Free and Easy Speed Calibration


	Free and Easy Speed Calibration
	I recently installed the LMS belt drive conversion kit on my Harbor Freight Mini Mill/Drill and it works really well. As a bonus, I was able to use the 30-tooth gear, removed in step one of the kit instructions, together with a fluorescent light as a "tachometer" to calibrate the speed knob on the mill. Mount the gear on the spindle and illuminate it with a fluorescent light. Now, as you slowly increase the speed from 0, you'll see that the gear teeth appear to stop every so often. At the first "stop", you'll see what looks like a 60-tooth gear -- twice the real tooth count. This corresponds to a speed of 120 RPM. (= 7200 / 60). At the second "stop" you'll see a true image of the 30-tooth gear. This corresponds to a speed of 240 RPM (= 7200 / 30). At the third "stop" you'll also see what looks like a 30-tooth gear but it will have a more fuzzy appearance because it will really be a double-image. This corresponds to a speed of 480 RPM (= 7200 / 15). The 30-tooth gear also has 6 "spokes" which you can use in the same way for calibrating higher speeds. At the first stop you'll see 12 spokes -- 600 RPM. At the second stop you'll see 6 spokes -- 1200 RPM. At the third stop you would see 6 fuzzy spokes, but 2400 RPM is outside the range of the mill using the low speed belt position. Before you start, highlight the teeth and spokes with a black marker to make them easier to see (as shown in the photo).

	You can also use gears with different tooth counts in the same way, thus getting more speed readings. A fluorescent light flashes 7200 times a minute (in the US). If the number of teeth (or spokes) is T, you'll see the three main stops at 7200/(2T), 7200/T, and 7200/(T/2) RPM. If you are old enough to remember phonographs, you may recall that some turn tables used the same trick for accurate speed adjustment. It works and is free. You can calibrate the speed knob and always know where you are.
	Richard 2004