Cutting a slot in a 304 stainless steel part?

J

javaduke

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Well, I know, it's a rookie question and there are multiple answers to it, but I just wanted to get a second opinion (and probably a third one too :)
As a part of my project, I need to make a few custom slotted screws out of a 304 steel rod. I made the screws on my lathe without any issues, but when I moved to my mill and tried to cut a slot with the slitting saw, the screw work hardened instantly on me, the saw started howling like a banshee and didn't cut anything at all.
So my plan B is to get some .040" carbide endmills, but I know these things break instantly when you just look at them from a wrong angle. So what would you guys recommend in terms of feed and speed for this? I assume I'd spin it as fast as I can (something like 750 rpm?), take .005 deep cuts and feed very slowly, and constantly pour oil on it. Is it a good plan or is there a better one?
 
The mistake was using 304 stainless, If you used 303, there would not be work hardening, besides that I suspect you were running the slitting saw way too fast. If you use an endmill, 750 rpm would be way too slow, should be on the order of many thousands of RPM, and likely it will break anyway. A slitting saw is the way to go, operated at a conservative cutting speed with cutting oil or tapping fluid.
 
304 can be a PITA. I still think the slitting saw is the right tool for the job, but calculate and apply the feeds and speeds for that material. You need to get under the cut with enough feed, and you might be running too much RPM and rubbing teeth which leads to work hardening in a hurry. 304 is machineable, just a bit picky about how it's treated.

In the future, "303, she's for me. 304, she's a...."

Actually, 304 is a nice material once you learn how to treat it. Always leaves a great finish, too.
 
On the subject of SS steel--- I have acquired some misc. shorts of SS. No markings.
If SS work hardens how do you go about annealing it? Temperatures/times? I looked on the net but the information was geared to having way more facilities than what I have.
 
Stainless is often sold annealed if it is martensitic (4xx series). It costs extra and when bought new there will not be a question about its state.

I also have a lot of unknown stainless drops, and Murphy says if it's austenitic, it's probably gonna be 304. If I'm lucky it's 308 or 312, and I will know it almost instantly on the lathe or mill.

Austinetic stainless does not heat treat at all, so it can't be annealed- but it sure work hardens! It's a different mechanism of hardening and needs a high infers rate. It will no let you take skin cuts unless you use a very acute tool, which is a tradeoff for finish. As I said before, 304 finishes beautifully if you feed hard. Small lathes will struggle with the tool pressures needed.
 
303 is really the best bet for such parts, think of it as a screw machine free cutting material, it has very little tendency to work harden, in my business, I used it for all machines stainless parts.
 
pontiac428 said:
Austinetic stainless does not heat treat at all, so it can't be annealed- but it sure work hardens!
Click to expand...

"304 stainless steel readily work hardens. Fabrication methods involving cold working may require an intermediate annealing stage to alleviate work hardening and avoid tearing or cracking. At the completion of fabrication a full annealing operation should be employed to reduce internal stresses and optimise corrosion resistance. Solution treatment or annealing can be done by rapid cooling after heating to 1010- 1120°C (2192±ºF.)" Info from Thyssenkrupp
 
I'm not going to argue with Thyssen-Krupp, but as usual there is more than meets the eye. Nonmagnetic steels aren't hardenable. Work hardening is a different mechanism that occurs at the cut from mechanical disruption to the microcrystalline lattice, it's skin deep. Putting things in ovens is referred to as annealing, but those steels don't anneal like carbon steel does around critical points, stainless doesn't have the same allotropic time-temperature transformations, they normalize where the surface has work hardened. Not an allotropic change in the usual sense as with heat treatment. Not enough carbon in 300 series to do that.
 
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