Thread: How to bore this tool holder?

Yes if the bit is sticking out the back that would be an issue. If I were faced with that situation I'd probably take a small piece of aluminium angle, grind out a notch if required for the back of the tool bit, and use that on the back of the boring bar. In my experience it is the incremental adjustments that I'm interested in, rather than an absolute measurement per se, and I think that would be more than accurate enough for this type of operation. Mind you take that for what it's worth, I haven't tried doing that, just trying to quickly think how I'd overcome that easily ... before I finish my coffee and have to do real work

Did you make the boring bar up yourself? Another tip you are probably aware of, but just for the heck I'll mention it anyway. The hole to hold the bit in this type of boring bar is ideally offset such that when you insert the tool the tip will be at the 90 degree tangent to the bar ... if that makes sense. ie you want the tool tip at bar centre, and not the centre of the tool at bar centre. If the hole is simply drilled on centre the tip will be off, and all your clearance angles etc will go to heck. It's then the same situation as having a boring bar in a lathe set way off centre height. With a conventional round boring bar in a toolpost you can rotate it to get centre height (well unless it has flats in it, then you hope they're ground in the right position!!), but if the bar is rotating with the bit sticking through the middle of it, that's obviously not possible, so it needs to be properly centred when made.

Another 2 cents, that makes 4 cents in one post, for what it's worth. Hope it all made sense, as my coffee is now well and truly finished!

Pete

The boring bar is Lock brand that fits in a forged holder. The hole is centred in the bar but the holder is adjusted to set the cutting edge at lathe centre height. I guess the geometry wouldn't have been ideal when used as I did here, but it seemed to work OK.

Originally Posted by jack620

The boring bar is Lock brand that fits in a forged holder. The hole is centred in the bar but the holder is adjusted to set the cutting edge at lathe centre height. I guess the geometry wouldn't have been ideal when used as I did here, but it seemed to work OK.

If it works it's good enough. I've made a few 'field expedient' boring bars where I've drilled straight through and used a broken centre drill as the cutter bit (I have a lifetime supply thanks to a TAFE teacher I used to know). You have to grind a lot of the cutter bit away to get your angles but it's do-able.

However it's a lot better to do as Pete F says & drill the cross hole off the centre line so there's a lot less grinding needed. But then you need a bigger diameter bar, which might not fit through the pilot bore, so around you go.

But, if it works for a one-off, which is all I cared about at the time....

PDW

That would also hold true for a boring bar in a boring head. As I got no bars with my Wohlhaupter I was going to make some from 22mm dia O-1. Are any of the high end boring heads bars broached off center or are they all like the cheap ones and on center? (sorry Chris bit of a hijack here.....)
Cheers,
Ew

Originally Posted by jack620

The boring bar is Lock brand that fits in a forged holder. The hole is centred in the bar but the holder is adjusted to set the cutting edge at lathe centre height. I guess the geometry wouldn't have been ideal when used as I did here, but it seemed to work OK.

I just mentioned it for future reference, not so much for yourself, but for future searches people may do.

A number of times I've been stopped with a "What the ..?" moment when boring because the tool bit was rubbing, especially on small diameter bores, and a rethink of how I was going to establish the required clearance angles was required.

Pete

Originally Posted by Pete F

I just mentioned it for future reference, not so much for yourself, but for future searches people may do.

A number of times I've been stopped with a "What the ..?" moment when boring because the tool bit was rubbing, especially on small diameter bores, and a rethink of how I was going to establish the required clearance angles was required.

Pete

Ditto, and I've ended up with some very fragile cutting tools after I've ground enough away for clearance on those small bores. Fortunately you're rarely taking an aggressive cut with a flimsy boring bar.

PDW

Originally Posted by Pete F

With a conventional round boring bar in a toolpost you can rotate it to get centre height (well unless it has flats in it, then you hope they're ground in the right position!!), but if the bar is rotating with the bit sticking through the middle of it, that's obviously not possible, so it needs to be properly centred when made.

Pete

This is an interesting comment.

I've done a lot of centre boring over the years and find that I always get a better cut when I lift the cutter above centre (particularly with small boring bars which are subject to flex).

I know this is not what they say to do, but it works best for me.

Otherwise I find smaller bars can skid and not go in, or chatter.

Just my observation.

Rob

If you use a 4- jaw chuck as recommended by Gavin you could offset the bar to adjust the position of the cutting edge. I didn't because the boring bar was relatively large compared to the bore.

Edit: Gavin, not Graham.