Posted by BillS on September 20, 2011 at 11:28:15:
In Reply to: Re: reverse engineerin a helical gear posted by Ron V on September 15, 2011 at 21:11:07:
Ron is absolutely right.
The minimum data to define a gear is:
Teeth
Pitch (DP, Module, or Circular Pitch)
Helical angle
OD
If you want to know one of these, you need the other three to define a nominal gear.
Dave, you could find the pitch by rolling cutters or profile gauges in what's left of the teeth. Even the tooth bottoms (and sometimes complete profiles are left at one of the faces) are usually still there.
Extended addendums will make things a little more complicated. Since you say you have teeth and OD, find DP (or module) and calculate helical angle assuming nominal gear specs. See if helical seems right or wrong using a protractor, etc. I used to use an Optical comparator with a pin in the teeth.
If helical seems wrong then suspect extended addendum. There are different ways that designers typically have added extended addendum. An old-fashioned way was to add 1 to teeth to find the enlarged OD. Newer way is the adjustment to addendum using tables (or formulas) based on number of teeth.
If you can get the mating member, you will enhance the chances of a good outcome. This will get you the center to center distance at least.
Try these approaches and see if you can get addendum and the four basic specs to agree.
Reversing a sample is a challenge, and there is a software tool that can help. Basically, it allows you to enter and do understandable "What-ifs" which can be difficult with paper and pencil- and usually with errors that lead to bad conclusions.
Call Scott at Ash Gear and talk to him about his GCP program. He designed it with reversing samples in mind.
I'm sure there are other PC programs, but I'm only familiar with Scott's.