Cutting teeth on a spur gear

[Eric WhiteParticipant @ericwhite]

Hello one and all,

I am building an attachment to fit to my lathe to enable me to cut the 8 teeth on a 5mm dia shaft which will end up being an armature pinion on a Hornby O gauge Locomotive.

My problem is I cannot find a supplier of the circular cutting blades for this small a tooth form, All the single and sets of tooling to cut tooth form are all much to large.

Can any member suggest a supplier who would stock the tooling I am looking for

Regards,,,,,,,,,,,,,Eric

[Eric WhiteParticipant @ericwhite]

Problem in finding tooling for the job

[JasonBModerator @jasonb]

The problem is that with a small number of teeth such as your 0.5MOD gear is that the teeth need an undercut which can't simply be done with an involute cutter that is whey they tend to start with ones for 12T upwards. It may be possible to make a couple of offset passes with the utter but it won't be as good as a gear cut on a proper machine.

[John HaineParticipant @johnhaine32865]

You have two problems here Eric.

First, you need a cutter with module 5/8 = 0.625 mm which is non-standard at least for conventional involute gears.

Second, I believe that anyway an involute pinion with 8 teeth cannot be cut using a normal circular gear cutter because the teeth are undercut, at least for standard pressure angles. Looking for example at the Davall Gear Catalogue they jump from 0.5 to 0.7 mm module and therr standard range has a minimum tooth count of 12.

Pinions with small numbers of teeth are used in clocks with a so-called "cycloidal" form which has no undercut. As far as I know the only supplier around is PP Thornton who could supply you with an 0.6 mm module cutter but it will cost you £68.50.

If you could post a photo of the mating gear it would be easy to see if it used a cycloid or involute form. If it is cycloidal then you can make a flycutter quite easily.

[JasonBModerator @jasonb]

John do you not think it is 0.5MOD

(8 +2) x 0.5 = 5mm OD

Though I wonder if Hornby used DP in which case 48DP would be close to 0.5MOD which is equivalent to 50.8DP.

Also some of the very small gear counts can use a different formula for OD such as T+3 rather than T=2

 

Edited By JasonB on 17/01/2020 20:51:30

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 17/01/2020 20:23:55:

.
The problem is that with a small number of teeth such as your 0.5MOD gear is that the teeth need an undercut […]

.

A quick check on ‘Easy Gear’ suggests that no perceptible undercut is needed for 10 teeth at 0.5MOD

MichaelG.

.

https://apps.apple.com/gb/app/easy-gear/id1197863906

Edited By Michael Gilligan on 17/01/2020 21:06:18

[John HaineParticipant @johnhaine32865]

Yes you're right Jason, I forgot the addendum. Still the undercut problem with 8 teeth. For Michael's 10 teeth the module would be 4.2 mm at the same shaft diameter, or if the shaft was made 6 mm diameter you could go for 10 teeth. But it depends on the tooth form and module of the mating gear which presumably exists.

[Michael GilliganParticipant @michaelgilligan61133]

Sorry, John … Brain-fade on my part

Must pay more attention !

I will check 8 teeth

MichaelG.

.

Edited By Michael Gilligan on 17/01/2020 21:14:58

[Pete RimmerParticipant @peterimmer30576]

Eric the first thing I'd do is measure the OD and tooth count of the larger mating gear so you can get a proper idea of the tooth pitch. This thread claims that Hornby used 40DP.

[speelwerkParticipant @speelwerk]

Sauerbier **LINK** lists in there "tandwiel"catalogue **LINK** with order number AE5844-08 a brass one of M0.5 mm with 8 teeth and OD of 5 mm, heigth is 2 mm, for only 3.30 euro. Niko.

Edited By speelwerk on 17/01/2020 22:30:07

[JasonBModerator @jasonb]

I don't think it is easy to measure the mating gear as most trains have the axle mounted gear at 90 degrees but then again don't they use a worm on the motor to drive a mating wheel?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 18/01/2020 06:55:42:

I don't think it is easy to measure the mating gear as most trains have the axle mounted gear at 90 degrees but then again don't they use a worm on the motor to drive a mating wheel?

.

That does appear to be true of ‘most trains’ Jason … but Eric was quite specific :

“ … which will end up being an armature pinion on a Hornby O gauge Locomotive.”

I know very little about Hornby O gauge Locomotives … but must assume that at least one model used spur gears [unless Eric is building a special]

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

At a wild guess … Eric may have something like this: **LINK**

https://thestationmastersrooms.co.uk/catalogue/product-details.aspx?id=J37671&t=Hornby+O+Gauge+Electric+20v+Motor+Mechanism+3+Rail+Original+Full+Working

MichaelG.

[John HinkleyParticipant @johnhinkley26699]

Another source of 8-tooth pinions, albeit in steel is:

L'Octant

Bit pricier than Speelwerk's link and of course you'd have to add postage, too. And yes, I do realise that the OP wants to cut the pinion directly into the armature shaft, but as discussed at length above, a separate pinion gear is another way to do it, if the original request can't be met. L'Octant can also supply matching larger gears, although you'd have to work out if the centre spacing is correct for the application.

John

P.S. Their various catalogues are worth downloading, if only to pass the time of day.

[Anonymous]

There are two different issues regarding undercutting. One, is when is an undercut needed to clear the apex of the mating tooth? Two, at what point will a hob form an undercut, whether you need it or not? The need for an undercut decreases with increasing pressure angle. The need for it can also be removed by increasing the diameter of the smaller gear (same number of teeth) or altering the tooth shape. It sure ain't simple!

I've cut 12 tooth 20PA gears with an involute cutter and they seem to mesh fine. On the other hand 10 tooth 20PA bevel gears machined on a 4-axis CNC mill have an undercut, and need it to mesh without interference.

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

I learn something new [to me] every day !!

**LINK**

https://www.brightontoymuseum.co.uk/index/Metropolitan_underground_locomotive,_110V_(Hornby)

MichaelG.

[Author]

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