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Lathe design

Saddle gibs - front or back?

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Kiwi Bloke13/05/2019 11:19:56
227 forum posts
1 photos

Many older inverted-dovetail bed lathes (EW, EXE, Portass, Grayson, and more) are arranged with the saddle's gib strip at the rear. This seems to have fallen out of favour, although I note that Cowell's still has the gib strip to the rear. Presumably, it's because it's a fossilized, ancient design.

Locating the gib strip to bear on the rear face of the bed's dovetail means that, apart from being difficult to access for adjustment, cutting loads are taken by the gib strip, not a rigid face. I assume this is why modern designs have moved the gib strip to the front. I see no reason why the Cowells lathe couldn't be made with the gib strip at the front of the saddle. Perhaps a re-design was just too much bother - or, perhaps there's a reason I've missed.

Does the location of the saddle's gib strip matter? Why the fashion for the 'wrong' location in the past?

Hopper13/05/2019 11:43:47
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3657 forum posts
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Maybe the cutting forces on a little Cowells are so small that they can be borne ok by the gib strip?

The Drummond M-type, dating back to early 1920s, used similar type bed but had the gib block at the front in the "correct" place. And it was a block too, not a strip. Piece of iron or steel about 1" by 1" by the width of the carriage. One edge was machined at an angle to match the inverted dovetail on the bed. No idea why others did different.

Kiwi Bloke13/05/2019 12:36:01
227 forum posts
1 photos

Yes, in normal use, the forces the Cowells has to resist will be small, but we have a tendency to push the limits, don't we? Clearly, the Cowells seems to satisfy its users - it has an excellent reputation - so it must perform OK. It would, however, be easy to re-engineer the Cowells in accordance with current standard practice. It might allow it to perform better when pushed hard. Would it be an improvement?

My question is really about whether I've misunderstood the situation and the underlying principles of machine design.

Edited By Kiwi Bloke 1 on 13/05/2019 12:37:18

John Haine13/05/2019 12:42:20
2591 forum posts
133 photos

If the cutting forces are mainly on the gib, presumably that means that it will wear more? Isn't that a good thing, since it's replaceable?

Michael Gilligan13/05/2019 12:50:11
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13827 forum posts
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Posted by Kiwi Bloke 1 on 13/05/2019 12:36:01:

My question is really about whether I've misunderstood the situation and the underlying principles of machine design.

.

You might enjoy the 'bedtime reading' link that I posted on this previous thread: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=120557&p=2

MichaelG.

Kiwi Bloke14/05/2019 08:42:54
227 forum posts
1 photos

Thanks Michael, an interesting link, which I've skimmed (I'll read it fully over a suitable number of bedtimes...), but it doesn't seem to address saddle design. It does, however, illustrate how early machine designers thrashed around, trying this and that, probably without much science being involved.

John, no. An adjuster-screw-supported gib strip (as opposed to a taper gib or 'block-type' gib) will certainly wear more than a 'rigid' bearing face - and unevenly too - if only because it's flexible, and really only reacts forces around the adjustment screw locations. But you don't want the major guiding face to wear, do you? Neither do you want to have to continually adjust it. Also, the aim is to make the guiding face to be as rigid as possible, so it should not be the gib-strip side. You will see that cross-slides and top-slides are (always?) arranged 'correctly'.

I'm still puzzled by some early designers having made what seems to me to be a fundamental blunder. I expect that they had their reasons, but what were they?

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