Machining Aluminium- Centrifugal force and yield strength
Machining Aluminium- Centrifugal force and yield strength
(OP)
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.
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.





RE: Machining Aluminium- Centrifugal force and yield strength
Mike Halloran
Pembroke Pines, FL, USA
RE: Machining Aluminium- Centrifugal force and yield strength
Aluminum thermal coefficient of linear expansion is 22.2x10^-6 m/m C. At about 1.0 m dia, .22mm/10degC
Ted
RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
Cheers
Greg Locock
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RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
We could look it up if an actual alloy spec had been revealed.
Mike Halloran
Pembroke Pines, FL, USA
RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
As in, do the parts meet spec when you use new/sharp tools?
Are you using tools optimized for aluminum?
Mike Halloran
Pembroke Pines, FL, USA
RE: Machining Aluminium- Centrifugal force and yield strength
NX 7.5
Teamcenter 8
RE: Machining Aluminium- Centrifugal force and yield strength
I don't have much Experience with aluminium , and certainly not at this size and wall section so I am trying to understand what's going on . I have a finishing operation aftermthis which takes the wall section down further and i am expecting some more material movement although hopefullymnot as much. I will look into the spinning of aluminium
Is it just stress relieving itself from stresses created from the ring rolling forging process as a lot of material has been removed.
The tools we use are sharp positive rake carbide inserts recommended for aluminium.
Hope this all makes sense
RE: Machining Aluminium- Centrifugal force and yield strength
I would use Von Mises Hencky equation Sigma=sort(3) P K^2/(K^2-1)for K=OD/ID. This is the wall stress experienced by the piece. Then your factor of safety is that stress divided by yield strength. If the result is less than one, then you are stressing the ring by turning. A number greater than one, unlikely this is the problem.
It could be thermal, even coolant wouldn't completely have the piece at ambient temperatures. Do the calculation based on a ring of equal circumference and see what temperature is required to grow the ID by 0.75 mm. This is a small amount, I suspect a few degrees.
Good luck with it.
Regards,
Cockroach
RE: Machining Aluminium- Centrifugal force and yield strength
I can believe Machining an un-stress relieved forging could be responsible for The change in diameters.
One easy test is to repeat the machining of an inside diameter or bore painted with dykem immediately after the first cut. The dykem will be untouched if the part distorted during that cut.
the change in length of a solid ring is interesting and harder for me to understand.
Is any kind of stress relief called for in the forging processing?http:
If there are no stress relief options then rough machining steps can help a lot. A sequence will have to be developed.
RE: Machining Aluminium- Centrifugal force and yield strength
Is the inside diameter also too big, or is that one ok?
What is the order in which every step of the turning takes place?
NX 7.5
Teamcenter 8
RE: Machining Aluminium- Centrifugal force and yield strength
Tmoose- no there is no stress relieve allowed on this material unfortunately. I will look at changing the method of manufacture and will probably be able to overcome most of the problems, however, if I could understand the reasons why there is so much movement then I can go about overcoming it. This is the only material that I have experienced such a change in diameter. The strangest one for me is what I said in an earlier post, the location diameter which does not get cut, grows by 0.75mm also- This is why is was thinking centrifugal forces etc( or maybe it is just stress relieving itself)
Walterke-It is actually around 800mm diameter and finishes up about 3mm wall thickness and 150mm long. It is contoured and has pockets etc so the wall section varies but generally no more that 4mm. both internal and external get bigger. The diameters internal and external are machined before the pockets so I can understand how they may move etc. As I keep saying though,the bit that gets me is the location diameter grows 0.75mm. To try and explain, this is an external diameter and is held to 0.002" on the previous operation, is 5mm long and has a groove directly above it locate clamps on. So the part is located in the fixture, clamped and it still grows 0.75mm??. We basically had to jack the part off with great difficulty. As an experiment we opened out the loc dia on our fixture for the next part and after the operation took the part off of the fixture and sized the loc dia and it had grown by 0.75mm???? Either it is thermal, stress relieving itself or the centrifugal force due to surface surface speed.
RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
RE: Machining Aluminium- Centrifugal force and yield strength
Residual Stress Analysis of Aircraft Aluminum Forgings
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Residual Stress in Aluminium Alloy Forgings
http:
Applications: Cold-Compressed Aluminum Forgings
http://www.lanl.gov/contour/forgings.html
Effect of Forging and Heat Treating on Residual Stress in Aluminum Forgings
http://w
RE: Machining Aluminium- Centrifugal force and yield strength
So it could be a complex combination of things, how you are chucking the raw material, depths of cuts turning 20 mm thick wall to 2 - 3 mm, imparted stresses from cutting thereof, etc. My suspicion is that several other commentors have given you some form of verbal solution, no need for repetition here.
Kind of hard to comment on or provide a numerical solution to the phenonema without more input datum. But I think that would fail to adequately address your situation. You are removing a lot of material and ending up with a very thin ring of soft material age hardened for strength. Machining stress are most likely the source of your deformation. You need to consider Poisson's Ratio in two planes, diameter and length in order to figure out the way material flows to each term in the strain relationship.
Kind of a neat problem, I suggest holding back on your cut depth and possibly stress relieving the ring intermittently during your manufacturing process. Perhaps jigging this across the span of length would help stabilize the OD somewhat.
Regards,
Cockroach
RE: Machining Aluminium- Centrifugal force and yield strength
This link discusses the measurement method for residual stress:
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Can this part be hydroformed and trimmed, or superplastic formed?