Tribrach adjustment

Nice work. I'm curious why you chose this approach over a commercial adjustment ring. One advantage of the latter is that it accommodates all makes and models of Wild-type tribrach. For me it was worth the 50 bucks or so I spent on one.

Why did I make it? Just in the habit of making anything needed I suppose - there's a cupboard full of special jigs and targets made as one offs, but which re-appear on occasions at short notice to be "just what is needed".

We have standardised on the GDF6 as being a very robust and accurate tribrach, so over the years quite a few (16) have been acquired. I normally trade back the tribrachs on any new instruments for useful things like prisms.

I'm not sure what the comercial offerings are in the USA, but to be honest I'd only seen a couple of rather poor options in the UK when I made the cap.

Nice, I like it.

I've never used one of those adjustment rings. But after doing it for so many years the old way, I could knock off checking/adjusting 16 fairly quickly. Well, unless someones bent some adjuster screws, or there are any busted parts.

My old boss had a simpler solution, as an FYI. Involved one well adjusted tribrach. This never left the area where he had it mounted. Exactly above the 5/8th screw , he had an X on the ceiling tile. Put in by Plumb Bob. When he had to adjust a tribrach, he simply had a 5/8 threaded pipe attached to both of them, so the adjusted one was upside down. Focus cross hairs on the X, adjust bubble, and calibrated.

MitchPTD, post: 412124, member: 10124 wrote: he had an X on the ceiling tile. Put in by Plumb Bob. When he had to adjust a tribrach, he simply had a 5/8 threaded pipe attached to both of them, so the adjusted one was upside down. Focus cross hairs on the X, adjust bubble, and calibrated.

(I think you mean "adjust crosshairs" rather than "adjust bubble.")

This is similar to the technique using the Seco adjusting ring, which is simply a thick-walled aluminum cylinder machined so that it's OD is the size of the tribrach interior and its two ends are machined square to the cylinder axis. You set the ring inside a tribrach on a tripod (this tribrach doesn't have to be in good adjustment, it's just serving as a holding device), then set the tribrach to be adjusted on top of the ring upside-down. Place a target roughly 5 feet above (typically on the ceiling) and center the whole affair beneath the target using the optical plummet of the upside-down tribrach and the adjusting screws of the bottom tribrach. Rotate the upper tribrach, noting any movement of the crosshairs on the target and adjust accordingly until no movement is noted during rotation.

It's easier than it sounds!

A field method is to trace the outline of the tribrach with a pencil on top of the legs. then use your pencil and mark the ground or a piece of paper taped to the ground. Rotate it align it to the pencil outline, level and mark another spot on the ground. Repeat it the third time. Then adjust. Then repeat.

chris mills, post: 411920, member: 6244 wrote: 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.

I really appreciate the ingenuity of our profession. I also applaud the craftsmanship. Today we have a variety of tools available to bring our ideas to a real product.

I was wondering if a 3D printer would have done the job for you? Granted you would have to spend significant time designing the template. But it would be reproducible for the future. Maybe use ShapeWays to do the actual printing. The cost for one of these are now well within the range for all of us. The FlashForge 3d Printer Creator Pro is a model that I was recently looking at. I have a few ideas too. Thanks for sharing.

makerofmaps, post: 412250, member: 9079 wrote: A field method is to trace the outline of the tribrach with a pencil on top of the legs. then use your pencil and mark the ground or a piece of paper taped to the ground. Rotate it align it to the pencil outline, level and mark another spot on the ground. Repeat it the third time. Then adjust. Then repeat.

Same method that I use, to adjust align crosshairs to "centroid" of three marks on floor and adjust level bubble. Run through rotation procedure again to check.

RRain

I personally prefer the SECO #2002 tribrach adjuster with the tubular level vial. Said adjuster is also very handy for leveling appliances such as front-load washers, by taking five readings on its top in an "X" pattern and doing a least-squares eyeball adjustment once the results are within reason.

Actually if you remove the target screw, then you would have something very much like the commercial jig.

Are all these techniques assuming a tribrach with an optical plummet? What procedure is used for tribrachs without the plummet? If my total station has recently been calibrated, can I put it in a to-be-adjusted tribrach, level the TS carefully (including using the compensation screens), then adjust the bubble accordingly?

If there's no OP, then you just need to level it up using the instrument bubble, and then adjust the tribrach bubble by its adjustment screws.

Here's what I do. I believe this to work as well or better than the hockey-puck style adjusting tools, and it uses items that most surveyors already have. All you need is a rotating tribrach adapter and a fixed tribrach adapter. Remove the screw from the center of the fixed adapter, flip the adapter over, and screw is onto the rotating adapter. This way you have 3 brass feet sticking up in both directions, and each side rotates. Attach tribrachs to each one, so one tribrach is upright and the other one is upside down. Screw the lower one onto a tripod. Then use the eyepiece on the upper tribrach to sight a point on the ceiling. There's no need to level either of the tribrachs. Turn the upper tribrach 180 degrees and adjust the crosshairs accordingly. Repeat if necessary. Then you can flip them over and adjust the other tribrach the same way, and repeat with as many tribrachs as needed. Once again, there is no need to level the tribrachs, as you are simply working around an axis. This is the most accurate method I have ever used.

I would post pictures of my setup, but I can't seem to post any pics on this site.

To post pics etc. you have to "embed" the image using the little camera icon. Click the icon then roll down a bit and click on the "upload a file".

I think these instructions were what you were referring to.