm thinking the side nearest the movable jaw is lifting causing the part to lift as well.. Most old timers use a dead blow or lead hammer to seat the work piece after the jaws are tightened.

 

[markba633csi]

Maybe the cutter is moving up or down in the collet? Or vise jaw lift as Projectnut mentioned
-M

 

[Maplehead]

Sounds to me like the work piece is lifting when the vise is tightened. Take a piece of stock with a known thickness, set it on some parallels in the vise and measure the side to side height. Then close the jaws and measure again. I'm thinking the side nearest the movable jaw is lifting causing the part to lift as well.. Most old timers use a dead blow or lead hammer to seat the work piece after the jaws are tightened.

I do the same every time I set a piece in the vice. I hammer/seat it in and check that both paralells are not moving.
Also, I trammed the Y axis on top of the vice and I got zero movement.

 

[Maplehead]

Maybe the cutter is moving up or down in the collet? Or vise jaw lift as Projectnut mentioned
-M

Possibly, but how would I test that? Also, I set my roughing end mill all the way up into the collet until it stops.
I could imagine that if you started facing a piece along the x axis from left to right, and the end mill moved up into the collet as it went along to the right, that it would stay in that spot when brought back to the left side, making it not touch.

 

[markba633csi]

OK sounds more like a vise issue- check with an indicator for jaw lift

 

[akjeff]

I do the same every time I set a piece in the vice. I hammer/seat it in and check that both paralells are not moving.
Also, I trammed the Y axis on top of the vice and I got zero movement.

After you tap it down on the paralells, make some cuts and check the parallels again. Odds are, you'll find that one of them now slips. Just because you tap the part down, doesn't mean it will stay put.

 

[Papa Charlie]

Check the end play of your spindle under force. Which means install a end mill and attach the test indicator to the spindle and apply downward pressure on the end mill to the vice. Do this in several places tramming along the top of the vice.

 

[Maplehead]

So I checked if I'm getting "lift" after vise tightening.
I measured the width and thickness of three paralells at their ends and middles. All good. I then set them in the vice.
I tapped the parallel down until I got no movement in the supporting paralells.
After indicating I got a 5 thou difference. Why?
I checked the movement afterwards and the supporting paralells were still tight.

 

[akjeff]

That parallel could possibly be bowing under the clamping forces, if you really reefed on the handle? Sweep across the parallel in the Y axis, and see if has a hump in the middle.