The main principle behind the form tool is to adapt the radius of the involute to the form tool. Looking at one tooth on a gear it is obvious that it has the same radius both sides to form the tooth. So if we take two disks of known radius and present them to the tooth so that they fit snug all we need to know is the distance apart and the distance fed in to duplicate the tooth. All this information is in Table 2.

Some Useful Formulae

TO FIND METRIC IMPERIAL

PCD Number of teeth x Mod No of teeth / DP

O/D [No of teeth + 2] x Mod [No of teeth + 2] / DP

DP 25.4 / Mod Pi [3.1416] / CP

MODULE mm CP / Pi 25.4 / CP

NO TEETH PCD [mm] - Mod PCD x DP

CP Mod x Pi Pi / DP

Pi can be taken as 3.1412.

A quick note here on the difference between DP CP and module. DP which stands for Diametrical pitch is the number of teeth per inch measured on the pitch diameter. CP which stands for Circular Pitch is the distance measured between two teeth measured on the pitch diameter. The module is the metric equivalent of the circular pitch and is the distance between two teeth measured on the pitch diameter in millimeters.

DP gear data is found by dividing the figures in Table 2 by the DP and the results will be in inches. Module gear data is found by multiplying the figures by the module and the results will be in millimeters.

Using the diagram in Figure 1 and Table 2 we will lay out an example for a 24 DP gear with 20 teeth.

From the table we need to make a No 6 cutter to cover 17 - 20 teeth, the diameter of the pins [D] needs to be 5.81 divided by 24 =0.242" dia. The distance apart [C] will be 6.864 / 24 = 0.286". The infeed distance [F] will be 2.543 / 24 =0.106" given that the blank [W] is 4.0 / 24 = 0.167".

To make the cutter first of all decide what bore size you will need to fit your machine. To economise on material if you select a bore size of 3/4" then the cutters can be made out of 1 1/2" silver steel or drill rod. Sizes above this are hard to find. To make these cutters you will need an arbor. Make up an arbor that can be used in the lathe as well as the miller.

To make the form tool turn up two pins in drill rod or silver steel as shown in Figure 2 and mount them in a holder to fit your lathe toolpost.

These pins must be hardened and tempered after turning. The top face then needs grinding flat to give a cutting edge. The distance apart [C] is critical and is best don with the holder mounted in the toolpost at an angle of 5 degrees and the distance measured using the crossslide dial.

The cutter is the next job. Turn up a blank of drill rod with a bore of 3/4" and a width of 0.167" [W]. Mount this on the arbor and present the form tool to it as laid out in Fig1. Using a slow speed and lots of coolant wind the form tool in to 0.106" [F]. this will then give you a disk cutter with the right shape but no cutting edges or clearance, also called form relief.

Remove the cutter from the arbor and mark eight equal radial lines on it, mark four lines 'A' and the other four lines 'B'. Refit the cutter to the arbor and mount in four jaw chuck and set to run 1/4" offset. Set the cutter so that one radial line, A, is on the centre at the point where the eccentric is nearest the tool, See Fig3.

Bring the cutter in and clean up the form until the cut extends between one pair of 'B' lines. Note the crosslide reading. Rotate the blank to the next 'A' line and repeat. Do this four times and you will have a blank with four equal lobes. Remove from the arbor and mill the four spaces out between 'A' and 'B'. mark the cutter details on one side. You will need the cutter number , the DP and the depth to cut. This is not the feed in depth from table 2 but the full depth plus clearance. This will have to be obtained either from a hand book or from the formulae 2.25 divided by the DP.

Harden and temper the cutter to light straw. To harden tools made in silver steel or drill rod, heat up evenly to a cherry red and quench in water vertically. Clean one face and put it on a steel plate with the clean face up. Heat the plate from underneath and watch the colour of the cutter, when it reaches light stray colour remove and requench as quickly as possible. Clean up and grind the four cutting faces taking care to keep the faces radial. Provided that the cutter is reground equally and radially it can be reused until it is worn away.

To make a cutter for a one off job or to make a quick job, the cutter blank can be mounted on the arbor and offset an 1/8" as described above and the form turned on in one go. Instead of rotating round and repeating the process, remove from the arbor and cut one gash in at the point of maximum eccentricity. Harden and temper as above and this will give you a serviceable fly cutter that is able to be reground many times.

The set out for a module gear is exactly the same the only difference is the working out of the form tool sizes.

As an example we will take a 1.5Mod pitch gear with 13 Teeth.

From table 2 we need a number 8 cutter. The pin diameter [D] is 4.100 x 1.5 = 6.15mm. The distance [C] is 5.267 x 1.5 = 7.90mm. The feed in [F] is 2.251 x 1.5 = 3.37mm and the blank width is 6.0mm. The cutting depth to be marked on the cutter is worked out from the formulae 2.25 x mod which in this case is 2.25 x 1.5 = 3.38 mm.