My method, has been to use dies, from Coventry die heads, Photograph 1, not just
to round off the tips, which they could be used for, but to cut a full form thread
completely. The Photograph shows the holder I have developed. Of course, when used
in the die head they are in sets of four but used for screw cutting only a single
die is required.
These, presently, can be obtained from Tracy Tools who supply sets made up of single
dies over a range of pitches. However, firms that deal in used machine tools are
likely to have obtained die heads and quantities of dies so enquiries in this direction
may provide a source for them. Also, they are widely available on eBay where used
sets can be had quite cheaply, new sets will though be far too expensive.
How then is the thread cut? Well, firstly, I must point out that this item is not
intended to be a detailed description of the process, only an introduction to recent
developments, that is, the use of indexable tips. It is assumed therefore that the
viewer will be conversant with setting up the changewheels and using the thread dial
indicator, etc.. For the viewer who is a total newcomer to the subject then other
reading will be necessary. Do be aware though that the thread is not cut at one pass
but by repeatedly deepening the cut until the required thread results.
There are two methods of applying the tool to the workpiece. The first is to traverse
the tool into the workpiece at right angles to its axis causing the cutter to cut
equally on both edges, Sk. 4 . In this case, theoretically, the tool should not have
a top rake as this will alter the angle of the tool and the thread cut. Whilst the
tool will work, machining most materials without any top rake is not ideal, especially
on a light weight lathe.
The viewer may therefore ask, why not add some top rake and adjust the angle ground
on the tool to compensate. Whilst in industry this may be practical, calculating
the required angle would be complex and the resulting angle too complex to set up
in the home workshop, a compromise has therefore to be made. This is to make the
top rake much less than would be normal, say 3 to 4 degrees, but even at this value
machining will be significantly improved and the effect on the thread angle, minimal.