Coventry Die Head Chaser Holder,
for screw cutting on the lathe. Harold Hall
Tools for screw cutting on the lathe largely fall into four types.
1. Multi tooth chasers
2. Square shank single point cutters, frequently ground in the workshop, but can
be purchased ready made from high speed steel, or with carbide tips brazed onto a
3. Indexable tips, which cut just the root of the thread and its two sides, but not
the radius on the thread's tips. Known as “partial profile”
4. Indexable tips, similar to those in three but which also create the radius on
the thread's tips. Known as “full profile”
Bypassing type one at this stage, the advantages of type two is that they can be
ground in the workshop and will produce any pitch, also can be used for thread forms
for which the angles match. Typically, Metric and Unified threads both have a 60
degree included angle.
Type three can also cut any pitch and thread type having the same angle, but is much
more expensive. This largely due to the cost of the holder that the tip requires,
but an individual tip will also cost more than the shop ground item
Type four's advantage is that it produces a full form thread but only for one specific
pitch. One tip is therefore required for every thread pitch to be cut, making it
even more expensive than those in item three.
Going back now to item one, these are similar to those in type four but rather than
a single thread cutting tip have a number of cutting edges spaced at the pitch for
which it is to be used, see, Sk.1.
The sketch shows that the chaser is fitted with a file type handle indicating that
it is used hand held. I have read that they can be used to fully cut a thread from
a plain diameter but feel that there must be an element of skill required that many
would not be able to acquire due to limited use.
Their main purpose therefore, is for most, to add the radius onto the outer diameter
after cutter a thread using either methods two or three. If this is not done, then
threads cut using these methods have to have an outer diameter a little smaller than
the norm. This to compensate for the absence of the outer radius, other wise the
inner and outer threads will not fit.
Using a single Coventry die head chaser
Moving now to the main purpose of this item, that is, using a single die from a Coventry
die head set of four, to perform the same features as for the hand held chaser, also
for cutting a thread fully.
Availability of Coventry die head chasers
These of course, when used in the die head, are in sets of four but used for screw
cutting on the lathe 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, typically
those that advertise in the Model Engineer and Model Engineers' Workshop, are likely
to have obtained die heads and quantities of dies. Because of this, enquiries in
this direction may provide a source. Also, they are widely available on eBay where
used sets can be had quite cheaply. New sets will though be far too expensive for
the home workshop.