Wow, boehme........What do you mean by "clarify" things? ............
.........or, as Ronald Reagan used to say, "There you go again ...". My wife would have said, "I just wanted to know the time ... not how to build an atomic clock". I am afraid that it is a chronic condition that afflicts many engineers.
My
FIRST POST on this question probably summed up things the best.
As I mentioned, I also have one of those "lipstick on a pig" projects. If you love to tinker with mechanical things, it can happen and there is nothing inherently wrong with that. Another engineer in our club, the Woodturners of North Texas, modified his mini lathe into a full-blown ornamental lathe that also has the capability of functioning as a rose engine lathe -- if you look hard, you can still see the mini lathe at the heart of his machine. Our
January 2008 program featured his ornamental lathe.
The bottom line is that converting a lathe to a drive system that has a variable speed motor
will work, but you might be disappointed in the results after spending a lot of money and time because the results will not be dramatic. For the benefit of those who don't want to earn an engineering degree in the process of modifying their machine, here are a few tips that may be of help:
- If you are modifying the lathe to eliminate the inconvenience of shifting the drive belt to change speed, you may find that you will still need to have at least two pulley speed ranges, if not more. Take a look at the commercially available machines and you will see that there are very few that do not still have multiple speed ranges. The good news is that on a well thought out machine like the Powermatic 3520, you will find that you can stay in the low speed range almost all of the time.
- Below what is known as "base speed" (motor speed at 60 Hz), power decreases linearly with decreasing speed for AC induction motors and for permanent magnet DC motors. For universal AC/DC motors, the power decreases much more rapidly (as the square root of the speed). By contrast, the power available at the spindle from a stepped pulley speed control arrangement is always constant for all speeds. This also means that the torque increases with decreasing spindle speed. That is a very important consideration to take into account when designing your variable speed drive system. You would like to still have the same or better low speed torque.
- In order to accomplish the above, one of the things that you ought to consider doing is to use a motor with a higher HP rating. Doubling the motor HP is what I would suggest as a simple solution. The other thing would be to change the drive ratio so that you "gear down" the spindle -- meaning that the motor must run faster. Without going to very expensive motors, there is not much leeway in that option as the typical AC induction motor doesn't like to be operated much more than 125% or so above the motor's base speed.
- There are some electrical and mechanical compatibility issues to consider -- the average three-phase AC induction motor may not fare very well in a variable speed drive. Motors need to be rated for this type of service and here are the reasons why --
- At slow speed the motor will not receive much internal cooling so it must be able to withstand higher temperatures than standard motors.
- You may need a motor to operate at 200% or more of base speed which reduces efficiency (more heating) and exceeds the bearings maximum speed rating.
- The voltage input to the motor from a VFD controller has high amplitude spikes that may exceed 1000 volts and this may lead to internal arcing between the windings in the motor. This is a very gradual incipient type of failure mode and may not be noticed for a long time until it is evident that the motor seems to be "weak" and running hot. Therefore, it is strongly recommended that motors rated for inverter duty (class F or class H windings).
- Design the pulley drive ratio so that the motor will be operating most of the time between 50% and 125% of base speed (base speed is typically 1750 RPM).
- For anything other than a mini lathe, design the drive so that the top end spindle speed is around 2500 RPM (and definitely not more than 3000 RPM).
- If you have what is known as a "sensorless vector" type of VFD, you can probably design the low speed limit to be around 100 RPM and still get reasonable power output. For a more basic (lower cost) Volts/Hertz VFD, performance at low speed will not be as good so the lowest practical speed might be around 300 RPM at the spindle.
- You can hardly go wrong with a big motor! (except that it will cost more).
- "A man's gotta' know his limitations ... ". Remember that your little Jet 1236 and similar lathes just are not very strong mechanically and you can't turn one into a Oneway 2436 by adding a big honkin' motor and using risers under the headstock and tailstock.
- I have put more hours of design work into my project than it would ever possibly be worth, but then, that is what hobbies are for! Why did I do it? Well, I am cheap, my time was free, and I got an great deal on some industrial surplus equipment that was nearly new. Sometimes, I think that I do woodworking just so that I can twiddle with my machines. Next on my list is my drill press. It is an ideal candidate for variable speed because changing belts on it is a real PITA.
I hope that I did a little better job of answering your question this time, but, if not, I will try to help with whatever questions that you have.
Bill