Saturday, 23 May 2020

Cutting the Gears

Now we know the main details of the 2 gears to be made, the next step is to select a material and cutters before machining.

Bearing in mind that the gears need to be hardened and tempered, it is by far the easiest for the home restorer to choose a steel that can be through-hardened by simple heat treatment rather than case-hardening or other hardening methods such as nitriding. I have chosen to use O1 tool steel  for making all my gears, including those in gearboxes, and so far this material has performed extremely well and can be finished to size before heat treatment. It is also readily obtainable.

The first step is to make a gear blank by cutting off a suitable size piece of steel from round bar and bore it to the finished size that will eventually be a press fit on the shaft. A mandrell is then made to support a gear blank and the OD of the gear blank is machined to its final size (1.375” or 2.625” in this case) on the lathe. The whole mandrell + blank assembly is then then set up in the dividing head on the milling machine in preparation for cutting the teeth.

Selection of the appropriate gear cutter depends on 2 things: the DP of the gear and the number of teeth of the gear to be cut. Here, we need:

20 tooth pinion: 16 DP, #3 cutter (for 17 – 20 teeth)

40 tooth gear: 16DP, #6 cutter (for 35 – 54 teeth)

These cutters can either be bought individually or form part of a set of 8 cutters. It is also worth pointing out that the gears in these older motorcycle engines (and gearboxes) invariably have a gear form with a 14 ½ degree pressure angle, but you don’t need to know this to cut the gears.

It is useful to use a small end mill first to remove material to save wear and tear on the gear cutter. The picture below shows the setup ….but be careful not to remove too much!

The next step is to cut the gear. The cutter is set up exactly in the centre of the gear in the vertical plane and then each tooth is cut in turn. The depth-of-cut is given by (D + f) and can be calculated by the formula:

D + f  = 2.157 / DP  =  0.135”

It is also engraved on each cutter, so you don’t need to calculate it but it does provide a check.

Each tooth is then cut to within .005” of the final depth, ie 0.130”, the tooth depth is checked with a vernier and then the milling machine table is fixed at the final depth of cut and one last cut on each tooth is made. The setup with the gear and the pinion is shown below.

Any final machining, such as oil transport slots, can be put in before removing the gear from the mandrell and mounting it vertically in a rotary table.

The gears should now be assembled on dummy shafts to ensure that the machining has been carried out correctly and that they engage without backlash or binding before heat treatment.

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