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Harold Hall

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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 square shank.

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.

Metalworking

Workshop Projects

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.

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Drawings