Machining stainless steel (304 & 316)

[mlucek]

After a bit of searching here, I couldn't sort thru the various postings about cutting stainless steel, so I figured I'd ask to get a clearer answer.

I need to make a connecting rod/piston for small steam engine. Plans call for stainless - 316. It's about 1.1" long, 3/16" square head - 1/4" long on one end (w/hole to connect to crank disk) with the rest of the stock turned down to 0.172 (piston end)

Not having any experience with stainless till a few days ago on a full-sized lathe. I put the 3/16" sq 316 stainless bar stock into a collet, center drilled the other end, promptly broke off the end of the #1 center drill in the stock. No prob, left it plenty oversized, & attempted to part off the end to get past the embedded tip of the ctr drill. That didn't go too well either LOL .... what I'm assuming was a HSS cutoff blade, after it got part way thru the 3/16" stock, it flexed a bit and left a little nub on the end of the stock. Carbide lathe tool took that off with some trouble. Figured 1000 RPM with carbide & 3/16" would be ok, still got quite hot for what little there was to face off. Guess the 316 work hardens pretty easily. Oh well scrapped that part so got to practice with a bigger center drill and managed to center dril and then turn a decent dia on the remaining scrap. Practice practice.

OK, enough background. Went to the local metals shop, got some 1/4" 304 stainless bar stock. So now I need to mill it down to 3/16" and then turn the longer part down to 0.172 o.d.

Cutting speed of 50 sfm for 304 from a couple of places on the net ?

Using a mill, what speed is the best for say a generic 1/4" (HSS I'm guess) E.M. ? 800 according to (4 * 50)/0.25 ? How about feed (manually) ?

Then on the lathe - similarly, (4*50)/(3/16") = ~1000rpm. How about for a carbide bit - usually 2-4X faster ?

Do those number sound about right ?? Lots of cutting oil I take it to cut down on the heat ?

Thanks folks !

Mike

[sconisbee]

I cut a hell of alot of 316,304 and 303 stainless all the time, as for speeds well i usually go by feel and chip colour, with and without coolant depending on the job, so im not much hel there....but, what ever you do dont let the tool dwell and rub it will work harden in an instant which is why you had troubles with centre drilling. Stainless can be a bit gummy too so watch out for a built up edge on the tool.... cutting stainless isnt as hard as it seems as long as you dont dwell.

goodluck

[wierdscience]

You need the lube to prevent edge buildup(chips sticking to the edges).Like said already don't dwell it will work harden and then your in trouble.

I personally like cobalt tooling for stainless,HSS doesn't last too long and carbide isn't the best choice either.

[lane]

Use 303 cuts lot better than 316. 316 is not bad if you know how but beginners should stay away fro it if they can. Also stay away from carpenter 20 SS.

[hdj80]

Lane and others

A mate turned up with some 316 bar that we wanted some trailer roller pins made for. He claimed it was a machining grade 316. I know there is the low carbon welding version but no reference to a machining grade.
Well anyway this stuff cut remarkbly well. Tool life was good with carbide with no work hardening. I was cutting with 1/16th DOC repeatedly and the work was hot as heck.
Now I wonder if it was actually 303. It is autenistic but other than that it's shiny stainless metal.
Man it machined nice though. Lovely finish off the tool....not doughy nothing. Great runs of golden swarp pouring off the job and a good candidate for a coolant feed.

[sconisbee]

Machining stainless parts was my first ever job even before i worked at this full time i was doing bits on the side was kinda like being dropped in the deep end when you cant swim . I dunno i guess theres alot of lore around stainlesses and sure some are a real pain but they arnt as bad as they sound. its dwelling thats trouble. i cut primarily 316 and it cuts like butter with the sandvik ccmt inserts bought for the purpose. oh while i remember there is one issue with it, unless you keep up the feed you can end up with some real nasty stringy chips, i mean like the largest i have had was around 3 foot or so before breaking. but as craig says with carbide it turns beautifully though it can be run dry i do recomend coolant not necicarily for tool life but to save the burns on your hand when you take the part out the chuck