Boring head questions

[FanMan]

A couple of noob boring head questions in the "correct me if I'm wrong" format:

Take a typical boring head that says "max 5/8" offset, I assume that's the center hole so that means that usinga boring bar that goes down to 9/16 gives a max diameter of .625x2+9/16 or 1.8125? Then the outer hole is, say, another 5/8 off so that'd be an additional 1.25 for a max 3.0625 hole diameter? At which point you can use the cross hole and go still wider?

If you use the cross hole do you use the same boring bar or do you need a different one with different rake for end cutting?

I've also been told that one can use a boring head for OD turning (I don't have a lathe) by turning the boring bar around. To do this you either need to reverse the mill's direction of rotation (not immediately possible on mine, though I haven't looked at the wiring) or a reverse cutting boring bar. I've never seen such a "backwards" boring bar advertised; are they actually available?

What about using a boring head for facing/flycutting or trepanning? OK or inadvisable?

 

[Tony Wells]

Well......sort of. The offset amount is the distance the bottom part of the head can be safely moved on the dovetail away from center, starting the measurement with that section running true with the main part of the body. For illustration sake, let's say you have a boring bar that will cut a 0.000" diameter hole. If you then move the lower movable section out the full 5/8", you gain that much in radius, not diameter, so your 0.000" becomes 1.125". Using your 9/16" starting point, you can add the 1 1/8" gain and get a hole 1 11/16" max hole using the central bar holding hole. Moving the bar to the outer hole, you can move the lower section in also if needed to allow a little overlap from where you left off with the previous operation, or just quite can't take that first cut because it's too heavy a cut for a single pass.

The hole on the side is used with a bar with a different grind, as you noted. Of course, you must use caution and judgment on the real limit as to how large you can safely bore. Obviously there is a limit. 36" would seem a little large, right? Well, the boring head is designed to take only so much torque, and of course, there is the issue of rigidity of the bar size used. It's not been a common use, in my experience. I've only done it a couple of times. But it can be done.

Facing can be done only if the head is designed to feed across the part. Most are not; some are. "Turning" the OD of a boss on a part is done in basically the same way as boring. Just flip the bar around. If you can't reverse the spindle, and that can be risky with a threaded arbor anyway, grind a tool that will run the right direction and it will work fine. At the bottom of the turned boss, you will need to use caution not to allow full width contact of the bar, because at the very least it will chatter, at worst, probably grab and toss something.

Flycutting is best left to flycutters in my opinion. I've seen people do it with boring heads, but it always seemed abusive, since most flycutting is interrupted. But it's yours, if you want to do it, go right ahead. As long as you don't have the head extended past the max, and are really getting rough with it, you'll probably get away with it just fine.

 

[FanMan]

Thanks... can one buy tools already ground for reverse running or side hole? Or is it just a simple thing with a bench grinder?

I see myself doing mostly smaller work, and inch or two, but I could see the need to very occasionally bore a 5-6" or so hole in aluminum, an inch deep or thereabouts.

When I spoke of facing and flycutting, I was thinking they were much the same thing except for size... but from what you said it sounds like there's more to it than that?

 

[mmcmdl]

Turn your existing tools 180 degrees and run in reverse . No special tools are needed . Yes , you can use your sidehole for whatever purposes you need . Grind tooling for clearance on a grinder if needed for required results . Yes . You could use your head as a flycutter , BUT , if you wind it up to 3000 rpm or such , your mill may walk across your shop . It is out of balance and not made to run at a high rpm .

Edit . No reverse ? Yes , bars are sold left hand or right hand .