Once the magic smoke comes out, things don't work any more.
John Kasunich
jmkasunich@fastmail.fm
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Wed, 20 Dec 2006
The old lathe motor, in approximately the correct location. Note that it normally sits a couple inches higher, held up by the tension on the first belt, which runs almost straight up to the countershaft. It sits high enough that the motor capacitors interfere with table travel. In the picture the table is as close to the headstock and as far back as it will go. If the motor was in its normal position, the table couldn't get where it is now without hitting.
The new motor is bigger and heavier, but I have no intention of pivoting it. It will be mounted in a fixed location, and an idler pulley used to set the belt tension. Note that eliminating the pivot leaves the space between motor and machine base open, which will make it much easier to keep clean. With the old motor, chips and crap would pack into there, and the pivot mechanism made it impossible to get them out. Another advantage of not pivoting the motor: the old motor shaft entered the "belt box" thru a hole big enough to pass the entire step pulley, which allows the motor to pivot. It also allows chips to enter the box. The new motor will have its mounting flange directly fastened to the back wall of the box.
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With the motor setting directly on the benchtop, there is about 1-1/8 inches between the top of the motor fins, and the bottom of the table stepper mounting bracket. Also, with the motor directly on the table, the bottom fins on the motor are useless. I have to decide what is the best approach to cooling the motor. Low speed high torque operation is the only time I'll be pushing the motor very hard, but since its rated for peak torque, and a spindle wants continuous torque, cooling will be important.
I spent more time than I want to admit screwing around with spindle drivetrain ideas, looking at different types of belts, pulley costs, etc. Spent some time talking about it on IRC, and a couple of the guys there had some useful input. "ejholmgren" pointed out a good source for pulleys, EconoBelt has a wider selection of HTD stuff than McMaster Carr does, at better prices. He also mentioned a design idea that I initially didn't like, but after banging my head against the wall for hours, I think I'll be using his idea after all. Instead of building expensive and deep "step pulleys" so I can change from the low range to the high range by moving only a belt, he suggested that I swap the pulleys too.
I did a little spreadsheet work, and came up with the following as a possibility:
On the motor is a 22 tooth HTD8 pulley, with a 20mm belt permanently installed driving a 40 tooth HTD8 on a countershaft. The motor speed range is 400 to 2000 RPM, and the countershaft range is 220 to 1100 RPM. Next, both the countershaft and the spindle are equipped with identical hubs. On one hub is a 32 tooth HTD8 pulley, and on the other is a 72 tooth one, connected with a 30mm belt and tensioner. The hubs need to be designed to allow the pulleys to be swapped easily. When the small pulley is on the countershaft, the spindle speed range is 98 to 489 RPM, and when the big one is on the countershaft, the spindle range is 495 to 2475 RPM.
One complication in design of the hubs is the lathe spindle. The spindle shaft is 35mm (1.378") in diameter at the pulley location. The existing pulley is a slip fit on the shaft, then a nut and locknut are used to both hold the pulley on and adjust the tapered roller bearings. Torque is transferred from pulley to spindle by a key, 6mm (0.236") wide, about 4mm high, and 19.5mm long. There is a spacer sleeve, 1.379" ID, 1.731" OD, and 0.868" long, between the pulley and the inner bearing race. So the spindle hub will need to have a keyway cut in it. In addition, the hub or something else at the spindle needs to drive a spindle encoder. Perhaps a narrow toothed belt pulley can be fit to (or replace) that spacer. However, the spacer only protrudes 0.200" beyond a seal plate of sorts (which is itself 0.320" thick). It looks like the narrowest MXL timing belts are 1/8" and 3/16" wide, so maybe that would work. I need to make an accurate drawing of the spindle end and surroundings, and work out the details. But that can wait - its going to be months before I have the variable speed drive for the new motor anyway, and I will need the lathe working to make the parts. So I should stop screwing around with the spindle and get on with the rest of the conversion.
(posted: 20 Dec 2006 13:01) (permalink)