compound and differential indexing/hobbing

Hi to the group. This is my first post here and is a request for information. I have the chance of getting a 127 gear cut if I can figure out the correct differential settings for a commercial gear hobber. Please does anyone have a paper or link to information discribing how to do differential indexing/ hobbing. All that I can find on the web at present are links to engineering book sales.

Regards Bob

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On 15 May 2007 11:07:16 -0700, bob wrote:

Why don't you just buy one - the Sherline 127 is less than $20 US, and the 100 is about the same (the 50 is under $10). Buy 'em and bore 'em.

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wrote:

Yes except that I need 18DP for a boxford(southbend clone)and ordering from south africa is not so cheap. Freight and inport dutys will more than double the cost. If I do it this way , I learn about gear hobbing and differential stuff and the cost is the scrap steel. And besides I'd rather save my money for the things that I can't make or scrounge.

Bob

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Greetings- never used a commercial hob, but think you will find the hob, assuming it is single lead, must rotate exactly 127 times as fast as the gear blank. Speed of spindle depends on material, of course. FWIW, I use a 47/37 compound gear for cutting metric threads. I know it is not accurate as the 127 gear. Regards, Jim

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On 15 May 2007 14:30:48 -0700, bob wrote:

Why do you need the 18dp?

Just key the 127 to one gear and the 100 (or 50) to the other.

Or do you want the 127 for somehting other than metric/imperial conversion?

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I would think that the required gearing would be unique to the gear train of the particular hobbing machine and would be found in the operating instructions for the machine. You might have better luck if you identify the machine. Compound and differential gearing for the Brown and Sharpe type dividing head is shown in some machinists reference books but I don't know if it would help any.

Don Young

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Irrespective of what make of gear hobber you won't need differential indexing to do a spur gear. Differential indexing is for helical's on a hobber. To do a 127 on a hobber you need a 127 in the train, usually supplied in the set.

However if you are talking about doing it on a dividing head then yes you do need differential indexing to do a 127 as 127 isn't native to standard plates. Ower Gert sells a set of generic plates on Ebay under the name of Marypoppinsbag that do have 127 on them and this allows a gear to be made by single indexing.

However as you are in Africa it's probably not convenient or cost effective to buy a set.

There are two ways to differential index. One is where the input from the selector arms drives the spindle thru a set of gears chosen for the number of divisions. Looks much like the setup for spiral milling but not connected to the bed. And the second is where you move round so many divisions on one circle and move the plates forward or back so many divisions on another circle, according to a chart specific for that head.

So the short answer is it depends on what equipment you are doing it on and we need more information. -- Regards,

John Stevenson Nottingham, England.

Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /

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bob wrote:

Differential indexing is AFAIK, separate from setting up a gear train to use a hobbing machine.

Are you using and indexing head on a mill, and cutting one tooth at a time, or a hobbing machine, which is a pretty much continuous movement process ?

Cheers Trevor Jones

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