Pinion dimension confusion

[Andrew Knowles 2Participant @andrewknowles2]

Hello

I am a beginner clock maker trying to make a lantern clock from Malcolm Wild's article in Model Engineer.

I am having trouble getting the clock to run and I am almost sure that this is due to the pinions I have cut.

I am using a Thorntons cutter so I do not think that is the issue (tried making my own first) I am confused about the pinion blank diameter and depth of cut.

The pinion in question is M 0.9 x 7 teeth

In the ME article the pinion blank OD is shown as 0.343" whilst in Malcolm Wild's book the OD is given as 0.309"

I am also unsure about the depth of cut Should one cut until the OD diameter of the blank just disappears or should it be possible to see the original OD on the tips of the teeth?

If I start with an OD of 0.343 and cut until the OD almost vanishes the leaves are much too narrow and mesh very poorly with the wheel (M1.0 x 72)

Any advice would be much appreciated as the pile of scrap pinions is growing.

Many thanks

Andrew

[Andrew Knowles 2Participant @andrewknowles2]

Help for a beginner clock maker please

[roy entwistleParticipant @royentwistle24699]

0.343 inches is about right diameter. Are you using a 7 tooth pinion cutter?

Depth of cut recommended by Thorntons is 2.7mm about 0.106 inches

I find it easier to work in metric when cutting wheels and pinions. Any other time I tend to work in imperial

Roy

[Clock polisherParticipant @clockpolisher]

Andrew,

I am also new to clockmaking and in particular gear cutting.

The first thing I did with regards to gears was to purchase a copy of J. Malcolm Wild's Wheel and Pinion cutting in Horology. It's a bit long-winded with maybe a few too many photos of historical workshop equipment but there are some gems of information to be found.

As I read it, in the vast majority of cases for clocks the module of the wheel cutter should be the same as that of the pinion cutter. All cutters should be cycloidal in form.

Wheel and pinion blanks should have the relevant faces painted with marking blue. Cutting should remove all but the finest line of this blue at the top of the teeth.

Also, as already posted, a different cutter is required for each different pinion based on tooth count and module.

I suppose this is why there are companies who supply pre-made pinions and wheels. Based on the price of an individual cutter and the need to own several it can get expensive.

David

[bernard towersParticipant @bernardtowers37738]

It was always my understanding that when pinion teeth counts go under a certain number the dims do not follow the mathematical sizes. If you look at HPC or DAVALL they change the size from ideal to slightly smaller, they do explain why in their catalogues.

[Michael GilliganParticipant @michaelgilligan61133]

Fair comment, Bernard … But those will be involute

MichaelG.

[bernard towersParticipant @bernardtowers37738]

I think you will find it applies to cycloidal as well to stop the root becoming extremely fragile.

[Martin KyteParticipant @martinkyte99762]

Couple of points.
You say that you are cutting a M0.9 pinion and then say it doesn’t mesh with a M1.0 wheel. The modulus of the pinion and the wheel should be the same.

The blank diameter of 0.343” is correct for a M1.0 7 leaf pinion and the 0.309” is correct for a M0.9 7leaf pinion.

Do check that the parameters of the pinion you wish to cut are correct.

I am assuming you are aware that pinions below 12 leaves have to be cut with pinion cutters for the specific number of leaves and not with wheel cutters. (I have inadvertently cut a 72 tooth wheel with a pinion cutter by accident and was puzzled for a while why I could not get a good mesh).

As for depth of cut I go deep enough to leave a very small witness at the tip of the teeth, I often use a marker pen to make it easier to see the original surface and a jewellers loupe is also a boon.

Keep at it it will all come right.

regards Martin

[roy entwistleParticipant @royentwistle24699]

The module of the pinion cutter is usually 0.05 smaller than that of the wheel cutter. The dimensions of the pinion are worked out using the larger wheel module. This gives slightly thicker roots to the pinion tooth for strength.

Roy

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for posting that, Roy

I didn’t want to appear argumentative, but was just about to show this:

**LINK**

https://www.m-p.co.uk/muk/acrobat/tech/cutters.pdf

MichaelG.

[Martin KyteParticipant @martinkyte99762]

Posted by roy entwistle on 04/11/2022 19:47:38:

The module of the pinion cutter is usually 0.05 smaller than that of the wheel cutter. The dimensions of the pinion are worked out using the larger wheel module. This gives slightly thicker roots to the pinion tooth for strength.

