Vertical pattern when turning on CNC converted mini lathe.. can anyone help?

Hi!

I've converted a micro lathe (SIEG C1) to CNC. Originally I had two 6mm micro ballscrews on Z and X.

Recently I adapted a 10mm (2mm pitch) ball screw with double nut (back to back) on the Z axis.

But my X axis, is stilll the single nut 6mm (1mm pitch) ball screw. I plan to use the nut from the old 6mm Z ballscrew into the X to make it double nut also.

But I'm experiencing a vertical line pattern, close to 2 - 2.5mm each vertical line when turning a part in aluminum which is what I use most. The cnc works ok, but I don't remember having this pattern in the past.

I Adjusted the Z and X gibs the most I could to get the most rigidity possible without making the axis' too hard to move.

Would this be a problem in the x axis or in the Z axis?

I suspect it's something in the X axis ball nut.. . If I keep hand pressing the QTCP forward (in the X- direction) the lines seem to disappear....

Could this be related to this single 6mm ball nut ... allows some axial movement? If I use the remaining 6mm ball nut from the old Z screw, in a back to back way, could this be fixed?

Thanks for reading, regards, Rodrigo

You might try increasing the feed a bit. This is pretty common on manual lathes also. Those lines generally go away when the tool is loaded a bit more.

This is not abnormal, especially in lower end machines.

Suggestions to look at:
- the balls crews should have proper angular contact bearings on one end. Make sure they are properly preloaded. The other end of the ball screw must be held with ball bearings that allow axial movement.
- Make sure the ball screws aren't bent
- these Cheap Chinese machines don't always have flat, machined ways and beds. When you adjust the gibs, do the adjustment in the part of the machine that will actually see contact during machining, even if that jams the machine in other areas. This isn't a great solution though.
- adjust toolpaths and tool selection as has been suggested to minimize the problem.

You can touch it up by hand with some sandpaper or scotch bright while it is spinning in the lathe.