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Lathe – height of cutting edge

When turning the height of the cutting edge of the cutting tool is critical. The problem is slightly complicated because many tools are made so they can cut when moving in more than one direction.

If we consider an cutting edge moving at right angles to the bed of the lathe we can see most of the factors that affect the height of the cutter.

fig

For the cutter to cut it has to have clearance at the front. If one considers what happens as the cutter approaches the round surface no cutting will occur. The first cut happens because the cutting edge at the point of contact is extremely small and the cutter is pushed into the material. The size of this initial cutting point increases dramatically as the clearance angle is reduced. When this angle is zero it is not possible to push the cutter in and it will not cut. Even if there is a clearance angle if the height of the cutting edge is at all above the center height, it will not cut.

The top of surface of the cutter usually slopes downwards as it goes back as shown. It will cut without this but it will often cut more freely if this is the case. This is called rake.

For most materials the angles of the clearance and rake have to be within a certain range of values to machine efficiently and get a good finish. These angles vary for different materials but for most purposes it is possible to find angles that will work for a range of materials.

Usually the aim is that the height of the cutting edge is exactly at the height of the axis of the lathe.

If the height of the cutting edge is not at the height of the axis of the lathe then the angles that would exist if is was at the right height will be different. If the diameter of the workpiece is large this error can be so small as not to be significant. But as the diameter gets smaller the angles dramatically change.

Suppose the cutter is too low as the cutting edge gets closer to the middle the cutter will just pass under the center without being able to cut at all. worse than this the workpiece can “suck” the cutter under the workpiece. If the cutting edge is part of a carbide tip, then it is quite possible for the front of the tip to be sheared off.

As the height of the edge is raised as the cutter reaches the middle the amount above the cutter gets smaller and smaller. Eventually the workpiece will be cut off leaving a small pip. The diameter of this pip will be twice the error in height. In fact one way of adjusting the height is to set the cutter starting just below the center and raise it till the pip disappears.

If the cutting edge is too high the edge might not cut at all. In this case it might be possible to start cutting but as the cutting diameter gets smaller the cutter will quite suddenly stop cutting. If the cutter is lowered very slightly it will be found to start cutting again.

It is often said that if the height is wrong when cutting a taper then this will affect the angle of the taper. And so it does but so long as the height is reasonably accurate the error is minimal.

There is a design for an adjustable height gauge in:

“Lathe Tool Height Gauge”, Harold Hall, Model Engineer’s Workshop, June/July 1991 p28