Drilling 7076-T6 aluminium

[mangocrazy]

Problem solved... And take a bow @Hood and @Rrobson (and anyone else who pointed the finger of blame at the drill). I was using a crap drill, it turns out. It was new, but crap. After establishing that Screwfix was open today I bought a DeWalt cobalt 8mm drill and it just breezed through. The proof as far as I was concerned was when I dived in at the deep end and drilled one of the 10mm holes. No problem, no sticking and nice chip sizes once I'd got the feed rate/pressure right. If a 10mm drill was problem-free, the culprit was clearly the original 8mm drill.

So my two blocks of 7075-T6 now have holes of the correct size in them (in the correct places) and now all I need to do is to make them look like the plywood template (#4 in the picture is the Chosen One). The others were tests that for one reason or another weren't quite right.

Thanks again to all who helped or suggested.

 

[mangocrazy]

For aluminium, you really want polished drill bits and with lubrication.
Chip clearance is the critical thing for aluminium, so on a manual drill, lots of pecking to ensure chips are kept clear.

Speed wise, a quick run through a speeds and feeds calculator using a 10mm HSS drill, gives an optimum speed of 2718RPM.
However, that requires nearly a full HP, so I'd probably go for the second fastest speed, and go a bit slower, but make sure the drill always has pressure on it when cutting.

My Startrite drill motor is only rated at a quarter HP (single phase) so I think it would have struggled at the next fastest speed (1840 rpm). Appreciate what you say about pressure, as soon as I started leaning on it a bit chip size was much better and everything felt good. I'd say (from my very limited experience) that 7075 is quite a nice grade of ali to drill/machine.

 

[mangocrazy]

All I can add is
Wd40 only use is for machining ally!

Yesterday I was doing 8mm slots in gum ally so just tried a 5mm pilot even though I knew it was a bad move but I didn’t want to clog slot drill
It pulled the new carbide 3 flute in like a plasterboard fitting then snapped it!

No magic no fancy feed charts , just good drill, no pilot and wd40

Up till now I've struggled to find any use for WD40. Whatever the job, there is something far better. But as you say, its forte is in machining ali. I'll be buying a few cans when they're next on offer in Aldi or Lidl...

 

[Hood]

Aye, it is a harder alloy and they drill nice and should chip nice. It is mainly 6082 and 5083 I work with, the 6082 is quite nice but the 5083 is much nicer, probably due to the lack of or minimal silicon.