Here is the simplest one:-
You can loosen and tighten the wing nut and bolt.
You can enlarge the sketch as shown. Click on images to see them more clearly.
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Here is the best most accurate one I made. This is cardboard template first, then one can make it with wooden ruler or whatever length timber you wish. The cardboard is too wobbly to use.
Diagram describes proportionality.
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Longcalipers.eg.
This model is not accurate at large sizes. The pincer tips make the error.
I think it is better to make a caliper dedicated to one proportion only, eg x8. This is the one used by sculptors who make enlarged final sculptures x8 times the size of the original maquette.
I got the idea when I saw Tom Bass' retrospective Exhibition at the Sydney Opera House, some years ago. His instrument was expertly and elegantly made in wood and brass.
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Here is my attempt to model the tip of a seashell:-
The calipers shown were fairly but not really accurate.
The tip of one pincer broke off and I had to try to glue a bit on to fix it.
Measure the width of the whorls, north, south, east and west. It made a flat diagram.
To make it a cone, find the height and put a similar cone underneath, and slit the sides all around to make the cone. Stickytape it together. Next I have to figure it out lengthwise so that I can plan how many crochet or knit stitches are needed to make a more accurate tip to my seashell models.
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