Saturday, December 29, 2007

The Countershaft

The lathe's countershaft is an improvised replacement. It's some manner of hard steel rod. It was perfectly suitable except that it was 5" too long and was protruding from the pulley that connects to the motor (hereafter known as the drive pulley.) To make matters worse, the last 6" of the rod was threaded. I decided to address this unsightly safety issue.

The smooth part of the shaft wasn't long enough which left the drive pulley sitting on threads. This didn't seem sensible to me. I determined the shaft's proper length and cut the excess off. That was a good start but it left the threads under the drive pulley. I removed them by turning the last inch or so of the shaft down to 1/2" from 3/4". Then I made a sleeve that would bring the diameter back up to 3/4" by boring a 1/2 hole in an aluminum rod. I pressed this to the shaft. It was a tight one-way fit. Finally, I turned the sleeve to the proper diameter, faced the end of the shaft, and broke the corners.

This was the first press fit of this size that Marco and I had ever done. It could be likened to monkeys doing math.

If I'd had the steel, I would have made the sleeve from that. We'll see how long the aluminum lasts.

Here are pictures of the countershaft in place.

While I had it all apart, I decided I didn't like the retaining collar on the right end of the countershaft and the pulley on the left end rubbing against the casting. Didn't seem too make a lot of sense. As an experiment I cut some washers from a plastic jug. We'll see how long they last. I don't expect they'll have too much pressure on them.

Saturday, December 8, 2007

Repairing the bull gear and cone pulley

Probably due to the previous 'fix' to the lathe, there is a considerable amount of wear on the hubs of the bull gear and cone pulley. As far as I can tell, there was no bearing between these parts. They were pressed hard together by the force of the spindle and frequently spun at different speeds. The Zamac hubs rubbed and eroded themselves down. Eventually, the outer edges of these parts started rubbing together and caused scoring. A bronze bushing in the cone pulley stopped the damage to that part, but the bull gear kept getting chewed up, if at a slower rate.

I decided to make room for a bearing between the bullgear and cone pulley. I'd size it so the edges ended up close but wouldn't touch.

The bull gear's right hub has seem some mysterious wear. I don't know what caused it. It was unsightly so I removed it. It also prepares for a possible retainer bearing similar to the one I put by the change gear. In addition, there are 60 holes at the edge of the bull gear for indexing. As is all too common, the retaining pin got pushed in while the lathe was running and tore up the holes. I smoothed the hub and indexing holes.

I turned the bull gear over and took about 90 mils off the the hub area. Note the damage to the edge of the piece - this isn't machined. That's where it wore against the cone pulley.

I turned my attention to the cone pulley. It's hub was not nearly as damaged due to the protection offered by the bronze bushing. I ran over it with Marco's mill. Again, note the damage towards the edge. That's where it rubbed against the bull gear.

This picture shows the bull gear with the sintered bronze 'oilite' bearing in place. The bearing is proud of the surface to prevent the wear around the edges of the parts.

Here is it all together. The cone pulley and bull gear are protected from one another by the bearing. Since the bearing is thicker than the amount of material I removed, the edges of the parts no longer touch, as seen in the following picture.

I need to file a few flats in my spindle for set screws. But otherwise this completes the headstock repairs.

Sunday, December 2, 2007

More lathe stuff

I ordered parts from (McMaster-Carr). They have a superb web site. You pretty much describe what you're looking for, and the site presents you with a list of matching parts and additional selection options. Before long you're down to a part or two and you can select the one that suits you. It's painless.

On thing I discovered about my spindle recently is that it's not like the others I've seen on the web. The bearing shoulder on mine is fully 1/2" shorter. That, and the lack of a flange bearing, was why the headstock had that weird thrust path.

Here's the spindle. The part we're concerned with is the shoulder in the middle of the image. I have to get the force from that shoulder to the thrust bearing.

I ordered a remarkably thin needle thrust bearing and a sintered bronze flange bearing. My friend used his Sherline to part the flange bearing to length. In this picture you can see how the bronze bearing channels the thrust from the spindle shoulder to the thrust bearing.

The back gear isn't really secured now that I removed that huge bearing. Originally I was going to secure the gear to the cone pulley but I've nixed that. Instead I turned a UHMW retaining ring that consumes the empty space between the back gear and the thrust bearing. Here's the ring. Note how it rides on the flange bearing. The ring fits relatively loosely and, being UHMW is very slick. If all goes according to plan, the back gear will not be able to affect the bearing.

Here it is all together. The back gear has no where to go.

Now there is still a little work to do on the spindle. The mating hubs of the bull gear and cone pulley are abrading one another. This is a very high-wear part of the design. Perhaps over time a bearing between has been lost. But now the hubs have rubbed down and allowed the outer edges of the parts to touch and scar. I'm going to turn the hubs smooth then remove enough material from each so a 1/8" washer bearing fits between them. I'll leave the bearing a little proud so it prevents the outer edges from touching. This should be easy enough.

Stay Tuned!