Part coming out eliptical

[ChrisE]

We just started a new prototype job made of annealed 1 inch plate 4340 steel that was sent out to waterjet cut into rings (7.4" OD, 5.7" ID).
Due to the size of the part, we had to use a 3-jaw chuck. But because of the mechanical properties of the final part, traditional chuck jaws were bending it into a triangular elliptical shape on the 2nd op, so we bought and switched to pie jaws so the higher surface area would distribute the clamping force more evenly on the ID during 2nd op where most of the final dimensions are being cut. I even made sure the chuck jaws were indicated nice and center within .001" on a 100 degree sweep of the outside of the jaws.

Chuck pressure during 2nd op roughing is 1.5MPA (217.6 psi), and finishing pressure is 1.0MPA (145 psi). Roughing with .032R inserts, SFM is 550, DOC is .03", and .008" feedrate. Finishing is done with .016R inserts, 550sfm, .01 DOC, and .0035 feedrate.
Final OD is 7.125" +0/-.001", final ID is 6.0" +/-.01", but only .3" from the back of the part, the rest has a funky profile.

I was careful to measure along multiple points while the part was still in the jaws after it had finished, it measured nice and round and i managed to land it nearly dead-center of the tolerance. But when I took the part out to measure it, it was elliptical by .004 and undersized to boot. I'm not sure what caused this, but I had plenty of coolant, so I can't imagine it was heat buildup, and the material was supposedly annealed. Thoughts?

edit: specified that 1st op didnt come out elliptical and that problems arise on 2nd op

 

[guythatbrews]

If the part isn't round on the first op and you force it round with pie jaws, it will finish out of round.

Can you reduce 1st op chuck pressure to minimize distortion? Can you put a spider in the ID opposite the chuck jaws and only bore down to the jaws, roughing that part on the 2nd op? Make the spider with 2 fixed points and on adjustable. Adjust with enough force to counteract distortion.

 

[ChrisE]

If the part isn't round on the first op and you force it round with pie jaws, it will finish out of round.

Can you reduce 1st op chuck pressure to minimize distortion? Can you put a spider in the ID opposite the chuck jaws and only bore down to the jaws, roughing that part on the 2nd op? Make the spider with 2 fixed points and on adjustable. Adjust with enough force to counteract distortion.

We currently don't have a spider and the 1st op didn't come out elliptical at all due to the fact that I only cut .35" into the material on both the inside and outside, the rest was clamped by the jaws and the excess stock material seemed to stabilize it enough. as for chuck pressure, i ran as low as it felt safe to do so given the cutting forces and narrow clamping surface (water jet belled out a bit on the camped side)

 

[guythatbrews]

So op one is round. Can you chuck for op 2 and run an indicator on the op 1 surface to see if it is still round after chucking?

 

[Houdini]

Ok, so the part is 1" plate (Z) you cut OD and ID in Op1, but only .350" deep(Z).
Then flip and do OP2, correct?

Initial stock is .275" oversized.

Are you using the same pie jaws for both OP1 and OP2? or are you switching?

You cant use the same ones for both OPs.

 

[EmGo]

I don't know why people think pie jaws are magic. If you hold parts in a chuck, they come out three-sided. Even a manual chuck. Even a six-jaw. Even pie jaws that hold the whole thing all around with only tiny gaps between the jaws and have been bored to .001" over the finished size. Even light pressure by hand (power is always higher pressure, can't help it).

Parts that have been chucked are not round - and probably not flat. It just depends on how accurately you measure them.

 

[GiroDyno]

We deal with this every day, raw material be it bar stock/waterjet/laser cut is not going to be perfectly round. If you're concerned about ovalizing or tacoing large diameter/thin parts (think frisbee or dinner plate) its helpful to add an operation to true up your starting material. Grip the raw part and skim cut some of the face and OD to match your pie jaws, flip it and grip on the now round/flat surfaces and do your roughing, drop clamp pressure and regrip for finishing. Taking the time to really dial in your pie jaws to match the finished diameter goes a long way to maintaining your finished part shape as well, if the diameters don't match then you're pretty much back to gripping on 3 points just like traditional jaws.

 

[Mtndew]

I don't know why people think pie jaws are magic. If you hold parts in a chuck, they come out three-sided. Even a manual chuck. Even a six-jaw. Even pie jaws that hold the whole thing all around with only tiny gaps between the jaws and have been bored to .001" over the finished size. Even light pressure by hand (power is always higher pressure, can't help it).

Parts that have been chucked are not round - and probably not flat. It just depends on how accurately you measure them.

What about parts that don't have a hole?

 

[Houdini]

So you just need to get a mag chuck then

 

[rjwalker1973]

If it was nice and round before you took it out then you have too much check pressure. put an indicator on the part as you unchuck it and see how much it moves.

 

[Houdini]

I have a buddy that has to machine some large injection molded plastic ring parts to tight round tolerance.
They have an Aluminium fixture disc with threaded holes in it.
they load it up, skim the fixture front flat, then mount the disk with clamps, first op outside dia, flip clamps, op 2 inside dia. no face .
looked like a pain the ar$$ to me.

 

[MotoX]

We deal with this every day, raw material be it bar stock/waterjet/laser cut is not going to be perfectly round. If you're concerned about ovalizing or tacoing large diameter/thin parts (think frisbee or dinner plate) its helpful to add an operation to true up your starting material. Grip the raw part and skim cut some of the face and OD to match your pie jaws, flip it and grip on the now round/flat surfaces and do your roughing, drop clamp pressure and regrip for finishing. Taking the time to really dial in your pie jaws to match the finished diameter goes a long way to maintaining your finished part shape as well, if the diameters don't match then you're pretty much back to gripping on 3 points just like traditional jaws.

