READJUSTING THE SPEED ON A HARBOR FREIGHT
MINILATHE WITH ELECTRONIC SPEED CONTROL
My Harbor Freight (also available through other importers) 7x10 Mini lathe with electronic speed control is a very good little machine capable of some good work. It is of Chinese manufacture and as with many such machine tools it lacks in the area of cosmetics. Internally, it is also rough where it doesn't affect the performance. Otherwise, it is a good little machine that is just perfect for the home shop model maker. At the present time it is available at a rock bottom price from several companies and makes it one of the best bargains on a small to medium capacity bench top machine.
Besides its features, such as auto feed, reversible motor and feed, built in heavy duty cross and compound, 2 and 3 morse taper tail and spindle and many others, the thing that clinched my decision to buy it was the price and the fact that it sported a two speed variable speed gear box and electronic variable speed control. With the lever thrown for the lower range it provides speeds from 220 to 1500 rpm, with the higher setting giving speeds from 440 to 2900 rpm. All speeds are adjustable on the fly by just turning a knob but you must turn off the unit before switching from the low to the high range or you will destroy the gear box. The control is not just a rheostat but an electronic controller that slows down the speed while maintaining full torque at the spindle. Since it comes equipped with a set of change gears (Delrin plastic) and a threading dial on the carriage, you would think it would be easy to conduct threading with it. The sad fact is that even at the lowest speed setting of 240 rpm, it's still too fast for anything but the higher TPIs. It can cut threads from 12 to 56 tpi with the gears provided and will also cut left handed threads if you really need it too by turning the work in the normal direction but with the carriage and tool moving from left to right. At 220 rpm and set up to cut a 16 tpi thread on a 3/4" spindle where the thread ends at a shoulder, the carriage simply advances too fast and can really get away from you if you are the least prone to look away.
A much slower speed was needed for some of the larger threads, but I just didn't know how to achieve that in a convenient manner. The answer came to me as a result of a random inquiry to a couple of comments about the lathe from owners of the lathe in the Internet. These individuals were discussing the very same lathe, although a previous model of it which at the time sold for much more than presently. Wanting to get acquainted with other owners of the tool, I sent an e-mail response to them. One of the replies had to do with a way to adjust the potentiometers inside the controller housing. Although she didn't specify how to exactly go about doing so, I took a deep breath and removed the cover and was met by a common circuit board with 5 potentiometers, 4 of which were lined up on the bottom edge of the board. To begin turning their adjustment screws mindlessly would have been disastrous. I painted a tiny witness mark with some with enamel located at 12 o-clock so in case it didn't work out, I would have a way of re-setting everything back the way it was. After some tinkering I realized that the second pot from the left, looking at the back of the board so it is upright, was the one that would reduce or for that manner, increase the low end speed limit while maintaining torque. The first pot on the left would also change the speed but you would loose all your low and middle end torque. I adjusted the second pot so the spindle stopped turning, then slowly increased the speed until it was about 70 rpm. That seemed to be the optimum setting and provided so much torque that I could not stop the spindle by gripping it. So you can see that it takes all of these individual pots, each of these adjusted just right to allow the controller to work properly. Modern model electric train power sources operate with the same principle of high frequency high voltage pulses to be able to turn the motors very slowly without having to overcome inertia. After the motor's rotor is moving and the speed is slowly increased, the pulse frequency gradually drops until it is running as a flat line rather than spikes. This allow model locomotives to start very slow and very smoothly instead of the old fashioned jerky or jack rabbit start that if done on a real train would put all its cargo in the caboose in an instant. Once I got everything running nice and slow, smooth and with maximum torque, I fabricated a plastic cover for the electronics as they are fully exposed to any swarf that will for sure find its way in there. A simple gasket on the cover wouldn't work because there is a large semi circular opening for the lead screw to pass through and I could just imagine a nice aluminum "curly" getting in there and shorting out on the board and the screw. Any way, now I have a lathe that has infinitely variable speeds from 70-2800 rpm (high) and 30-1450 rpm (low). Thread cutting is a breeze now and cut off work done at a very low speed now produces chatter free cuts. I found it very difficult to part off anything in steel and now I can. I recommend the use of the Internet, The World Wide Web and the metal work related newsgroups as a huge source of information for our metal working questions as well as great forum for any of our ideas or metalworking question.