Making Spur Gears

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Making Spur Gears – Phil Oles 2/14/17

Information about the gear making workshop Two sessions for each pair of registrants are required to make a set of two gears.

Your commitment will be two, 1 - 1.5 hour sessions to make two gears. If you miss one session you may be out of luck to finish both gears.

This is a brief description of how to make functioning spur gears

Preparation: 1. Decide how the gears are to be used, ie, power or load placed upon them. This determines what diametral pitch (DP) you’ll need.

This workshop makes this part easy. I’ve chosen gears with a DP of 31.75 or Module 0.8 (metric designation) for you. This corresponds to the set of gear cutters I have available.

2. Decide what diameter of shaft you’re using for the gears

I’ve chosen gears with a central hole of 0.313” or 5/16”.

3. Choose gear material.

I’ve chosen aluminum because it’s easy to work with and I have it.

4. Choose how many teeth to make for each gear. This is determined by the ratio you want to change rotational speeds.

You’ll be making two gears, a 20 tooth and a 40 tooth.

Step 1: Start by turning blanks on the lathe and drilling/reaming the shaft hole. The blanks should be at least 0.030” oversized. Blanks will be provided.

Gear blanks.jpg

Step 2 Mount the blank on a suitable arbor (I’ll provide that) and turn to the exact diameter for the gear which is determined by the number of teeth and the pitch (DP). The formula is:

Diameter = (N + 2)/DP OD = 22/31.75 = 0.693 for 20T and 1.323” for 40T

Use a collet chuck on the lathe because its runout is 0.001” or less. This will be set up for you

Once the OD is turned do not remove the blank from the arbor until all teeth are cut.

Step 3 Set up the dividing head (DH) on the mill table using a dial indicator so that the face is perpendicular to the mill table. This will be set up for you

Dividing head setup.jpg

Step 4 Adjust the cutter to be at the exact centerline of the DH spindle using a dead center in the DH spindle. This will be set up for you but you’ll need to confirm it before you start cutting.

Setting the cutter height

Step 5 Cut the profiles between the teeth remembering that the cutter rotates right handed and only cuts in one direction. After each cut, retract the cutter before you rotate the handle of the DH by the required amount. The depth of cut is 2.157/DP = 0.068”.

Repeat this process until all the teeth are formed. Take care to NOT rotate the DH in the wrong (counterclockwise) direction which will introduce backlash.

If you want the central hole to have a keyway I can do that in my workshop. It requires a heavy duty arbor press even to cut a small 1/8” keyway. Mine is a 3 ton, weighs about 125 pounds, therefore, not very portable.

Step 6 When your gear is finished, test it for accuracy by placing it on a fixture which will be set up for you. If your gear meshes with another at the appropriate distance apart, congratulations.

Here are some gears I made for a recent project. Fourteen were good and four were rejects, and I’m not sure why that happened. Be prepared for this.

Note on dividing heads in general:

They are usually supplied with a 40:1 worm reduction which means one full turn of the handle moves the gear on the arbor 360o/40 = 9o

Therefore, making a 40T gear is easy because you just need to advance the blank by turning the DH handle 1 full turn.

Likewise, making a 20T gear is straightforward because you just need to advance the blank by turning the DH handle 2 full turns.

That’s why you’re making 20T and 40T gears!