I assembled a calibration jig over the holidays, mostly using AliExpress parts and a few bits and bobs I had lying around. The design is based on various other designs that are floating around on the interwebs.
Frame:2x 2020 aluminum extrusion 600mm (the length depends somewhat on the length of the scale and spoke; the image shows a ~280mm spoke)
2x 2020N aluminum extrusion 200mm (2020N (as opposed to regular 2020) has one solid side wall; I chose it for extra rigidity but plain 2020 probably would have worked as well)
4x corner brackets with T-nuts and screws
Center section, from left to right:M8 threaded rod (I bought a 1m rod and chopped it into pieces)
M8 nut (I used a flanged nut but a non-flanged one should work just fine)
thrust bearing 8mm inner diameter, 22mm outer diameter
M8 cap nut
nipple + spoke
M8 'I-type' clevis end - for J-hook spokes; for straight-pull spokes you'd use a second cap nut
M8 rod
2x M8 clevis end (aka yoke end or Y-knuckle on AliExpress; attaches to the scale)
crane scale in the 150 to 200kg range; I chose the most compact model I was able to find
M8 rod
M8 nut (thumb nut in this case but a regular nut will do just fine)
Fabrication:drill 2020N extrusions for M8 rod
drill cap nut so that a spoke nipple can fit through
grind short groove into the 'I-type' clevis end so that the spoke head can be slotted into it (as mentioned, for straight-pull spokes, use a second cap nut)
I also had to grind the 'Y-type' clevis ends slightly so that they fit into the scale without interference
Note: under tension, the J-hook spoke head will deform - in my experience, it'll remain in its groove securely but count on one sacrificial spoke for calibration. (I'm guessing straight-pull spokes don't have that problem.)
Sources:2020(N) extrusions and brackets - Zhuhai store (cheap but very slow shipping)
https://www.aliexpress.com/store/1100992109thrust bearing -
https://www.aliexpress.us/item/3256806942964649.html'I-type' clevis end (M8 version) -
https://www.aliexpress.us/item/2251832818258210.html'Y-type' clevis end (M8 version) -
https://www.aliexpress.us/item/2255799845781219.htmlcrane scale (150kg version) -
https://www.aliexpress.us/item/3256806675357153.htmlEverything else I got locally.
Calibration:You'll need two wrenches to put tension on the spoke, one to hold fast the cap nut and another to turn the left-most (flanged) nut. (Adding tension is a little fiddly but using a closed-end wrench for the flanged nut (or even a ratcheting wrench if you have one) makes things easier.)
Tension your spoke to a specific value (e.g. 70kgf), then check the reading on your tension meter and write it down. Repeat these steps for e.g. 80/90/100/110/120/130kgf and create yourself a tidy little table with kgf + tension meter readings. Check everything a few times.
Disclaimer: if you kill yourself using this jig, don't blame me. If you kill yourself using a wheel built with this jig, don't blame me.
Cost:Roughly USD70 - I had a few pieces already so that helped.