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Many projects call for variable capacitors that in have 3/16-inch shafts. This is unfortunate, as most knobs and dials take only ¼ inch rods. A coupling is required, but one that will mate a ¼ inch shaft with a 3/16 inch shaft isn't sold commercially (as far as we know). It's not too difficult, however, to machine a coupling from a brass rod.

Obtain a ¾-inch length of brass rod, 3/8 inch in diameter. Drill a 3/16 inch hole down the axis of the shaft through both ends. With a ¼ inch bit, enlarge the hole to ¼ inch in diameter half the length of the rod. This can be done best on a lathe, but a drill press will give satisfactory results. Be sure to clamp the brass rod in a vise if the work is done on a drill press. Drill 1/8 inch holes for the 6-32 × 6-inch set screws about 1/8 inch from each end of the coupling. Tap the two holes with a 6-32 tapered tap and then with a 6-32 bottoming tap. After deburring the holes, screw in the set screws. Don't forget to use oil during all drilling and tapping operations.

Fig 1
Fig. 1. Cross-sectional view of 3/16 inch to ¼inch shaft coupling.

William C. Bakewell, WBBGHB