There are a large surface grinder jitter point about several issues

Wheel balancing is not in place, when grinding a slight jitter at the surface will produce a regular chatter. Loose  cutting tools , the machine used for too long, axial gap increases due to wear, resulting generated when grinding chatter. Workbench loose, move around the jitter  grinding wheel  lead to the workpiece surface chatter. There is also a rear wheel spindle  Diamond blade , I forgot what is a bearing, can also cause wear and tear after chatter. The level of machine did a good school, general work NTU Chuangzai problem could occur, affecting small insignificant.