Post edited 4:29 PM – March 14, 2011 by Jerry

I'll see if I have any suitable high carbon steel and look into this for you.

But for now, consider the relationship between speeds and feeds. In this instance that's the speed the workpiece is rotating at, and the feed rate at which we apply the tool.

Because of the fact that any given RPM the 'periphery' speed of a workpiece is determined by the diameter on a small narrow taper the speed of the surface passing a cutting tool can be very low indeed. 

A problem of perception can then occur. Whilst our chuck may be spinning at 500 rpm and would be fine on sensible diameter, with the point of a thin taper this could place needs of RPM's in the thousands to achieve the same cutting speed for a 1" bar say. So what can happen is folk feed in the tool at a 'normal' rate, but the material is not passing the tool quickly enough so digs in and it all goes pear shaped. Tiny diameters need huge RPM's.

Tools have to be sharp, very sharp, and with as small a contact patch as possible. So rough tools, forming tools, and finishing tools are usually out, we need fine sharp points. This reduces the friction, and subsequent force needed to make a cut.

Also consider, in the case of this particular need, the nature of the point design. In the pictures it appears to be a single taper to a point, whereas most commercial scribes have a secondary angle on point. This secondary angle forms the actual scribing tip, and is usually ground on, the same way a centre punch should be ground, with the grinding direction in line with the axis of the tool. I'll cover that some more when I've had a fiddle.

Finally, think out of the box.

If you're using the the whole taper length protruding from the chuck and expecting it to work, revisit that. A lot of machining tricks are about the application of common sense. So, in this instance, try this:

If our taper is, say, two inches long. Allow the first 1/4" to stick out of the chuck and turn it down to a fine point, leaving it slightly oversize. Then loosen the chuck minimally and pull out a further 1/4" without rotating the workpiece, turn down this section to the taper to marry up with the first piece turned. Follow this with a 1/2" bite and repeat, finally perhaps the last inch can be done to finish the roughing down. To finalise it and blend the sections finish with abrasive cloth over a file in the normal safe way. By using this method the vulnerable overhang is reduced to a place where it won't matter so much due to the increasing diameter being cut, if at all. Some experimenting with the size of bite may be needed, but it'll work far better than trying to turn the whole thing down in one setting.