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Harold Hall

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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.    

 

The Process

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.

Metalworking

Workshop Processes

Screw cutting chaser

All pictures can be clicked on to provide a larger view

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