Chatter finish when facing a 10" diameter .375 thick 7075 T 651

[moonlight machine]

It has a 2.75 Dia. hole on center. faced flat to a diameter of 5" Then a 2.5 degree taper out to 9". Chatter was bad at any speed or feed. I made big pie jaws and that helped but not enough. I solved it by cutting 2 wood blocks with a small taper. I put them through the center hole and between the part and the chuck with just enough pressure to keep them from falling out when the spindle is stopped. Results were instant, no more chatter, beautiful finish.

 

[Mtndew]

A part like that doesn't seem like it should chatter.
The wood is a nice trick though.

 

[moonlight machine]

A part like that doesn't seem like it should chatter.
The wood is a nice trick though.

I have run into it before but never this bad.

 

[DrHook]

Do you have pictures? I'm have trouble visualizing the where the problem was, and how the wood changed that. I have mystery chatter issues on oddball stuff now and then, and ain't too old to learn...

 

[Joe Gwinn]

It has a 2.75 Dia. hole on center. faced flat to a diameter of 5" Then a 2.5 degree taper out to 9". Chatter was bad at any speed or feed. I made big pie jaws and that helped but not enough. I solved it by cutting 2 wood blocks with a small taper. I put them through the center hole and between the part and the chuck with just enough pressure to keep them from falling out when the spindle is stopped. Results were instant, no more chatter, beautiful finish.

So, it's a 10" diameter disk made of 3/8" 7071-T651 aluminum, with a 2.75" diameter hole in the center, clamped in a pie-jaw chuck on the outer diameter, which disk chatters when being faced unless the center hole is corked with a wooden plug?

I suspect that what's happening is that the inner edge of the plate is resonating in shear (transversely to the plane of the plate), with very low damping at frequencies at which a standing wave can form (an integral number of waves fit in the inner circle).

Here is a picture of the mode shapes (look at figure 2, ignore the surrounding math):

https://www.extrica.com/article/21116

The wooden plug would be perfect to suppress those standing waves, due to attenuation as the inner metal edge tries to move axially back and forth against the wooden plug.

Making the pie jaws from wood might also work. Or using leather between pie jaw and workpiece.

 

[moonlight machine]

Do you have pictures? I'm have trouble visualizing the where the problem was, and how the wood changed that. I have mystery chatter issues on oddball stuff now and then, and ain't too old to learn...

I don't know how to post photos, the wood was between face of the chuck and the part. there is a 1.3" gap. The wood is tapered about .050 and fits in through the hole in the center of the part after chucking the part. It is a very light fit, centrifugal force keeps them in place.

 

[moonlight machine]

So, it's a 10" diameter disk made of 3/8" 7071-T651 aluminum, with a 2.75" diameter hole in the center, clamped in a pie-jaw chuck on the outer diameter, which disk chatters when being faced unless the center hole is corked with a wooden plug?

I suspect that what's happening is that the inner edge of the plate is resonating in shear (transversely to the plane of the plate), with very low damping at frequencies at which a standing wave can form (an integral number of waves fit in the inner circle).

Here is a picture of the mode shapes (look at figure 2, ignore the surrounding math):

https://www.extrica.com/article/21116

The wooden plug would be perfect to suppress those standing waves, due to attenuation as the inner metal edge tries to move axially back and forth against the wooden plug.

Making the pie jaws from wood might also work. Or using leather between pie jaw and workpiece.

The wood does not cork the hole as I need to finish that diameter. it goes between the part and the face of the chuck

 

[Mtndew]

I have run into it before but never this bad.

I glossed over where you said it was 3/8 thick. Yeah that's chatter territory.

 

[Joe Gwinn]

The wood does not cork the hole as I need to finish that diameter. it goes between the part and the face of the chuck

That would work to suppress the transverse oscillation as well.

 

[Joe Gwinn]

So, one prescription would be to cement a disk of leather or sorbothane to the pie jaws (while closed) while leaving an open area at the outer radius, razor cut the leather/rubber to free the three jaws, and proceed: firmly push the workpiece down onto the leather/rubber layer, close the pie jaws, and proceed with the facing operation. The soft disk need not make it to the outer radius of the workpiece, as all the action is near the edge of the inner 2.5" hole.

For more precision, one could use a wood layer instead of leather/rubber, and turn the wood face after the cement has dried

 

[Booze Daily]

I worked with an old timer that used 3 tennis balls to do that.

 

[moonlight machine]

So, one prescription would be to cement a disk of leather or sorbothane to the pie jaws (while closed) while leaving an open area at the outer radius, razor cut the leather/rubber to free the three jaws, and proceed: firmly push the workpiece down onto the leather/rubber layer, close the pie jaws, and proceed with the facing operation. The soft disk need not make it to the outer radius of the workpiece, as all the action is near the edge of the inner 2.5" hole.

For more precision, one could use a wood layer instead of leather/rubber, and turn the wood face after the cement has dried

Not sure why I would deal with the mess of gluing and the clean up when I can just put a little wood block held by centrifugal force and especially when I got such spectacular results.

 

[Joe Gwinn]

Not sure why I would deal with the mess of gluing and the clean up when I can just put a little wood block held by centrifugal force and especially when I got such spectacular results.

The only reason is to achieve precision of making the plate perpendicular to the rotation axis, if such precision is needed.

 

[moonlight machine]

The only reason is to achieve precision of making the plate perpendicular to the rotation axis, if such precision is needed.

The 2 faces are within .0005, plenty close for a disc brake bell that allows the rotor .002 side to side float,