Download Drawing
I designed this accessory mainly as an aid in screwcutting, where it is a requirement after each traverse to return the cross-slide to a pre-determined position ready for the next cut. This applies specifically when using the 'set-over' technique for screwcutting where the cut is put on with the top-slide. The main design considerations were that it should be sturdy enough to resist the feedscrew as far as possible thus bringing the cross-slide to a dead stop without any over-run, repeatability within 0.001" was required. Also, there was the small problem of where to fit the thing! I don't like intrusive attachments that get in the way of normal usage - I like them to be there when I need them and out of the way when I don't.
I think this attachment is fairly unobtrusive, occupying as it does a single T-slot, and in any case the main component can be rapidly removed from the lathe by releasing two hex socket grub screws (set screws). The other component (the stop lug) is permanently mounted on the carriage but is unlikely to get in the way of other machining operations.
From the photos and drawing you can see that the support piece is an elongated T-nut, the overhanging end being threaded to take an adjustable stop rod. Consideration was given to maintaining zero backlash in the screw to eliminate slack. The thread I used was of 40 TPI, and I thought hard about selecting this particular thread. The advantage of using such a fine thread was that adjustments would be more precise, and at 40 TPI each full turn would be a convenient 25 thou (convenient if you are working in imperial at least). The disadvantage was that adjustments would be rather slow in operation because of the 40 full turns to move but an inch. In the end the need for accuracy won and I selected the fine thread. I had considered using a 40 TPI threaded bush with a plain rod running up the centre locked with a screw fitting, this would have offered both rapid adjustment and fine setting capability, but I suspected that such an arrangement would be unlikely to resist the cross-slide feedscrew and would slip under load, so the idea was abandoned.
The first job is to machine the long T-nut, a simple milling operation on a piece of 3/4" square b.m.s., carried out either in the milling machine or the lathe vertical slide. Machine this to a good fit in the T-slot, the dimensions given provide for a precise fit in the Myford T-slots on my own Super 7 lathe - check the dimensions on your own machine. All exposed surfaces were then 'surface ground' using the disc grinder (see here for further info on this) because this part is very visible and so it may as well look good!
Holes were drilled and tapped 1/4" BSF for the two locking grubscrews at the points indicated, the serrated ends of the screws being ground off to prevent chewing up the base of the slot. Only very moderate pressure is required for the screws to firmly lock the part in place, the main job of location being accomplished by accurate milling such that the nut fits the slot precisely without any slack. Next, the support piece is mounted in the milling vice and the hole for the stop rod drilled 9/32" and tapped 5/16" x 40 TPI right through. At the same setting the tapped hole is counter-bored 11/32" to a depth of 5/16" and tapped 3/8" x 32 TPI with a plug tap.
Backlash is eliminated by having a screw-in bush threaded internally 40 TPI and externally 32 TPI. The action of tightening this bush will remove any play in the matching stop rod thread. This small turning (1/2" dia., 7/16" long and preferably of gunmetal) is made having an external thread of 3/8" x 32 TPI x 5/16" long, drilled and tapped right through 5/16" x 40 TPI. A knurled ring 1/8" long is left on the outer end, and a small (4 BA) grubscrew passes through the end of the support piece to hold it in place after adjument has been made (use a copper slug or, better, a piece of fibre washer under the grubscrew to avoid damaging the threads - and there is no need to overtighten this screw). Make sure the outer and inner threads are concentric by completing the work at one setting.
The stop rod is made from a length of 5/16" silver steel (drill rod) and should be screwcut in the lathe to give a free fit for the matching thread in the support piece and bush. You can use a die as a thread chaser later but screwcutting is necessary to produce a true thread first. Dome the end face of the rod using a file and emery cloth to provide a good contact with the stop lug. Fitting this piece can be a bit fiddly as the threads in both the support piece and threaded bush need to be aligned before the rod will screw in all the way, and this is best done by 'feel' rocking the bush from side to side. Up to one full turn of the bush may be needed before the threads align correctly. When in place backlash can be completely removed by turning the bush slightly and then locking it in place with the setscrew. Further adjustment is unlikely to be necessary for a long period. As extra insurance against the stop rod moving from it's desired setting a locking ring is indicated which is tightened after setting the rod to depth.
The stop lug on my machine is fitted onto the carriage at the inner edge of the nearside bedway. I feel this position provides a sufficient range of adjusment for most uses. Take extra care when drilling and tapping the two 4 BA holes for the securing screws that you do not inadvertantly penetrate right through to the bed - use a drill with a depth collar. As an added safety measure, if you have a powered cross-slide, use a threading tool to turn a V-groove just under the head of the 4BA cap screws - down to a diameter of about 1/16". If you inadvertently engage the power cross feed with the stop set the heads of the screws will (with a bit of luck) break off without damaging the power feed mechanism. The threaded portion can then be removed quite easily as it will be above the surface of the carriage top face.
Little needs be said about the use of the attachment. It's probably easiest to set by moving the stop to the approximate in-feed position required, and then the final position of the toolpoint set with the cross-slide feedscrew. When feeding the cross-slide in to the stop turn the feedscrew gently - the feedscrew offers such great leverage that no matter how stiff the stop mechanism you can always 'spring' it by a couple of thou. (In practice, with the version I made it takes pretty rough treatment to spring the stop by just 0.001", and it is quite easy to maintain an exact setting according to the cross-slide micrometer collar). So, with just a little practice you will now be able to reposition the lathe tool accurately, without having to look at the micrometer collar, and avoiding that mistaken 'extra' turn (Yes, I've done it too - a 0.101" cut when you wanted that last 0.001"!)
(c) Chris Heapy 1996.
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