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Specifying/Inspecting conical surface

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dgallup

dgallup

Automotive
May 9, 2003
4,712
We buy machined and hardened 440C blanks that we grind to the finished dimensions. Inside the blank is a conical surface that becomes a valve seat. We have to grind this to a very tight roundness and surface finish. If the blank part has grooves or ridges on the conical seat it causes our grinding wheels to break down and we don't meet the finished part requirements.

We currently specify an Ra value of 32 microinches on the blank part seat, however, because of the small size of the parts we have to use the shortest (.003") sampling length to measure them. At this short sampling length the tool marks are filtered out. However, it is the peaks & valleys of the tool marks that cause us problems with our grinding wheels breaking down. If the part was bigger and we could use a longer sampling length these parts would fail the surface finish specification. Reduceing the Ra value is not effective because the measurement is still ignoring the tool marks.

So what I need is some other way to specify the short conical surface in the blank part. If it was a flat surface I could use flatness or if it was cylindrical I could use total runout. What can I use on a conical surface?
 
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Have you thought about changing you measurement parameter? Since this is a critical surface you may want to look at Rq(RMS) and possibly even ask for a print change that would ask for a tighter tolerance, or a note that would specifically ask that no nicks, sctraches, or ..... be permitted in this area?

Either of these ideas would probably cause the price of the blanks to increase though.

Mike
 
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The problem with notes (no nicks, etc.) is that they are totally subjective. I need a numerical criteria. Any surface finish parameter Rz, Rmax, etc, is going to suffer from the same high pass filtering due to the short cut off length. I need something that will not filter out the tool marks.

I'm thinking that profile of a line is what I want, possibly in conjunction with profile of a surface?
 
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An issue that comes to mind with the profile of a line or surface would be the specialized equipment that you would need to accomplish what you had asked.

From my research on surface finish, I found that on Rq(RMS) IS sensitive to peaks and valleys on a surface and the reading accounts for the extreme peaks or valleys whereas Ra averages them into the reading.

Mike
 
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I'll try Rq and see if we get sepparation between the good & bad parts. We have equipment to do profile so that is not an issue (but our recieving & inspection people may not want to use it).
 
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