Once the magic smoke comes out, things don't work any more.

John Kasunich
jmkasunich@fastmail.fm
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Tue, 19 Dec 2006

Spindle Speeds

The existing lathe spindle drivetrain leaves a bit to be desired. The motor is 3/4 HP, single phase, capacitor start, capacitor run. The start capacitors are not very good - frequent starting lets the magic smoke out of them. (I've done that twice in the time I've had the machine.)

The belt drive system is a nuisance as well. There are 16 speeds ranging from 120 to 2400 RPM, using two vee-belts and a step pulley countershaft, along with step pulleys on the spindle and motor. The motor to countershaft belt isn't too bad, since the motor is pivoted, and its weight provides tension. Just lift the motor and switch the belt to a new step. The countershaft to spindle belt is a nuisance though. To change steps, you need to loosen the countershaft bolt and slide the shaft to loosen the belt, and then slide it again and tighten to set the tension. It's impossible to get it really tight, or maybe the vee-belt just isn't strong enough, because when working on large parts at the lowest speed I've made the belt slip many times.

I have a large AC servo motor that I'd like to use as a spindle motor. It is a three-phase permanent magnet synchronous motor, with an encoder, rated at 2000 RPM and 250 lb-in (28.2 Nm) peak torque. By comparison, the existing 3/4HP 1725 RPM motor has a rated torque of only 27 lb-in. The servo motor (with an appropriate drive) will give me variable speed, so I shouldn't need so many belt changes. The much higher rated torque is the key - when you reduce speed with a belt or gear, you gain torque. When you reduce speed with a variable speed drive, you don't gain torque, so you need a lot more torque to start with.

First, what spindle speed range do I need? I do the vast majority of my lathe work with high speed steel tools. For HSS on tough steel workpieces, I might want to go as low as 50 surface feet per minute (SFPM). HSS on mild steel is about 100 SFPM, and HSS on aluminum can be 200 SFPM or more. Carbide is a whole 'nother world, with speeds of 400 SFPM or more on steel, and 1000 on aluminum. But for most of my work, 50 to a two or three hundred SFPM is about right.

The other factor is workpiece diameter (actually cutting diameter - boring a 1" hole in a 12" diameter workpiece is the same as turning the OD of a 1" part). The maximum swing of the machine is a little over 16", but I've never actually made cuts at that diameter. The biggest parts I've spun on the lathe so far were 10" x 10.5" x 3/4" aluminum plates (14.5" across the diagonal), but I was only boring a 1.5" hole in them. I think the biggest cutting diameter I've done to date is about 8". The vast majority of my work is under 4". On the small end, I don't often work on things much smaller than 1/2", and almost never on things less than 1/4". (Except for drilling holes of course - I routinely drill holes down to 0.100" or so from the tailstock.)

The table below shows the spindle speed in RPM based on cutting diameter in inches, and surface speed in SFPM:

The top left corner of the table is why real lathes have back gears. The area under 100 or so RPM is pretty much out of the question for a belt-driven spindle. My goal is to have the equivalent of at 1/2 to 3/4 HP at all speeds - 3/4 HP at 11.9 RPM is 331 lb-ft of torque, which is the output torque of a nice V8 automobile engine. That kind of torque simply doesn't travel over belts (at least not belts that I want to use on this machine.

Most of the bottom right of the table is likewise insane. The spindle bearings in the Shoptask are tapered roller bearings. The original machine was rated for 2400 RPM, and I could probably push them to 3000 or so, but the thought of running a 6" 3-jaw chuck at 5000 RPM or more is just scary. Thats not a real problem though, since the last two columns of the table are for carbide tooling only, and I don't use carbide that much. I also rarely work on parts smaller than 1/4".

The net result of this exercise is that I'm going to aim for a full power range of about 100 to 2500 RPM. At 100 RPM, 3/4HP means 472 lb-in of torque, so I'll have to step the motor speed down by at least 2:1. "At least", because the 250 lb-in rating of the motor is a peak rating. Unfortunately I don't know the continuous rating, but I'm sure its no more than half the peak. A 4:1 step down would mean about 400 RPM and 120 lb-in continuous at the motor, while delivering 3/4 HP at 100 RPM to the spindle. With a 4:1 ratio, and the max motor speed of 2000 RPM, that low range would get me up to 500 RPM on the spindle with no belt change. Then I could switch to a high range with a ratio of 4:5, which would take me from 500 RPM to 2500 RPM.

The next step is to figure out the pulleys, belts, and such needed to deliver those ratios. But that is something for another time.

(posted: 19 Dec 2006 13:40) (permalink)