When I did my Fowler I found it was cheaper to buy some of the smaller gears and modify to suit than buy the material and cutters. That used a mix of 12.10 & 8 DP much like the full size used different size gears.

You only need 5 cutters not a whole set 2, 3, 6, 7 & 8

I would think that 5 separate cutters is likely more expensive than buying a complete set (probably from China). Quality can vary, of course. One needs to take into account extra import costs, but these things often avoid taxes (especially if ordered at busy times of the year) for small postal packages.

Perhaps a visit to one of the second hand tool dealers at a model engineering exhibition might provide a good deal.

I’ve bought cheap gears of ‘just too large diameter’ for recutting teeth for a smaller gear. Altering, or even changing, the boss is easy enough – especially if they are not on view. Thinking more of change wheels for these.

11T is availabe to buy which would be correctly cut for about the same price as a cutter. It would be 20pa so at least the mating gear would have to be the same pa or all. The rest of the sizes are all available but maybe not from the cheaper sources and even they are not that cheap for DP gears as China prefers to make metric ones at far more reasonable prices.

A lower pressure angle gives a more parallel looking tooth that becomes more triangular as the angle increases

The lower 14.5pa is especially useful on models that have a sprung rear axle as the gap between the teeth (backlash) does not change as much as the two gears move in and out of mesh.

I'll wait to see if Andrew comes back to explain the undercut, if not I'll do so later. I think he has some photos of gears he has cut that illustrate it well.

Edited By JasonB on 19/09/2019 16:58:43

Although convenient the diagram above is slightly misleading as the pressure angle is not referenced to the horizontal. As correctly described it's the angle between the normal at the contact point and the tangent to the pitch circles at the intersection of the centre to centre line of the gears. The important point to note is that as the teeth roll past each other the pressure angle stays constant and hence the angular velocity is constant.

For a gear with a small number of teeth during the rolling action the tip of one tooth does not clear all the sides of the opposing space. So the bottom of the tooth profile has to be cut away to give clearance. Although a bevel gear these 10 tooth pinions illustrate undercutting:

Look at the top tooth on the left pinion. From the top of the tooth the curve each side comes down in a smooth manner until about half way down the tooth. At this point there is a sudden change of direction and the width of the tooth gets narrower rather than continuing to get wider. This is undercutting. If you think about it the undercut cannot be formed using a rotary cutter cutting one space at a time. Undercutting is not inherent to involute gears, but is a solution to interference problems with small numbers of teeth. The need for it is less with higher pressure angles. As mentioned there are also other ways of reducing the need for it by increasing the OD or tweaking other parameters. If a spur gear is hobbed with a standard hob you will get undercutting for small numbers of teeth, whether you want it or not.

Since the bevel pinions as designed are undercut they were cut on a 4-axis CNC mill to avoid the issues of using involute cutters:

Andrew

Edited By Andrew Johnston on 19/09/2019 18:57:19