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I have some aluminum / aluminium strips that are about 3 mm thick and about 20 mm wide.

I need to drill some holes in them. The holes will be 3.5 mm.

What type of drill bit do I need? What speed should I run the drill; fast or medium or slowish? Should I use a cutting compound or anything similar or just drill dry?

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Thanks for the question. I assume you've used a spade drill bit for this? –  mbrownnyc Dec 9 '12 at 20:42
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2 Answers 2

up vote 4 down vote accepted

Drill dry. You need a carbide tipped bit that's rated for metal; this will be obvious on the outside of the packaging. I'd drill fast.

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Also, use a nail, nail punch, or some other pointed tool to hammer an initial dent into the surface. Otherwise the bit will ride all over the surface rather than making a hole where you want it. –  BMitch Aug 31 '11 at 15:19
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As Karl recommends, drilling dry with a metal drill bit (HSS or Carbide) is the basic information that you need.

As far as drilling speed goes, you can refer to a cutting speed chart and do a bit of math. Aluminum's cutting speed is 350-400 fpm or 106,680-121,920 mm/minute. The distance travelled by a point on the outermost edge of the bit during one revolution is D*pi = 10.9955743 mm.

This gives roughly 9,700 to 11,100 RPM for your drilling speed. Most handheld drills can't get over 2,500 RPM, so you shouldn't have to worry about drilling too fast if you're using one.

This website has a drilling/cutting speed chart along with a calculator.

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