Martin,

If it helps to visualise what Jason has proposed, I attach a portion of a drawing showing a 70 tooth 20° PA 10 DP gear ( the lower gear portion in black ) with a gear portion of the same specification, but 14.5° PA ( in red ) superimposed, meshing with a portion of a 17 tooth 14.5° PA 10 DP gear ( above ).

It was generated using GearDXF program and manipulated in QCAD Professional.

John

Martin,

There is an alternative and that might be to cut the gears as a metric set, 2.5 Mod and 20 degree PA.

The cutters might be more competitively priced for those and your build notes would record the change from plans for the benefit of someone further down the line.

I'll now retreat to the defended bunker

Brian

You can of course choose any module size you care to define, not just nice round numbers. You wouldn't be able to buy an off the shelf cutter for that exact size perhaps but for the purposes of making a functioning scale model, you could surely use the nearest "standard" size which may be only marginally different. That way you wouldn't need to mess with the centre distances.

To calculate your custom module size, you'd divide the reference diameter by the number of teeth. If there is a std module size close to that, you can take a view on whether it is close enough not to worry about.

As an example, I recently specified some gears in a mixture of mod sizes including 0.98. As the mould was going to be specially made anyway, it was no skin off anyone's teeth if they didn't all come out as exact integers.

Murray

Edited By Muzzer on 15/01/2018 12:07:45

In practice pressure angle only needs to be matched for each pair of gears, but it will look better if the whole train share the same pressure angle.

In practice older equipment/machines typically had 14.5 degree gears so Jason is probably correct that they will actually be more correct in appearance.

Pressure angle does not change any gear calculations, except it affects when you need to make allowances for undercutting when making pinions with small numbers of teeth (14.5 degree gears get undercut at 32 teeth, 20 degree at 17 teeth).

I'd guess 20 DP is specced because your pinion is 17 teeth. If you use a 14.5 degree cutter you can get around this by making the pinion slightly oversize (typically by about 1/4 of a tooth depth) and either increasing the gear spacing or making the mating wheel undersize by the same amount and accepting 'squarer' teeth.

This works better with hobbed than form cut gears.

Neil

No, it requires a hobbed or generated gear to make an undercut, but if you use a pitch circle shift you can cut a usable gear with a form cutter.

Neil

That's generally what I use to create STLs for 3D printing.

Thanks to that diagram, I now know what the difference is.

Fantastic looking job Martin, I feel reading the entire thread you did the right thing, I will try to follow the same.

Keep up the good work, and keep posting I'm sure it helps lots of people, not just me.

Just ordered the first few bits from Bridport so now in committed.

Regards Stephen.

One final question martin.

What did you use for the rear axle,

Is obtaining the required length in silver steel the way

Or is a simple MS bar polished up enough to run smooth

In the bearings. I can imagine a ground bar being pricey especially as its longer than 13" the standard for silver steel.

Any help appreciated from anyone.

If you did want to go the the expense of silver steel then 1m lengths are also a standard length just not from the usual ME suppliers, PGMS would be another option but a bit OTT

Like Andrew I went with bright bar though it took a couple of attempts to get a decent bit, first was bent and the second has quite a heavy spiral pattern down the outside more like you get on con cast bronze.