Roy

That need not be the case when making a clock from scratch. The advice to use a slightly smaller module is applicable when making replacement pinions to mate with existing wheels in old clocks which can have an altered tooth form. This advice does seem to have been taken up as general practice in new clockmaking where it is not necessary. Practically speaking it doesn’t really matter either way in a new clock except to confuse new clockmakers (and seasoned clockmakers too if I’m honest).

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

Andrew

You might find it useful to have a look here: **LINK**

https://www.csparks.com/watchmaking/CycloidalGears/CycloidCalculator.html

Obviously, you will need to run the calculator twice … once for each module

… on each occasion, ignore the irrelevant answers.

The final ‘physical’ optimisation of the mesh is [with cycloidal gearing] done by depthing.

Note [and this might prompt some debate] that I believe Involute gearing should not be so-adjusted, because it is designed to run on specific centres.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

I tried Dr Hessmer’s page …

it’s not really optimised for viewing on the iPad, but I did manage to grab this:

.

.

Hopefully, that looks reasonably familiar.

MichaelG.

Edited By Michael Gilligan on 05/11/2022 06:53:17

[Andrew Knowles 2Participant @andrewknowles2]

Gentlemen

Firstly thank you all for your help with my issue

Just to confirm the ME article clearly shows a M1 x 72 tooth great wheel meshing with a M0.9 x 7 tooth pinion

The great wheel was cut with a M1 Thorntons wheel cutter and I am using a M0.9 x 7 Thorntons pinion cutter.

The OD of the pinion blank is 0.343" on the drawing. This ties up with the value from Wild's book for a M1 pinion and also Roy's comment that the pinion dimensions should be based on the module of the larger wheel module.

The outstanding point is now deciding if I should try a M0.9 x 7 or a M1 x 7 cutter to try. I would rather not have to buy both so would welcome suggestions on which one to try.

Once again many thanks for all your help

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

deleted because the forum told me it had failed to post [but it lied]
 

Edited By Michael Gilligan on 05/11/2022 15:33:03

[Michael GilliganParticipant @michaelgilligan61133]

For reasons best known to itself, the calculator that I linked falls-over with Module=1

I have therefore used Module=0.99 for the bigger cutter.

.

.

A quick check through those numbers should inform your choice of cutter, Andrew

MichaelG.

[roy entwistleParticipant @royentwistle24699]

Andrew. Could you post a photo of the offending wheel and pinion in mesh to let us see what's happening. Are you using a depthing tool. Or trying to mesh them in measured holes. You say that you are using an M1wheel cutter and a M0.9 x 7 leaf pinion cutter. It should work

Roy

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 05/11/2022 15:26:08:

deleted because the forum told me it had failed to post [but it lied]

So now you have to make 25001 posts before we break out the champagne!

[Andrew Knowles 2Participant @andrewknowles2]

Hi Roy

I am using a depthing tool and I will get some pictures next week.

Regards

Andrew

[ChrisLHParticipant @chrislh]

Michael, your point re. Involute gearing is, I believe, actually the other way round.

Involute gearing will tolerate some variation in centre distance with no change in velocity ratio.

I believe this is not true for cycloidal gearing but because speeds are low in typical applications (clocks) variation of velocity ratio through the meshing cycle at any other than the theoretically correct centre distance is undetectable and therefore tolerable.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the observation, Chris

I am struggling a bit with this at the moment, and my reference books are not accessible, so I’m relying upon the internet for evidence of whether or not I have lost the plot [a risky situation!]

Could you please have a look at this concurrent thread, and the link I posted at 10:52 this morning:

**LINK**

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

I am basically using this statement, and the explanatory animation, to support my case:

[quote]

If the center distance differs from the standard center distance, then there is no backlash-free pairing of the gears and the line of contact is shortened (see dark blue gear in the animation below).

[/quote]

.

But yes … it is quite likely that I have ‘lost the plot’

MichaelG.

[ChrisLHParticipant @chrislh]

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 ?

[Michael GilliganParticipant @michaelgilligan61133]

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.

[Michael GilliganParticipant @michaelgilligan61133]

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.

[John HaineParticipant @johnhaine32865]

For the reason I mentioned above.

[Author]

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