tapping stainless steel | Practical Machinist

If you have lots to do, the best taps to use are serial taps. They come in sets of three, but each of the first two taps cuts a progressively deeper acme-like form

If you want to do it in one pass, a lot depends on whether it's a blind or through hole, and how close you need to tap to the end if it's blind

Need more info to answer your question, also: will you be using a tapping head, if not, what machine/ method? Ox wins the prize.
Spiral flute modified bottoming tap is the ticket.
The spiral flutes pull the chips back and out,and the modified bottom feature gives 3-4 leads of thread (thus dividing up the chip load).
Make sure you use a series designed for tapping stainless, this is very important.
Stainless geometries are ground with more "hook" in the rake face giving it a free cutting action to shear the stainless. They also have more radial relief to prevent friction hardening and binding.
Greenfield EM-SS taps are very popular. Here's some trivia for you: Any high performance tap from Greenfield will have "Japan" etched on the shank. This is because Greenfield buys them from YMW Tap in Japan. YMW is the old Yamawa brand, the same one old timers refer to as "ya-mama".
Here's your tap:
http://www.jlindustrial.com/CGI/JIS...=&scrNtt=gsfh&x=0&y=0&Ntk=Keyword+Search

See the price?
Here's what I sell locally for about $7.50
[edited]

For the money it's hard to beat YG1 taps. Find a local dealer and try some.
Oh yeah, Castrol Moly-D tapping fluid works well in stainless.
Good luck,
Curt jims

Apologies to all for recommending something which evidently is not easy to source over your way, these days, at any rate (the old timers probly have them tucked away)

Neither McMaster Carr nor MSC appear to list them. They are sometimes referred to as "progressive" taps, but Machinery's Handbook describes them, under "serial" taps.

The best ones I have were made by SKF before they joined up with Dormer, and there was one guy here in NZ who worked for SKF who could access them direct from Sweden (none of his colleagues even knew SKF made taps !)

I bought a lifetime supply from him over twenty years ago, because I do a lot of hand tapping on stainless of unknown provenance which is where they REALLY come into their own (in situ, marine stuff mainly), and good ones last forever, so it's a long time since I tried to source any.


But occasionally I've seen them at specialist tooling supply houses here in NZ.

Google also throws up a lot of hits in India, like dozens ! I guess hand tapping has become a third-world speciality ...
I bet you didn't know NZ was 3rd world, but in some respects we are :-)

Dormer have subsequently been swallowed up by Sandvik, but may still make them (one Google hit suggested that might be the case)

If you know any whizz-kids in that organisation, they might be able to help. I tried to download info from their interwobble site, but my Acrobat installation is acting up and/or their website was written by a sociopath.

Rallying your serve in the "I've never heard of that" game, I've never heard of roll-forming taps which were rated for stainless steel, or even titanium. So thanks for the OSG website recommendation, where I see both material rated as do-able.

I need to fit a cable gland to the top of my mast. There's a 6mm stainless steel plate bolted over the top & I need to tap this.

I have cobalt drills to drill the holes but I'm assuming I can't use my normal taps since they'll be too soft & I don't want to risk beaking one inside the hole.

Any ideas on what taps to get & where to get them? Can you get cobalt taps for instance.

Or, for that metter, would a stainless self tapper cut into a stainless plate? 'Normal' taps will cut threads in stainless steel without any problem. 300 series stainless steel is as soft as butter, the only problem with it is that it work hardens rather easily, such as by drilling with low loads and blunt drills.

Taps are hardened to a very high hardness and provided they are in reasonable condition will cut threads perfectly well. I have just finished tapping about 30 holes in stainless steel, using bog standard taps. Use plenty of oil, for both the drilling and tapping. [ QUOTE ]
Use plenty of oil,

[/ QUOTE ] Preferably soluble oil? Or will any light oil be just as good.

Centre punching the holes over enthusiastically can cause problems due to work hardening I believe. I also found that it makes it easier to clean the thread a little more often i.e. 1/2 turm clockwise followed by full turn or turn and half backwards gets rid of swarf . Soluble cutting oil is preferable, as being water based it has a greater cooling effect. However, I don't have any and only use engine oil.

I don't understand the paraffin statement, why would oil boil before paraffin? Boiling point of paraffin is 300C, boiling point of machine oil is 380 - 400 C. Be sure not to harden the stainless when drilling it. Then use a proper set of taps - ie 1st and second taper and plug. If going right through you wont need the plug but using the two different tapers mkes much lighter work of it and hugely reduces the risk of breaking a tap. parafin is good for alumin not stainless. Any oil is ok, proper stainless cutting fluid is best but expensive for one off job. Use a good quality HSS tap not a HCS. 1/2 turn in and 1 turn out will remove tap! Good to clear swarf if hole is deep and blind. Drill hole square and tap square. If you are buying taps specially, and don't mind the extra cost, go for high speed steel, (HSS) ground thread taps from an Engineers' Suppliers, rather than carbon steel from DIY outlets etc.. They work much better in stainless steel, but perhaps a bit extravagant for a "one-off". Also, drill the hole a few "thou" over the recommended tapping size. Your suggestion of a self tapper doesn't sound very good for 6mm plate if you want any strength. The right drilling and cutting fluid is tepmatic or similar. Your success depends on drilling. If you work harden the SS while drilling, you can forget cutting thread.

Paraffin is used for aluminium.

All special thread cutting fluid consists mostly of acetone with some oils added. Acetone evaporates and so cools the work, no work hardening can take place.

I make my own mix, but have used pure acetone for cooling in a fix.