Adventures with Stainless Steel

[Stepside]

The task is some 1/2-20 threads in some Stainless of unknown heritage. I know it was a propeller shaft from a 5 litre hydroplane. I also know it is magnetic.

So I drilled a 29/64 hole 1.25 deep in a couple of pieces and proceeded to tap with a brand new quality tap. As they are blind holes it was tap a bit then remove and clean the tap and the hole.
Part way down there were some problems with the quality of threads.then it seemed to get better. Obviously it did repair itself but it cut with ease.

So here are the questions/thoughts. 1) Does some Stainless have hard spots possibly caused by getting hot and if so what rough temperature might it take? 2) I made some soft jaws for my vise that are 1/2 inch Baltic Birch plywood held on with "Super Magnets". Would it be conceivable that the magnets were affecting the clearing of chips?

Your thoughts and questions.

Thanks in advance

Pete

[lakeside53]

It is possible to work harden spots in "stainless" when drilling... you need to keep constant pressure and cut, but clearance needs pecking. I find aggressive pecks work best in 316/303. Is your stainless magnetic or non?

[Richard P Wilson]

It is possible to work harden spots in "stainless" when drilling... you need to keep constant pressure and cut, but clearance needs pecking. I find aggressive pecks work best in 316/303. Is your stainless magnetic or non?

Second line he says its magnetic.

[lakeside53]

ha! he did too. That pins it down to precipitation hardening stainless or 400 series


416 is often used for prop shafts, but not in annealed state. It is heat treated after use and it's possible for the heat treating to be uneven, and in any case, makes for pretty tough stuff. I have some old 1 inch 416 prop shafts. Not nice to machine without annealing.

[Stepside]

Lakeside

Thanks for the information. Mystery Metal is no fun.
BTW I used my larger lathe to drill with, not your Drill Press.

Pete

[old mart]

The best way to drill ss is to run slow and feed fast with a sharp drill to reduce work hardening.

[mattthemuppet]

a spiral flute tap would have worked wonders here too - no need to remove tap to clear the flutes, just tap all the way to the bottom. I've read of people putting aside taps just for use on stainless, I'm guessing to keep them as sharp as possible.

[Willy]

416 stainless, if this is what it is, is wonderful to machine in the annealed state. It can be hardened thru heat treatment but it does not work harden like 304 or especially 316.
It is often used as the material for making rifle barrels. It's corrosion resistance is greatly enhanced by polishing. I've used it several times for making plungers for use in pressure washers.

[Ries]

Actually, 304 stainless will get mildly magnetic if work hardened. Its noticeably less magnetic than mild steel, but it can certainly attract a magnet a little bit. Usually its pretty much non-magnetic fresh from the mill, but I have gotten it to be magnetic when I am heavily forging it with a power hammer.
300 series stainless is annealed by heating it to red and quenching in water.
Counter-intuitive, I know, but unlike carbon steels, water quench makes it softer.

[Stepside]

The rest of the story: After being informed that the shafting I was tapping was Aquamet 19, (Spec sheet says it is a modified Type 304 Stainless that is non-magnetic) the boat owner arrived for the parts. I stuck a magnet on the shaft and referred him to the Spec sheet. This jogged his memory as to the source of the shaft and its dubious pedigree.

My thoughts are 1) it is a "mystery metal" in that we don't know what or where it came from. 2) It might have had some hard spots do to drilling 3) the magnet attached soft jaws caused the chips to get caught in the tap. It is a 1/2-20 thread.

The job is finished and the parts delivered.

thanks for all the ideas.

Pete