Gear cutting/buying question

[Metal Butcher]

In preparation for future builds of Hit-n-Miss engines I started to look into the 2 to 1 gear reduction needed on this style of engine. Usually 20 tooth and 40 tooth gears. I looked into buying and making a cutter. I looked into buying ready made gears that will most likely need modification. As usual, after 2 days of searching the internet and unable to make a decision all that I have achieved is a minor head ache. I fixed that with an old fashioned aspirin.

My question is: What are the pros and cons of 14.5 degree vs 20 degree? It seems that the 14.5 is usually specified on the prints that I looked over just recently. Is there a reason that I should avoid using the 20 degree? I found some reasonably priced 32 pitch, 20 degree gears. A 20T and a 40T would be about $15 for the pair.

-MB

 

[gbritnell]

Hi Rick,
You can use either pressure angle. A lot of gears are changing over to the 20* pressure angle. It's a stubbier profile but for model work I generally use the 14.5* for no particular reason.
You can buy manufactured gears from these people. They have a good selection and the prices are reasonable.
https://sdp-si.com/eStore/
There's really no need to buy a cutter, or in this case 2 cutters, one for each diameter gear unless you're going to be making a lot of gears. You can make your own. There has been quite a bit posted on making home-made gear cutters.
George

 

[Metal Butcher]

Hi George. Knowing that the 20 degree are OK puts my mind at ease. Boston gear has some good prices through Amazon on brass 20T and a 40T with free ship if I buy a 2nd set, ($25 min to qualify).

I decided to shorten the build time by not making an Ignition, Spark Plug, or Gears. Hats of to you, and others that have the ability and patience! :bow:

-MB

 

[Mainer]

14.5 degree pressure angle gears are somewhat more forgiving of minor depthing errors than 20 degree. If you can accurately space the shaft centers though, that doesn't matter.

20 degree PA gears have less tooth undercutting with small numbers of teeth.

20 degree PA gear teeth are somewhat stronger.

 

[precisionmetal]

In modern gear design, 14.5º PA has fallen out of favor many, many years ago. About the only reason to use 14.5º (IMO) is if the goal is to replicate a scale model of an old design that used that PA. (a valid reason of course)

Using standard numbers, the undercut limit on 14.5º PA gears is 32 teeth whereas it's 17 teeth on 20º PA.

PM

 

[Mainer]

I've heard that the 14.5 degree PA originated because sin(14.5 deg) is very nearly 1/4 (0.2503), and that fact made making patterns for early cast gears a bit easier.

 

[Groomengineering]

Hi all!

The story I was told was that 14.5°PA was chosen because it matched the 29° included angle of Acme threads. Acme tools could be used to make hobbs, and the gears would mesh with Acme screws. Not sure if that was the real reason, but it makes sense. In our use as long as the mating gears have the same PA it doesn't make a lot of difference what the PA is.

Cheers

Jeff

 

[Bernd]

MB,

Take a look at this thread. Might give you some ideas.

http://www.homemodelenginemachinist.com/index.php?topic=1652.0

Bernd