Pinion dimension confusion

Michael,

I believe the statement to be true but it is dependant on what is meant by standard centre distance. This is all part of a sea of confusion that has built up around involute gears with standard gears, corrected gears, standard centre distance, pitch circle, etc. all of which mean different things depending on who you are.

If the "standard centre distance" is taken to mean half the sum of the blank diameters minus the tooth depth then backlash can be negative or positive depending on the gear details. In manufacture this is allowed for by making adjustment to the cutting depth. Complex formulae are available for calculating these corrections.
In brief, with a given pair of gears if you extend the centre distance, backlash inevitably increases but, as long as the line of contact is longer than the base pitch, an unvarying velocity ratio will be maintained.

But I am not sure of your drft, is this what you wanted to know ?

What I wanted to know, Chris was whether I was right or wrong in saying that Involute gears only work ‘correctly’ [whatever that might mean] when the two pitch circles roll precisely in contact.

I always thought that to be the case, but both the “essay” and your observation tell me otherwise.

If your answer is “well it all depends what you mean by correctly” that’s fine … but if your answer is “no, Michael you are most definitely wrong” then I need to retract my previous comment.

MichaelG.

I have just spent some time skimming through the archive of the Horological Journal, and I am now convinced that my statement was wrong … every reference that discusses depthing indicates that the Involute form is more tolerant of variations in depthing than the Cycloidal form.

MichaelG.

For the reason I mentioned above.

.

I must be going blind as well as daft, John

… Where ?

MichaelG.

.

Edit: __ It’s O.K. John … don’t bother answering … Your words of wisdom are on the other thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=183863&p=2

Edited By Michael Gilligan on 05/11/2022 21:46:43

Michael,

You were wrong but, congratulations, you now have the opportunity to be right !

A pair of involute gears can work "correctly" over a range of centre distances.

"Correctly" here means without change of velocity ratio whatever the rotational position of the gears. Just to make clear, that would not be true if you continuously changed the centres as the gears rotated.

If you needed to maximise the range of centres for some reason, involute gears have been designed that could tolerate a 7% change in centre distance and still work "correctly" at either extreme.

Edited By ChrisLH on 05/11/2022 22:26:52

Well I'm glad I didn't take up Michael's invitation then

.

It was actually a request, not an invitation

but anyway, it’s nice to know you’re happy, Duncan

MichaelG.

Edited By Michael Gilligan on 06/11/2022 18:04:50

Gentlemen

With some embarrassment I have to report that I have solved my pinion cutting issue.

After reading all the advice it appeared that pinion blank size and cutter were correct.

I then realised that the tooth space on the pinion was wider than the cutter teeth. On checking the wheels I had previously cut the cutter fitted perfectly into the tooth space.

On checking the cutter arbour I found it was bent. This must have happened after I finished cutting the wheels.

New arbor made and now the pinions look much better

Once again many thanks to all who replied to my question

Kind regards

Andrew Knowles

Good result, Andrew

… and some of us managed to exercise a few brain-cells on the journey

Win-Win

MichaelG.

Andrew, did you solve your problem? I'm just composing a PM to you.

Gentlemen (I made this posting on 11/11/22 but it does not show up in this thread but it does in My Postings)

With some embarrassment I have to report that I have solved my pinion cutting issue.

After reading all the advice it appeared that pinion blank size and cutter were correct.

I then realised that the tooth space on the pinion was wider than the cutter teeth. On checking the wheels I had previously cut the cutter fitted perfectly into the tooth space.

On checking the cutter arbour I found it was bent. This must have happened after I finished cutting the wheels.

New arbor made and now the pinions look much better

Once again many thanks to all who replied to my question

Kind regards

Andrew Knowles

And after the posting above the original now appears

Very strange

Andrew

Considering that the hole in a Thorntons cutter is 7mm, how on earth have you managed to bend the arbor?

Roy

Hi Roy

I have no idea but bent it is. I just do not understand as it was fine whilst I was cutting the wheels.

The mill has no power feeds so not that.

I will post some photos of the results with the new arbor. I would be most grateful for any comments

Kind regards

Andrew

Hello All

I have been meaning to post a picture of my pinion cutting set up for the last few weeks

I have now managed to produce some reasonable ( in my opinion ) results with the help of all the comments I have received. Today I have got the going train completed and it runs !!!

Many thanks for all the help

Andrew

Just realised I do not know how to insert an image but I will find out and post the picture in a minute

That was not so hard was it

Andrew, I like that v-groove "tailstock" support.

And the direction of cut, even if the spindle is running in reverse!

Or are my eyes deceiving me Andrew?

Sam

Sam. Well spotted I run the spindle in reverse so I can have the cutter in front of the work to make it easier to see what is going on.

Easy to do on the BCA just put the belt on the motor the other way around

Andrew