Pseudo-roundness is best defined as any shape not perfectly round, which exhibits constant dimensions when measured in any direction between two parallel planes, ie, with a 2 point micrometer.
- A non-round (pseudo-round) workpiece can mike as if it were round using only customary two- point, two- plane gage;
- A pseudo-round can be detected with a three point gaging system;
- Given the number of lobes from using a 90 degree V-block and indicator, a savvy shop hand can choose the proper included angle V-Block to determine the difference between R and r;
- Salvage of individual parts is possible given sufficient stock to remove;
- Bars exhibiting lobing can be ground to the next incremental size and the lobing eliminated with attention to details of wheels settings, support plate location and geometry, and rotational speed.
In centerless grinding, a geometric condition can be encountered that puzzles most shop hands and often stumps the old pros. That geometry condition is called pseudo-roundness, lobing, or the triangle effect.
Pseudo-rounds have odd numbered crests , which because of the exact opposition of their high to low spots always have a constant “Miked “ Dimension. This is explained by the fact that the gaging planes of R and r are tangent to the arcs of the profile, and perpendicular to both radii which belong to the same common center. That is:
Miked Dimension = R (center to apex) + r (center to low spot)=Constant value
I first encountered this when I had a batch of material that miked fine but didn’t pass the customers roundness test. That material was lobed.
I’ll spare you all the geometry lesson, but here are a half dozen facts that I know from my 30+ years experience in the bar business :
- Everybody will tell you it can’t be fixed. They would be wrong. Round gage pins are held to millionths of an inch of roundness, and they didn’t start off life at that level of perfect roundness before grinding
- Yes, it seems impossible to be able to fix it if you know that the centerless grinder has the high spot hitting the regulating wheel while the pressure from the grinding wheel holds the work down against both the regulating wheel and the blade support, since the diameter obtained in centerless grinding is determined by the distance between the regulating and grinding wheel. But as I mentioned above, the fact that a constant diameter results does not assure perfect cylindricity.
- In order to assure a more perfect circle, adjustments of the geometrical arrangement of the grinding and regulating wheels, support blade need to be made. In my experience, increasing the speed of rotation also seems to help.
- While you can regrind to eliminate the lobing, that doesn’t mean that A)it will come in both round and B) within your originally desired tolerance. Unless you have left a great deal of stock on the workpiece, there is usually insufficient material to true up to your hoped for diameter.
- Use a three point micrometer if roundness / cylindricity is a critical feature on your product. The sketch above shows you how a constant diameter solid workpiece can in fact not be a perfect circle. A two plane gage will not reveal this to you.
- A standard 90 degree V-Block and indicator will do, but if you want to measure the difference between R and r (R-r) you will need V-block of the angle given by the following :
2A = 180-(360/N)
where N is the number of high points or lobes. For a three lobe configuration, A becomes 30 degrees, (the half angle) and the V-Block should be 2*A or 60 degrees included angle.
