Controlling Warpage in Aluminum

Hi, I'm not a machinist or a mech eng...in fact I'm a Cat , so this may be completely bonkers.

But I'd say the warpage is due to stress in the billet. If you machine out all the pockets slightly under-size this should allow the warpage to take place and relieve the stress. Then machine to the final finished dimensions.

The implication of this is that the reference plane (assuming there is such a thing) itself may warp, in which case it would need to be machined flat again before the final finishing...I realise that this will vastly increase your machining time.

Apologies if I'm talking out of my backside.

I've just re-read it and noticed the material is only 5mm thick... in which case I'd think it was going to to be hard to hold any tolerance at all as it isn't going to be very stiff .

Del

Dude! That's a tight tolerance on 5mm sheet. You might try soldering (low temp solder) the aluminum to a base, but that seems like a lot of work. I don't understand your shape (are these through holes), but can you use a laser?

What is you workholding method? If you are holding from the sides with too much force, doing the pockets and then taking a skim, as soon as you unclamp it, it will warp! Look into your workholding, and the order of machining!

All the above answer are right on. The warping is cause because the stress is coming out of the part as you remove material.

You could try to send this to a heat treater for stress relieving before you machine it.

But seeing that it's such a thin part, surely the part is restrained during assembly and it stays restrained during it's life.

Maybe you could get your customer to agree to inspect the part as it would be used, restrained. Then build a fixture that would mimic the part as if it was assemble. Some times that flattens out the part, hold it steady and then it passes inspection.

Good luck

Good answer...questioning the final use is a great idea, you may well find the tight tolerance isn't necessary.

Get together with the customer and solve the problem together.
Del

Add this to the other good answers which are all valid. First make sure you are using top quality cutters that are designed for aluminum. Cutters designed for aluminum will have a higher heilix angle to the flutes, more clearance angle and also more back rake angle. This will cause less stress to the aluminum and generate less heat when cutting. Of course the cutters must be sharp. You cannot test the sharpness with your thumb! If you cannot look at the cutter through a microscope or at least a 10 power eye loupe then Just start with new cutters. Siince it is most likely you are doing this on a CNC mill you should consider high speed machining techniques, which involve high rpm, high feed, and shallow cuts. This is not just fast, but generates less stress into the material.

I experiment with a lot of different cutter brands. Here are some that I like and use:OSG (Japan) Kennametal (USA/Germany) Accupro (?) Hanita (Israel) Micro100 (USA) Harvey (USA) Promax (USA) Destiny (USA). My main criteria for these choices is performance. In fact there are several old standby American companies whose quality and technology ahve not improved in the 35 years that I have been cutting metal and I won't use them no matter the price. I make such a big deal about cutters because it is likely the most important choice you make when cutting aluminum or any other metal for that matter.

As someone else has mentioned, how you hold the part is critical, and if your part or your fixture is not flat to begin with you are wasting your time. If you can, holding the part down with a vaccuum fixture is a good idea. If there are enough available holes in the part then pre-drilling them and completely deburring the part before screwing down to a flat fixture is your next best bet. After that you may have to use small clamps, or build in hold down tabs on the part for machining which will have to be cut off later. For a part that thin, holding in a vise is out of the question.

If these suggestions do not yeild the tolerence you require which is very tight (.020mm=.00078"), you may want to consider leaving about .03mm stock on the part and lapping it flat after milling.

Please let us know how you make out or if we can be further help. It would be better to know more details to be able to better advise.

Thanks to everybody who answer my doubts.Actually i have seen the sample workpiece from my customer,and it really controlled <0.02mm warpage.But when i done it myself ,i just cannot get it flat.Let me explain how i do it,i order the material 50mm bigger outside dimension,then i clamp it on a flatbase;i mill all the pocket with new carbide endmill and 20000rpm speed.After complete all the slots and holes then only i mill the outskirt to size.Is that any process that could flatten the workpiece after machining?

What flatness are you able to hold? It may still be that lapping as a final operation will be your best bet, or are you trying to hold flatness within the pockets? If that is the case I would need to know more about the part, the program, and the cutters.

Hi,

A.: which aluminum alloy in which temper?

B.: stretched or not? The aerospace industry is facing a similar problem with parts where extended milling (up to 95% removal) is existing, they use stretched material (5 to 7%) to remove internal stress.

C.: Aluminum is said not being able of being stress relieved by tempering. This is true if T6 temper is needed, but if T7 or slightly lower strength can be tolerated then extended tempering is considerably doing good in stress relieve. We are designing and prototyping highly stable parts from aluminum (high strength) alloys used in airbearings and measurements. Tolerances are much tighter there and possible to achieve.

D.: Above comments on cutting conditions are very important and best clamping is by a very thin layer of hot-melting glue that is often used in optical parts (prisms) grinding and polishing.

Have success

RHABE

All good answers so far. Are you making this part from sheet? If you are machining the skin on one side only it may induce warping, machining both sides is better even if you have to start with a thicker piece of raw material. Solution treatment before you start will help to relieve internal stresses & rough machining then leaving the part to settle will help.

All the earlier comments about work-holding, cutters & solving this with your customer are sound advice.

I would resort to chemical milling where you need to machine number of packets and avoid warp age caused due to mechanical machining stresses on aluminum sheet. The process is amenable to remove material on intricate surface such as integrally stiffened wing surface of a high performance aircraft. Sodium hydroxide base or other suitable alkaline based solutions are generally used to chemically mill aluminum. The process is carried out at elevated temperature. Metal removal or dissolution is controlled by masking, rate of immersion, duration of immersion and composition and temperature of the bath.

The chemical milling is a preferred process for the mass production of intricate multi step removal on aluminum sheet components. The process is employed for taper reduction of metal to fine tune aero dynamic surface efficiency of helicopter tail rotor of Aloutte II & III designed by Eurocopter. The process is also used extensively on many passenger aircraft component manufacture to remove unwanted weight, most economically

Hi,

all processes and procedures that do not relieve internal stress to a low level will produce unstable parts: with time and temperature.

With time because the internal stress is relieved slowly by creep.

With temperature because creep is activated by temperature and because the elastic modulus is temperature dependent.

RHABE

A drawing would be needed to actually tell what might move where. I don't know if the pockets are through pockets or if the tollerence problem is do to side walls taperieng or bending. If depth is the problem you must start with a completely flat part while held evenly with a fixture that doesn't stress the part when you hold it. I've used a vacume fixture to hold parts as thin as 1.5 mm thick like someone meantioned, i've also used double sticky tape. It may be nessessary to to rough cut the part then lap the bottom flat this would neutralize what ever bending of the part took place from the basic removal of material, before making the finishing cuts. Cast aluminum (mic6) stays flatter than aircraft aluminum but is more difficult to cut if you don't know how.

If the pockets are through pockets with thin walls then the order you cut the pockets has to be considered and you may have to leave some material on the bottom to hold it together until it's done and then skim that material off the bottom after.

Hope that is of some help without a drawing.

Peter Rosenholm

treelaw45@yahoo.com