Between centres boring bar bit grinding

Amazing where a question on cutting tool geometry can lead!

I'm with fowlers fury, have just bored a redwing cal for a friend with angle plate on crosslide and did it with 21mm b/c boring bar as per GHT and worked perfectly . well worth the time and effort.

I bored out the body of my Versatile Dividing Head with GHT's recommended boring bar with the angled tool bit and it also worked perfectly. I could see when I made it how having the straight hole and the grub screw hole all in one plane would make a very weak point. Plus I could measure the tool position with a standard mike and get it right where I wanted it.

Edited By Hopper on 25/05/2022 10:28:00

Since our hobby is emulating industrial engineering practices at smaller scales, it's often worth looking back at both trade and model-engineering text-books.

Doing so I found that boring-bars generally hold the tool-bit at about 45º to the axis, close to the end so it extends a little beyond that; allowing use in blind or stepped bores. It also gives more support to the bit.

Sometimes the grub-screw gripping the bit enters from a corresponding chamfer on the bar end, so is correctly perpendicular to the tool shank. In others it enters concentrically from the end, particularly for screw-cutting bits more easily arranged at right angles to the axis.

Another form uses a tubular holder in which a long, close-fitting draw-bar cross-drilled to match the bar and bit, is tightened by a nut on its threaded end beyond the lathe's tool-holder.

The latter pattern can be used in between-centres mode if the draw-bar itself is suitably centre-drilled at both ends Its advantage is that the boring-bar is cross-drilled with only the bit-hole.

A boring-bar can be double-ended: angled bit at one end, right-angle at the other. I think I have a specimen of this pattern somewhere.

'

E.T. Westbury suggests in his Lathe Accessories - how to make and use them, that a hollow boring-bar bar be made from steel tube, which would have to be fully-drawn not ERW, with the tube and closely-fitting draw-bar as mutual stiffeners. (Drilling such a length, many times drill diameter, concentrically for full length, is feasible but not easy. I wouldn't risk it!)

Sometimes the work allowed the boring bar to have a smaller-diameter spigot as a pilot in a hole ahead of the main bore. Particularly suiting chuck and faceplate work, where it may be possible for the pilot to run through a bush in the lathe spindle, such a bar can of course be centre-drilled also for use between-centres.

The above comments also apply to internal recessing, grooving and screw-cutting tools in which the cutter itself is a small bit in a cross-drilled holder. Really, this is the HSS-bit version of the now-familiar carbide-tip tooling; and I can't think of any reason why pilot and between-centres type internal-cutting tools of larger diameters cannot be made for carbide tips. Fit a small, straight-shank indexing-tool through the boring-bar itself.

For setting the tool's height from the boring-bar, if you envisage a lot of such machining or a particularly critical operation, it could be worth making a suitable depth-micrometer or micrometer-thimble device based on a Vee-block.

For gauging the bore, make a ring-gauge that rides on the bar, perhaps parked on the tailstock nose or centre while the cut is in progress.

Between-centres boring-bars assume the use of a lathe with T-slotted saddle, or a horizontal mill. On the lathe, the principle can be extended to internal recessing and threading as second-operations without disturbing the work-piece setting.

I have seen a horizontal-borer, which is basically a development of the lathe, set to face, bore and internally thread a brake-ejector casting for a full-size locomotive. I don't know, but would think, the cones were separate parts fitted in parallel bores.