Aluminium Material Dimensions before Machining

[stresscalcs]

Hi

In the Aircraft Industry it is normal practice to choose the size of an Aluminium block to be machined to be well oversize compared to the final machined part required, so that all the block outer skin/surfaces are totally removed whilst machining.

Could anyone point me in the direction of a description of the logic of this and the unwanted consequences of using the absolute minimum size block possible to produce the machined part.

Any help would be appreciated.

 

[racookpe1978]

Part you just wrote.

Aero parts are VERY expensive (final dim's and heat treat or strength and NDE requirement and QA control and QA/accountability paper lineage) ...

The "cost" of the paperwork and QA regularly exceeds the cost of the Al block and often the original block and the machining. SO, spending the time to cut off potentially bad material to get the good material at the right heat treat condition inside is cheaper than the cut-off material.

 

[EdStainless]

Some end users have specification requirements for minimum stock removal, and yes it seems like a lot of material. I have seen prints that dictate the starting material size (and orientation of rolling directions).
But as Racookpe said, their are good reasons behind it.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

The amount of stock removal is usually different for cold worked vs hot worked materials. The hot worked materials usually need more surface material removed.

If the ratio of stock material weight to finished part weight is very high and the part is large, you might consider using a piece of material hand forged closer to the machined part shape. Often this will reduce both raw material cost and machine time enough to more than offset the added cost of the hand forging.

 

[stresscalcs]

Thank you all for your comments but the information I am looking for is unfortunately not mentioned.

Racooke writes of "the good material at the right heat treat condition inside".
EdStainless points out that Racookpe said "there are good reasons behind it".

I am looking for some starting point of documentation (and hopefully eventually some concrete material values) of those good reasons.

I have tried searching the internet without success, my keywords "aluminium, dimensions, machining, prior to, before" are simply not effective to isolate the subject.

Hence my request for any help.

 

[racookpe1978]

Best then - for this particular case (now that you've heard some of the excuses (er, valid reasons) for a over-sized chunk of good metal to be chopped up into a much smaller final part) - is to go into the shop and look at the process; listen to the machinists (they may know) and listen to the older engineers/QA people/foremen in the shop.

Don't be too surprised if "that's the way we've always done it" is the only reason.

 

[Tmoose]

Some of the ASME specs for pipe and tubing have guidelines used when buying material based on finished size requirements compared to the size, roundness etc tolerances for the material.

 

[tbuelna]

stresscalcs-

I don't know of any industry standards for determining minimum raw material size for components machined from wrought aluminum stock. However, I have seen internal company process specs that provide guidance on selecting and specifying raw material sizes based on factors such as part size/shape, amount of stock removed during machining, accuracy required in the finished part, application where the component is used, etc.

To keep things simple, we'll assume your application is a part 100% machined from aircraft quality wrought aluminum plate or bar. With large aircraft structural components that must be light weight, it is common for 75% or more of the raw material to be machined away. This amount of machining can produce distortions (bending, twisting) in the finished part, especially when the stock removal is not uniform. The distortion in the finished part usually does not become apparent until it is removed from the machining fixture restraints. Some combinations of aluminum alloy/heat treatment are less susceptible to machining distortions than others. Sometimes, when there is significant amounts of material removed, the part will be rough machined and stress relieved prior to finish machining. Often the primary consideration for selecting the size/shape of your raw material will simply be cost. You need to provide enough stock so the finished part can be manufactured to meet tolerance requirements, and you should consult with your manufacturing people to determine what is necessary after they have reviewed the engineering documentation. The added cost of a small amount of extra raw material is usually insignificant compared to the cost of scrapping a finish machined part that is out of tolerance.