Notes on Workshop Techniques

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WORKSHOP HOW-TO

9. Cross drilling in the lathe - Barry Harrison

Most methods for cross drilling round bars are either somewhat 'hit & miss' or require the production of a more or less complex jig, usually requiring a separate drill bush for every size of hole required. Accuracy is dependent upon the construction of the jig, or the precision used to set up the job. The method to be described is for cross-drilling on the lathe, and you've probably already got the necessary components - it needs only an angle plate and a vee block. It can be applied to any size of bar and hole (it even works when the hole is bigger than the bar!) within the capacity of the equipment and produces the hole to an accuracy limited only by the equipment. As long as you take reasonable care in assembling the kit, i.e. cleaning all bolting surfaces, making sure they are in full contact, etc., then accuracy is automatic. It's easy, it's quick, and I've never seen it described in print.

If we position a vee block on lathe centre height, with the vee towards the headstock, then any round object in the vee block can be drilled exactly on centre line from the headstock - how to set the vee block at centre height is the problem. read on.

Clamp the lathe tailstock to the bed, extend the barrel and clamp the barrel. Position the saddle under the barrel and place an angle plate on the cross slide, with its vertical face against the barrel. Hold the angle plate against the barrel and bolt the angle plate to the cross slide. Now withdraw the cross slide, place the vee block between the angle plate and the barrel with the barrel located in the vee. Advance the cross slide to trap the vee block between the angle plate and the barrel, secure the vee block to the angle plate. The vee is now located at lathe centre height. If you doubt the alignment of your tailstock barrel, use a long between-centre mandrel or boring bar and go through the above procedure, but with the cross slide, angle plate & vee block right up against the headstock end of the mandrel.

Now turn the angle plate to align the vee block across the lathe bed, square to the mandrel axis - use any appropriate method to ensure that it is as square as you want it, but bear in mind the following points:-

  1. If you use a DTI on the angle plate face, and if the lathe is correctly adjusted, there will be a small alignment error. This is because the cross slide should be aligned to face off concave, so it's travel will not be exactly square to the headstock axis.
  2. You can hold the top edges of the vee block against a face plate whilst clamping the angle plate, but any alignment error between the vee and the top edges will come out as an error in the drilled hole. Better to use a large diameter bar in the vee and hold that against the faceplate - assuming the bar is round, of course!
  3. If using the faceplate, remember that it's concave - if you carry out the alignment exercise on one side of the faceplate, then the work axis will again not be square to the headstock axis. This problem is overcome by always aligning the vee block against the centre of the faceplate, i.e. with the edges or the round bar protruding equally either side of the headstock axis. This is actually the method that I always use.

Now with a drill in the headstock mandrel and the round bar in the vee, the drilled hole will be exactly on the centre line of the bar - as I said, depending upon the accuracy of the lathe, angle plate, vee block, etc.

My own vee blocks were made somewhat differently to most commercial units - every face other than that with the vee is ground flat and square, and all have tapped holes in them. This makes securing the vee block to the angle plate a very simple matter indeed. For conventional vee blocks, you'll either have to make some suitable clamps, or drill and tap holes in the edges of the blocks - well worth doing, and it doesn't affect the function of the block in any way.

So now you can hold a round object at exactly lathe centre height, you will soon realize that this idea is not limited to cross-drilling, not by a long way!

Turn the vee block to 45 degrees off-axis, fly-cut or mill the end of a round bar to 45 degrees, drill and ream for a tool bit, all at the same setting. You've just made a nice boring bar.

Align the vee with the axis of the lathe, mill a slot across the end of the bar, and that's the first operation in making a forked end - with the slot exactly in the middle of the bar.

Align the vee across the lathe, and keyway milling is both accurate and easy.

If you have a milling/drilling spindle with a circular barrel (I use a Unimat SL headstock for this purpose) that too can be held in the vee and used to cross drill work held in the chuck, in conjunction with some means of dividing for the headstock. Or you can mill a keyway along a bar protruding from the chuck.

Aligning the milling spindle axis with the lathe axis, you can drill the face of chuck mounted work (e.g. cylinder covers) or mill slots across the face. Mill slots across the face of two components, fit a square/rectangular bar in one of them, and you have a drive dog/clutch assembly, beautifully true-running.

But the operation that gave me most satisfaction was cross-drilling the eccentrics for the Stevenson gear on my 5" loco. The eccentrics are held in position on the axle by radial grub screws, and if these are not exactly on the axle centre line (not, of course, necessarily the eccentric centre line, but preferably so) then the action of tightening the screws can cause the eccentric to shift slightly around the axle, upsetting the valve settings. Finish machine the eccentrics, and bore for the axles. Turn up a mandrel to an exact fit in the bores, and of sufficient length to hold in the vee block. Align the vee block across the lathe axis and clamp the mandrel in the vee with sufficient protruding to allow mounting an eccentric. Mount the eccentric, secure it with a toolmakers clamp to hold the eccentric against the end of the vee block, and any cross-drilled hole will be on the axle centre line. To get the hole on the eccentric centre line, mount a DTI at lathe centre height (use a flat foot to eliminate any centre height errors) and position the DTI against the edge of the eccentric. Rotate the eccentric until the DTI reading is a maximum or minimum (depends which way round your DTI works) and the hole will also be on the eccentric centre line.

As you can see, the method described is not only simple and accurate, it is also extremely versatile. I've only given examples of its capabilities, I've done many other jobs using the same principles. When I made the vee blocks, it was fortunate that I made a matched pair, as one of them is almost permanently attached to an angle plate!

And if you have a matched pair, make a mounting plate to take both of them - then they can be bolted to the mounting plate, which in turn is bolted to the angle plate - then even long bars can be adequately supported.

I hope you find the method of some use, if you have any comments, suggestions, operations, etc. that you work out, then no doubt we could include them on these pages for the benefit of others.

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(c) Barry Harrison 1997.

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