There are a number of reasons that may cause the grinder to chip the glass surface. Some of the things to check are:
Too much pressure
It may be that you are pressing the glass into the grinder head too hard. The grinder head should do the work. Firm rather than hard pressure should be applied. If the grinder slows, it is an indication that far too much pressure is being applied.
Insufficient water supply
There may be too little water reaching the head to lubricate the diamonds and keep the glass cool. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply.
Worn or damaged grinder bit/head
Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. Any dents or smooth places on the bit cause a vibration that is similar to a tiny hammer tapping the edge of the glass.
It is possible that you may be using too coarse a grit on the grinder bit/head. The more coarse the grit is the larger the chips will be taken off the edge surfaces. Smaller grits take smaller chips off the edges, and so are less obvious.
|Examples of the range and grit differences in grinding bits|