Elliott Omnimill Gib

[Martin Seymour 2Participant @martinseymour2]

I've had this machine now for quite a while. When I got it it was missing the able gib. I got one made up but didn't realize it was tapered – would anyone have a drawing or dimensions for one?

Cheers

[Martin Seymour 2Participant @martinseymour2]

[RobCoxParticipant @robcox]

Give me a while and I'll pop the gib out of mine and measure it.

Rob

[Clive FosterParticipant @clivefoster55965]

Thats a tricky measurement without dedicated metrology equipment.

It is said that typically taper gibs on machines made to imperial standards are tapered at 1/4" to the foot.

Even non typical ones will be a sensible fraction ion an inch per foot. Slope is much easier to set out accurately than angles.

Metric countries use 50:1, 5 mm in 250 mm. Close enough to be confusing if you try to do it by angles.

Allegedly you can measure direct on the machine using two bearing balls of different, suitable, sizes. Push the slide over so the flat sides are in contact and insert the small ball from the narrow gap end and larger one from the wider gap end.

Measure how far down they go, calculate the separation at the centre line running transversely across the gib gap and calculate the slope. The maths is simple trigonometry of similar triangles.

Obviously the greater the difference in ball sizes the easier it is to be accurate. Also easier retrieve the balls if they don't go in very far. Really helps that the result is known to be a sensible fraction of an inch per foot.

If you can mount things on a mill table, or similar accurate leadscrew controlled thing, so that both the straight and taper dovetails can be got at with a dial gauge its possible to measure the taper directly. Straight side acts as a reference so you don't have to worry about getting things uber accurately aligned with the travel.

Clive

[RobCoxParticipant @robcox]

 

It turned out a while was a couple of days! Still, here are some pics of the measurement technique and the figures.

Here's the gib coming out. Not sure if you've got the screw or not so I've measured that too:

After a cleanup, here is the item in question on the surface plate:

A fair bit of wear on mine. Also, it's not straight, which complicated the measurement procedure as I had to press it flat against the surface plate to get a consistent measurement. Firstly, length (all measurements are imperial as my gauge blocks are imperial, as is most of my workshop… and this mill):

Neither end is particularly square so about 18" is as good as it gets. There are a couple of drill holes in mine. Leftovers from fabrication perhaps:

First hole is 0.5" from the thick end, second is 9" in. I don't think they do anything.

Width of the flat bearing surfaces was gauged with a vernier. The bevels dont rub on anything so I don't think this dimension has to be accurate to the thou. Both sides are 0.80":

For the crucial measurement of the taper, I blued up 3 areas each 6" apart, lightly scribed some lines in the blue spaced as accurately as I could judge off a ruler 6" apart. These are the three measurement positions. Pressing down hard on the gib to make sure it was in firm contact with the surface plate, I zeroed a DTI with a few thou of preload. Once I had a consistent reading at the scribe line, I found the thickness of gauge blocks to also zero the DTI:

Readings were 0.4405" about 1 inch from the thick end, 0.3670" at 6" further on, and 0.2940" at 12". I make that between 0.146"/ft and 0.147"/ft taper. If you're making one I'd make it a few thou thick and scrape it in to fit anyway.

The top and bottom edges of the gib are anything but flat, so precision just isn't possible here:

Above it's clamped to a 123 block, there's contact at the first and second measurement lines, and a 12 thou gap at the third:

So to measure the broad width taper I sandwiched it between two 123 blocks and did the best I could with the callipers:

At the three measurement lines the widths were 1.158", 1.107" and 1.044". Prob about 100thou per foot allowing for the 12 thou out of flatness of the bottom edge.

In cross section the two bevelled sides are 45degree bevels:

And finally, a pic of my notebook page, with the gib adjustment screw dimensions too:

 

Edited By RobCox on 09/04/2023 12:06:55

[Michael GilliganParticipant @michaelgilligan61133]

Very thorough, Rob … extracting the important details from the surrounding obfuscation

MichaelG.

[Martin Seymour 2Participant @martinseymour2]

Rob I can't thank you enough for the time you've taken to do that

I don't know why this didn't come up in my earlier searches but I found this –

https://www.model-engineer.co.uk/forums/postings.asp?th=118480

Adding your dimensions into MarkC's idea that the measurements should be taken at 45 degs to the thickness 's you gave me makes sense as the calculation comes out at a taper of 0.985 degree – near enough to one whole degree – it did take me a while to work out what he was talking about though!

The link that MarkC gave of his drawing isn't available (to me any how!), I have messaged both him and Ian but have had nothing back as yet.

I had a gib made not understanding how it worked – it's parallel at 0.375 thick !

Cheers

P.s. Rob – the only bit I don't understand on your drawing is the thickness dimension 0.4410" – 0.01mm ?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Seymour 2 on 10/04/2023 17:05:20:

[…]

The link that MarkC gave of his drawing isn't available (to me any how!), I have messaged both him and Ian but have had nothing back as yet.

[…]

.

I can’t speak for Mark C, Martin … But I would be surprised if he sees your message.

He was [in my opinion] an extremely valuable contributor to the forum … with considerable expertise in Finite Element Analysis, and a willingness to share that.

But, unfortunately, some of the responses to what he was doing were [shall we say] unappreciative.

I may be wrong, but my understanding is that he gave-up contributing here, as being ‘a bad job’

MichaelG.

[RobCoxParticipant @robcox]

Hi Martin,

Yes, my notes were being written on the fly as I was doing the measurements, so they're not exactly, er, comprehensible. Being of a certain age I tend to be fluent in both metric and imperial and have few qualms about using the most convenient units for the measurement at hand, hence the weird dimension.

I was comparing the thickness of the gib with a selection of imperial gauge blocks, using a metric DTI. As the DTI was being used as a comparator it shouldn't have influenced what I wrote down, but for the thickest part of the gib the DTI wasn't zeroed but read +0.01mm. Had I selected some of the "tenths" gauge blocks I expect I could have zeroed it but if I had dropped the gauge block stack by 1 thou the DTI would have read -0.015mm.

So, a gauge block stack of 0.4410in was 0.01mm too tall, hence the notation. The value in the text in the posting above is correct, to the nearest half a thou, ie. 0.4405in.

When you make the gib I'd be interested in your fixturing as I could do with a new cross-slide gib on my lathe and one day may get round to making one!

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