Machining Aluminium- Centrifugal force and yield strength

[sydneyjongleur]

Hi Guys

I am machining(turning) an aluminium alloy ring, just under 1.0m diameter x 150mm high. The ring starts with a wall section of around 20mm and it gets taken down to about 3-4mm.

The issue that I am having is that after the operation, whilst still on the machine, the ring grows by 0.75mm on diameter over the full length of the ring and also grows on length by around 0.3mm.This only ever seems to happen with aluminium.

My initial thoughts is that this is caused by centrifugal force and that the yield strength of the material must have been reached for the ring to expand by this amount. Can anyone confirm or tell me otherwise if this is correct.

I have worked out the centrifugal force based on a 20mm section and then with 3-4mm section to get an answer in Newtons(800 and 140 respectively), however, I am struggling to decide how to relate this to the yield strength of the alloy. If I can get to the bottom of it I may be able to slow the machine down hence reducing centrifugal force.

The alloy is precipitation hardened and the yield strength is around 400 Nmm^2.

I may be completely on the wrong path here so any guidance would be appreciated.

 

[gearcutter]

Is the ring, while on the machine, being held by the jaws of a chuck or is it clamped to a faceplate? Jaw pressure can distort a weak part.
I don't have experience with aluminium but if the part was made of steel or CI then I'd suggest that the part is undergoing a form of stress relieving.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[sydneyjongleur]

I was thinking that stress relieving/ residual stresses were playing a part but it is consistently 0.75mm approx. big on every dia. feature. The ring is being held in a fixture with around 5mm grip. The surface speeds have been reduced and it seems to be helping a little bit which makes me think its the speed at which it is turning that is causing at least some of the problems

 

[gearcutter]

One thing I forgot to mention is the heat generated as a result of the machining process.
We’ve found that it’s important to first rough out the part, let it cool down and then release the part from the fixture, re-clamp and then finish turn.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181