Thread: machining stainless

G'day Gavin,
304 stainless is not one of the free cutting grades and can be dificult to macine.
You're right, it will work harden so dont let your tool rub on a single spot.
You don't need to take particularly agressive cuts just keep the tool moving when in contact with the work.
If you have carbide tooling, use this for all your roughing and if you need a fine finish use HSS for the finishing cuts but keep these fine or your tool will wear quickly.

Surface speed (HSS) for 304 is around 120feet/min from memory so you are probably looking at around 450 RPM for your finishing cuts, about 4 times this for carbide.
Of course these will depend in many things such as size and power of your machine so start with this and if you get chatter reduce the speed.

Single point threading stainless can be very tricky as it tends to tear and preventing work hardening is dificult because the rear of the tool can rub. Trial and error might be your only option. Try cutting a thread in your stock before you machine to size and see what combination works best.

As always, have your tool sharp, stainless tends to stick and build up on your tool tip so keep an eye on that too.
Good luck.
Cheers,
Greg.

I have just finished turning quite a few bushes in 304 (16mm rod), before that I used 303, which is much easier to machine and get a good finish on.

Now, I am by no means an experienced machinist. I have 9x20 lathe with a QCTP, using 10mm HSS, running at 800-900 RPM, taking 0.5mm cuts (off diameter). Some bushes go down to 10mm others to 13-14mm. I found I could get quite a few without sharpening th HSS in between and still get a good finish, particularly when using autofeed.

Be very careful of the swarf, I was getting fairly long ribbon swarf coming off, SS doesn't chip as well as 1214 etc, but that could be my grinding angles on the tooling. It can cut flesh quicker than you realise (DAMHIKT-just had some stitches removed)

Having said all that, if I had the choice I'd use 303 any day, it has slightly lower corrosion resistance, not an issue in my application, but is much easier to machine.

Thanks for your input guys - I'm off to the workshop now to see how it goes. I tried to get 303 but it's unavailable in SA, the 304 I got from Surman Metals is supposedly an improved grade of 304 with machining characteristics close to that of 303, the guy there said that 304IM is supplanting 303 - we'll see!