drill a 7/8" hole in SS for a 1/2" NPTM nipple

danw

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Oct 2004
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midwest, USA
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I need to drill a single hole (to pass a 1/2" NPTM pipe nipple) in a stainless panel/enclosure (already installed in the field, not in a shop).

Suggestions for a drill technique/tool?

I suspect a bi-metal hole saw might not drill all the way through before it dulls.

Web reviews on the carbide tipped hole saws at Home Depot have terrible reviews.

The guy at the local tool house suggested a cobolt twist drill but my experience with large diameter twist drills in sheet metal is frustration when the cutting edge digs in and gouges the sheet metal.
 
A quality hole saw will work. Where everyone goes wrong is the speed. SLOW. About 100 RPM or less.
Keep a steady pressure on the tool. When you release pressure the material work hardens so when you push again you have to go thru that hard stuff and dull your tool. Have someone continually squirt coolant on the cut.
You can also use a unibit but the same rules apply.

We do this all the time.
 
I have a set of Greenlee "holecutters". They are a one-piece hole saw with a replaceable pilot bit. I have had them since '98. Although I haven't used them daily since then, they have lasted a long time.
The key to using them for stainless is like gas said in his reply--very, very slow speed. Use a lot of cutting oil. I have cut many holes in stainless over the years, sometimes 10 to 20 holes in one day. They're still good. I have the ones that will make a hole for 1/2", 3/4" and 1" conduit or pipe fittings/conduit fittings. Hope this helps.
 
Another trick to remember, especially on thicker gauge steel -

Drill the pilot hole with a good drill bit, then use the pilot bit on the hole saw.

Too many times when the pilot bit penetrates, the hole saw will slam into the panel hard, bending pilot bits and chipping teeth.

EDIT: Plus, for sheet metal I have a Bosch hole saw set that has flanged lips on the saws to prevent it going through the panels.
 
Thanks for all the advice.

Since it's one hole and I don't know when I'd do another, the knock-out punches lost out to a carbide tipped hole saw. The Ideal TKO brand has a number of good testimonials/reviews both in this thread and on Amazon about their decent quality.


And excellent advice from Aabeck about drilling the pilot hole with a separate drill to avoid driving the hole saw into the material when the pilot drill passes through the metal. That had not come up in any other review.
 

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