Tapping 303 S.s.

[drill]

Was wondering whats the best method a cut tap, roll tap, or a hi spiral.
The tap that I need to use it a 4-40 gh3

[mark c]

I like to roll stainless. Especially small holes. Use a good tapping oil

[widgitmaster]

What is the problem, 303 SS is a soft stainless, which usually taps easily with a two flute spirial point cut tap using brown threading oil! If the print/project allows, simply use a slightly larger drill to make tapping even easier!
I would not use a hi-spiral tap, as they are weak and tend to snap in stainless steel!
If you choose to use a roll tap, I would suggest using a light spindle oil instead of thread cutting oil, because roll taps displace metal instead of cutting it and need lubrication!

Are you power tapping with a tapping head, or hand tapping each hole?

Hope this helps!
Eric

[gibbsman]

There are a few factors that need to be taken into account, are we talking about a blind hole or a thru hole. If it is a thru hole a spiral point tap is typically a good choice, if it is a blind hole then a hy spiral tap should be considered. If the tap depth is relatively deep around .30" dp. or more then you should tap drill to the top of the tolerance - see the machinist handbook. A tapping compound as opposed to coolant may be preferable. I have heard that you can use a roll tap in steel but I personally have never done it. In any case, do yourself a favour and spend the extra money to get a tap designed for use in steel, I have had good luck with 'OSG HY PRO TAPS' for all materials.