calibratedindexhead

There are many hole-ring based index heads for lathes, and these may be perfect for standard index steps and for quick set up of those steps. However, my need is for an ability to choose rotational steps that may not be supported by a standard set of index holes. For instance, in setting up for marking a knob ring or dial for a VFO (Variable Frequency Oscillator) I calculated that the tuning will not be straight-line, but instead will follow a pre-defined curve. This necessitates the dial markings to be at specific rotation degrees, but not at equally spaced degree settings. This can only be accomplished on the lathe by having an index head that is marked and adjustable in precise 0 to 360 degree markings. This index head will also have to include a locking mechanism that allows it to be locked to a precise setting without changing the setting and that will have enough holding torque to stay in place while the dial being manufactured is scribed with the necessary marking.

Some sort of cam-lock mechanism seems to be the optimum rotational lock, so first method to be explored will be much like the caliper brakes on an automobile wheel.

Next obstacle seems to be finding a suitable method for making the initial calibration marking on the index wheel. Ideally these would be from 0 to 359 degrees, in one degree increments. Since the drip-tray under the lathe is 9 inches wide, this seems like an optimum width for the index wheel, as it will allow the cam-locking support structure to fasten across under the drip tray, with it's vertical parts to be outside the drip tray width. The distance around this 9 inch diameter index wheel will then be 9 X 3.14159 = 28.27431 inches circumference. Dividing that by 360 degrees gives us 28.27431 / 360 = 0.079 inches per degree. If we are to put stamped index values every 5 degrees, this means that the stamped markings will have to be placed every 5 X 0.079 = 0.39 inches around the circumference. Since my inexpensive metal stamp set is 0.250 size, this seems to provide adequate space for each stamped marking.

OK, now we know what we are up against for marking the index wheel. How do we measure and mark it accurately. I happen to have a fiberglass backed measuring tape that is marked off in 1/16 inch increments. Since 1/16 inch is 1 / 16 = 0.0625 inches per marking, it seems that I should be able to interpolate the needed 0.079 inch markings with fairly good precision (a calculator or better yet a computer spreadsheet program will be invaluable here). By wrapping the measuring tape tightly around my index wheel, making sure that it crosses over itself at the 28.27 inch mark, and then guessing at the 0.079 points it should be possible to come up with adequately precise 1 degree minor marks, 5 degree major marks, and 10 degree primary marks all around the wheel.

=========

Nov. 24, 2007: In looking at this again, it appears that making the wheel have 36 inches circumference may be worthwhile because it would be easier to measure and apply calibration markings, and the increase in accuracy it should provide. Some cardboard mock-up testing indicates that this larger size is not excessive to the point of being unwieldy when mounted on the lathe. After further consideration, I think the first index wheel will be made from 1/2 inch thick plastic, as this can be obtained in several material types from the local plastics store junk bin at only a couple of dollars per pound. Since I already have a vibratory engraver, making the necessary numeric entries on the wheel should not be a big problem. one-degree spaced index lines will simply be scribed in the plastic with a sharp tool. A metal hub for this thick plastic index wheel can be made by truing a 1/2 inch pi;e floor flange casting and using a short 1/2 inch pipe nipple as the mount shaft. Screw the pipe tightly into the floor flange, clamp this pipe in the 3-jaw chuck, and turn until everything rotates with no wobble. This provides a very accurate faceplate for mounting the plastic disk. The 7 X 10 lathe does not have capability to true up the 10.5 inch plastic disk, but this can be done on my wood lathe which has 12 inches of throw over the ways. Since I have a 4-jaw chuck for the wood lathe, this should be rather easy to accomplish.

=========

January 9, 2008: Since my last entry this project has gone through several thought and design sessions, but to-date no actual fabrication has been done. At this point I think my direction has changed from a simple index wheel to using a worm driven index wheel with ability to set the worm in or out of contact with the index wheel via an over-center arrangement. This would allow it to remain installed but out-of-gear when doing normal turning activities on the lathe. The worm drive itself will be along the lines of one I already constructed for ham radio purposes. With the resolution available from a worm drive, some sort of micrometer indicator knob arrangement may be built to help set specific rotational positions.

Please be patient...this is actually getting closer to implementation as I try various incantations as paper drawings prior to actually building the device.

=========

March 16, 2008: This project is getting closer to actually building something. A drawing has now been generated for the actual construction.