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View Full Version : Between-centres boring bar - a tip



rohart
04-06-2012, 05:00 PM
I'm talking about those boring bars which consist of a bar with centres, one diagonal hole for an HSS bit and an adjustment screw, and another diagonal hole at right angles to the first for a locking grub screw.

Well, I'd made one a while back - about 30cm long, 17mm diameter, 3/16" diam bit.

I took it out the other day and used it. The locking screw was on the same side of the bar as the bit, with the adjustment screw opposite.

So far so good. But I needed a bigger and longer bore this time. I merrily went ahead and made a bigger one, 50cm long, 20mm diameter with a 1/4" bit. To get more purchase for the locking screw I put the adjustment screw and the locking screw on the same side of the bar. That was my big mistake.

If the screws are on the same side of the bar, then if they work loose in the bore one will have its leading edge rubbing the work and the other will have its trailing edge rubbing the work. One of them will have a tendency to unscrew in the work, and you can guess what merry hell that will produce.

As it happens, it was the socket setscrew locking screw that unscrewed, and by the time everything calmed down there was no trace left of the hex socket part of the screw.

I was lucky that I still had a couple of millimetres to go, so the bore was recoverable.

So one lesson is that the adjustment screw and the locking screw must both present either their leading or their trailing edges to the work, no mixing the two. Even then, you must make sure that the direction of rotation will not allow the screw to unscrew if they touch the inside of the work.

Another lesson is that there is probably an extra facet to everything you design, although that lesson would mean you never got down to anything in your search for perfection.

R W
04-06-2012, 06:24 PM
Any chance of a picture or diagram of you boring bar set up.

Carld
04-06-2012, 10:46 PM
On a bar like that I remove the adjusting screw so it won't back out. The locking screw should not back out unless you failed to tighten it. Chatter may be the only thing that would loosen the locking screw. It sounds like your boring bar may be to big for the bore.

Evan
04-07-2012, 01:10 AM
I am having trouble visualizing the setup too. I have maybe 4 various size boring bars like that and have never had problems with the set screws loosening.

One thing I do is to run the spindle end in a collet instead of between centres. I have a very good set of collets from back when they were made in England instead of China. They are plenty accurate. It also gives more room around the headstock end and easier setup.

Duffy
04-08-2012, 10:37 AM
I saw a used boring bar for sale last week. It was LARGE, (maybe 1 1/2" diameter and 30" long.) The imortant point is that one end had a large Morse taper, (4 or 5.) I would think that this would give even more usable space than Evan's method. Whether it would need a threaded end to the taper is arguable, but I think not since the other end is supported on a center.
This bar, by the way, was pierced for four bits, presumbably for different boring operations.