Best tap for stainless steel?

I used to use spiral flute machine taps on various stainless steels, all about 30 HRc hardness. They worked OK even as hand taps but I would always use a drill press or lathe if possible. Just tighten the chuck with enough force so the tap will work, but then spin in the chuck when it hits the bottom of a blind hole and not break.

 

Many stainless steels work harden and so must be drilled and tapped with HSS tools.
Some require cobalt high speed steel = HSS-Co, or carbide tooling.
It is essential that you use a cutting fluid. http://www.bssa.org.uk/topics.php?article=192

 

Ideally use starting and finishing taps .

For hand use the HSS TiN coated progressive cutting series of taps gives excellent results on difficult materials .

This sort of thing : http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Taps/Serial-Hand-Taps-Metric-Coarse

 

I'm on the other side of the planet and I buy unusual taps from Victor Machinery Exchange in Woodside, New York, USA. They have the contacts to get special taps made to order if needed. http://www.victornet.com/departments/TAPS/100.html

If the stainless steel you have does not finish well then you might consider drilling an oversize hole, then using an STI tap and an insert.
That will give you a very good finish on the "recoil" thread insert. It also uses a larger tap that is less likely to break.

 

Cheap taps are commonly made from Carbon Steel. More reliable, harder taps are made from High Speed Steel.
HS is almost certainly “High Speed”, the steel goes without saying.
G H3 is probably the finish and oversize. H3 may mean 1.0 to 1.5 thou oversize.

Find some way to support the tap in line with the hole. Experiment with cutting fluid, oil or soap. That will almost certainly reduce breakage. Some taps in some materials produce the best finish by cutting continuously. Others require a slight advance, then a reverse to break off the cuttings. You will need to experiment to find the best technique.

Corrosion is usually the most important parameter, but if possible select a grade of stainless that is free machining.

 

One thing that is important when tapping difficult materials is getting the pilot hole the right diameter .

Using pilot holes slightly larger than the theoretical correct size eases the load on the tap considerably and reduces chances of breakage .

 

Use a tap lubricant specifically for stainless steel. Tap Magic is commonly available. And never turn the tap more than 1/2 turn without backing off at least 1/4 turn. Other than that any decent quality tap will work. It's also a good idea to have a starter tap and a plug tap. The starter tap makes it easier to start the threads "square". If you start at an angle you will not be able to complete threading, and the tap will break.

 

Sorry if you know this but... Are you just trying to screw the tap in or are you doing the normal half turn in and a quarter turn out?

 

I used ridgid for most purposes, but "working a tap" not (over working it) usually produces the desired results . Use a LOT of cutting oil and if you get in a bind stop, HSS an CHS taps will snap just as quick if not quicker than any of the cheapo taps you can buy if you over work them in stainless.

These are pretty good:

http://www.northerntool.com/shop/tools/product_200624059_200624059

 

Different stainless steels work harden differently. If you “work” the tap you may harden the material, then break the tap when it hits the previously hardened wall. If you continuously turn the tap the chips will be hardened but the material being cut will be fresh and normal hardness. The speed of cutting is important because the hardening process is a chemical phase change. In some materials, if you back off torque momentarily you may hit a hardening wall.

Avoid machining old, salvaged or unidentified stainless steel. You will probably need carbide tooling.

There are different flute profile taps, spiral flute taps are designed to cut continuously while straight flute are designed to be reversed to break the chips. Don' be mislead by “spiral point”, in hard materials they are often as bad or worse than straight flute because they clog with chips, especially in blind holes. The spiral path of the flute decides the curl of the chip and the quality of the thread finish.

There are many different stainless steels. What grade material are you threading ?
How long or deep is the thread you are cutting? Blind or through hole?
Is your material new or aged? Has the material been subjected to cold temperatures? for what periods?