How do you find the correct PA of an existing gear

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stioc

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I recently cut my first test gears (https://www.hobby-machinist.com/threads/my-first-gear-cutting-adventure.78446 ) and now I'm thinking if I had to make a gear to mesh with an existing gear how would I go about finding the correct PA? For example my 9x20 Grizzly lathe's change gears seems to be DP25 which I'm thinking is actually Module 1, I figured this out by using the formula DP=N+2/OD. Now I can find M1 gear cutters but they come in different PAs, 20 seems to be more common. How do I find the PA of an existing gear?

Also, has anyone made their own single point gear cutter by forming the correct profile so you don't have to buy expensive gear cutters for any oddball size that you run into?
 
The easiest way would be to compare to gears with known pressure angles. An on-line search will turn up numerous examples of the two.
 
Thanks, you mean print it out and then lay the gear on top to see which one it matches up with? I was hoping there was a way to calculate it given the OD, teeth and the DP.
 
No way to calculate it but there is a visible difference between a 14-1/2º and 20º pressure angle. Wikipedea has a good primer on spur gears. Other sites have outlines of various mod or diametral pitch gears.

Spur Gear Terminology & Formulas | Pitch Diameter, Pressure Angle,

What is a Spur Gear Everything you need to know about Spur Gear Terminology and how to Calculate and define common terms pitch diameter, module,
www.grobinc.com www.grobinc.com
 
That was a good read, thanks. So if there are really just two main standards i.e 14.5 and 20 then it shouldn't be too bad comparing and eyeballing it.
 
As an experiment to see if I could, I ground a tool and set it up in a fly cutter to duplicate a change gear on my lathe. I installed the resulting gear on the lathe and quite to my amazement it ran fine. It was made of aluminum and done with a spin indexer. I now have a rotary table and intend to try more gear cutting.
 
stioc said:
Also, has anyone made their own single point gear cutter by forming the correct profile so you don't have to buy expensive gear cutters for any oddball size that you run into?
Click to expand...

20 years ago. I bought an old leBlond that turned out to have two teeth missing on the feed gear. I brazed it solid then cut new teeth.

Started by buying "gears and gear cutting" by ivan law. all the math for making an involute cutter is in there.

But i ended up just grinding on an HSS bit in a fly cutter and comparing it on an optical comparator to the gear. Spend days making the cutter, maybe an hour cutting

Still got the lathe, runs great.
 
Browning sold a set of gauges. Each gauge was about 1/32 thick and had a 14.5* rack on one side ane a 20* rack on the other. There was a separate gauge for each DP. I don't know if they ever offered gauges for module pitches.
 
I had similar questions when trying to replace a gear in my lathes back gear. Found out it was pretty easy to see the difference. 14.5° pa has square teeth while 20° pa teeth appear more rounded.
 
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