15 foot-lbs to tap 3/8-16 in Stainless

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Bill Kahn

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I needed to tap 304 stainless

https://www.ebay.com/itm/322563718105

With 3/8-16. Even after oversizing to an R hole c.f. https://www.physics.wisc.edu/ishop/tapdrillchart.html my cheapo-tap after the starter cuts simply stopped. No matter how I backed out and came in or how much Tap Magic it was like hitting a wall. Would not budge.

I ordered https://www.ebay.com/itm/282830021598 and it too simply would not go further.

Well, I had nothing to lose. I wanted the threads. So I just put a cheater bar on (everything was perfectly aligned as I set it up in the lathe) and simple cranked hard. Lo and behold tap did not break. I was able to tap the 8 threads I needed. But oh how I would have sworn that you can't abuse a tap like that. I measured it as 15 foot-lbs of leverage.

This is a good lesson. A good (not great) new 3/8 tap can take 15 foot-lbs (on a slightly oversized hole). And, if you are in stainless, that seems to be what it takes. (Maybe, just for the fun of it I will try again with a Q hole and measure the leverage required. The R really was too big--with 8 threads the Home Depot bolt was a bit sloppy in it.)

-Bill
 
Kind of make you squirm just hoping it will not shatter! Many SS will work harden and use of correct lube like Anchorlube can make the difference plus a spiral point tap if possible.
 
a GH8 class fit is also super loose - most are typically GH3. GH8 is at least 3x the clearance than a GH3. Good if you have oversized bolts (eg. zinc plated), but it'll give a sloppy fit with anything else.

Stainless can be a bear to work with. Best off getting drills and taps that are coated/ ground for stainless and then putting them aside so you don't use them on anything else. Use cutting fluid and drill with low rpm/ high feed. Tap with an appropriate tapping fluid for stainless. even then you might get screwed!
 
Stainless is a bear to tap anyways, especially 316. I would power tap using that kind of tap getting it started. Plus you need some good cutting oil like Mobilmet 766 or one of the others made for stainless. I know Emuge taps are expensive, I would recommend them for stainless, too. As matt said, GH8 has a much larger PD and my add to the torque needed to tap.
 
We had to tap a SS fluid end one time with a 3" pipe tap in a 4' Lucas horizontal boring mill. After about two threads in the tap would not go any farther, the nitrided spindle on the Lucas would torque about 20 degrees before I would get chickenhearted and back off. We had to single point as much as we could and try the tap again. We could do straight threads but not tapered threads.
 
I agree with Ken on the Mobilmet 766, stinky when it gets hot but it gets the job done on SS. For taping stainless I switch from Tapmagic to 766 and if I think about it I add a drop or more of TCE solvent in with it, makes it slippery as can be. Although TCE is widely thought to be no longer available, Brownells sells it as a solvent, wonderful stuff if used prudently.
 
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