Gear tooth pressure angle

[firbikrhd1]

Thank you all again for your help. The link provided by Harold Mulder is most helpful as it has silhouettes of spur gears in several DPs in both 14 1/2 and 20 degree PAs. I have an old Boston gear catalog with similar silhouettes but it omits the 24 DP which is what I need to cut. I've been able to verify the DP mathematically and more or less assumed the gear to be 14 1/2 degree PA because the machine using it is an older machine, however it's better to verify it before wasting time making the wrong part as has been mentioned here previously. I believe I can compare the broken gear I have to the silhouettes on the linked part and get some assurance of the PA.

My initial post made an assumption that if one were to have a set of Acme thread pitch gauges and happened to find one that fit a particular DP gear that the gauge would represent a rack of that DP with a 14 1/2 degree PA so that if the remaining teeth on a broken gear gear were to be able to run on the gauge smoothly with no visible clearance between the gauge and the tooth forms one might be certain that the PA is indeed 14/1/2 degree PA. If a 20 degree Pa gear were run on the same gauge it would seem that there would be clearance where the angles were not the same. Perhaps my thinking is flawed in this matter, however the suggestions above should help me verify what I am working with.

[Paul Alciatore]

Many gear tooth profile gauges are of teeth for a rack; that is to say, teeth with straight sides where all of their length is at the pressure angle. So, an Acme thread gauge or an Acme thread, with 14.5° flanks would, in theory, provide a check of the pressure angle. The problem is a standard Acme thread has a height that is 0.5 times the linear pitch of the thread while a standard involute gear tooth has a height that is greater than 0.7 times the circular pitch. Note that the circular pitch of a gear becomes a linear distance when talking about a rack. So, the Acme thread gauge or Acme thread will not fit all the way down into the space between the gear teeth. There would be some extra space between them and judging the match up for the pressure angle may be difficult. While this difficulty would negligible for a rack, it will become greater as the number of teeth on the gear becomes lower.

And it would not necessarily be a good check for the rest of the features of the gear tooth's profile.

In my searches for an inexpensive gauge, I ran across a good set of three views that show a proper fit, an incorrect pressure angle, and an incorrect pitch in the McMaster Carr catalog. Here is the link.

https://www.mcmaster.com/gear-tooth-gauges/

I believe with a bit of imagination (for the reduced tooth height), it also shows the difficulty of using an Acme gauge or thread to check a gear.

[djc]

Could not find it before, but now relocated:

https://www.artec-machine.com/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=2&moduleid=1470&articleid=166&documentid=113

A Google search with the words "janninick base pitch" will also find multiple copies of it.

[metalmagpie]

I bought my gear tooth gage set from McMaster years ago. They were a little expensive but not too bad and they are dang useful when needed.

metalmagpie

[Hopefuldave]

Hmmm, as there are really only two pitches to choose between, I roll the gear on a bed of Plasticene (TM - a modelling clay) and it generates a rack form with the flanks at the pitch angle...

Dave H. (the other one)

[AntonLargiader]

No, you’re trying something that isn’t working. Make up your mind on who likes Starrett (since it seems so important to you).

.

You have answered your own question.
Given To Me.
It is unlikely that you would buy them new at full retail price, they are expensive for the same reason that people will pay more for BRANDED products then the same product without the name.
The same basic product,
This one can be bought for $95.00 rather then $1800.00
https://www.penntoolco.com/precise-g...yABEgJGjfD_BwE

[firbikrhd1]

Once again, thanks to all who responded with assistance. In addition to the information provided I've had some luck. I found a set of gear tooth gauges in both 14 1/2 and 20 degree, used, for less than half of what McMaster Carr's price. That price I can afford and justify the expense for my hobby shop. Between the information provided and these gauges I should be able to cut my first spur gears.

[AntonLargiader]

Interestingly enough, some of the gears I deal with are either 15° or 17.5°. These are on older BMW motorcycle transmissions. Newer stuff is all 20°.

[firbikrhd1]

Interestingly enough, some of the gears I deal with are either 15° or 17.5°. These are on older BMW motorcycle transmissions. Newer stuff is all 20°.

Fortunately, those seem to be the unusual case and not the norm. The gears I'm making are for older American made machinery so I expect them to be 14.5 degree however 20 degree is possible since I don't really know at what time period 14.5 degree PA fell out of favor.

[Rich Carlstedt]

"since I don't really know at what time period 14.5 degree PA fell out of favor."

I started seeing 20 degrees in the 1960's and then about 1975 they really seemed to take off
This is a Non-Automotive observation , but one from industrial applications
...Just my opinion
Rich

[J Tiers]

.............USA hobbyists adore Starrett tools however.

Not this one.... I do not like many Starrett tools, they do not feel right when using.

I would prefer Lufkin mics, or B&S, maybe even Slocombe. The Starrett to me have too much of the "loose" feel of Scherr-Tumico, or "General" brand. Some of the no-name imports are fine, but some are not accurate when checked against gauge blocks (they may "zero" OK, but drift "out" at larger distances).

[AntonLargiader]

BMW changed from 15 to 17.5 in the early '80s for their traditional air-cooled boxers and kept that until production stopped in the mid '90s. However, I don't know if they used 15° all the way back to 1923; I only work on '70s and newer. The other model lines (water-cooled inline from the mid '80s, later boxers, etc), as far as I know, always used 20 although I don't have any need to verify it since there is no possibility of mixup as there is with the boxers.

As for measurement tools, the ones I use most often are Mitutoyo (electronic depth gauge and mechanical micrometers), a Snap-On electronic caliper (no idea who actually made it), and a dial indicator made by some German company, I think Käfer.

"since I don't really know at what time period 14.5 degree PA fell out of favor."

I started seeing 20 degrees in the 1960's and then about 1975 they really seemed to take off
This is a Non-Automotive observation , but one from industrial applications
...Just my opinion
Rich