It is a kind of pair of calipers that allows to scribe a median line on a piece with parallel edges.
The calipers have two sides, one allowing to take the measure of the width, and the other allowing to mark in the center of that measured width, after having flipped the calipers over.
The design of these calipers is based on rigorous geometrical ratios that must be respected while building in order to get an accurate tool.
I will show below an accurate method for making these calipers, respecting these geometrical constraints.
The figure below allows to understand the design principle for these calipers :
B=2A ==> b=2a
d1 = a-r
==> d2 = 2 d1
Using the calipers
Example 1 : Drawing a median line
On the longer side, I measure the width between both pins.
I flip the tool over and with the shorter side, trace using a mechanical pencil through the 1mm hole located at the exact center.
Verification, from one side.
And from the other side.
Example 2 : Drawing the center lines of a tenon.
I verify that the pivot screw is well tightened, so that I don't loose the measure when flipping the tool over.
Building the calipers
Here are the steps for building this tool, the process respects accurately the geometrical constraints.
I trace with a pair of calipers 3 segments of equal length on one of the branches. The axes are punched.
I clamp together both branches and drill the three axes with a 1mm bit.
The holes for the pivot and 3 out of 4 ends are over sized.
Turning the tenons of the pins, and the pivot screw.
The pins are silver brazed. The set is then polished.
My excellent friend Jean Paul Dautais, from forum-lutherie.org, made me remark , not without malice, that sometimes, we do not need a sledgehammer to swat a fly.
Here is his own version :
Just great ! .... (but I still like my calipers anyway).
Thanks Jean Paul and thanks Rick Sparber.
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Category : - Workshop
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