Tool-bit Shape

[gunboatbay]

I have a requirement to cut very fine external threads on a small diameter portion of a workpiece. Because interference with the workpiece is a problem, I want to use a small HSS toolbit; 5/16”. Searching in older machining books, I’ve found two toolbit shapes that seem be suitable (A & B, below). Can anyone comment on the pros/cons of these toolbit shapes for what I want to do?? Thanks.

 

[DaveH]

I don't think there is a lot in it - one does appear to be stronger, tool bit A but it is a fine thread not very deep.
I would choose the one that is the easiest to grind when the point chips.
Dave

 

[Maryak]

Many threads finish against or close to a shoulder. Offsetting the point closer to the leading edge allows this to happen without the body of the tool fouling the shoulder.

Hope this helps

Best Regards
Bob

 

[steamer]

Tool "A" for a shoulder on the left

Tool "B" for a shoulder on the right....tailstock end.

Dave

 

[Stan]

The first consideration is how far from the chuck will you be working. Neither of the examples you show can get close to the chuck, which you usually want on small diameter work.

There is no real reason to start with a 5/16" tool bit and grind away most of it when you can start with a 3/16" bit. The first grind would be something like a 45° angle on the end so that the tool post can be rotated away from the chuck.The second grind would be on the left side just slightly longer than the depth of thread you are going to cut. The third grind is on the right hand side to get your 60° thread angle.

See the one on the top right as an example although it is a bigger bit for a coarse thread

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