Those of you who own a Taig micro lathe or any of the variants such as the Richard Simms or Bonnie Kline versions, will be familiar with the very good six step machined pulleys that are made for it. They are machined from solid aluminum bar stock with bored spindle holes for high speed, vibration free performance. The most common way to power a Taig, since they do not come with a motor is to use a 1/2" shaft medium size fractional horsepower 115 volt induction motor with a no load speed of 1725 rpm. This setup will give you a speed range of 550 to 5500 rpm in six geometric progressions. The "V" grooves on the pulleys are small so a very thin and rather flimsy 3M brand "V" belt is used to link the motor and lathe spindle together. If the two pulleys are not running exactly co-planar to each other, rapid belt wear is the result. The constant changing of pulley positions to change spindle speeds does not help the belt wear situation either.
In order to provide the very desirable variable speed feature to this lathe, you would need to adapt a universal type motor connected to a speed controller of some sort. The problem is that most universal motors normally run so fast in normal operation that when slowed down to a manageable speed, dramatically loose their low end torque. I have an old Sears variable speed scroll saw I purchased re-conditioned for about $115 that uses a DC motor controlled through an electronic speed controller. These units maintain full slow speed torque and are ideal for this type of modification. Those who own a Sherline tool, know very well the virtues of this system. The saw, when new was very good, but now it has worn out pivot arm bushings. I know they could be replaced but a much better designed saw model is now available for a few more dollars so replacement is now a sure thing. I used the motor and controller from the old saw to convert the Taig to a variable speed lathe. As I mentioned, one of the most popular small (4" swing ) lathes and milling machines is the Sherline. A large proportion of its popularity may be due to the excellent and very desirable electronic variable speed system it comes equipped with. No wanting to spend a further $400 to $500 for a new Sherline, it was a simple matter to just mount the motor / controller unit where the regular motor was. I did have to make a bushing to enlarge the smaller metric sized shaft to fit the 1/2" bore motor pulley. The controller can slow the basic motor to 400 rpm as well as giving you a top no load speed of 1725 rpm. If we remember that the lowest speed with the original single speed, 1725 rpm motor was 550. not quite a 1:3 reduction, you can see that with the motor running at 400 you can get a low spindle speed of around 130 rpm. Great for parting off work, almost impossible with the stock Taig except with 1/4" and smaller aluminum or brass and a very thin cut off tool. By turning the speed dial to full, you can get about 550 rpm. Top speed at the highest pulley setting will also reach 5500 rpm as with the original motor, but can be steplessly slowed down to around 1660 rpm. Of course, there will be a lot of overlapping of speeds and in reality, you would only need to use the first, third and sixth pulley steps to get the total range of stepless speeds. If most of your work will involve roughing operation, you can use a speed that falls within the lowest range of around 130 to 550 rpm. Finishing cuts can be taken at speeds within the middle range and work on very small diameter stock held in collets can use the higher speed range with great advantage.
Please don't try to operate the three or four jaw chucks any faster that about 3000 rpm. Running at 5500 rpm can produce such forces that if the tool jams, the work can fly out becoming a very effective and impressive projectile. The chuck could then unscrew itself right out of the spindle at 5500 rpm. I wouldn't want to be in the same zip code with something like that coming toward me. The higher speeds can be used on arbor or collet held work. If you perceive that you will not be doing any small diameter work requiring high speeds, you can make or buy a pair of two to three step pulleys so long as the shaft holes are compatible. The ratio between two pulleys is what dictates the final lathe spindle speed. The larger the actual diameters of the pulleys dictate how much force is transmitted from the motor to the lathe spindle. In other words, a 1" diameter motor pulley will not be able to transmit as much belt force as a 2" or 3" diameter pulley. Assuming that the pulley circumference ratio is the same between the two pulleys. The final speed at the spindle are the same but the larger pulleys will transmit much more force or torque and much less belt slippage. Just the thing when turning larger diameter work.
Some of the actual benefits of changing over to a DC electronic variable speed unit is that with an induction motor the rpm at the spindle can be made to slow down under a heavy enough load. The electronically controlled system tends to maintain the rpm even as the load increases. This is not limitless of course, but it merely maintains a steady rpm during various work loads. DC or AC universal motors generally produce a lot more power for a given size. So they take up a lot less room within the setup. The disadvantages include somewhat higher level of motor noise and maybe some time down the road you will have to replace the motor brushes.
I made this modification only because I had to replace my old variable speed scroll saw with a new one. Since the motor was still in perfect working order, it made the modification a very cost effective one. Now my Taig is a pleasure to use for any cutting situation I may ever need to put it through.
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