Machinery's Handbook, 31st Edition
Surface Plates 731 Clamp the straight edge to the surface plate in the position shown in Fig. 6 below. The straightedge will always be offset from the center of the line so that the centerline of the mirror will travel directly above the center of the reference line. Place the interferometer and retroreflector on the reference line closest to the laser head. Direct the beam through the interferometer optics and, using the vertical and horizontal adjustments on the tripod head, adjust until the beam returns through the optics and regis ters a strong beam strength on the beam strength indicator. Slide the retroreflector to the far end of the reference line and use the rotation adjustment on the tripod head to swing the beam back into alignment with the laser head until once again the beam strength indicator shows a strong beam strength. Using this method of Translate Near / Rotate Far you will be able to align the beam with the path of the reference line. Patience will be required, and the manufacturer will also have information regarding the best method for the specific model being used.
Clamp
North
Straightedge
Movable Mirror
West
East
TI
Clamp
Straightedge Face
South
MFG’s Namplate W/SER. No.
Fig. 6. Placement of the Straightedge for Measurement of the First Reference Line
Once the alignment of the first reference line has been completed, take the measurements at the stations recommended by the software calculations. The number of stations will change according to the size of the surface plate and the length of the footpad used for the retroreflector. Record the values at each station along the first reference line then return to the first position and verify that the reading repeats before proceeding to the next line. Great care must be taken to ensure a clean plate and footpad, free of residue or buildup. A very small amount of dirt or dust can cause an out of tolerance condition and result in many wasted hours of failure impact analysis and possibly recall of parts to be reinspected unnecessarily. When the data from all the reference lines has been collected, the interferometer software will calculate the overall flatness of the plate and make the determination as to whether or not the plate meets the specific grade to which it has been assigned or needs to be lapped to meet tolerance limits. If there is a failure determination, the plate will need to be lapped and reshot to collect the “As Left” data and close the calibration event with an acceptance of the calibration data. Lapping a surface plate is a very specific skill and should not be attempted by an amateur or enthusiastic technician however good the intentions may be. Lapping requires special ized tools and compounds as well as skill. An inexperienced technician attempting to lap a surface plate may well cause more damage than is already present. Always consult an experienced and proven professional in this field.
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