Hi Vic,

I don’t know too much about machining SS, but most certainly some grades work harden more than others. Cutting with blunt drills may be hardening it and seeing off your taps?

I don’t often break taps and most of mine are really old (but not threading SS). The last one I broke recently was simply due to me being too blasé. Not starting straight is another recipe for breakage – the tap slowly tries to cut into the side of the drilled hole – and may be something that happens by not taking 100% care.

I don’t buy cheap taps – unless for a ‘use-once’ job. Same with dies – I bought one from the net and it was just not up to the job. Same steel, different threads on each end (imperial and metric), and the old imperial die coped easily but the metric one chipped – too brittle and not tough enough.

edited to add that if the twist drill is not cutting straight for some reason, the tap going in ‘straight’ might be giving it a hard time – similar to cutting at a crooked angle.

Edited By not done it yet on 22/12/2019 13:39:37

Some types of SS can be problematic to drill and tap. The tooling needs to be sharp so that it gets a chance to cut. Drills and taps gradually loose their sharp edges and should then be relegated to less demanding metals. Ideally, a selection of drills and taps of a given size should be kept in stock, and swapped around to find the best performing ones for each job. I have tapped a tungsten alloy called Densimet which behaves very much like SS for 2.5mm holes. I bought new Europa taps for hard steels, and they didn't work well. I am in the fortunate position to have a large stock of donated taps to try out and an ancient one finished the holes easily. I think that taps designed for aluminium might work better in work hardening types of metal, but have yet to test the theory.

I'm impressed by Volkel brand serial taps for difficult materials. I get mine from Davethetools via E-Bay. 2.5 mm set would be £20. Not cheap but I think worth it. especially as having had the pleasure of shifting 3 broken 2.5 mm ones in sadly grabby aluminium!

Clive

What type of cutting compound are you using? I find that the thick stuff, the type with the consistency of grease will hold the chips and clog small taps. Try using a very light spray of something like wd40.

When I had to work for a living we had to make a small sample part from 316 SS. The shape of the part suggested that plate should be used, fortunately some was found. The material work hardened so fast that it was virtually impossible to machine. Some enquiries were made of the supplier and I was told that plate was made to be weldable rather than machineable. Bar was made to be machineable, and not weldable, and to make it machineable a small measure of (I think) sulphur was added. So some bar was obtained and found to machine beautifully.

This was up here in Australia, so it may only apply here, although apparently Australia hasn't made stainless steel for decades, or so I have been told.

Happy New Year and cheers!!

Bill