This is a good tip. Garbage in is garbage out.

My entire family of parts is thin walled tubing. I use mostly three jaw chucks. Most of my operations are in the 40psi to 120 psi in chuck pressure***.

***Equal chuck pressure does not equal gripping force across different machines. Machines vary in cylinder size, stick-slip of drawtube, angle of jaw travel mechanism in the chuck, and chuck condition, etc etc..

Reduce the chuck pressure, then smack the part with mallet. Did it move? if not, then reduce again.

Repeat until you find the line, then go slightly higher. Takes a bit of trial and error or experience to find the relationship of machining forces as compared to mallet whacks, but I'd say you can drop you pressure quite a bit.

10 PSI can be a large change in gripping force, and distortion.

 

[BT Fabrication]

We just started a new prototype job made of annealed 1 inch plate 4340 steel that was sent out to waterjet cut into rings (7.4" OD, 5.7" ID).
Due to the size of the part, we had to use a 3-jaw chuck. But because of the mechanical properties of the final part, traditional chuck jaws were bending it into a triangular elliptical shape on the 2nd op, so we bought and switched to pie jaws so the higher surface area would distribute the clamping force more evenly on the ID during 2nd op where most of the final dimensions are being cut. I even made sure the chuck jaws were indicated nice and center within .001" on a 100 degree sweep of the outside of the jaws.

Chuck pressure during 2nd op roughing is 1.5MPA (217.6 psi), and finishing pressure is 1.0MPA (145 psi). Roughing with .032R inserts, SFM is 550, DOC is .03", and .008" feedrate. Finishing is done with .016R inserts, 550sfm, .01 DOC, and .0035 feedrate.
Final OD is 7.125" +0/-.001", final ID is 6.0" +/-.01", but only .3" from the back of the part, the rest has a funky profile.

I was careful to measure along multiple points while the part was still in the jaws after it had finished, it measured nice and round and i managed to land it nearly dead-center of the tolerance. But when I took the part out to measure it, it was elliptical by .004 and undersized to boot. I'm not sure what caused this, but I had plenty of coolant, so I can't imagine it was heat buildup, and the material was supposedly annealed. Thoughts?

edit: specified that 1st op didnt come out elliptical and that problems arise on 2nd op

depth of cut and feed is smaller then the tip radius, you are basically pushing material around and burnishing it and not actually cutting anything with a 16 thou tip rad and only taking a 10 thou cut.......
this causes stresses in the material and causing it to warp when released.

 

[kevin66]

Without knowing the part profile and the amount of material removed on 2nd/last op I would guess that the part is relieving itself during machining and springing out of round when the pie jaws release it. Grain direction in plate is not ideal for a round part without roughing it complete then finish turning it in a separate op.

 

[guythatbrews]

Reduce the chuck pressure, then smack the part with mallet. Did it move? if not, then reduce again.

This is a good tip. For more than thin rings.

 

[GiroDyno]

This is a good tip. Garbage in is garbage out.

My entire family of parts is thin walled tubing. I use mostly three jaw chucks. Most of my operations are in the 40psi to 120 psi in chuck pressure***.

***Equal chuck pressure does not equal gripping force across different machines. Machines vary in cylinder size, stick-slip of drawtube, angle of jaw travel mechanism in the chuck, and chuck condition, etc etc..

Reduce the chuck pressure, then smack the part with mallet. Did it move? if not, then reduce again.

Repeat until you find the line, then go slightly higher. Takes a bit of trial and error or experience to find the relationship of machining forces as compared to mallet whacks, but I'd say you can drop you pressure quite a bit.

10 PSI can be a large change in gripping force, and distortion.

The lower your pressure the less consistent your chucking will be as well.
I've done a lot of measuring at varying clamp pressures are we can see a variation of 30% between rechucking when running at the low end of the pressure gage while mid range pressure is much more consistent ~5%.
We have replaced all of the draw cylinders on our VTLs with 1/2 or 1/4 size units to allow us to grip light enough and still be in the sweet spot on the gage.
We basically clamp the part and try to spin it by hand, drop the pressure until it slips, then add 10PSI and that will be our pressure for finishing operations.

Another annoying thing when running at the low end is your chucks actuate really slowly, which makes hand offs in dual spindle machines painful to watch. Smaller draw cylinders also help with this.

 

[MotoX]

The lower your pressure the less consistent your chucking will be as well.

Right on here.

I've been meaning to refit my machines with lower range of gauges, but just never had the time.

The regulators can struggle with the lower pressures as well. I can dial to a low pressure, rap my knuckle on the sheet metal and watch the gauge move. What really matters is what the gauge settles on when the jaws stop moving.

A couple back and forth checks while cycling the jaws can help one get consistent.

 

[Superbowl]

I would try making a face plate larger than the part with threaded holes both inside and outside of the part. Then using the threaded holes, face clamp the part against the face plate. Switch clamps from ID to OD as needed.

 

[Houdini]

I would try making a face plate larger than the part with threaded holes both inside and outside of the part. Then using the threaded holes, face clamp the part against the face plate. Switch clamps from ID to OD as needed.

I already mentioned, but, yup!

 

[Superbowl]

I already mentioned, but, yup!

Sorry I missed that