While the example uses 10 holes a larger multiple of holes will cover the same number of divisions and several more. The following formula is used to determine the holes required for a number of divisions. So the notion about having to have a special ratio is horse puckey ! I have been developing the holes patterns for the plates using the simple formula Harold Hall provides in his dividing head info on his web site, so my plates will work with my head and the 30:1 ratio. Every manufacturer has proprietary plates because of their size, or the way they mount, or where they mount. There are several ratios that have been used over the many years. The first thing we are going to do is dispel the notion you must use a 40:1 ratio because most use it and the available plate charts use it. The latest issue of Home Shop Machinist magazine has an article covering how to do just this, but it arrived too late for me as I got a nice 30:1 worm gear set donated to the project by a friend. I was researching how to drive a spur gear with a worm cut on the lathe as several dividing head projects do this and it will work well. Sometimes they can be found reasonable on sites like ebay, but not often. While most dividing heads use a 40:1 ratio worm gear set, it is way too costly to plunk down $150 to $250 for a new worm gear set. As many know, I like to build my own tooling and try to show the home hobby machinist how to make decent quality tooling without the big cash outlay required to purchase it. There was another thread I had started to discuss the possibility of building a dividing head.
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