The traditional tool for cutting threads on the lathe has been from the earliest
home workshop days a single point tool ground in the workshop from tool steel and
is still a method used by many. Its advantage being that a single point tool can
easily be ground and will then be suitable for all pitches, and across thread forms
where the angle is the same, typically 60° for both metric and unified. I said the
tool can easily be ground, however, a grinding rest with some form of guidance will
help to accurately establish the angle.
Whilst the method would also have been common in industry in the past, tooling is
now becoming available to make it largely obsolete in this situation. However, the
system is relatively expensive making the single point tool still a favourite for
many home workshop owners.
Before going into detail regarding the modern approach, the main purpose of this
article, I will comment briefly on screw cutting in general.
Having ground your tool to the angle required it will have to be left with only a
very small radius on its tip so that it can be used at the finest pitches. This will
mean that at coarser pitches the core diameter will be smaller than the thread form
demands, see Sk. 1. The result of this will be that the component will have a
smaller core diameter and be less strong than one having the precise thread form.
The effect will though be minimal, especially at larger diameters, and is therefore
more a theoretical problem than actual.
Since first publishing the above, it has been pointed out to me that, when the tool
tip reaches the thread's normal core diameter, the tool can then be traversed sideways
to achieve the required width, see Sk. 1A. The amount could of course be calculated
but I think it would be a case of a little at a time and tested using the internal
thread already made.
Another disadvantage of the single point tool is that it cannot produce the radius
on the crest making it necessary to cut the thread from a slightly smaller diameter
and leaving a flat on the top on the thread, Sk. 2. The main drawback of this is
purely aesthetic, it doesn’t look good. This may have been corrected in the past
by making the thread to the correct diameter and the crests of the thread rounded
using a chaser. These would be hand held, Sk. 3. Having found them only a few years
back when I considered purchasing one, I was surprised that they no longer appear,
no doubt made largely obsolete by modern methods. Used ones are though to be found