Written by Murali Krishna M
Thursday, 12 February 2009 12:32

If a body or disk mounted upon a shaft rotates about it, the center of gravity of the body or disk must be at the center of the shaft, if a perfect running balance is to be obtained. In most cases, however, the center of gravity of the disk will be slightly removed from the center of the shaft, owing to the difficulty of perfect balancing.

Now, if the shaft and disk be rotated, the centrifugal force generated by the heavier side will be greater than that generated by the lighter side geometrically opposite to it, and the shaft will deflect toward the heavier side, causing the center of the disk to rotate in a small circle. A rotating shaft without a body or disk mounted on it can also become dynamically unstable, and the resulting vibrations and deflections can result in damage not only to the shaft but to the machine of which it is a part. These conditions hold true up to a comparatively high speed; but a point is eventually reached (at several thousand r.p.m.) when momentarily there will be excessive vibration, and then the parts will run quietly again. The speed at which this occurs is called the critical speed of the wheel or shaft, and the phenomenon itself for the shaft-mounted disk or body is called the settling of the wheel.

The explanation of the settling is that at this speed the axis of rotation changes, and the wheel and shaft, instead of rotating about their geometrical center, begin to rotate about an axis through their center of gravity. The shaft itself is then deflected so that for every revolution its geometrical center traces a circle around the center of gravity of the rotating mass.

## Nesting the parts while machining using CAM

While working on a recent CAM project using CAD model, we found few interesting techniques. In the first job, we requested the operator to use a 1/4″ Ball nose cutter of 1″ long. We were expecting there could be some tool flexing because for the first cavity profile cut. But he used almost 1 1/2″ long cutter. Finally, once the machining is done there is a clear cut draft left on the part which is almost 3 degrees. Also, there was not enough overpass of ball nose at the bottom of the part leaving a radius there. It was not too bad other than doing some manual cleanup of the part after machining.

In our second batch of CAM, we nested four parts as shown on image from two separate CAD models. So, we have to make a single pre-machining block to make two separate parts of quantity two each. Also, we tried to machine only up to 4 mm around the part using 1/4″ cutter. This saved us almost 50% of machining time as we eliminated machining unneeded stock. Also, we ensured the cutter overhang is only 22mm just more than nose overpass. This didn’t cause any tool flex and squeak.

Nesting the Parts example

Another interesting thing is after doing the rough cut with 1/4″ cutter, we lost the datums where we set original zero. So, for finish cut we have not changed X and Y coordinates. But, asked the operator to reset Z with respect part top surface which was intentionally left unmachined before. This helped us in quickly running the finish cut using 1/8″ cutter without much time on reference point set.

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