Engineering/gear cutting

[Deleted User]

Can two gears with the same number of teeth and the same DP have different pressure angles

[Deleted User]

Yes

[Deleted User]

Hi Dave,Thank you for the information. Is there an easy way to measure the pressure angle?
Roger

[Deleted User]

The more or less industry standard pressure angle in the UK is 20° and in the USA 14½° seems to have been favoured. The trouble is that there are all sorts of variations to this, some for entirely justifiable reasons, that can confuse the issue and to the Mk.1 eyeball there is little in the way of obvious differences.

You could however try this:

Firstly you need to know the DP of the gear. To find it add two to the tooth count and divide that by the O.D of the gear.

Take a measurement over any number of teeth with whatever equipment you have, multiply the cosine of the assumed PA by 3.1416 and then divide by the DP. Add this to your measurement, this should be the measurement over one more tooth if its not the same, change the PA and try again.

Example:-
10dp 14.5 pa, 30 teeth,
Measurement over 3 teeth .776
Cos 14.5° = 0.986147

(0.986147 x 3.1416) = 3.04

3.04/10 = 0.304

0.304 + 0.776 = 1.080 which should be the measurement over 4 teeth if the gear is 14.5° PA.

[Deleted User]

I think efr125 sums up the situation precisely.

[Deleted User]

Thankyou David. When I was a mere youf I had a maths teacher,Maurice Edlestone,he was the captain of Reading FC and captain of England Amateur football team. He was a great teacher and pounded logarithms into our thick skulls ad infinitum.When we complained at having to learn how to use all these tables,little did we know that one day (65yrs later)we would have cause to use them again,so thankyou David,I now know the pressure angle of a gear.
Roger

[Deleted User]

I only confirmed efr125s solution, that is the person to thank.

[BRIAN MARSHALL]

I often think of the years spent doing differential and integral calculus, trignometry, logarithms, and general maths and algebra, and shudder… When I first started work, (1963), I was introduced to slide rules, including Otis rotary ones, but there were no computers or even pocket calculators. Everything had to be done the hard way….. Playing darts was the best way to hone my mental arithmetic however ! Happy days…
Brian

[Deleted User]

“I wish I was cleaver like you Brian.” (think baked beans!) When our maths bloke wrote dx/dy on the board the shutters clanged down never to rise again . . .

I remember those cylindrical slide rules, they had the equivalent of a 5ft scale if I remember correctly, bought my best mate one when he went to university.

Our first calculator with scientific functions was fitted into a tailor made welded steel case which was bolted onto a 7ft x 5ft pillar which in turn
held up the College so everyone could use it without getting it nicked. We got an especially cheap discount on it because we were a University –– a mere £127.00!!!

[Deleted User]

Hi Dave (djcb) I was in fact thanking (efr215) David !!! But really it was thankyou to everyone who has contributed to this little discussion. You learn something new every day,so they say. (Don’t know who “they” are ???)

[Deleted User]

Happy daze indeed. As a callow youth of about 21, I watched fascinated as a low loader delivered a strange looking machine to the loading dock of our former family business. It was a Drummond Maxicut 3A gear shaper, but my peace of mind was shattered when I was told to get to know how to use it and make the tooling. Once I got used to it, I really liked it but it helped to have a good quality steel that cut nicely and especially accurately made components.
We ended up with a couple of 2A machines also but apart from the gears I made for the Silks, we only made for ourselves, and, of course my 4 speed Scott boxes. I still have the cutters in a box but no room for a machine and perhaps no time to use it if I had one, but I know folks who will cut gears for me when I need them. I was so lucky to have had the opportunity to work so many machines and be central to production planning, so that making things is not really a challenge. For those who have spent time with machine tools, I was fascinated (again) by the picture of Scotts cylinder barrel machine for skirt end ops. A hefty turret lathe but with an extra cross slide under the turret so that when the blind head form tool was at depth, they could feed sideways to undercut the main bore. I would like to make barrels at a price that was reasonable, but have not cracked that one yet! Cheers Roger

[Deleted User]

Oh! Dear! Roger, you’ve raised an unwelcome spectre from my forgotten past!

Long, long ago I got my hands on a very, very ancient Johnson opposed twin outboard motor, it too had one piece cylinders+heads. While attempting to salvage the rest of it I decided the safest place for the pistons was in the cylinders. At some point I moved one of the cylinders and when I put it down there was an audible “click” . . .

Yup! You’ve guessed it. The aluminium cylinders had iron liners and I had jarred it enough for the piston to move and the top ring to drop into the “undercut” formed by the head end of the liner. The ring was still enough in its groove, (Sodds law), to prevent removal of the piston and the only point of access was through the spark plug hole.

Several weeks of off and on increasingly desparate fiddling later I did manage to compress the ring back into its groove and extract the piston.

I found another “safe place” to store the pistons after that!

Sadly the engine was never a total success and today it lurks at the back of my shed as another of those projects that: “I’ll get around to sorting yet!”

[Deleted User]

Knowledge. What we gain from all the painful experiences in our past.
A reminder of the uncomfortable truth that we are human and thus fallible. R

[BRIAN MARSHALL]

Like many of us I was a graduate of “The School of Hard Knocks”. It was, and remains, a tough school, but its lessons are usually (not always !) remembered, and remembered better than things learned elsewhere….
Brian

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