The one in the photo doe snot look that sharp to start with

I would grind away some of the curve so that you get a tool more like your other HSS ones and similar to a lathe tool. The angle will depend somewhat on the diameter of the hole you are cutting as the smaller the hole the more acute the angle will need to be to stop the tool rubbing.

Peter

You may find this useful when assessing whether you have sufficient clearance at the bottom of the tool.

Particularly when dealing with holes of relatively small diameter in relation to boring tool depth its easy to end up with interference at the lower side of the tool jacking the cutting edge away from the work. I guess diameters less than 5 or 6 times tool depth are most likely to be troublesome.

Above that 70° clearance is probably good enough.

I drew this out after deciding that proper investigation was more productive than workshop esparanto. Generally, over the years, I'd found my "by eye" estimation to be fine but then there was that "one bad day".

Clive

As so often, there is good information in GHT's Workshop Manual.

Bill is quite correct to chide me for oversimplified diagrams when compared to best practice book clearance angles.

However, at the modest cuts most home workshop machines constrain us to use, the simple form works well and has the great advantage of being relatively tolerant of small errors in height setting. As ever tip on centre height is best but a bit up or down is fine.

If you use a shallow 12 to 15 degree cutting rake angle the tool tip needs to be lifted above centre height to ensure the work clears the junction between the bottom of the cutting rake and top of the clearance rake. Interference there can really ruin the finish.

A relative neophyte taking their first baby steps into tool grinding and setting has quite enough to worry about without the added complication of figuring out how much to set the tool above centre height to avoid interference where the two angles meet. The smaller the hole the harder it all gets.

Fact is the clearance angle below the cut is in the wind and its exact value largely irrelevant to folk like us. Over the years I've slowly come to the conclusion that, for our small cuts, dead sharp (and not negative rake) is much more important than angles to the book.

Top rake is needed because a flat top tool is effectively working in negative rake as the work approaches the tool from behind the edge.

Clive

Hopper

Glad you like it.

If anyone wants a copy of the original PDF, much better definition than the compressed jpg posted, PM me your e-Mail and I'll shoot it over.

That was going to be part of a write up on boring tool clearances for ME or MEW but it went daft trying to deal with actual top rake angle and the effects of lifting the tool above centre to accommodate shallow section at the proper, book, rake angle as described by Bill. After thinking over Bills comments I may have figured a simple way to show folk whats actually happening at different bore sizes.

Clive