Both the Spherometer and Optical Flat where covered in the 3-Plate method sections. Both of these inspection tools where critical in the cast iron lapping process. In this section we'll cover the other 2 critical tools which are more specific to granite surface plate inspection: the Autocollimator & Repeat-O-Meter.
Autocollimator
Right out of the gate I want to say that the Differential Level by Federal or Mahr is a better (faster, easier) tool to use to inspect granite surface plate overall flatness. Unfortunately, these tools are very expensive and even used units on Ebay command high prices together with a high risk of buying a damaged product. An autocollimator, although more time consuming to use, is easier to obtain.
The unit I picked up was a used Davidson Optronics D-638 (pictured below) which is a very solid unit fitted with easy eyepiece viewing and a well-positioned easily read micrometer drum.
The unit I picked up was a used Davidson Optronics D-638 (pictured below) which is a very solid unit fitted with easy eyepiece viewing and a well-positioned easily read micrometer drum.
An Autocollimator projects a beam of collimated light onto an external reflector. This reflected beam is then focused and detected by a photodetector. changes in the inclination angle of the reflector can then be recorded in multiple positions to obtain a straight-line profile of the surface. Notice in the above images the "Mirror Mount" (or "Sled") has a specific "Base Length" and the "foot to foot" matching is critical to understand and respect during the straight line 1D measurement.
Inspection of the 2D flatness is done by following the "Moody Method" which collects the 1D straight line profile data of 8 lines across the plate in a pattern as shown below. This data can then be plotted manually or inputted into software to output the surface topology.
Inspection of the 2D flatness is done by following the "Moody Method" which collects the 1D straight line profile data of 8 lines across the plate in a pattern as shown below. This data can then be plotted manually or inputted into software to output the surface topology.
Some design elements are required to perform the inspection together with the autocollimator.
The Mirror Mount (or Sled) should be chosen based on the size of the plate you're inspecting. In my case I'll typically be inspecting plates on the smaller size so the foot to foot placement I chose was 2.125". This allows the required 2.6" minimum distance from the edge of the plate while maintaining the minimum number of 6 straight line measurements positions for the Sled. So for an 18" wide plate... 18-2.6-2.6 = 12.8 (round down to 12.75) and then 12.75/2.125 = 6. The Vermont Photonics demo software explains this very well and also provides some calculators in the software to help. Now this Sled size is perfect for an 18"(x24") plate but of course will also work with larger plates. Anything over 24x36" I'd consider making a longer Sled to reduce the number of data points.
The Mirror Mount (or Sled) should be chosen based on the size of the plate you're inspecting. In my case I'll typically be inspecting plates on the smaller size so the foot to foot placement I chose was 2.125". This allows the required 2.6" minimum distance from the edge of the plate while maintaining the minimum number of 6 straight line measurements positions for the Sled. So for an 18" wide plate... 18-2.6-2.6 = 12.8 (round down to 12.75) and then 12.75/2.125 = 6. The Vermont Photonics demo software explains this very well and also provides some calculators in the software to help. Now this Sled size is perfect for an 18"(x24") plate but of course will also work with larger plates. Anything over 24x36" I'd consider making a longer Sled to reduce the number of data points.