Chapter 6: The Great WheelDecember 24, 2004
My great wheel isn't finished yet, but so far making it has been loads of fun. I cut the blank (see left) from a sheet of 3/16" brass, just like the front and back plates. A nice chunk of metal, with some good heft to it. Thinking ahead, I scribed some of the lines I knew I'd need later when it came time to spoke the wheel. I drilled a 1/4" hole in the center, planning to bore it to size, and then discovered belatedly that my boring tool was too large. Whoops. I ended up grinding down an extra one I had on hand, and it worked like a charm. Plus, now I have both small and large boring tools! I was worried that the hole was a bit too large, since the wheel does wobble a bit on the arbor. However, there's absolutely no sideways freedom (perpendicular to the arbor axis), so I think it will be fine.
Here's a shot of my setup about halfway through cutting the teeth on the great wheel. I was actually pretty worried about this. First, I had never cut gear teeth before, I knew they had to be very accurate, and the great wheel has 108 little teeth to make a mistake on (which would scrap the whole part). And second, it appeared that the wheel was too large for my mill! There wasn't enough room for the diameter of the wheel blank on either side of the spindle! I was considering removing the stop bolt for the y axis (which might have given me just enough clearance), installing a headstock spacer and trying to machine the gear from behind the spindle, or possibly building a milling column to attach to the lathe (like the book recommends). But - a helpful friend pointed out that I actually already had a headstock spacer installed! I didn't remember ordering one, and the Sherline site doesn't indicate that it's one of the upgrades included with the 5400 series, but removing it left me with plenty of room to spare, and cutting the teeth on the great wheel was no problem at all.
I ended up cutting the teeth slightly deeper than I intended, and the sliver of layout blue on the ends of them completely disappeared... but as it happens, my chuck must have been slightly off center as well, since one side of the finished gear has clear bands of layout blue (larger than I would have liked, really), and the other side had none at all. My experimenting at the beginning while trying to find the correct depth to cut the teeth leads me to believe that the chuck would only need to be one or two thousandths of an inch off center to have that affect. I expect that if I want to machine truly accurate gears I need to do it with the gear blank mounted on the lathe, just after turning the diameter. This would hopefully ensure that it's perfectly centered. I'll probably try to build a small milling column that mounts on the lathe cross slide before machining any more gears. Hopefully it will yield better results. I expect my great wheel will probably work fine, given the reasonably small error, but unfortunately I won't know until much later in the project. :-/ This will be even more important on later projects, where I plan to design the mechanical works myself, and also use smaller gears (which require greater accuracy).
Cutting the slip washer groove went swimmingly. I had to grind yet another custom tool shape, but it worked quite well, and I have a groove to be proud of. I'm actually pretty glad I've gone to the effort to grind all these tools. Not only do I have a growing assortment of useful tool shapes, but when faced with a tricky machining problem I'm now confident enough to just grind myself a custom tool, and count on it to do the job well. I installed the great wheel on the arbor, locked it in place with the slip washer, and dimpled the washer just enough to remove the extra space. The wheel turns easily, but the slip washer doesn't allow any sideways movement along the arbor. I must have been lucky with the hammer, because it took almost no adjustment to obtain apparently excellent results.
Next up is crossing out the wheel, and filing it to it's final shape. However, I've decided to postpone any more work on the great wheel until I'm sure it will mesh nicely with the pinion. If the teeth are off center enough that it won't mesh well, I'll need to cut another great wheel. :-/