Shop Doc – Chatter while turning on 20mm Swiss CNC Lathe

Today’s Machining World July 2008 Volume 04 Issue 07

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

Dear Squeal,
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.

4) The cut exceeds the driven bushings’ rigidity: Lathe spindles and driven guide bushings are rated by the manufacturer with a figure known as the “maximum chip section.” It is usually given as area in square millimeters. You calculate this figure by multiplying the depth of cut by the feed rate. On a typical 20mm Swiss this can range from 0.3 square millimeters to 0.8 square millimeters for a heavy duty driven bushing. Let’s assume a 4mm DOC. Take 0.3mm (0.0118″) and divide by 4mm (0.1575″) and the result equals 0.075mm (0.003″), which is the maximum feed rate possible before the bearing in the bushing becomes overloaded and chatter results. Repeating the calculations for the stronger guide bushing you will find that you can feed up to 0.2mm (0.0079″) per revolution before you will overload the bearings. This may also be the source of your problem while over-tightening the bushing is compounding the problem.

Dan Murphy
Regional Sales Manager
Tsugami/REM Sales