Machining thick Aluminium 2

[corrosionman]

Does thick aluminium plate have any " locked in stresses"
From a disc 30" dia and 5" thick we wish to make a saucer to very precise curvature. If we machine the concave face first to exact curvature will there be any changes when we remove a lot of metal from the other face? Its only the concave side that critical so maybe we machine this side last and then it will be as per planned curve, but it would be a lot easier to have done it first. Hope someone can advise.
Regards CM

 

[CoryPad]

Any plate regardless of material and thickness can have residual stresses. If dimensional accuracy is important, then you need to ensure the material is stress free by specifying a stress relief heat treatment for the initial material or during your machining operations.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

An alternate option is to roughly machine the part leaving little material to remove for final dimensions. Let the part rest for two weeks and then final machine using light fine cuts.

 

[unclesyd]

You might want to checkout this paper and do a little follow up on the NASA Tech Briefs website.
Even though this procedure is for Cryo mirrors I've seen very similar procedure for terrestrial mirrors.

http://www.techbriefs.com/content/view/479/32/

 

[IRstuff]

Aluminum is not particularly stiff, so you also need to be concerned about how this structure will be mounted. Tightening of screws on the mounting interface can also distort aluminum structures.

I'm not sure what you mean by "very precise." Are you talking 1/4-wave, 1/20-wave? You might need to interferometrically check the surface at each step of the process to ensure that the previous processing hasn't changed the surface figure.

TTFN

FAQ731-376

 

[corrosionman]

Gentlemen all your comments are appreciated and helpfull. Our 750 mm reflector has a dimension tolerance of 0.02 mm but the coordinates of the concave curve are computer generated and given to 6 decimal places. There are many specifications for aluminium and for tempering it but what I needed was confidence that the finished item will be the same size next month as it is today - - - here is where real Engineers come in. The finished reflector hasn't measureably changed in 36 hours so its now en route to the Client.Thanksfor your inputs. CM