Tip #17
Disc Sanding (continued)
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Sanding Curves and Circles

Figure 17-19. Use a light, sweeping motion when smoothing outside curves. Don't hesitate at any point or the disc will sand a "flat".

To sand curves, move the work-piece in to contact the disc and then use a sweeping motion to maintain the work-to-disc contact throughout the pass (Figure 17-19). Feed should be light and smooth even when a great deal of material must be removed. Several light passes are always better than a single heavy one. The disc has a fast cutting action, so excessive pressure can cause burn marks and will lead to premature clogging of the abrasive.

Figure 17-20. Use a guide to sand curved workpieces to a uniform width. The edge that bears against the dowel must be smooth and true.

Sanding Curves to Width
You can guarantee that curved workpieces will be of uniform width throughout their length if you follow the procedure demonstrated in Figure 17-20.

The guide is clamped in place so the distance from the dowel to the disc will equal the width of the workpiece. The stock is then slowly passed between the dowel and the disc. There are two important factors: (1) The curve of the workpiece where it bears against the dowel must always be tangent to the disc; and (2) the inside edge of the workpiece must be smooth and parallel to the outside edge, something you can accomplish with a drum sander. If there are bumps or hollows in the bearing edge of the workpiece, you will not get good results. The construction details of a guide you can make are shown in Figure 17-21.

Figure 17-21. Construction details of a guide for sanding curved workpieces to a uniform width.

Figure 17-22. A special pivot fixture that you can make to sand perfect circles.

Sanding Circles
Circular workpieces can be sanded freehand. But you will be more accurate, especially if you need duplicates, by using the pivot method of guiding the workpiece. The miter gauge, locked in place and with a pin threaded in the hole that is at the end of the bar, can be used as the pivot. You can also make a special fixture, like the one shown in Figure 17-22.

When setting up, place the workpiece on the fixture and posi-tion the worktable so the edge of the workpiece will be about 1/4" away from the disc. Advance the disc so it will start sanding the workpiece; then secure the disc's position by using the quill lock. The workpiece is then slowly rotated a full 360° (Figure 17-23).

Figure 17-23. The workpiece is mounted on the pivot post and slowly rotated agains the disc.

Use the same procedure, but with the worktable tilted to the right, when you need to bevel the edge of a circular workpiece.

Figure 17-24. The pivot guide method can also be used to round off the ends of straight pieces.

The same arrangement is useful when you need to round off the ends of straight pieces (Figure 17-24). Drill a pivot hole at the center of the workpiece; then proceed to sand as if the work-piece were fully circular.

Construction details of the pivot fixture are shown in Figure 17-25. Notice that you can make pivots that are straight posts or are pointed.

Figure 17-25. Construction details of the pivot fixture. Notice the two types of pivot posts; one has a point to be used when the workpiece does not have a center hole.

The short, pointed one can be used when the workpiece does not have a center hole. The L-shaped lock can be used to secure the sliding bar if you remove the table insert before putting the fixture in place. The pivots, if threaded deeply enough, will also serve to secure the bar in a particular position.

You can pivot sand exceptionally large circles using the following setup. For Model 500, place the rip fence on the extension table and mount the lathe cup center in the hole used for the mortising hold-down. For the Model 510, drill a 5/8" dia. hole in the top of a rip fence extension. Mount the extension to the rip fence and mount the lathe cup center in the hole. Set the height of the extension table so the point of the cup center will be slightly above the surface of the worktable (Figure 17-26). Extend the quill so the distance from the disc to the point will equal the radius of the workpiece. Set and lock the depth control dial at “0.”

After the workpiece is in position, advance and lock the quill (the amount of extension will be controlled by the depth control); then slowly rotate the workpiece until its entire circumference is sanded. Remember that the cup center point is just a pivot guide; the workpiece must rest solidly on the worktable.

Figure 17-27. Corners can be rounded off by sanding to a line. The bulk of the wast should be removed with saw cuts before you begin.

Sanding Round Corners
One method of sanding round corners is shown in Figure 17-27. Prepare the stock by sawing off the bulk of the waste material and then finish the shaping by using the disc sander. When the radius of the corner isn't very large, the entire job can be done by sanding, a procedure that is especially applicable when you need many similar pieces. Set the miter gauge at 45° and use the miter gauge stop rod as a backup for the workpiece. Secure the workpiece by holding the safety grip and, with the depth control set to limit the disc's extension, feed the disc forward to sand to a line that is tangent to the curve (Figure 17-28).

Figure 17-28. This is a setup that can be used if the corners are not too large. First sand to a line that is tangent to the curve. Finish rounding off by working freehand.

After all corners have been sanded in this manner, finish the job freehand. There will be very little material left to remove.

Figure 17-29. The ends of dowls or larger rounds can be pointed or chamfered freehand by angling the miter gauge.

Pointing or Chamfering Rounds
Pointing or chamfering dowels or rounds can be done freehand by setting the miter gauge to the angle you need and then using it as a guide as you rotate the workpiece against the disc (Figure 17-29). If you want more precise results or need to shape duplicate pieces, work as follows.

Use the miter gauge stop rod or a long extension with a stop block to back up the workpiece. Advance the disc to the point where it will form the chamfer or point you need while rotating the stock against the miter gauge.

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