Minimum recommended SFM for carbide inserts

[AlbertNakaji]

Given stainless steel (nothing exotic) on a lathe what is the minimum recommended "speed" (SFM) for carbide inserts. Certainly, carbide can be used at slow speeds, but what is considered the optimum SFM range?

[GlennW]

There are lots of variables there, but the insert manufacturer should be able to provide specifics for the grade and edge prep of the particular insert.

There are many different grades of stainless, some machine easily, and some don't.

Work hardening can be a huge issue with some alloys, and the use or non use of flood coolant or oil can have a huge effect.

[johnfreese]

There will be a minimum speed needed to obtain a good finish, especially with negative rake inserts. You will probably need to experiment a bit.

[dbstoo]

Given stainless steel (nothing exotic) on a lathe what is the minimum recommended "speed" (SFM) for carbide inserts. Certainly, carbide can be used at slow speeds, but what is considered the optimum SFM range?

I have threaded using carbide inserts while turning the chuck by hand. That's SLOW.

The OPTIMUM SFM depends on the metal that's being cut and the trade off of faster production VS tool lifespan. Spinning fast and digging deep will produce more parts per hour, but you may need to change the insert before the part is done. Slower and shallower can mean fewer parts per hour but the ability to use the same insert for 10 times longer. In essence, if you only need to make one part a week, you may want to optimize for tool life.

In a home shop, I use the middle SFM from the charts for the material that I'm working on. I only change inserts occasionally.

Dan

[pete]

Far too many variables involved as Glenn's already said to give a set number for rpm. Carbide grades, surface coatings, honed or unhoned edges and likely a lot more I haven't thought of yet. Optimum slowest speed there designed to work at would involve what surface finish is tolerable if you go lower. Since any carbide I know of is designed for high speed metal removal rates it's probably a fairly narrow range for the high / low recommended rpm. At least it seems to be for any of the tip manufacturers tables I've seen. Most of the well known industrial carbide tool manufacturer's should have those sets of tables for each type of tip design, depth of cut, coating, chip breaker, and what rpm range and material it's good for. If you need lower then going to HSS or even High Carbon home heat treated tooling would be the most logical. I've read that HSS can be sharpened to a higher level of sharp than carbide can, High Carbon can be honed a bit sharper than HSS. Carbide was developed due to it's resistance to failures at high heat and high cutting tool pressures at those high speeds. Some of todays micro grain factory honed carbide edges may approach what HSS can be sharpened to. But the sharper the edge the less resistant it is to remain at that level and for much shorter time periods. There's a few older Youtube videos showing high magnification shots of how the chips start to form with carbide. At it's optimum speeds the metal due to the heat and pressure starts to plasticize just like John said about getting a good finish. My best guess is it's mainly that result that then gives the best surface finish. Without it then the surface finish degrades. The metal tears or is pushed off instead of it flowing.

And that work hardening Glenn mentioned usually happens due to low feeds and depths of cut. The tool starts to rub instead of cut and then the work surface hardens. The following cut just gets progressively worse since it will work harden even more. So higher feed rates and / or depths of cut are needed so it can't start. In a home shop carbide is likely more of a convienience than a real necessity. Chilled cast iron castings or exotic alloys maybe being the exceptions. Hardinge HLV lathes are designed to do single point threading at real high speeds. Carbide would probably work well on them.For our more normal manual machines threading is done at far slower speeds. HSS threading on my machines gives a lot better surface finish than any carbide I've tried. So if I need the low rpm I don't even use carbide.

I have a number of industrial grade replacable tip and braised carbide boring bars. I've still bought some of the A.R. Warner HSS tips that fit those replacable tip bars. With our lighter machines those HSS tips can give better surface finish results than the carbide can. Honed carbide might work just about as well, but I haven't tried those for boring yet. Your question was a bit open ended so I don't know if any of us have properly answered it.

[SteveM]

I have threaded using carbide inserts while turning the chuck by hand. That's SLOW.

SHPM - Surface Hands per Minute

Steve

[AlbertNakaji]

Thank you all for the comments.
I think the answer is that I check the manufacturer's specs. Never thought of that.