Wednesday, August 3, 2016

A dividing plate for the dividing head

If you look back on the archives (under the label "DividingHed" (misspell) you will see my homemade dividing head, made out of a printer salvage and some (literally) bedstead scraps. The way a dividing head works is based on a worm and gear. For each turn of the worm, the gear advances one tooth. I happen to have a 75 tooth gear. So if you had a 75 tooth gear to cut, life would be easy: one turn per tooth. In real life not all gears have 75 teeth. Sometimes more, sometimes less. So you have full turns or no full turns plus a fraction. The fraction of a turn is supplied by a dividing plate. It is a circle full of a bunch of holes that give you the fraction. I carefully worked out the holes needed by my clock and concluded I could get away with 50 and  40 hole plates. Commercial dividing heads come supplied with all kinds of plates with a wide variety of holes.

The first step in making one of these plates is to run my PostScript program that lays out the plate. I could have used a CAD program but I am afraid that their conversion to print format will distort my plate. That done, I glue the template on to a Lexxan circle. Now the fun begins.

Above I am using my optical center punch (Veritas) to centerpunch each hole. Very tedious work. Next to this is a box of very tiny Morse drill bits. By hand, with an Archimides drill I will go through each hole. Also very tedious. This gives me a pilot hole. Now we can get going.

 Using still a very small drill -- about 1mm -- we drill each hole a bit larger, and so on till the last size, around 4mm. For this kind of work opti-visors are really wonderful. See below.

Next job is to mount the plate on the dividing head. After all that work, this is easy.

This dividing plate has worked extremely well, and revealed a design flaw in the head itself. Deal with that later! Meanwhile I have an escape wheel to cut, shown below.

The escape wheel is not really a gear. It is a starfish-shaped wheel that regulates the rate at which the clock ticks. So the article says I should use a slitting saw but nobody makes one small enough for my mill. I use a Dremel abrasive disk instead. This is about half a millimeter thick. Bit thick,  but the scape wheel came out very nicely.

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