Hardness and Surface Roughness Testers for Shafts

Right now, we are looking at improving our ability to test parts, and a big part of that will be bringing production of test fixtures (shafts, press-in bores, etc.) in-house. Our customers provide the spec of their intended production shaft for size, roughness, and hardness. We would then be responsible to manufacture a shaft that meets that specification on one end, and mates up with the test rig on the other.

What do all of you use for measuring surface roughness and/or hardness on shafts? Immediate need would be for round shafts from about 1/2" up to about 4" and matching bores. Typical requirements for hardness in the 30s HRC, and surface finish under 0.5 µm Ra for shafts, and 1.5-3 µm Ra for the bores.

Ideally, I would only want one set of equipment for both ID and OD measurements. It seems like a lot of test stand type setups would have a difficult time with both of those, seemingly being ideal for flat surfaces. I'm well aware that indenters that are off-center of a round part can give false readings because of the pressure being applied on an angle to the surface. Is careful fixturing all that is required, or would a more specialized test stand be needed?

Thanks in advance for any and all feedback.

Are you through-hardening, or just surface? If through, use the ends of the shafts for indenting.

For roughness, I'd check out the Mits surface roughness testers:

Surftest SJ-21�- Series 178-Portable Surface Roughness Tester

There's cheaper copies out there, but if the budget is available get a good one...

For hardening, my plan was to choose a plain carbon steel that would "max out" somewhere near the target value. I saw a really nice chart showing what % martensite was possible in a hard quench for a given carbon content and the associated hardness that would produce, but I can't seem to find it again. My plan is to use that value, so for a min 35 HRC, I would select 10xx that can achieve 40 HRC maximum, heat the end that needs to be hard, and keep the end that needs to be stable cool. Quench, check, and then finish grind to size. Without finding that chart again, I might have to go with 1040 or 1060 and learn how to accurately temper things...

So in that case, using the end would be just fine, as long as the tester can accommodate a ~9" long part.

Thanks for the recommendation on the roughness tester. Looks like they have an adjustable v-groove setup for shafts as an accessory, which would be perfect. Size range is right around what I need, too.

BoxcarPete said:

Immediate need would be for round shafts from about 1/2" up to about 4"

It seems like a lot of test stand type setups would have a difficult time with both of those, seemingly being ideal for flat surfaces. I'm well aware that indenters that are off-center of a round part can give false readings because of the pressure being applied on an angle to the surface.

Click to expand...

Take care with the small diameter shafts. Even when on center, you will get a false reading. The smaller the shaft, the greater the error.

Gobo said:

Take care with the small diameter shafts. Even when on center, you will get a false reading. The smaller the shaft, the greater the error.

Click to expand...

So, given that the indenter is pushing against a surface that is "incomplete" in coverage of the point, there will be less material to push back against it and the reading will tend to be softer than the actual value, correct? What would be the typical error for a 1" or a 2" diameter?

The tolerance on the hardness isn't very tight; it is just to ensure that the shaft is going to have somewhat comparable wear properties to the actual part that enters production. If the shaft is completely unhardened and wears away causing a leak, the test isn't valid because the leak was caused by a failure mode that isn't relevant to the device under test. Conversely, if the shaft is ultra-hard and exhibits zero wear, a potential failure could be missed. A few points either way won't invalidate the results.