Beautifully illustrated, Robin

MichaelG.

I don't think the chuck forces caused it to bulge. More likely the 3 jaws supported the work infront of then and the rest distorted away from the tool leaving them slightly thicker as they spring back. Though you may find the hole is distorted where the chuck has closed up the thin spigot in 3 places.

Lighter cuts, sharper tool or some form of rear support would cure it, collet would just see a conical shape as the whole outer unsupported area get pushed away for the whole 360deg.

looks like bronze or GM which can do that more then other metals

Edited By JasonB on 05/09/2022 07:17:35

So a collett is the way to go

apparently

Making a block that holds the part and supports the back of the face being cut is important if the flatness really does matter. You have done a very good illustration of the distortions that can happen when facing a part.

Agree totally with Jason, that the meta, being thin, l is likely to have flexed away from the tool due to cutting forces.

Think in terms of trying to face a sheet of paper !

For metal, of that thickness, the tool needs to be REALLY sharp, set dead on centre height and the cuts need to ,be very shallow to minimise cutting forces.

If that doesn't work, make up a thick backing washer to support the disc being faced.

Howard

The pictures show almost conclusively that the chuck jaws must have had an influence on the surface profile of the part. Whilst I agree with that statement I am curious as to the actual mechanics or nature of the influence.

If the part had the proportions of a sheet of paper then support would be essential but I dont consider 1/16" at about 2" diameter to be anywhere near 'thin', brass is a rigid hard metal and it surprises me that, especially for a finishing cut, it flexes enough between each chuck jaw to leave a witness on the face.

I wonder if the part had been chucked with gap left between the jaws and the workpiece, whether the part would have ended up flatter? (as no influence from the jaws). My money is on (as speculated by the OP) that it was the radial pressure of the chuck jaws on the spigot that caused the distortion. Having said that the spigot itself looks quite substantial and I would have thought that it easily coped with the cutting forces involved.

Just food for thought

Ian P

Robin

Generic FEA of the part you are trying to machine. First picture shows the model with jaw forces applied. Second and third pictures both show the way the parts deflects, Values are not important it is the indicative pattern we are interested in. Blue is small, red is large relatively.

Regards

Dave

I agree, Bernards bung would have prevented the distortion.

 There is nothing better than learning by your mistakes, I'm getting to the point where I remember the best way to do something after doing it the wrong way.

Edited By old mart on 06/09/2022 21:09:23

Robin

The part deflects along the axial plane. So the red maximum deflection zones bend backwards towards the headstock.

With CNC, MG's varied surfaces are probably possible.

We had some flywheel housings CNC machined

The code called for the mountimng to be milled round, by interpolation.

But when unclamped, the housing was oval by 0.004"

"No problem said the stter. He rreprogrammed the machine to mill the aperture 0.004" oval in the other plane. When unclamped, it sprang to round!

Howard