Cutting a Gear

Now you have a cutter. You use this very much like a single point cutter, cutting one tooth (gap) at a time.

the cutter is mounted in the mill, in its collet chuck. As it was made in that chuck, it should run perfectly true with all the teeth having an equal chance to cut. A gear blank should be mounted on an arbour in a dividing head or rotary table – whatever suits you. The blank diameter is (z+2)*mod or (z+2)/DP. If I wanted a 48 tooth, 0.5mod gear, the blank would be 25mm diameter. It is paticularly important that finer pitch gears are mounted truly to the dividing spindle. If necessary, you can finish turn them to size on the lathe while mounted on the spindle to be used in the dividing arrangelment. You hairy types cutting traction engine drive gears at 2inch scale can probably worry more about dropping it on your foot.

Adjust the height of the cutter so that the middle tooth is exactly at the centre height of your gear blank.

Setting up the hob to start cutting

I wanted a number of gears of the same size so I turned a bar to the appropriate diameter and cut quite a long way along it. Index around all the way and be sure to go an extra three teeth. this ensures that the full profile is properly cut and that you have got all the steps correct. If the last cut is still removing much metal you may have a tooth out of place. For example, I added an extra hole when stepping round one gear. It looked OK until I got back to the beginning!

The infeed is whatever your gear standard requires. consulting a number of eferences gives slightly conflicting values for the whole depth of the teeth. Involute forms a fairly forgiving but, if you cut too shallow, you will not be able to make the gears mesh at the calculated centres. I use 2.25*module (0.044 in). I believe this should be 2.4*mod for gears finer than 1.25mod

Here is the large blank finished and ready to be transferred back to the lathe to part of the individual gears.

Blank ready to part off individual gears

Gears generally need a hole in the middle to be useful so be sure to maintain concentricity when sorting that out. For individual gears, put the hole in and over size blank, mount to a true arbour and finish turn to size at that setting before cutting the teeth.

These aluminium gears were from my first ever attempt. They mesh with each other and run pretty well against commercial gears. However, if you look at them carefully, you will see that not all the teeth are the same size and that the teeth are too shallow. There is also sign that I crashed the cutter into the blank while indexing to the next tooth

Early samples

Well, practice makes perfect and so I made a few more. Here is a more recent steel pinion. The End view reveals that the tooth profiles are actually made up of straight lines but the difference is very small and I suspect that a bit of running in will even things up to a degree.

A steel pinion
pinion gear end view

Gears made like this will work perfectly well for all but the most critical of tasks. They run a little more noisily but if there were no noise, how would you know it was working.

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