Gear cutting basics help needed.

There's a handy little computer program called GearDXF where you can input your MOD/DP/Tooth count etc. and it will output the various dimensions and visualisation:

Ivans Laws book is on order although I am more practical, I need to see and get my hands on it to understand it, books just don't do it for me.

JW machined his gears screwed to wood as a backplate, so I will use a similar method.

thanks for the link to GEARDXF Andrew.

Ian, if the clock is to a Wilding design and as Michael's post indicates uses module cutters, then those are the ones to use, don't faff around with DP cutters. Also if the wheels work on lantern pinions then you need "cycloidal" teeth to work properly. Get the right cutters, or make them.

Another site on the lines of the one that Andrew posted designs cycloidal gears and is useful to show what they look like. You can also download dxf files of the designs, and I have used these to produce g-code to cut them with CNC.

Ah, that explains quite a lot.

Andrew

Suffolk born and bred, strong in the arm thick in the head.

Gear design is difficult. The designer not only has to cut the right number of teeth, but the teeth have to mesh with another gear without jambing, and, especially in a clock, with low friction. The other problem with poorly shaped gears is they wear rapidly.

Low friction and wear requires the flanks and valleys have to be curved in a particular way, the idea being the teeth roll over each other rather than bump and grind.

In days of yore, the gear wheels used in mills, were shaped approximately and they ground themselves to a moderately efficient shape. On first fit, the mill would be stiff and underpowered. After running for a while, the teeth would wear closer to the right shape and the mill would run free at full power. Unfortunately, the teeth rarely settle into the exact shape needed, so they wear rapidly. For that reason the teeth in traditional mills were usually replaceable.

Clockmakers were first to put effort into finding an optimum curve. Mathematically, it's a hypocycloid, and understanding what that is allows really good gears to be made. The maths are difficult and were cracked in the 16th Century by the Italian genius Gerolamo Cardano. Still a steep learning curve for non-mathematicians.

Later, it was realised that the Hypocycloid curve is inferior for most purposes to the Involute, described by another genius (Huygens), a century later. Apart from clocks, almost all modern gears are of Involute form. Clockmakers firmly believe hypocycloids are best for gearing up light mechanisms that don't transfer power. In contrast, involutes have several advantages: including being best for transmitting power and easier to make accurately. Unfortunately another steep learning curve.

Controversial view: I think satisfactory clocks can be made with involute gears. (SoD puts Tin Hat on and retreats to bunker.)

Ideally, a clock design would provide an exact recipe!

• Hold a disc of diameter x in a rotary table,
• set the dividing plate and clock-hands up to count the required number of gears
• Put a module cutter of given size in the milling spindle
• Cut n teeth

This is OK as long as the tools are affordable. ( Just because a child needs shoes 'She Who Must be Obeyed' often makes a silly fuss about 'Man in Shed' spending another few hundred quid on more toys!) However, most of the theory can be avoided unless you have to roll your own using one of the other, cheaper, methods.

I think most of us have to mix book learning with hands-on before machining makes sense.

Dave

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Not particularly controversial, Dave … that view has been exercised several times on this forum.

BUT __ Involute gears don’t play very nicely with Lantern Pinions

MichaelG.

Edited By Michael Gilligan on 02/04/2022 13:18:36

Don't forget that in a clock the wheel is driving the pinion. Not an ideal situation.

Roy

Note to Ian. If using Thorntons or Bergeons cutters, the bore of the cutter is 7mm.

Edited By roy entwistle on 02/04/2022 13:55:39

Am I right in thinking that the reason involute gears aren't used by and large in clocks is that the very small pinions (6 and 8 teeth) beloved of compact clock designs need undercut teeth to follow the correct involute shape. Undercut teeth are weak and impossible to cut with a rotary milling cutter, they have to be planed.

Fact or fiction?

By the by, I built a clock using 20 DP involute gears - that was the set of cutters I had available. To be strictly accurate I was building the 12:1 gear train from the minute hand to the hour hand on the local church clock, so space wasn't an issue. The minute hand is driven by a synchronous motor rotating once per hour, and obviously the hour hand is coaxial. Thus the 12:1 reduction driving the hour hand is equivalent to the back ear on a lathe headstock a la Myford etc. As I say, space was not a consideration but longevity is a point of honour!

Have a look at this thread, Simon : **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=141778

It’s one of several that include a link to an excellent essay on the subject.

MichaelG.

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Ref. http://www.csparks.com/watchmaking/CycloidalGears/index.jxl

… and be sure to follow the link from there to Richard Thoen

http://www.csparks.com/watchmaking/CycloidalGears/RichardThoen.xhtml

Edited By Michael Gilligan on 02/04/2022 17:48:22

What you describe is a gearbox rather than a clock. I would say the use of involute gears for your application is totally appropriate. In a clock the train from the power source to the escapement is geared up not down which is where the issues of friction arise. Tower clocks in general have lots of spare power and space and have the luxury of massive frames. Involutes could be usefully employed on the lower end of tower clocks as can steel or cast iron gearing.

regards Martin

My thanks as ever to Michael Gilligan, whose recommendations for further research by the student proved to be fascinating and educational.

Best rgds Simon

PP Thorntons site askes you to select module and leaves? Putting "Gear leaves" into google just shows green leaves is "leaves" the tooth count? The site also doesn't show bore size, anyone know what it is?

Not a clockmaker myself, so not expert on their jargon. However pretty sure a leaf is what an engineer would call a tooth.

I dimly recall the term comes from the petal shape gear teeth become on small diameter gearwheels, what clockmakers call pinions.

My memory is terrible, I should have mentioned before that another book worth buying is Stan Bray's "Making Clocks".

Dave

As stated earlier bore size is 7mm. ( 02/04/2022 ) You only need leaf count for pinions, all wheels use one cutter

Roy

thanks so I need an R8 arbour that will take a 7mm bore cutter, more expense.

thanks so I need an R8 arbour that will take a 7mm bore cutter, more expense.

Sorry double post for some reason

Edited By ian voller on 03/04/2022 20:19:39

As you mention R8 I assume you have R8 collets. No need to buy an arbor (note spelling), you can make your own. Start with say 12mm steel, turn down a length to 10mm, stepping down to 7mm for the cutter, either with an M6 threaded portion for a clamp nut or an M6 threaded hole for a clamp bolt. Make sure the 10mm and 7mm parts are to accurate size and turned without removing from the chuck. Part off from the parent stock, mount in an R8 10mm collet. I'm sure you can work out the lengths.

Oh, and you'll need to make some arbors to mount the gear blanks on too.