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Tribrach adjustment
A recent thread asked about calibration and tribrach adjustment, so I thought that a few pictures of the aid I made might be of interest.
Adjusting one or two tribrachs isn’t a hassle, but when you have a lot to do just turning them through 120 degrees and ensuring they are in exactly the same place takes time.
I made a simple “tripod cap” with a central hole cut to the EXACT size of the baseplate on our tribrachs (Wild GDF6’s). Place cap on top of tripod. Drop tribrach into the hole, level up, mark plumb point, turn through 120 degrees, repeat twice, hopefully leaving three marked points on the floor, close together and forming a neat equilateral triangle. Mark centroid of triangle, Adjust optical plumb nuts until crosshairs are on centroid mark. Turn tribach again to check adjustment is correct.
Holder is made out of ABS sheet 1mm. thick. I first made a thin card template by upending a tripod onto the card and tracing the EXACT outline of the head. Cut out card; check by placing card on tripod head and lightly held with a spot of glue. Use a set square to check that the whole of the edge is exactly over the edge of the tripod head. If it is too big, then trim lightly – too small then cut a new template.
Use card template to cut the shape out of the ABS. Cut fractionally bigger than the template. Place shape on tripod head and repeat check with set square. Rub down any area which protrudes. You want to end up with a perfect fit.
Remove from tripod head and trace outline of tribrach base onto the ABS. Cut out carefully, JUST inside the marked line. I used a fretsaw with a fine blade (6/0 size) Try tribrach for fit to hole (it won’t, since you cut just inside the line). Gently work around the hole filing or sanding a little at a time until the tribrach just slides in with a little finger pressure. It should now be an exact fit: check in all three positions.Once happy, cut a length of ABS about 3/8″ (10mm) wide, long enough to wrap right around. Carefully fix strip around the edge to make a “lid). This is best done by sticking a shortish length at a time, starting on a fairly straight section. That way the strip is anchored when you try and bend it round the tighter curves. Give each dose of solvent plenty of time to go off before bending and fixing the next area. As you get near the end you can trim the strip to exact length and stick the final part down.
Once the joint has hardened, try it on the tripod head. Hopefully it will fit on with just a touch of the finger.Try the tribrach and check that each footscrew base goes down onto the tripod head. As it stands this would work, but it is likely to get damaged, so I strengthen it up with another couple of layers.
Cut another ABS head, this time to overlap the edge strip you added. Stick to top of cap and when dry sand the edge to make a perfect match over the original edge strip. Now add another edge strip, which will overlap the edge of this new cap, this time 1mm wider than the original strip (to allow for the added thickness of the new top layer). Once set hard use the original centre cutout to cut through the new top to leave the tribrach space. Again, sand and file this to give a perfact fit to the tribach base.Finally (yes it does end! it actually takes little more time to make than it does to read) cut yet another top plate to overlap the latest edge strip, fix on and repeat the hole in the centre for the tribrach.
It might seem long winded, but cutting the tribrach hole separately in each layer as you make it gives a very good fit and enables you to “adjust” things if you accidentally take off too much material in one of the earlier stages. With care the tribrach can be rotated and replaced to better than 0.2mm.
Overall time spent making it – around 2 hours – spread over two days to allow solvent to harden off between each stage.
The photos show top and bottom of the cap, the cap in place on a tripod and a tribrach and traverse base in place, ready to adjust the tribrach.
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