Harold Hall


The alternative is to feed the tool in at half the included angle of the thread as shown in SK. 5. In this case, the tool can have significant side rake and machining will be appreciably easier with cutting taking place on one edge only as the sketch shows. However, setting the top slide to precisely the correct angle is not easy and if an error exists the side of the thread not being machined can become jagged.


If you are using a tool without side rake then the angle can be set with a deliberate error such that it just shaves the right hand side, avoiding any possibility of the face being jagged, see Sk.6.


I have not mentioned matching the tool to the helix angle of the thread , but in most cases this can be ignored. Providing the tool has a generous side clearance on the leading  side, and you are not cutting a small diameter coarse thread, or say a two start thread, there should be no problem.


Using Indexable Tipped Tooling

We now come to the main purpose of these pages, Indexable Tipped Thread Cutting Tools. Their only disadvantage in the home workshop is expense, as like other tipped tools holders, they seem overly costly compared to other tooling items. The tips are also more expensive than those for more straight forward turning operations. I have though found an appreciable difference in the cost of holders and tips between suppliers so it is worth shopping around, though be cautious if the items on offer are far cheaper.


The Tips

The tips are basically triangular and made in a number of sizes though for outside diameter usage a tip with a nominal side length of 16mm is by far the most common. However, some makes call these 3/8", for reasons I have been unable to determine. It would because of this be worth being doubly sure that the item you are considering purchasing is what you want before actually placing an order. Smaller tips are often for internal threads. Photograph 2 shows some typical tips the smaller one being able to cut internal threads down to an initial bore size of 6mm diameter.


As well as size, tips are available in two forms, full and partial profile. The partial profile tip is really the equivalent to the tool ground from tool steel mentioned earlier. Its advantage being that one tip will cut all the pitches require by a given thread system. Whilst this may be advantageous in a commercial set up, in the home workshop it would seem pointless due to the cost of the holder,  and grinding your own  would seem sensible.


Workshop Processes

Screw cutting using Indexable tips