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Harold Hall

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Thread Cutting using Tipped Tools       Harold Hall

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 on eBay.

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