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All this device does is allow you to set the top-slide to the same angle as a tapered shaft held between lathe centres. I emphasise Morse Taper turning because I think it's here that it will find most use, but it's equally applicable to copying any tapered item (of a reasonable length). Turning Morse tapers seems to be something many folk shy away from due to the difficulty of setting the topslide accurately enough.
The gauge (well, may as well give it a fancy name) is just an accurately machined slab of bms which is set truly parrallel to the top-slide axis, at precisely lathe centre height, and is of a width and length to enable it's working edge to contact the side of a taper shank held between centres without fouling the tailstock. It must of course be mountable in exactly the same position on the top-slide each time it's used. So, instead of using a DTI to set parallel, tapping it this way and that, winding it up and down each time whilst checking for run-out, one merely adjusts the top-slide until the gauge sits truly against the side of the taper shank you wish to copy. Simple enough?
There are a couple of caveats here - there must be a true inner edge to the top-slide to act as a datum for putting the gauge in position, and it's easier to make the gauge if your top-slide will rotate so that it's pointing towards you parallel to the cross-slide (i.e., handle away from you). Many won't do this so you might have to modify the construction accordingly. The new S7 (with the top-slide which is machined all over) is ideal, older S7s will need the top part of the inner edge machining flat - as will ML7s (but as the latter don't rotate far enough you'll have to un-bolt it and use clamps to hold it in place - a bit awkward). ML10s will roatate but the handle fouls the boring table. None of these problems are insurmountable, if you read on you will get the general idea of how it works and will likely come up with your own solution - my method is not the only one.
Right then. First thing is to get a piece of bms 2.5" x 2.5" x 1" thick. In the lathe or milling machine true one side, then machine a shoulder 1" by 0.25" deep all the way across. This shoulder will sit against the top-slide datum edge. Aim for a good finish. Now you want to ensure the gauge face is at centre height. Mount the gauge on the top-slide shimmed up with a 1" x 1/4" parallel under the shoulder, and facing the headstock. The parallel is the right thickness if you are using a 1/2" end mill (a 3/8" mill would need a 3/16" parallel). You want to cut underneath the working edge of the gauge to a distance of about 3/8 to 1/2" from the working edge. Don't try to cut it all off in one pass! You can see that by doing this, then removing the shim, the bottom edge just has to be at centre height after making the cut. For best accuracy hold the end mill in a collet.
You now have a piece of bms, one edge with a step 1" wide to sit on - and be clamped to - the top-slide, the bottom of the opposite side is exactly at lathe centre height. You need to thin this opposite side to taper down to about 10 thou to give a clean working edge for the gauge.
Next job is to ensure that when in place the working edge of the gauge is truly parallel to the top-slide axis. This is where you need to set up the top-slide as described above. Probably the easiest way is to clamp your DTI in the toolpost and run it across something flat mounted on the mandrel nose (like the faceplate) until no movement of the pointer shows. This done, clamp your gauge under the toolpost - make quite sure the shoulder is registered squarely on the top-slide datum edge - and take a fine cut across the working edge. It shouldn't need much, just enough to true it up.
And that's just about it. To turn a Morse taper, mount a taper shank between centres (I use a drill chuck shank as it has female centres each end, and is a bit longer), slacken off the top-slide clamps and move the cross-slide while feeling for a good contact between gauge face and the taper shank. Clamp up the top-slide and cut your taper.
How successful this method is depends entirely on how accurately you make the gauge, it should be good enough to use the turned taper straight off without major surgery with file and emery (never very successfull for correcting anyway).
One final advantage with the gauge is that it allows you to rapidly re-set the top-slide to turn parallel - simply by substituting a parallel shaft for the tapered shank set between centres.
Addendum
There is another quick 'n dirty method of setting the topslide for short taper turning - if you have the latest incarnation of the Myford S7 (with the ground-all-over top-slide body). In this version the body is accurately ground parallel with the dovetail slides, the older models of course have a rounded-top body casting and the following method won't work. With the later version mount a taper shank between centers and place a piece of white paper beneath, shine a light onto the paper and shade the top-slide and taper shank. Now, with a bit of careful positioning you can sight vertically down onto the edge of the top-slide against the edge of the taper shank, the lighting allowing for a very fine knife-edge slit of light to be used to set the two parallel. I set my topslide using this method and cut a 2MT shank without further adjustment. I was very pleased (and with some disbelief) to get a good wringing fit first time. I checked using the pencil mark method and the taper was a good fit for virtually the entire length of the taper. Maybe I was just lucky on this occaision, but I'll certainly try it again next time to find out. This method is especially useful for rarely used tapers, it worked equally well on the small JT taper on the end of the ram of the sensitive drilling attachment.
(c) Chris Heapy 1996.
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