Do you use the same bit?? If so, check if it is not dull or worn. Maybe it is time for a new bit. Additionally, I would check if the z axis mounts are tightened correctly and the router has no wiggle room. If that doesn't help, check all of the play that may be in the structure and where the bearings run along the rails and tighten all loose connections that may be present.
So confused....is backlash and chatter the same thing or are they completely unrelated?
Chatter and backlash are different. Backlash is a condition where there is play in the mechanical parts (i.e. a nut not tight against the treads, or two gears not meshing together tightly). Chatter is a phenomenon caused by many possible conditions including vibration, resonance, springiness and flexibility in the system and certain types of end kill and machine movement scenarios.
Meant to say end “mill” at the end there.
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when cutting patterns with my cnc router there is significant chatter in the cut-I've replaced the router-what else should I try?