Increase Surface Roughness on Hardened Stainless Steel? 2

[leopardforest]

Hi all,

We have a piece of test equipment that has a hardened 440C stainless steel shaft with a 12 rms micron finish and a case hardness (about 0.080 depth) of about Rockwell C50-55.

I need to bring the surface finish to somewhere within 35-125 rms microns to meet the spec of a keyless bushing that will attach a rotor to the shaft. What are some methods that I could use to increase the surface roughness?

Thank you!

 

[EdStainless]

How was it case hardened?
I would look at grinding with the appropriate coarseness of abrasive.
It will be delicate, you can't remove too much material, but you will likely need to take off 0.005-0.010" in order to get a good uniform rough surface.

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Plymouth Tube

 

[leopardforest]

EdStainless,

I can't remove that much material otherwise I will be out of spec for the keyless bushing in the shaft diameter tolerance. I wouldn't be comfortable removing more than .001-.002". It is a 1.500" shaft. I am not sure on the casehardeing method, as it was purchased from McMaster: [URL unfurl="true"]http://www.mcmaster.com/#precision-shafts/=qh7fx9[/url]

Any other options? Sand blasting? Diamond sand paper?

 

[EdStainless]

You could try blasting. The trouble is that you might embed some abrasive.
You could roughen it with some SiC paper (or CBN, or Norbide) but you won't have much control.
Could you use Locktite to help fix the bushing?

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Plymouth Tube

 

[tbuelna]

A shaft made from 440C is probably thru hardened rather than case hardened. If you wish to alter the surface roughness from the existing 12 microinch RMS to a 35-125 microinch RMS texture without affecting the shaft dimension, then there are processes like belt polishing, honing or abrasive blasting that should get the job done. If you want to precisely control both roughness and "lay", then honing or belt polishing would be the best choices.

 

[mrfailure]

Would shot peening with very fine hardened metal beads be an option?

 

[leopardforest]

EdStainless,

Loctite is not a bad idea, but due to the environment (completely submerged in water that can get rather hot) that depending on loctite to hold might be risky. I would like to try using paper like you suggested, and I think we might be okay as we have a large range to fall within to meet spec.

Mrfailure,

Shot peening sounds like a great idea as it would increase my overall strength and the surface roughness. Is there anything specific I should be looking for in a company that would do it? How well could they control finish?

 

[leopardforest]

mrfailure,

Maybe I shouldn't have said that shot peening would increase strength, but rather reduce residual stress from the manufacturing processes?

 

[mrfailure]

Shot peening will impart compressive stress on the surface; just remember the trade-off is increased residual tensile stresses directly underneath the area affected by shot peening.

 

[tbuelna]

Since the application involves a bushing fitted onto the shaft that relies on clamping friction to transfer torque, I don't think using shot peen or abrasive blasting to modify the shaft surface roughness is the correct approach. At the micro level, shot peen or grit blasting would produce a random pattern of shallow depressions surrounded by narrow raised ridges.

A two stage "plateau" hone process would produce a much better surface for a clamped friction interface. Plateau honing would produce a micro surface profile where the individual tiny points of contact on the shaft surface are uniform and consistent. The plateau honing process involves an initial hone operation using a relatively coarse stone, followed by a final hone operation using a fine stone. This leaves a surface consisting of a controlled pattern of flat plateaus surrounded by tiny valleys. This surface texture of asperities having a controlled plateau profile would provide a more consistent friction result in a clamped connection.

 

[leopardforest]

tbuelna,

I am not exactly sure what "plateau honing" is but I will do my research as it sounds like what I am really looking for.

One of my colleagues has recommended that we chemically etch the surface to get the desired finish. What are your thoughts on this process?

 

[tbuelna]

I'd stay away from chemical processes, since they usually require a fair bit of development work to get the process right.

As I said, your best bet is a mechanical process like honing. It will give the best result and is easy to control.

 

[leopardforest]

tbuelna,

I agree with you that the chemical process sounds like it is difficult to control, but due to powers out of my control it sounds like we are going to do the chem etch anyways. So I guess we will see how it goes.....