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Relieving on the lathe

On some type of cutters where there is a series of teeth either forming a circle of a spiral such as gear cutters and gear hobbers then these teeth need to be relieved.

If these are being made on a lathe this means that as each tooth is cut, the depth of cut is increased as the tooth is cut then the cutter is returned to the starting position just before cutting the next tooth.

There are two main ways of doing this the first is the Eureka device. The second it the method uses by Giles Parks for cutting gear hob but can also be used for cutting gear cutters.

The Eureka device


Giles Parks’ method

In this method the cutter is moved in and out by the rotation of the headstock spindle. The way this is done here is by fitting a gear on the headstock spindle by fitting onto the back of the backplate.

fig chuck mounted with gear fitted

fig gear on back of backplate

This gear drives at least one other gear that is fitted on an arm fitted on the bed of the lathe. As the workpiece rotates then for each tooth being cut the cutter has to move in and out. If, for example, eight teeth are cut in one rotation of the cutter being made, then the ratio between the gear on the backplate and the gear driving the tool must be 8:1.

fig the two gears meshing

The last gear on the arm will be on a shaft that connects to a universal drive.

sometimes where this happens will need to be moved so extra gears might be in place on the arm. Most of bits for this just happen to be those that are used on my dividing head when helical milling.

fig gearing “moved”

The tool holder needs to slide in/out by the amount of the deepest relief say 5mm. It is driven into the workpiece by a cam driven by the universal shaft and then returned by means of some springs.

fig moving tool



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