Is that 0.731 mm, inches, feet, etc? 

If it's inches then you are at about 82DP so with 80dp being the nearest common size play with the tooth count. 39T/78T would be about right with a PCD of 0.73125" or make a custom 82DP cutter and keep your tooth count to 40/80

Edited By JasonB on 07/01/2021 17:24:32

Should have said the other alternative is to alter the ctr distance to 0.750" then you can use an 80DP cutter and keep to your tooth counts or buy off the shelf 40/80 gears in 80DP.

Edited By JasonB on 07/01/2021 17:40:17

Consider two gears at a centre distance D. The two gears touch on their pitch circles, so for 1:1 ratio, each pitch circle has has a radius of D/2 or 1/2 * D. That means each gear has a pitch circle diameter of D.

The total (sum) of the pitch circle diameters is 2D and the gears share this 2D in proportion to their ratio (i.e. 1 * D each for 1:1 ratio).

Now, for 1:2 ratio, one pitch circle will have radius D/3 (or 1/3 * D) and the other 2D/3 (or 2/3 * D). The corresponding pitch circle diameters will be 2D/3 and 4D/3.

Again, the sum of the pitch circle diameters is 2D and the gears share this in proportion to their ratio (i.e. 1/3 * 2D for one and 2/3 * 2D for the other).

Let us try a 13:27 ratio. One gear will have a PCD of 13/40 * 2D and the other a PCD of 27/40 * 2D, the sum of the PCD's again being 2D.

So the general rule is to take twice the centre distance and pro-rata it according to the gear ratio.

This gives you the PCD of each gear that will fit in the space and achieve the ratio you want. These two numbers are inviolate.

You then choose a tooth count and calculate a DP/MOD for this count or you choose a DP/MOD and calculate the number of teeth.

For any given space and given ratio, you can use big gears with few teeth or small gears with lots of teeth.

How I calculate these. 0.731ctrs x 2 = 1.462 x 80 dp = 116.96 total no of teeth in gear pair, 117/ 3 gives 39 and 78 as true gear pair for 2:1 ratio. Reduce 78 tooth gear to give centres required.

This should do you for the two custom 82DP cutters so you can keep the 40/80T ratio and 0.731" ctrs. Dimensions in thou. Based on Ivan Laws button method will work with a single point or multi tooth cutter. Click for larger image

Ignor the mm it was easier to work in a larger size but this is the two gears 40/80T 82DP

Edited By JasonB on 07/01/2021 19:57:39

Edited By JasonB on 07/01/2021 20:08:06

For Brian, or anyone else with a similar query …

I have just downloaded a very simple [and free] iOS App which does these calculations

… highly recommended if you have a suitable device

**LINK**

https://apps.apple.com/gb/app/spur-gear-calculator/id827110560

No fancy graphics … but it seems to get the sums right

MichaelG.

.

Edited By Michael Gilligan on 07/01/2021 23:39:28