Quote: If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means twice the the error that you could detect by comparing one disc with an optical flat as reference.End quote
There must be something wrong with my measuring. All three disks are showing to be concave. Two are consistently on all measuring locations 0.0003" concave. The third less at 0.00025". I checked it today again thinking there may be a difference in the way I measured, but it is the same result. grinding one on top of the other would produce concavity/convexity. I checked the specified sequence of which goes on top or bottom and I copied it OK. I kept check on the sequence of grinding by marking it off on a sheet. I counted the strokes for each combination. I washed the disks each time a sequence was finished. As you say, if the measurements are the same for all the disks, which they are more or less, I was splitting .0005" graduations to arrive at 0.0003 and 0.00025, then the disks are flat. That leaves the optical flat. Over ebay, a Russian one, Comes with a certificate
, could be the culprit. So I need to find out what is going on. I am making a bigger spherometer, cant do any more anyway
until the grit comes for polishing. I hope I can get it calibrated at the local machine shop.
The grit I was quoted for #1000 and #1200 is silicon carbide. Would I be wrong to ask for aluminium oxide?
Thank you Stefan