Tapping 304 Ss Help

boostin53

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I'm having a heck of a time tapping 304 SS. They are blind holes being tapped to m12x1.25 in 2" OD 304SS, 1.25" deep. I drilled to 27/64 for 75% thread, then failed. I then drilled up to 7/16 for 50% thread and failed again. I can get my tap started, but that's about it. It feels like the tap just wants to snap.

I know SS work hardens fairly fast. I step drilled up to the given sizes and made sure to use plenty of cutting oil and not let the bits dwell. The work never got too warm and neither did the bits.

What should I do differently? I know my taps aren't the best quality, but they cut threads in 303 SS with some effort. Am I just experiencing the less machinability of 304? Now might be a good time to warm up my internal single point threading skill (or lack of).
 
My recent experience is limited in ss304 to tapping about a half dozen through holes in some 1/4" material. It was a very slow process and. I'm glad they were through holes and only 1/4" deep. I had hand taps so I did the usual 1/4 to 1/2 turn and back-up. I'm sure I would have broken a tap in deeper holes as tough as the going was. I think it's just the difficulty with the material that is giving you problems. You mentioned using cutting oil when drilling. I'm sure you used it while tapping as well.

Maybe someone knows of a high end tap that would work better? I know some taps have coatings that are supposed to make them last longer, but not sure if they will make this specific job any easier.

My tap felt like it was going to snap as well but in my case I didn't have a lot to loose and twisted harder than I thought possible without breaking, but the same tap held up through 6 holes. In this case the tap was a 70's vintage craftsman brand. I'm sure there has to be some better taps available that may help.
 
Yes, I used oil while attempting my tapping operation. If it's this hard to tap 304, I may just give up and change material or internal single point.
 
If you have the patience to watch the whole video in the following link, he gives some valuable info on what type of fluid you should be looking for, IIRC the we used to to use heavy tapping oil to tap 304 :

 
The only thing I can think of is to buy a new, high quality tap. If you're having trouble with your existing tap in 303, I'd call that a clue indicating a dull tap.
 
I turn and tap 304 nearly everyday, a good quality sharp tap is important otherwise I have no problems with it.

By the way work hardening can happen with no heat generated, as its name implies it is deforming (working) the material that causes problems. Try machine tapping that size thread in one shot and use a spiral flute tap in a blind hole as they spit the chips out like a twist drill, a very nice tool for your application.
An example http://www.osgtool.com/Product-Group/threading
 
A new tap might be what I need. Along with the OSG brand, what other brands should I be looking at for a quality tap?
 
A new tap might be what I need. Along with the OSG brand, what other brands should I be looking at for a quality tap?
PTD, Dormer and Guhring come to mind, you will find them a bit expensive by hobby standards however, steer clear of what home depot/ lowes/ace hardware offers, these tools are not intended for this sort of work.
 
PTD, Dormer and Guhring come to mind, you will find them a bit expensive by hobby standards however, steer clear of what home depot/ lowes/ace hardware offers, these tools are not intended for this sort of work.

Yeah I expect a quality tap to be pricey for hobby use. But if I'm going to be using it often, which I will be, I rather pay the money for quality. Saves me future headaches. Thanks for the info Wreck!
 
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