Bowing aluminium when pocketting

[Lugy]

Hi folks, we've got a large contract coming up, 400 sets of 3 parts each month. The parts are 6082 Al and are 8mm(5/16") thick at most. Just now we are doing a pre-production batch for inspection by the customer.

At the moment I'm working on a cover plate, approx 95x125mm by 5.2mm thick. On the rear of the plate is a large pocket (85x115mm app.) machined 3.6mm deep.
The first operation is to face the material (8mm thick) and machine the profile of the part. Next the part is turned over, the excess material removed and the pocket is machined.
The problem I'm getting is that the parts bow when pocketed, I'd imagine being a cover it won't be critical. I'd rather it was perfect though, the sample part we were sent is only very slightly bowed.
I'm gripping the part in a vice fairly lightly, the bow runs parallel to the jaws however so I'm not convinced that's the problem.
I'm using a 6mm 2FA type cutter (fast helix carbide effort).

Any suggestions? Bearing mind the large quantity that will be coming shortly!

[Shane123]

if its not the jaw bowing it, use a block under it so when you pocket it, it cant push down.

[Shane123]

you could make aluminum soft jaws and pocket out the part OD in the jaws, and make it so that the jaws have about a 1/16 gap between when closed on the part.

[RICHARD ZASTROW]

Might be a stress problem. If the material is rolled or wrought, it has mechanical stress in it and will "bow" or curve when material is removed. The more material removed, the greater the bow.

Machining itself can introduce stress int the material. Does it bow after the first side is machined as well?

I've had some luck machining side 1, forcing the part flat in a vise then machining the second side. Seemed to compensate and return to original flatness.

Doesn't always work, but might be worth a try.

Incidentally, are these parts stress relieved before machining?

Dick Z

[Geof]

Originally Posted by Lugy ....Any suggestions? Bearing mind the large quantity that will be coming shortly!

The bowing is almost certainly due to stresses in the material, I have run into a similar problem using 4" x 1/4" 6061 flat bar.

Given the quantities you are looking at my suggestion is to find someone who can anneal and then re-temper your material before machining. This is not a difficult operation and involves heating to around four hundred degrees C followed by water quenching then re-heating to around three hundred degrees and holding it at this temperature for a few hours.

[Lugy]

This is the brand of vise we use;


I might look into making some soft jaws for it then, I'll need to make 2 sets as it's being run on a twin pallet machine, though that'd be laziness to complain of course .

The material is rolled. After the first operation the part is still flat, it's just when we remove the material from the pocket it bows up in the middle, this gives a poor finish too.

[Jaxbubba]

On that thin of material, we have had to take off .02 on one side, flip the stock over and remove .02 from the backside. Then release the vice jaw to let the material move where it wants too. Lightly tighten the vice back up and just take enough material off to get it flat. Flip the material over in the vice, but make sure you have a spacer under it to support the material in the center of the vice, cut it to thickness, pocket it, and it should come out pretty flat. The spacer on the finish passes under the stock is the key here. Hope this helps

[SORCHEROR]

i always cut things like that on a fixture plate helps draw the heat out and keep the bowing down,you should deck one side first,than flip it,im not sure you have a way to clamp it to a plate or not,some parts i have machined from thicker blocks and decked off the excess,its a crap shoot unless you buy better material and or stress relieved

[SORCHEROR]

oh,my bad,your parts are fairly small,are there any holes through this cover to fixture it?
if not i would buy thicker stock,ive had good luck keeping parts flat by side milling the excess stock off rather than the bottom of the cutter,i usually machine the whole part in one sitting from one side than deck off extra stock off,i usually leave 3/16-1/4 on it
just another way

[SORCHEROR]

the poor finish is due to the lack of support on the bottom,try a thicker block,you'll see a big improvement

[Lugy]

Well I've just run one with better packing under it, the finish is up to scratch now though it's still bowing, it's around .010" over 5".

There aren't any holes through it unfortunately.

[packrat]

Try doing one where the pocket side is cut first. Then if it should bow, the bow will be in the pocket. The finish/top side will be flat that way.

cary