Drilling Stainless Steel

Low pressure is not the key to drilling in stainless steel. 304 or 316 are both soft materials in an annealed state, as soft at mild steel or aluminum. Both materials work harden badly if rubbed but not cut. You start with a 35ksi material and work harden it to 80ksi, about the same as hard alloy steel. You must cut at least 0.001 per cutting edge, and preferable 0.002 to prevent this from happening. That is why steady feed (preferably power feed) from a drill press is highly desirable. In a CNC machine with very reliable feed rate I can drill hundreds of holes through stainless with an ordinary HSS or cheap cobalt bit without sharpening. SS does generate a lot of heat and you want to keep the tool cool, however most of the problems people have are in not cutting the material, but rubbing it and work hardening it. The only way to get sufficient pressure with a bit over about 1/4 is to step up the size incrementally so all the pressure is concentrated on a small cutting edge. If you use light pressure you may eventually wear through the material, but it is a lot of extra effort and will ruin the bit.

Nobody has mentioned water jet cutting. Would that be an option? I know nothing about it other than it's existence.

twistedtree said:

Nobody has mentioned water jet cutting. Would that be an option? I know nothing about it other than it's existence.

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I have had lots of metal cut on a waterjet. Piece of cake, but not that many of them around compared to plasma cutters.

I never tried to drill a hole that big in stainless. When I need something like that, I drill multiple small holes and finish with files or my Dremel chucked with the solid carbide bits.

Sounds awful, I know. I twice cut 3/8” x 5” long curved slots in 1/4” 316 stainless with this method. Looks fine, works as intended.

jleonard said:

I have had lots of metal cut on a waterjet. Piece of cake, but not that many of them around compared to plasma cutters.

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But a much cleaner cut with a water jet, right?

Codger2 said:

Having been a machinest/tool maker in a past life, I found the above advice to be right on.

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Personally, I would have used more cutting oil, cheaper than dulling the drill bit.
Also, start with a center punch. Watch the color of the chips. Dark blue, slow the drill down.... Just MY opinion.

twistedtree said:

But a much cleaner cut with a water jet, right?

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Waterjet does make a clean hole, HOWEVER the cut is not straight. The upper surface will be a slightly smaller diameter than the lower surface.
Similar to "die break" on a pierced hole.

A waterjet will make a clean cut. Except for the most sophisticated ones, you will get a slightly beveled edge (but you will with plasma too). The hole will be fairly accurate, as good as a hand held hole saw, but not as good as an annular cutter or milled hole. If you waterjet polished stainless, it will dust the area next to the kerf.

Phil23 said:

I need to drill a 30mm hole through a 4mm plate without the result looking like a beaver had a go at it. Tungsten Carbide or Titanium tipped/coated saws, wil either do it or is one better than the other. Cheers

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That’s a job for a ‘Peck Drill’. It’s basically a drill press, slow turning, and reciprocates into/out of contact. Oil is important. The main thing is keeping the stainless from getting hot at the drill site. Cobalt bit is also critical. Manually, with a drill press, go hard but for short bursts.

The critical thing is to drill slow, slow and keep the SS cool. Stainless steel work hardens with heat and increases resistance as a dull bit stops cutting. Slowest possible speeds on a drill press with tungsten carbide bits.

SS does not work harden with heat. It work hardens if it is worked. That is why you need to keep the bit cutting, and the chips in the 0.002 thick range or thicker. Otherwise you are working the SS and hardening it. Heat will ruin the bit if you let it get too hot, so slow isn't a bad idea, nor is coolant if you have it. But SS (in the 304 and 316 alloys normally used on boats) is not a heat hardening material.

Peck drilling will clear the chips and may let the bit cool and allow some lubricant into the hole. But if you are pecking SS, do not dwell at the beginning or end of the peck. This will work harden the surface and you will have to break through that to drill annealed material again.

I drill this stuff all the time.