Harrison L5 Cross-slide Question

I have repaired the worn-out nut on my L5 cross-slide, by boring it out and sweating in a threaded bush turned from leaded bronze. *

During that repair I noticed this oddity:

- the cross-slide screw looks and seems by measuring, biased to one side by about 1/8".

- the spigot on the nut, a rectangular bronze block, is central but the bore off-set, though the wear was so severe it was difficult to measure..

So my question is this:

:Is this off-set right - or am I miss-measuring it or falling for some strange optical illusion?

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I need know because I have bought an unfinished slotted cross-slide to fit, and I do not want to risk machining the nut seating off-line from the screw axis.

(I also have an original Harrison unit, albeit with a possibly-10tpi nut, but so wide it would hamper using the tailstock.)

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It came, by the way, via the For Sale ads on here - thank you Michelle W!

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* (How, with the lathe in bits? Screw-cut it in that lathe, which can handle very coarse threads, while it was still intact; then dismantled it to complete the machining, between centres on my ML7.)

Hi Nigel is if possible to put any pictures on as I find it difficult to see what the problem is. I have looked at the manual and can't see how you can get an offset to the nut from the leadscrew.

https://www.scribd.com/doc/126978356/Harrison-L-5-L5A-Manual-pdf

David

Edited By David George 1 on 19/08/2022 13:33:26

You're right - the manual dose not indicate anything odd here!

I've not been able to photograph it I'm afraid, and I've put it all back together; but visualise it this way...

The slide runs on dovetails with a central channel in which live the screw and nut, and allowing for the channel being still the as-cast finish, the screw looks unexpectedly off-centre to it.

At the apron end of the screw is a plain shank, and measuring from that to the edges of the dovetails, using a vernier caliper, gives an unsymmetrical result.

However, this is not an easy thing to measure; and it's possible the dovetail on one side is wider than the other, to accommodate the tapered gib.

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So let's look at the nut.

In original form this was a rectangular bronze block with a cylindrical spigot on top, locating in a hole drilled in the underside of the slide. A countersunk screw through a smaller-diameter continuation of the hole holds the nut to the slide.

One would expect the spigot to be above the thread's axis, but it is not; by eye and by measuring; although measuring to the thread was chancy because it was so badly worn I think the combination worked from memory. It had about 1/8" backlash - a complete turn of the screw!

Testing this by rotating the nut end-for-end, the side of the nut struck the edge of the entrance to the channel.

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So it would seem the screw is off-centre relatively to the sliding surfaces, and the nut thread is off-centred from its locating spigot, to match the screw.

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When I bored out the nut, by bar between centres, I centred it for height, cross-ways and in axial alignment in a vice on the Myford cross-slide, by centering the screw onto which I had threaded it, and aligning the vice to the nut. This kept the curious off-set intact.

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The mystery remains......

Well you can disregard the casting straight up. Castings are all over the place. Not unusual to be off to one side or the other.

If your dovetails are tapered or offset to allow for a tapered gib, maybe you should be measuring from the outer surface of that gib strip to the screw, or the actual bearing surfaces.

If you still get an offset result, it's possible the screw is offset so the handle clears something else on the lathe, maybe the topslide handle when it is swung around for screwcutting, or something like a cross feed lever on the apron etc.

Edited By Hopper on 20/08/2022 01:07:22

Edited By Hopper on 20/08/2022 01:08:05

Thank you - useful pointers!

I should have been a bit clearer perhaps.

The dovetails on the saddle are not tapered, and it was those I was measuring to. One of the dovetail in the slide is cut on an angle to take the gib, which is tapered from that side only.

I was trying to measure the saddle to screw distance; using the edges of its dovetails and a plain section of the screw, but it is not easy to use a caliper there, in a very narrow gap.

Something is unsymmetrical somewhere, and it makes the nut unsymmetrical, but it is not clear where or why.

The difference there is not enough to affect the topslide, although at a high angle of rotation its handle strikes the cross-slide hand-wheel so you can't swing it round to feed in rightwards (e.g. to cut a shallow cone on the end of the bar). The slide will rotate more than enough for any screw-cutting.

The apron controls are all well away from these.