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

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About Dan Murphy

Dan Murphy is a regional sales manager for Rem Sales LLC., a U.S. Tsugami distributor. He can be reached at dmurphy@remsales.com.

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

  1. avatarWilliam Shaffer

    The chatter problem can be solved very easily. The tools need to have an edge preparation applied. Due to the free-cutting you get with a sharp tool, you are experiencing the tool “digging-in” then releasing. The microgeometry on the cutting edge needs to such that it allows the tool to cut efficiently, with just the right amount of tool pressure, to eliminate the tendency of the tool to chatter. We can help…

    B. Shaffer — Conicity Technologies

     
  2. avatarDuane Evans

    ask the right ?????’s
    1- what is the stock ?
    2- what size is the stock ?
    3- is it ground stock & consistant ?
    4- what size is the guide bush, does it fit the stock and close parallel on it ?
    5- are your tools centered properly ?
    6- is there play or looseness in the revolving bushing assembly / bearings ?
    7- are the tools well matched to the material cutting needs ?
    8- is the tool nose or contact surface too large for the cut ?
    9- are the speeds & feeds matched to the work/tools/material conditions ?
    10- are the machine slideways worn and sloppy ?
    11- is every thing involved in the set-up properly tight and secured ?

    Duane Evans, mgr. eng. / lead prgmr.
    Wightman Engineering
    Santa Clara, Ca.

     
  3. avatarJim Parisse

    Tsugami NT-12-II misindexing and Other times will run fine for 1hr or more .Controlled by fanuc 3T

     
    +3

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