Brian.

You could try Brian Caddy of Slide way services at Nuneaton

**LINK**

Ian

Remember if you do grind/machine away the reference face of the saddle to slideway, you may need to adjust the alignment of the main leadscrew. There is limited freedom in the clamp screws which hold the apron in place. All possible with care.

Hi Brian,

And thank you Neil – yes Mr Radford's mod with a slight twist…

As your S7 is narrow guide, then measure the gaps (left and right) with feeler gauges between the back of the saddle and the vertical faces of the rear shear. On mine the gaps were <> 19 thou. They should be equal, most likely not – the fixed narrow guide which bears on the rear of the front shear is worn and the gib is unable to be set correctly causing the saddle to rock. Or the gib has been set incorrectly causing uneven wear on the fixed guide.

The bed on my S7 shows very little or even wear but I still converted it by removing the saddle to clean out any debris at the rear the saddle and inserting a 1/2 inch wide shim just a tad thicker than the measured gaps at the rear – no machining required. This effectively converts the saddle to wide-guide principle (after <> 1973) making use of the unworn rear shear. The shim (brass) must be just thick enough to move the saddle back to the unworn position (on mine the shim is 20 thou) creating a small gap between the rear of the front shear and the worn fixed guide on the saddle.

Readjust the saddle gib-strip, taking care to have equal pressure on all screws – my S7 now faces as good as flat, over 5" and I can move the saddle to the tailstock end without any binding, showing that the bed is not unduly worn. It took less than a couple of hours to carry out and has restored the accuracy of the S7. If you now measure the very small gaps which have been created at the rear of the front shear they will most likely NOT be equal, proving that this is the wear point.

The shim has stayed in place with stiction, but it does need the later "bridge" shaped wiper on the front of the saddle to replace the older "rifle" type. There is a small chance that swarf will enter the small gap that has been created, but I fitted a swarf tray anyway to minimise swarf on the bed in front of the saddle.

This mod. is completely reversible – just pull out the shim and readjust the saddle – no harm done.

I've obtained a spare saddle to carry out a more drastic machining operation to fit a 1mm thick Turcite strip in place of the shim but that has gone onto the to-do pile for now.

As a point of interest, does anyone know what the factory clearance was at the rear of the saddle? Having this measurement would help me restore the saddle to it's designed position and reduce any misalignment of the lead-screw and half nuts.

Regards

Phil

Edited By CotswoldsPhil on 16/09/2015 15:32:19

I believe the narrow guide was perceived as being better at resisting twisting. I seem to recall that when Tubal Cain got a new Super 7 he converted it from wide guide back to narrow guide It's in the ME archive somewhere!

And David Piddington has articles currently in MEW

Rod

The prismatic beds generally have huge width to length ratios. The faces of the vertical V can also provide large areas to resist wear.

The myford way with the front guide attempts to put the lead screw and rack central and beneath the guide as they all do which should lead to a lack of twisting forces as the saddle is moved. My ML7 which had hardened rails showed around 0.0005" wear on the rear edge of the front rail. In practice it could have been more because the only way I could measure it was with a straight edge – to remove any wear from the front which appeared to be ok.

I'd be very surprised if Myford didn't do all of the rail grinding in one setting but they may have ground the underside of the rails as a separate setting. i'd expect them to have a machine that would do all of it at one setting though. Perhaps they couldn't afford a slide way grinder.

Chis Heapy's, hope I have spelled that correctly, idea of improving the narrow guide on a myford was to add a gib strip to the rear of the front rail. I can't see that this would help other than making it easily replaceable and allowing the lead screw to be correctly aligned. I assume he got this idea from some one maybe Tubal Cane. He never gave any details, just mentioned it.

I can remember talking to David P and am pretty sure he was the one with a form grinder but it could have been some one else that worked there. I had a Raglan at the time. It's odd that the similar bed arrangement on those does wear better than a Myford – it causes less trouble just as prismatic V do.

There was a slideway grinder in Worcester as well. Brand new around 12 – 15 years ago. They offered to see if they could convert my ML7 to a prismatic bed or even fit it with wear strips. I wasn't happy about the amount of material they wanted to remove. I can understand their problem – set up time. They can grind the grinders table to make sure that is aligned – not keen to do that – then stand the bed on parallels upside down and grind the feet true and then turn it over, set up and do the grinding – all in one go. Actually in this case the saddle was carried on a sub table behind the bed with the V matched by off setting it so that was done too by the same diamond wheels.

John

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Hi Brian,

That's as expected – being that the machine is facing convex. I would expect the wear on the bed to be on the rear face of the front shear where the narrower fixed guide runs. What's the dimension across both shears?

Regards

Phil

Morning Mark,

I'm just about to go out…

When I get back I'll measure my bed and both saddles – the one on the lathe and the spare I picked up.

Based on your's and a previous set of measurements provided by another member in an earlier post I am getting the feeling that the saddle on my machine has been re-ground to correct the narrow guide face, ie I need only 20 thou for the shim, your machine will need 35 ish. to do the same job. Unless of course Myford did not bother about this dimension which I doubt. The leadscrew on my machine appears to be aligned quite well meaning that the saddle is not too far forward.

IMHO I think that the narrow guide design makes it very easy to set the tailstock end of the gib too tight (it's not pulling directly against anything) increasing the wear and skewing the saddle.

John's explanation of having the leadscrew as near the centre of the guided ways makes a lot of sense to reduce twist induced friction.

Go to go

Phil H

Edited By CotswoldsPhil on 17/09/2015 08:57:29

Brian, You ask why the saddle is so worn?

It is very easy to adjust the front gib strip screws so that they actually force the saddle to "twist" meaning that the pressure on that short face running on the rear of the front shear is pressing harder at one end that the other.

Because the front gib strip is longer than the under saddle rib you cannot easily feel that is the situation. IE, if it is tight and wrong it still does not rock to tell you it is not correctly aligned.

On one of my Super 7's I spent a considerable time measuring and correcting the wear, replacing the material taken off with a steel shim strip bonded to the short rib. Since then the facing performance has been well inside specification.