CNC Spindle Alloy ?

[Joe_CNC]

Does anyone know what alloy of tool steel is typically used for CNC machine spindles?

Thanks,
Joe

[HuFlungDung]

I doubt they use tool steel at all.

Machinery builders like C8620, which is a carburizing grade of steel. I wouldn't swear on it, but I would guess they might use it. It is particularly popular for gears. However, this grade requires carburizing and hardening after machining, so they expect to have to grind it straight and true after heat treating. This material is quite stable in heat treatment, if properly supported in the furnace. But, for the home shop, it might be easier to stick with the pre heat-treated steels, like C4140 or C4340.

See H3ndrix's thread on this same subject.

[Fastrip]

HuFlungDung is correct.
The average spindle is made of carburized 8620. All areas aren't necessarily carburized though. Generally any wear areas and areas that might be exposed to chips and the like are case hardened. The rest is usually hard enough (tough enough) at the 36-40Rc that 8620 hardens to all by itself. Any qualified heat treat or the proper process routing can achieve such spot case hardening.
That 36-40Rc is great for threads and fixed splines. Even some sliding splines are left 36-40Rc considering availible lubrication and the surface area and hardness of any mating part. Considering the extra potential cost to grind splines often times splined areas are not case hardened, finish machined and hobbed after the part has been hardened. As long as the core hardness is held to the lower level, even machine work with HSS is quite possible.
Another case treatment often used is gas nitride. A nitrided case has very many advantages and is reasonably availible. Heat treated 4140 would probably be the most availible material for any such process. As long as the material is drawn at a temperature at least 50 degrees F above the highest nitride temperature. I'm not sure but 28-32Rc is a requirement I have worked with for heat treat material. The draw for 28-32rc usually runs between 1075 and 1100 F. That leaves room to nitride (probably Floe or 2 stage) up to 1025F. A proper nitrided case of .025-.030'' (specified at 3 points Rc above core hardness) is quite attainable. The advantages to a nitrided case are its anti gall charactaristics, corrosion resistance and the potential for up to 30%increase in mechanical properties are a few on top of the list. Because a nitrided case is not quenched (shocked) the parts can be very near complete prior to nitride. As a matter of fact, considering the percipitive or progressive nature of a nitrided case the parts need to be near finished.
Another material that would work well in the case of nitride would be Nitralloy 135M. This material increases nitrided hardness primarily due to the addition of aluminum as a alloy.

Without knowing what you are building, what you budget might be or anything else, this is the best I can do.

I hope the whole thing isn't too confusing.

If you need anything else or clarification of anything I will try to keep an eye out for a reply.

Good luck,
Watch the slivers,
Fastrip