Guide Bushing Fit I

Guide Bushings: The Right Fit is Critical

I’ve heard it said more than once that the guide bushing is the heart of a CNC Swiss automatic. To take that analogy a step further, it’s my opinion that if CNC Swiss machinists were heart surgeons, we would have a lot of sick and dead patients. This is hardly surprising since you can’t go to school and get a degree in Swiss turning.

The most common mistake I see in the field is running the bushing too tight on the bar. This stems from the belief that if the stock is tight in the bushing, the machine will hold better tolerance. This is not the case at all. Having the bushing adjusted too tightly to the bar causes a number of problems including; chatter, poor tool life, inconsistent lengths, servo alarms, and premature bearing failure in the driven bushing housing.

In a typical driven guide bushing housing there are usually three angular contact bearings, two in the front facing forward and one in the back facing rearward (fig. A). When the bushing is adjusted too tightly, the force of the bar pushing forward puts excessive load on the rear bearing while pushing apart the front bearings, effectively removing their preload. Without preload the front bearings can’t withstand the cutting forces, resulting in high frequency chatter. This high frequency chatter will result in poor tool life. Most often the turning insert will show excessive flank wear. In more extreme cases it may chip out along the cutting edges.

The chatter may not be readily visible in the surface finish of the work with the naked eye but can be seen under magnification. In all cases it can be heard during cutting. Usually as a high pitch squeal, or whistling noise. In addition to causing poor tool life high frequency vibration leads to premature bearing failure in the driven bushing unit.

In Swiss machining the fit between the material and the guide bushing does affect the precision of the turned diameters, namely the roundness. As the turning tool cuts the material the cutting forces push the bar against the bushing opposite the turning tool (Distance “A”). Since this distance remains constant no matter the fit to the bushing, the resultant turned diameter remains constant. So the fit to the material to the bushing doesn’t affect the accuracy of the diameter being turned. But if the fit is loose, the turned diameter will be out of round. Also if the fit is loose it can cause poor finish due to chatter.

The fit between the stock and the bushing can also be poor if the bushing ID is too large or too small. If the bushing is too large then it will only contact the bar in a narrow band at the front of the bushing.

If the bushing is too small it will only contact the bar at the back of the carbide pads well away from the turning tool. In either case the support will be poor and the precision of the turned diameters won’t be as good as it would if the carbide pads fit snug to the bar along their entire length.

What can you do if you have to get a job up and running and the bushing doesn’t fit the bar properly? You can lap it to fit the bar better. In part 2 I’ll explain how to lap and fix up guide bushings so that you will achieve optimum results.