Boring Cutter Sharpening

I am still struggling to get boring to work well on the mill (SX1LP). I have a 30mm (ARC) boring head which came with a couple of tools.

The HSS tool which has a "square" profile works fine (apologies for the poor focus).

In search of a slightly longer tool to bore deeper, I acquired a few "German type" HSS boring tools. These have a semi-circular profile.

I am struggling to get them to work. They do not appear to have any side clearance, other than the curve of the basic shape. I am setting them with the top face parallel to the boring head dovetails.

I tried stoning the top rake and front clearance ( on another tool, but that snapped!) to see if it was simply a question of sharpness, but it didn't help.

Are they simply not properly ground, and I need to treat them as blanks and add appropriate front clearance, or am I misunderstanding the appropriate way to use them

Ref bottom picture, it looks like the ground face is above centre line so it will always rub, you will need to add clearance.

Tony

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.

Thanks, Out with the grinder and treat them as blanks. It's just that I couldn't find sharpening information for that shape tool, either in my books or on the internet.

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

Thanks again. I think I have my head around lathe tool angles, milling cutters are consumables - but boring head cutters are still giving me a headache!

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

The boring tool works on the same principle as a lathe tool cutting on the outside of the bar. The required 'rake' angle is determined by the tool and work material, and to some degree by conditions. The rake angle for mild steel is generally quoted as 12 to 15 degrees, other materials differ.

Referring to JasonB's diagram, grind that to the rake angle. I would disagree with Clive's diagrams as ar ake angle for cutting purposes, but would add a second 'clearance' angle to clear the bore, depending on diameter. This can be any suitable angle, as suggested by Clive's diagrams. The rake angle doesn't need to be very wide, a few mm for our purposes.

The top of the tool should also be ground at an angle, falling away from the cutting edge, perhaps as much as 15 degrees for mild steel, brass famously likes zero degrees. Don't remove too much material from the boring tool, enough to clear the work and allow swarf to collect (and periodically clear it out).

Bill

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

Now that's a darn handy chart. Never seen one like that before. Will be printed out and taped to the workshop wall by the lathe in the morning. Thanks for posting it.

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

I had similar problems of insuffient clearance on carbide tip bars that came with the common 50 mm boring head.

Dont assume that just because you invested money in something that it will work out the box. That's engineering.

Martin