Machining Aluminium- Centrifugal force and yield strength 1

[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.

 

[dvd]

Measuring the state of residual stress with strain gages and hole drilling method could confirm that there are locked-in stresses prior to machining. Accurately predicting how to machine the part to release the residual stresses and have the part stay within tolerance is quite a task. The stresses are very likely not uniform, so a machining strategy for a round, turned part sounds monumental.

This link discusses the measurement method for residual stress:

http://www.intertechnology.com/Vishay/pdfs/TechNotes_TechTips/TN-503.pdf

Can this part be hydroformed and trimmed, or superplastic formed?