Help tapping stainless steel

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jppostKW

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I built a diy version of derrins sanke conversion today all went well until I tried tapping the holes one was drilled to 7/16 for a 1/4 npt fitting the other 9/16 for a 3/8 npt I used thread cutting oil but it was really tough and ended up chipping my 3/8 npt tap the cap is 316L I was wondering if I need to use a specific stainless tap are my hole sizes wrong etc.

Thanks guys
Paul
 
sounds like the heat generated during the hole drilling has work-hardened the stainless.. unfortunately i don't know of an easy way to anneal stainless once it has turned into glass.
 
Do yourself a favor. Call some local machine shops and see what they will charge to do it or if they can recommend someone. You will destroy the taps that you can buy from Harbor Freight, Lowe's, and Home Depot. Then you're about $60.00 in, at least, and whistling "who da thought it". I'm sure everyone else on here, besides you & I, has been successful doing this. Maybe you can even coax them with some homebrew. The guy I used drinks Miller Lite, so it was even cheeper. Let me know what you do. Luck - Dwain
 
The hole sizes sound correct, here is a chart http://www.engineersedge.com/manufacturing/pipe-tap-drill-chart.htm. The most succesful method I have used for tapping stainless steel is to advance 1/4 turn then back up 1/2 turn to break the chip, then advance another 1/4 turn past previous ending point. If you try to go forward continously like you can with carbon steel the chips will sometimes bind and shatter the tap face. It is best to start with a new tap and pitch it in the junk pile after a couple holes as the stainless is really hard on non cobalt alloy taps.
 
The taps are snap on not cheap and kladue that's the basic method I used.
Dwain this was basically seeing if I could build something cheaper than buying.
20 bucks shipped for tri clover clamp cap and gasket
12 for the SS racking cane
I already had the compression fitting and barb fitting but they would add two bucks apiece
That's a total of 36 dollars I guess if a machine shop cuts and taps the holes I have to bank on the guys working for beer!
Thanks for the insight so far.
 
Yeah, I used an old Mac tap first and ruined it, then the succession of others. Most machine shops routinely cut and thread stainless. That stuff is harder than Superman's knee caps. Luck - Dwain
 
I built a diy version of derrins sanke conversion today all went well until I tried tapping the holes one was drilled to 7/16 for a 1/4 npt fitting the other 9/16 for a 3/8 npt I used thread cutting oil but it was really tough and ended up chipping my 3/8 npt tap the cap is 316L I was wondering if I need to use a specific stainless tap are my hole sizes wrong etc.

Thanks guys
Paul

you will find some tough going in stainless without using an npt reamer...:)
 
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