Turning advise

[dthomas16]

Hello to all.

I am looking for some advise, ideas or input.

I have a Minganti vertcal CNC lathe, it is capable of holding 2/10 tolerance.

I am turning heat treated parts that are somewhat distorted.

My problem is when I complete the turn some of the part's are out of round or sometimes it has a taper.

The parts that are more out of round before the turn are more out of round when finished turning.

Any part that is more than .015 out of round seems to be the ones that turn out bad.

Then part program takes the initial rough cut down to .018 over the final dimension. The initial raw part is .030 over sized. The wall thickness (raw) is .250.

My primary concern is how much stock to remove. In other words, if my initial is .030 over and I cut it down to .018 over on the first rough cut, could this possible a problem?

So I guess my question should be how much material can be removed before the metal is stressed?

I am trying to hold a 4/10 tolerance.


Thanks, Dave

[houstonsamuel]

Is this part a large ring? If it is what is the O.D.? Also I don't understand the 2/10 or 4/10 tolerance.

[Geof]

Your wall thickness is 1/4", what is the ID, OD and length? I suspect these parts may be biggish if you are doing them on a vertical lathe.

What you describe sounds to me like stresses in the material arising from the heat treatment. Heat treating normally requires quenching and with a ring or a bushing it is possible to put the part into the quenching bath end on so the entire circumference gets quenched at the same time with the quenching moving along the length as it is submerged. This means any stresses induced during quenching are evenly distributed around the part and it stays round and straight. However, if the part is quenched by putting it in the tank sideways the stresses are not evenly distributed; the region that contacts first is cooled and contracted first pulling the shape out of round. If the part is put in at an angle not only can it be pulled out of round by the uneven cooling it can be bent.

During the machining the stresses from the quenching are partially released and further distortion can take place in an unpredictable manner. It might be possible to minimize the distortion by taking many, many fine cuts but this can be difficult with heat treated material because often the cut has to be .01" or greater to get into the material and provide a good surface finish.

The ideal thing would be to stress relieve the parts after heat treatment and before machining but this may affect the heat treated properties so it would be necessary to find out if this would be the case from your heat treater.

[dthomas16]

Thanks for the replies:

The parts dimensions. OD= 4.545, Height= 1.000 wall= .250

tolerances = .0002, .0004 respectively.

As to the quenching process, I am told that the parts a submerged on the Z plane.