Hi,

can someone please suggest proper way to machine Al-7075-T651 alloy to minimize/avoid deformation?

Stock ~1000x70x25mm, was clamped to the 6 standoffs using one M6 screw per standoff, which were then clamped to the table. When I machined one side and released it from the table, the part was bent (~4mm about the middle of the part lengthwise).
I've attached some pics illustrating machining strategy and the result.
1. profile part using 6mm single flute carbide endmill, flute length 25mm (6/1/25 cem): DOC0.5mm, F3000mm/min, S13800rpm
2. clear ~550x70x8-10mm area in zig-zag pattern, using 6/1/17 cem, DOC8-10mm, WOC1mm, F875mm/min, S12000rpm
3. rough using 6/1/17 cem, climb only, waterline: DOC2-17mm, WOC,Feed and Speed variable too keep relatively constant MRR and the machine happy
4. finish using 6/3/18 cem, waterline climb in X direction + contour in Y direction combined, max DOC/WOC 2mm, step-over 0.1mm, F4000mm/min, S24000rpm
Material was never too hot to touch - it was warmest after #2 (and I suspect that was the problem). I used some WD40 on all cuts.

I've done similar parts in Al-6082-T6 ~800x80x25 and Al-6060-? ~800x70x20, using different roughing strategy (waterline 0.5mm DOC), different clamping method (normal clamps on top) and the parts turned out straight.

Thanx,
Mihael

FWIW,

All metal has internal built in stresses. So if you remove material from one side then remove the piece from a restrained position something is going to happen
ie; may cup, twist, deflect one way or another. You just never know what will happen as it is somewhat shape dependant! In a similar way, machining a rather deep slot  can in material may alter the ability of remaining material to maintain shape.

So material can be sent out to stress relieved, normalized,  can be aged, and even then it may or can spring some.

Just as a true example, we were machining large Al plates that would be sent out to be top and bottom ground ( 6' x 6' x 4" to 6" thick)
and were machined to be parallel and flat to about 0.003" before being sent out to be ground. They were stress relieved before machining. Took a lot of money, material, and time / testing to figure out the machining sequence and how much material should be removed.  

So I would suggest the following:
Rough out to remove the bulk of material, but leave some material to adjust post checking what is happening to the piece ... removed from hold down to check.
Then machine a new reference flat surface. Continue machining. The smaller the change in dimensions from machining one could expect smaller
change to the piece from flat reference. You may want to go in steps and refine as machining is progressed.
If the flat surface of the  piece will be bolted down to a mating  flat surface, the force to deflect the machined piece may be acceptable, so the sprung
part may be acceptable.

FWIW,
RICH
 
 

Thank you very much for your answer Rich. I have to machine the other side of the part, maybe it springs back