[How do I?] Thread 7175 Alu?

B

Bob Sorenson

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Just bought two feet of 7075 alu. to make a knob for my DR brushcutter. It took a looong
time to cut off a piece with my horizontal band saw. 2 1/2" diameter. No problem since we're using power machinery. Next is to bring it to shape on the lathe. Cuts great. Then the problem comes up.
I need to drill a hole in the bottom to tap a 5/16 hole. Does not want to drill . 1, drill a spotter hole, 2, start off with a 1/8" drill bit. Just does want to cut. 3, tried to drill with the proper bit for 5/16" threads. Still can't drill.
All of the bits I tried we're brand new. Other than changing to 6061 alu ,which I purchased couple of days later, I would like to make my first attempt work.

Can anyone help me out how to drill this hole.

Thanks in advance
 
What temper is the 7075?

Edit: Is the alloy and temper stamped on the bar? It will be something like 7075-T6 or 7075-T651, or 7075-T0.
 
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Slow the spindle down, maybe 400 RPM or so, no coolant (or maybe some WD-40), and a lot of pressure. I have had aluminum give me problems like that when using sulphur cutting oils.
 
Huh, I also use, and love, and recommend 7075 and 7050, never had issues like that, my stuff parted easily. I'm with Jim, don't use oil or lubricant, those alloys are amongst the hardest of alum. Lol is the spindle turning in the correct rotation? Jk.
Maybe face off .050 and try again
 
I have always found 7075 to be a dream to machine. What are you using for drills?
 
You said a spotter, as in center drill? Should be no need for any more than that and drill the size needed. Even new bits at times need touched up, especially if a cheaper variety.
 
Spot drill is a little different than a center drill, spot drills don't drill a pilot hole, just a cone "spot" you can't run a center in the hole it makes without further tooling changes, they are meant to make sure a hole will be drilled on location, huge reduction in deflection cause of the stubby flute design, but with good reach distance
 
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