Tips to get the Rmr superfinishing requirement on HVOF Tungsten Carbide coating(landing gear equipm) 1

[Jeremya]

Hello guys,
We bought a new automatic superfinishing machine after grind. We need to finish Tungsten Carbide (10% Cobalt, 4% chromium) to the following parameters:

Ra= 4 µin max
Rz= 40 µin max
Rp= 8 µin max
Rmr(Tp)= 70-95% @ .25 Rz deep

We are getting good results with all parameters but fail at getting the bearing ratio right.
We are using 15 & 30 micron diamond lapping film.

After grinding with a 220 grit diamond wheel and prior to superfinishing we typically get the following results:

Ra= 5 to 7 µin
Rz= 30 to 40 µin
Rp= 20 to 25 µin
Rmr(Tp)= 10-15% @ .25 Rz deep

Then, after superfinishing, typically results following:

Ra= 1 µin
Rz= 30 µin
Rp= 6 µin
Rmr(Tp)= 0-15% @ .25 Rz deep

We actually think we are getting some kind of too good surface finish after grind and\or our superfinish paper isn't agressive enough.

What do you think? Any piece of advice or solution direction to give?

Thanks

 

[WKTaylor]

Jeremya...

Did the equipment vendor PROVE to you [your company] that their equipment/materials/processes could meet Your severe TC superfinishing requirements BEFORE You purchased the package deal?

Are You following internal or customer or vendor supplied process documents for grinding the finish... or the SAE practice?
AMS2449 Grinding of HVOF Sprayed Tungsten Carbide Coatings Applied to High Strength Steels + AMS2452 Superfinishing of HVOF Applied Tungsten Carbide Coatings?

Which TC application process are You using?
Company proprietary?
AMS2447 Coating, Thermal Spray High Velocity Oxygen/Fuel Process
AMS2448 Application of Tungsten Carbide Coatings on Ultra High Strength Steels High Velocity Oxygen / Fuel Process

Which TC material are You using?
Company Propritary
A-A-59315/*
AMS2435
AMS5792
AMS7879
AMS7880
AMS7881
AMS7882

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[CoryPad]

I don't think your as-ground surface is too good, you want it smooth. What does the surface look like after the 30 micrometer film? Have you removed the ground pattern? When you go to 15 micrometer film, does that remove the pattern from the 30 micrometer film? You may need to go to a smaller diamond (9, 6, 3...) to meet that final requirement.