#5 - The Barrelled Arrow Problem
Barrelled and tapered arrows are a problem.
Basically, barrelled and tapered arrows protrude down in the middle by virtue of their shape, reducing the nominal half inch between the arrow and the central column by enough to significantly affect the spine reading.
I spent a long time wondering how Bearpaw got around this problem, and was totally baffled. Then I found an instruction manual online that contained a statement to the effect that arrows with non-parallel shafts could be measured and compared with each other but the absolute spine readings would not be accurate. So, Bearpaw hadn’t got around the issue at all, which made me feel a little better.
Now, barrelled and tapered arrows aren’t an issue for spine gauges with pointers or dial gauges as the gauges are so easy to zero that you can set the zero for each individual arrow. This doesn’t work for the Bearpaw as (I believe) it is calibrated at the factory, and a user can’t adjust it, so the problem is handwaved away. And it’s not great for my design either as my design is so difficult to calibrate. The pillar’s position is set with a grub-screw – if you loosen the screw then the pillar comes loose and drops down. Consequently it’s really hard to make fine adjustments. If every arrow that you measure is the same shape then you can spend time setting it for those arrows and forget about it, but small tweaks on an arrow by arrow basis aren’t really feasible.
If I was making it from scratch I’d thread the bottom of the pillar which would allow for easy, fine micrometer-like adjustments by turning it slightly. I think that’d be a worthwhile improvement anyway, but it’s still not an adjustment that you’d want to be doing a lot.
All in all, I’m not sure that there’s a good solution.