Boring Head Tooling Questions

[petertha]

Q1 - I have a set of Borite cobalt/Hss tools for my boring head. I managed to chip the cutting point (dropped it on vise). Figuring I didn't have much to lose, I slightly ground back the flat & the diameter relief angle just enough to restore a new cutting edge. That part looks good now, but just wondering - is there a big deal about the flat no longer being 1/2 the shank? (Maybe I'm getting mixed up with D-bits & such). Can I just compensate the slightly different tool angle (rake) relative to bore ID?

Q2 - now that I think about it, I always kind of tried to set these tools with neutral rake & had good results. But I'm not exactly sure where I got that from. Maybe positive rake can cut better but can deflect & cut larger diameter based on depth of cut or something? Negative rake... dunno!

Q3 - I've seen insert boring bars like pic for lathe toolpost boring, in fact I use some with CCMT inserts & more flat faced shanks, but still of round stock profile. I see some with totally round shanks too. Doing the price math, its actually cheaper to buy an insert type 0.500" shank boring bar & cut to length & use the variety of inserts I have. Is there any difference cutting action wise between how a boring head would hold the tool vs in a lathe? (Ie. both boring ID's).

 

[mikey]

I use Borite cobalt bars almost exclusively in my boring head - great tools!

If I understand you correctly, you ground the flat down and restored the relief under the cutting edge and you want to know if the tool will cut well, even though the cutting edge is lower relative to the centerline of the tool ... right? If so, then yes, the tool will cut. Your tip will be off the centerline of the spindle so it isn't ideal but it will definitely work - I've done it.

Boring bars normally are oriented so that the flat of the cutting tip is on the centerline of the spindle. The reason is so that when you dial in a 0.005" cut, that is what you get. If the tool is not oriented correctly then what you dial in is not what you get.

If an inserted tip bar is cut to fit and the tip of the insert provides adequate clearance under the cutting edge then there is no reason you can't use it. They sell bars for this very application and if you have a bar with similar tip geometry then maybe cut it down and give it a try. They may be heavier though, so you have to keep your speed down and the finish may not be ideal (which is why I only use cobalt nowadays).

 

[kd4gij]

like these


http://www.ebay.com/itm/PREMIUM-4-P...942249?hash=item1eb69fc2a9:g:MPwAAOSw65FXthAn

 

[petertha]

Very good points, thanks. Hopefully the (exaggerated) sketch elaborates on how I had to dress it down. Fingers crossed it wont be a huge impact. If I'm visualizing it correctly, removing tool face material should result in slight positive rake effect? So dumb question, but in order to sharpen these for normal wear, one should be dressing the our radial relief & leaving the face alone? I saw those Borite bars with TCMT inserts. Amazingly I don't use that particular insert but they look quite common & 3 cutting corners vs. 2 I suppose.

 

[mikey]

Very good points, thanks. Hopefully the (exaggerated) sketch elaborates on how I had to dress it down. Fingers crossed it wont be a huge impact. If I'm visualizing it correctly, removing tool face material should result in slight positive rake effect? So dumb question, but in order to sharpen these for normal wear, one should be dressing the our radial relief & leaving the face alone? I saw those Borite bars with TCMT inserts. Amazingly I don't use that particular insert but they look quite common & 3 cutting corners vs. 2 I suppose.

These bars are usually sharpened by honing the flats only. That gets them pretty sharp. Accordingly, there is some small offset behind the centerline so yeah, it should work okay. This is not rake. For you to introduce rake, you must rotate the tool in the holder.

 

[petertha]

Thanks.
BTW, I knew there was a reason I was cool on those Borites when I first looked at them a while back. If I understand correct, the TT inserts are somewhat uncommon, or at least more limited to these kinds of boring bars? At least vs. TP/TC type inserts of the same nominal inscribed circle & thickness which seem to be more prevalent? Apparently the hole & few other features are different.

 

[Tony Wells]

Normally, those are simply ground on the end, keeping the same front clearance under the cutting edge, and a slightly negative lead. The outside radius is left untouched, as is the flat face that is more or less vertical. Those are the two most critical surfaces with respect to rake and getting the cutting edge on centerline.

 

[mikey]

Problem with just sharpening the end of the bar is that much of the wear occurs at the corner and side cutting edge. If you don't hone the flat, much of the cutting action is lost.

Sorry, I don't use those inserted tip bars so cannot help there.

 

[Tony Wells]

The outside edge shouldn't contact the bore except at the very tip where the cutting action is. Those bars are not meant to cut on the side at all. They cut on the end, as though you were plunging with an end mill. That round part is relieved. The original round section is ground as a cone, smaller at the top. If you're waiting until you're rubbing on the side of the bar, you're waiting far too long to sharpen it, or you have it misoriented.

Take a really good look at a brand new one. Maybe even blue it up and take a skim cut on a already smooth, centered bore. Then look at where the blue is missing. Make sure the cutting tip is exactly on center.

 

[mikey]

The outside edge shouldn't contact the bore except at the very tip where the cutting action is. Those bars are not meant to cut on the side at all. They cut on the end, as though you were plunging with an end mill. That round part is relieved. The original round section is ground as a cone, smaller at the top. If you're waiting until you're rubbing on the side of the bar, you're waiting far too long to sharpen it, or you have it misoriented.

Take a really good look at a brand new one. Maybe even blue it up and take a skim cut on a already smooth, centered bore. Then look at where the blue is missing. Make sure the cutting tip is exactly on center.

I understand what you're saying. If oriented so that the flat is on the centerline of the boring head then the area just behind the cutting corner, which is relieved, should not contact the work. Unfortunately, this relieved area definitely does make contact because I have seen wear in this area many times. I don't know if it is the chip coming off that makes the contact but it does wear there.

The next time I use the boring head I will look at this more closely. Up till now, I have always thought that the end edge has little to do with anything because all the wear I've seen takes place at the corner and adjacent side cutting edge. I'll report back when I have something concrete.