Gear pressure angle question

[Suzuki4evr]

How do you calculate the pressure angle of a gear to buy the correct cutter?

 

[benmychree]

You do not calculate it, you compare an existing gear with a printed sample or a gear pitch gage of the same pressure angle and diametral pitch, then the number of the cutter is determined by the number of teeth on the gear to be cut. I am speaking here of diametral pitch gears, which are made in 14 - 1/2 degree and 20 degree pressure angle, but there are also module system gears which are common on European and Asian machinery, which unless quite old are liable to be 20 degree PA, also noted, for them, the cutter numbering system is backwards from the DP system, but tool catalogs will detail which cutter is in which number of teeth range.

 

[Suzuki4evr]

I understand and have the rest of the gear making under control, it is just the PA I needed to figure out.

This is the gear that drives the gear I have to make.


And this is the cast iron gear I must make. The teeth does not look like it actually has a involute form or pressure angle by the looks of it. I don't know if I must make a HSS toolbit to take the form of the old gear, because it almost look like an ACME thread form,but I know it is not, or get a gear cutter with the same DP and PA as the small gear.

As you can see from the pics below,the old gear is pretty chewed up.


What do you all recommended?

 

[benmychree]

I think you should do the math and calculate what exactly you are dealing with so far as DP, CP, or Module gearing. The larger gear would look more like an Acme due to the tooth shape being more straight sided than the pinion gear because the curve shape of the teeth goes flatter (less curved) as the number of teeth increases until it becomes a rack gear with straight sided (no curve). I would measure the approximate OD, PD and root diameters and try to match them up with the dimensions from formulas, this can tell you what system they are made under.
If you hold a rule against one of the teeth on the larger gear pretty much centered on the curve of the tooth and scribe a line on the gear, then use a protractor to compare the scribed angle with the center line that you can also scribe, the resultant angle will be the pressure angle.

 

[Suzuki4evr]

If you have two gears,one 39 teeth and the other 13teeth, with 5DP 14-1/2 degree PA, can you make the exact size gears with a module cutter equivalent, which coincidently also mod 5 with 20degree PA if that is the type of cutters available to you?

 

[benmychree]

Five module and 5 DP are not equivalent, DP is in the inch system, and module is in the metric system. A 5 DP set with 13 and 39 T would have a center distance of 5.2 inches. a similar set of 13 and 39 T in 5 module would have a center distance of 5.118". 5 mod = 5.1 DP, conversely,
5 DP = 5.08 mod. If the center distance could be changed to suit these dimensions, either cutters could be used, if I am reading the formulas correctly. To further complicate matters, I see that there are also standards for both inch and metric British Standard gearing.

 

[Suzuki4evr]

So if I understand you correctly it is possible if the center distance can be adjusted ? And I can change pressure angle from 14-1/2PA to 20PA?

 

[benmychree]

es, I think so, as long as both gears are the same pressure angle and the center distance is corrected, the difference being.082". Since we do not know whether or not your gear set is DP or Module, we do not know in what direction it would be, plus or minus, but the center distance can be easily calculated, my reference being Machinery's Handbook from 1964, that being when I started my apprenticeship in the trade.

 

[Suzuki4evr]

The current set is 5DP 14-1/2 PA. But I am struggling to get the correct cutter for the right price to be shipped to me without breaking the bank. I can get two cutters with Mod5 20PA on Aliexpress. According to the client the center distance is 135mm (5.314"), I haven't actually measured it myself.

 

[benmychree]

That center distance (5.314") is not correct for either Module or DP.