Heat treating machined aluminum gear case 1

[nsmith40]

I am getting ready to machine an approximately 8" x 12" gear case out of 6061-T6 extruded billet (3" thick billet). I will be removing a lot of material to create basically a shell case of about 0.2" thick, with stiffening ribs and bearing pockets.

I would like to maintain fairly tight tolerances for proper shaft alignment and proper sealing of the case halves. What kind of stress relieving processes should be performed? Should stress be relieved prior to, during, or after the machining process? How does this affect the T6 heat treat that has already been performed on the material?

 

[swall]

What you should have done was start with billet in the T4 or T42 condition. Then you could have rough machined, aged to the T6 condition and finish machined. At this point, you have three options. 1--Rough machine in stages, taking care to equalize material removal from opposing sides. Then finish machine and hope for the best.2--Rough machine, then stress relieve at 350F for 2-4 hours. You will lose some ultimate strength. Then finish machine. 3--Have a heat treater re-solutionize your billet to the T4 condition, then follow plan 1.

 

[israelkk]

Rough machine, leave to rest for two weeks then final finish with fine machine cuts.

 

[mfgenggear]

I am asking a question?

why not just rough machine
stress relieve
then finish up

T6 will machine easily

 

[swall]

The problem is the stress relief. The ageing temperature for 6061-T6 extruded billet is 350F, so for a stress relief to be effective it has to be at 350F or higher and you will start to overage the material, degrading its mechanical properties.

 

[nsmith40]

I looked into my material a bit more and it turns out that it is actually 6061-T6511

Does this make any difference?

What are advantages and disadvantages of rough machining, letting sit for 2 weeks, then finish machining?

 

[mfgenggear]

ok great

according to the old MIL-H-6088 which is now obsolete
replace by AMS-H-6088 -T6511 is aged extrusion.
Did you receive certifications? what is the requirement.
do you need -T6511 ?
can you return the material or is it rough cut?

it just time if you have it.

an other suggestion is to

rough machine
re anneal to T4 condition
re solution age T6
finish machine

there of course would be added cost for re heat treating.
best bet like swall said buy in T4 condition
then age to T6

 

[nsmith40]

Thanks for the replies.

Let's say I do method 1) from swall above (rough machine equally from both sides, then finish machine)

How many times should I flip back and forth? 2? 4? 8?

Any ideas on how much distortion I might expect? 0.001" change in flatness across the mounting face? .01"? 0.1"?

 

[swall]

I doubt that anyone on the metallurgy board can address the specifics of the machining sequence. I suggest you start a new thread on the machining board and give specifics about your section sizes, geometry and starting size of your stock.

 

[israelkk]

nsmith40

My recommendation to let the part rest for two weeks before final machining is based on ASM metals handbook version 8 (~1970). 30 years ago we used this method it for a very accurate gear pump aluminum case. Therefore, using swall recommendation for rough machining sequence and the two weeks rest may be sufficient.

Are you the machinist or you give it to a third party?
The clamping method is also a major factor in the machining process. The clamping method should be such that it will minimize the stresses on the part.