The hand wheel or outboard side of the lathe headstock is a good place for an index wheel on the Powermatic 3520a. The inboard side of the headstock is the best place but in this position it will usually interfere with turning. Having the index wheel on the hand wheel side of the headstock has one significant issue that must be considered right off. Make the index wheel too large in diameter and fingers may get pinched between the motor and the index wheel. I chose to make the index wheel the same diameter of the hand wheel at the suggestion of another turner.
The tools necessary to make this index wheel and the positioning latch included a welder, metal lathe with milling capability, drill press, # drill bits, tap and die set. I also found a 24 segment template in the book, Segmented Wood Turning, by William Smith made the job of aligning the indexing holes simple. The template was centered and glued to the surface of the index wheel then sprayed with Krylon clear to preserve the lines during the machining.
The index latch was made like a dead bolt. I chose to make my own to have the slide bolt spring loaded in the latched position and the bolt be made of ¼" rod to fit ¼" holes in the index wheel. Another good reason to have the latch spring loaded is to keep it from jumping to a latched position when the lathe jumps and shakes from normal or abnormal use. I drilled and tapped an 8-32 thread in the slide bolt for a short lever. The slots for the latch were milled on the metal lathe and welded to a rectangular mounting plate with counter sunk mounting screw holes at each corner. The index latch is mounted on the vertical surface on the back corner of the headstock. It must be mounted with the index latch slide bolt parallel to the radius of the head stock shaft. Also it must be mounted at 90 degrees to the head stock shaft. Since the head stock casting is not 90 degrees to the shaft a couple of washer under the far corners of the mounting plate and none under the close corners was all that was necessary to get the mounting angle correct. I found that final drilling of the cast iron head stock for the 8-32 mounting screws of the index latch was better done after the index wheel was drilled and mounted on the shaft allowing a positive check of the latch being at the correct angle to the index wheel.
One additional step in making the index latch, the slide bolt was shaped to a sharp point on the metal lathe at about the angle of a twist drill bit. This point was necessary to scratch a line all around the mounted index wheel for drilling the position holes.
Before marking and drilling the 24 position holes in the index wheel the index wheel must be drilled and shaped for mounting on the shaft. Mount the ½" round aluminum index wheel in a metal lathe chuck. With a tailstock mounted drill chuck bore a 1" hole at the center of the index wheel, I started with smaller bits and worked up to 1". To get the index wheel closer to the headstock the 1" hole must be enlarged just deep enough to fit over the threaded shaft collar. A 1/16" clearance from the headstock is good enough. I also enlarged the 1" hole on the other side of the index wheel to accept the hand wheel hub. I used a boring bar to enlarge both sides of the index wheel. Since I have a vacuum chuck screwed to the hand wheel I need to reduce the thickness of the hand wheel and its hub. I mounted the hand wheel in the metal lathe chuck and took ¼" off the hub. With the expanded hole in the index wheel the lathe shaft was now flush with the face of the hand wheel. I drilled and tapped 2 new ¼"-20 holes for the hand wheel set screws in the hub closer to the hand wheel since the original threaded set screw holes were partially buried in the index wheel.
To secure the index wheel to the hand wheel I drilled and countersunk three holes in the hand wheel for ¼"-20 flat head screws. Mount the index wheel and hand wheel on the shaft and using a ¼" drill bit mark the alignment of the 3 holes in the index wheel. Remove the hand wheel and the index wheel and drill and tap the index wheel for the ¼"-20, 1 ½" flat head mounting screws.
The next job is to drill the index wheel latch holes. Mount the index wheel and the hand wheel on the headstock shaft. With the 2 wheels mount on the shaft, hold the index latch in the approximate mounting position on the headstock with the sharp point of the bolt on the index wheel. Rotate the lathe shaft slowly by hand, scribing a line all around the index wheel. Take the hand wheel and index wheel off the lathe shaft and remove the screws holding the wheels together.
To drill the latch holes in the index wheel I put the index wheel in a drill press vice. I used a letter "F" drill bit which is 0.007 larger than the ¼" latch bolt for drilling the latch holes. With the index wheel in the drill press vise I visually aligned the center of the drill bit with the scribed line around the wheel and using a small right angle square on the top of the vise aligned the bit with one of the 24 radial lines on the template. Clamp the vise solid to the drill press table. Tighten the drill press vise to secure the index wheel. Recheck the alignment and adjust if necessary. The first hole can now be drilled approximately 3/8" deep. Set the drill press stop so that all the holes will be the same depth. Loosen the vise jaw and rotate the index wheel to the next of the template positions and drill as per the first hole. Continue until all of the 24 index holes are drilled. Remount the index wheel and hand wheel on the head stock shaft. With a number stamp set and hammer, stamp the number of each hole beginning at #1.
The last job is to permanently mount the index latch to the headstock. With the index latch bolt in the extended position, slip the bolt into an index wheel hole. Hold the index latch mounting plate with about 1/8" clearance from the index wheel in the selected positions and with a centering punch mark one screw mounting hole. Remove the index latch. Bore about ½" deep hole in the cast iron headstock with a #29 drill bit and thread with and 8-32 tap. Be careful not to break the tap in the hole. Remove the tap after making a few threads and blow out the cuttings with high pressure air. Continue tapping till the tap meets the bottom of the hole.
Remount the index latch and thread the one 8-32 screw into its hole loosely. The index latch bolt should slide freely in and out of the index hole. Mark the remaining mounting holes, drill and thread. Mount the index latch and use the index head as necessary.