Dear Shop Doc,
We are having trouble with chatter while turning on our 20mm Swiss CNC lathe. We recently replaced the guide bushing bearings and we have the bushing adjusted so tight that the bar is a press fit inside the bushing, but it still chatters. We have also tried different tool geometries and slowing the rpm down. Nothing works. Can you help?
Squealing in Wheeling
I’m pretty sure I see the problem. These are some of the possible causes of chatter while turning on a Swiss:
1) Incorrect Tool Geometry: Positive rake tools cut more freely and can reduce the cutting pressure, which in turn can help eliminate chatter. Smaller nose radii also generate less pressure. However, this effect is minimal compared to reducing the feed rate or the depth of cut.
2) Guide bushing is too loose: Obviously not the problem here, but when the bushing is too loose the bar can move about during turning causing chatter.
3) Guide bushing is too tight: While this seems counterintuitive, over-tightening the guide bushing can cause chatter. A typical driven bushing has two angular contact bearings in the front and one or two bearings at the rear. When the bushing is run too tight, the back bearings will compress under the load caused by the bar moving forward, and the front bearings will unload. With the preload on the front bearings being pressed out by the bar being forced through the bushing, the bearing set is unable to support the cutting forces created by turning. This is probably your problem. This situation will lead to the back bearing failing, followed soon after by the front bearings. There is no need to run the bushing as tight as you describe. If you are after better roundness use ground bar stock. The bushing should be adjusted snug to the bar but still be loose enough that you can rotate it by hand with the bushing locked. You should feel some drag on the bar when you rotate it.