# Calibration Process Step Clarity Please

I’m at the step where it says this:

Next we need to enter a rough estimate for the rotation radius.
This is the distance from the tip of the chain to the center of the router bit.

I have a measurement of exactly 139.5 mm from bit center to the centre of the chain pin that joins the chain to sled. Should I reduce my measurement by the difference from pin center to chain ‘tip’?

Why is this entered as a ‘rough estimate’, surely it’s a fundamental mesurement?

Thanks,
Miles

The idea here is that it will be close enough. When you run the calibration cuts is when you want to be very exacting with the measurements.

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Okay, cool, thanks for the speedy reply. I’m also looking at that chain pin center to link tip and it varies depending on which link your chain ends with. Alternate links are made from slightly different sized ‘eights’, so on my #25 1/4", 6.35mm (that is actually 6.346 mm) that pin center to chain tip is anywhere between 2.65 and 3.05 mm.
I appreciate that it can be worked out at calibration cut, but as I’ve largely built my own machine from scratch I’m trying to alleviate as many inaccuracies as I go, and this one has had me perplexed for a while, especially as it wasn’t that easy to measure accurately. My hand bent ring isn’t perfectly true either, so I’m probably clutching at any other variables to make up for my build inadequacies ;¬)

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what this measurement is is the distance from the center of the bit to the part
of the chain that is at the 12 o’clock position on the sprocket when you start
feeding out the chain.

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So a 3mm difference in measurement is kinda crucial, no?

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The assumption is that because this is a tough thing to measure we don’t ask you to measure it directly, but instead make a series of cuts and then derive the accurate radius by measuring the spacing of the cuts.

If you enter the rough measurement of 140mm it will be refined to a more accurate measurement during the cut test pattern.

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That’s some clever stuff! Awaiting a window to launch a full test cut, and then I’ve got to make something!! ;¬)

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I can’t take the credit, that was a community contribution

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