Measuring for the PA of an ordinary involute pinion gear

John yes indeed - it is out of a Meddings Drill but that does not help me with learning the principle for future replacement of different worn gears. Meddings themselves are treating the PA as proprietary information and not willing to divulge it.

Adam,

Have you had a go at the blu-tac method yet?

You don't have to faff around with mid points, as the gear should generate a 'rack' which has (more or less) straight-sided teeth. OK it will be a rather mangy looking and distorted rack but the difference between 14.5 degrees and 20 degrees should be obvious.

Neil

P.S. 'other angles are available':

.

Adam,

Just returning to this comment in your original post. ^^^

[unless the gear has chamfered ends, which could make it tricky] ... If you can photograph the gear exactly end-on, there should be no problem: The combination of perspective and depth of field will hide most of the confusing details, and it is a simple matter to identify the edges on the plane surface.

The worst you are likely to get is some lens distortion [barrel, or pincushion], but you can easily check the performance of the lens by photographing a sheet of graph paper.

Alternatively, as I said before; if it's flat, put it on a flat-bed scanner.

MichaelG.

.

P.S. ... Here is a photo that I took a while back; showing one tooth of a cutter ... The lighting was awkward, but the image is certainly fit-for-purpose.

Edited By Michael Gilligan on 19/03/2015 14:45:48

Dunno if this is of any use to you. It's imperial of course.

Bottom line, you are going to have to measure something and then determine if it's what you'd expect to see, or simply guess it's 20 degrees or whatever by eye and live dangerously. I don't suppose anyone would die if you got it wrong, worst case it may be a bit noisy perhaps?

Could you post the number of teeth and its module or DP?

Martin.

If it's Meddings it's 20 PA

Reason I know is I had a two speed drill once and it stripped one of the fibre gears and the other was about 1/2 worn.

Measured it up and looked in HPC catalogue and they did the small gear in steel but not the large one, can't remember how many teeth that was. Wasn't worried about the large one as it still had life in it.

Bought the small one in steel, sod the fibre, I know it's probably for silent running and a safety feature but I'm deaf [ what ? ] and a slipping drill is also a safety feature, not something that stops play for a week or so.

Machined it, bunged it in and job's a good un, still running AFAIK, in fact Meddings bought it back off me.

For gear ignoramuses like myself, here's a simple article on pressure angles. Doesn't sound difficult to measure really but looks as if JS has solved the mystery already.

John thanks for the confirmation it is 20 deg. I had been told that was most likely but I read that 14.5 deg , albeit uncommon post the 1950's,is useful if you preferred quiet operation of gears to strength, and so thought perhaps Meddings would have gone for "quiet" as their drills are so silky smooth when right.