Circles aren't round anymore

[Fretboy]

I've got a four year old CNC Shark that I use to make many things including fret boards. The fret boards have 1/4" diameter round x 1/8" deep pockets for pearl fret dots. I've cut dozens of fret boards with perfectly round fret dot pockets but now they are irregular - more like asymmetrical ovals. I'm using the same program, same bit, etc. Any idea what is causing this? Thx.

[tonydude]

Have you checked the coupling to make sure they are tight? They do come loose sometimes.

Tony

[thepuchman]

It could be a few things. Couplings, worn screw, worn nut, loose nut, bearing starting to fail, stepper starting to fail. Basically anything that can allow back lash. If you have a dial indicator you can check for back lash pretty easy. Mount the indicator to the table and place the tip on the Y axis gantry near the base. Gently push the gantry away from you and remove your hand. Zero out the indicator. Now gently pull the gantry toward you and let go. The reading on the dial is the amount of back lash on that axis. Repeat for the X axis. These do not use ball screws so by nature will have some backlash. I do not know what the amount is when the screw/nut is new on these. For a Bridgeport mill it is to be no less than .007". Any tighter and it causes premature wear of the components. 7 thou in wood is nothing and would only show up in very small details or tight tolerance work.