Tuesday, October 13, 2015

Divide and conquer. Maybe.

Well, it has been a while and I have not posted. Life gets in the way. In the last post I had tracked my wheel (gear) cutting problems down to to bent shaft in the dividing head. Since the dividing head is a Topsy project (she just growed) this is a retrofit and rather difficult.Below, I am checking the runout on one of the wheels. Just as large as ever it was, a whole mm. Nothing new. Nothing for it but to remake the shaft. Turning it is easy, but..

Thing about the shaft is the dimensions. One end has to fit the gear. That is 5mm right on the money; the printer I took apart to get these gears is that dimension. The other end has to be a duplicate of the Proxxon mill spindle. This is M8x0.75, also metric. So I remade the shaft. Here it is, parted off. Note I am using both my homemade steady rest and a Dremel tool to part off the 5mm end. This gives a nice clean cut and wil not distort the shaft. I claim a new parting-off method.

Now, the big job is to put a front support on the shaft. For this I used a piece of my lovely Aluminum block that I ordered from the Internet.

 The problem is that I have to bore the 8mm hole exactly in line with the original 5mm hole in the rear. Lacking a jig borer (they cost a fortune) I used a transfer punch, and the above lashup shows how I got it done.

A runout check shows that I am down to a runout of .30 mm or so. Not really good enough but better than the full mm I had to begin with. What I will have to do is to watch my depth of cut. The wheel is perfectly round as the lathe can get it; the dividing head has a wobble! By varying the depth of cut, maybe I can compensate for this. So the setup looks like this:

 The dividing head is bolted to the mill table and "trammed" i.e squared to the table. In the mill spindle is my homemade fly cutter.  Let's try a 50 tooth wheel, I have two of them to do. Result:

Eureka! I have a wheel. It was like pulling teeth. Literally. One mistake and the whole wheel is trash. And I made lots of mistakes. Acrylic is totally unforgiving of mistakes, say forgetting to lock the dividing head. It explodes.  So I have switched to Lexan, much more resilient.

In the next episode we will cut the big wheels.

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