# Re: tracing a helical gear on hobber

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Posted by BillS on July 28, 2014 at 18:29:03:

In Reply to: Re: tracing a helical gear on hobber posted by steve green on July 27, 2014 at 17:34:06:

Steve,
RonV gave you an example of what the formula should look like in his earlier post. Here it is again, but using NDP and the constant I have in the FAQ page for a Newark #3:

0.318309886 x Sin (Helix Angle) x NDP
Decimal Ratio = ------------------------------------------------------
Hob No. Starts

The decimal Ratio you calculate above should closely match the ratio of a set of differential change gears, to about 4 or 5 decimal places. Since you will be looking for 4 change gear sizes, this can be a trying process. It's best to have a very complete inventory of change gears, and to have a PC software program designed to find best match within a given inventory. You already have CPC-RATIO, so all you need to do is solve the formula for decimal ratio and enter it into CPC-RATIO.

Here is an example gear to cut:
35T     12NDP     10 Deg Helix Angle

Substitute these values into the formula above and you get a decimal ratio of:
0.66328533

Enter that decimal value into CPC-RATIO and get:

53 x 74        Driver x Driver
----------     ---------------------
73 x 81       Driven x Driven

This is just an example - your gear and gear set will vary.

If you don't have all of the sizes in a desired gear set, then click [F10] key to add your Newark machine to CPC-RATIO (if you haven't already). Enter the sizes missing from your change gear inventory and [Save]. Return to the first screen, click [F10], select your Newark, and run the decimal value again. This time you will get the best possible gear set that matches your decimal value.

To see other options (change gear mounting criteria, etc) that could be useful to you, please refer to the CPC-RATIO User Manual or send me an email.

I have always relied on the generosity of contributors who have, over years, provided the machine constants and formulas that you see in the FAQ. If anyone sees an error, please let me know.

If you have any problems getting your setup to work, email me and we'll work it out.

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