Boring Head Tooling Questions

[mikey]

I decided to go to the source and emailed Borite, asking how their cobalt tools should be sharpened. Here is what they had to say:

It is suggested that you hone the top and the face.
Or...you can send the tool in to us and we will sharpen it for you for $6.00 per tool.
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$6.00 per tool? No, I don't think so.

 

[dieselshadow]

For $6 per tool to get back factory performance? Where do I sign up?

Now this is coming from a guy who wants to spend time using the equipment, not grinding or trying unsuccessfully to sharpen a tool. This is why I really like the insert tooling system. Toss a bum insert, replace with a fresh new one when needed.

Seems like having a chainsaw chain costs $20 or more to have it sharpened. $6 seems like a deal for precision factory performance.

 

[mikey]

It's just two flats, one on top and one on the end, that are easily honed with an extra-fine diamond stone. Takes about 10 seconds to do both and the tool is sharp. Clearly not worth the charge, at least to me.

 

[petertha]

This is not rake. For you to introduce rake, you must rotate the tool in the holder.

Hmm.. now you have me wondering about this. Consider this marked up sketch. And I admit, its greatly exaggerated In reality it probably won't amount to much of anything. But if we say that 'zero rake' of a new tool means the flat of the boring tool contacts the bore circle ID at its widest point (the diameter), this makes a 90-deg angle between the ID tangent line & tool face (red line #1). Now if we re-grind the tool face back some & tool shank stays in same boring head hole position & new flat is parallel to the original tool flat, then the tool tip will come to rest on the ID slightly aft of the original position. And the new tangent line (black #2) forms a slightly larger angle, which is slightly positive rake, no?

Having gone through all this mental imaging, why is it that the majority of lathe boring tips predominantly have negative rake? What's the difference between a mill boring head which rotates the tool against a fixed part vs. a fixed lathe boring bar against a rotating part?

 

[mikey]

... why is it that the majority of lathe boring tips predominantly have negative rake? What's the difference between a mill boring head which rotates the tool against a fixed part vs. a fixed lathe boring bar against a rotating part?

The rake of an inserted tip boring bar for the lathe is what the owner chooses for his use. ALL of my boring bars save one is a positive rake bar. Moreover, most of my bars are also positive lead, with the remainder being zero lead. I do not use a negative lead bar on the lathe and the only time I use a negative rake insert is when I hog out a through bore.

As for the difference between boring in a mill vs a lathe, I'd have to think about that but, in principle, they are the same. The percentage of the three cutting forces may differ, though, but in what proportion I am not sure.