I was originally planning to use my own design of parts for this, but having taken the trouble to design them and send them off for quotes for laser cutting and folding, the cost would have been prohibitive, averaging out around £10 per component – and there were 18 components, 3 per leg.
For the initial prototype, it turned out to be much cheaper to use off-the-shelf Chinese components, which brought the cost down to around £40. I bought 18 sets of these pan-tilt servo mounts, nicely made in 2mm aluminium, check out Ali Express for the best deal! They also come with a bag of useful mounting hardware. Each set has a swing bracket and a servo mount.
6 pairs of the swing brackets will be mounted at 90 degrees to each other to give fore-aft movement and the first of two up-down movements.
12 brackets thus need to be cut down to minimise the distance between pivot points, thus reducing the load on the servos. Cutting the first bracket just below the first group of 4 x 3mm and 1 x 8mm holes gives a useful offcut that can then be used as a template for the rest of the marking out and as a drilling guide.
The guide template should be bolted to the bracket with a 3mm bolt, see photo for where this bolt needs to go.
It’s a good idea to precisely mark the centre of the 8mm hole as this needs to be accurately cut. Using the servo mount bracket as a template is useful here, as some of the cutouts on this have a 3mm central hole, rather than an 8mm hole. Bolting the 2 parts together as before, then using the central hole to locate a 3mm drill, gives a hole that can then be opened out using a succession of drill sizes – 4, 5, 6, 7 and 8mm in order. Attempting to drill the hole directly with an 8mm drill will be unlikely to give an accurately centred hole, as drill bits tend to wander, especially in thin aluminium.
Once the 12 brackets have been cut down and all the holes drilled, pairs of brackets should be fixed together at 90 degrees using 3 x 3mm pop rivets, using the existing holes. Some filing of the rivets may be needed to clear the servo mounting brackets on either side as they swing past.
And here’s the first prototype arm, complete with servos – 1 down, 5 to go…
I also intend to add ‘legs’ to the end of each arm, a short length of aluminium tubing with a flexible nylon inner part, sprung loaded, to operate limit switches. This is my initial try, but I think that something half this length will work out better!
Note that I have also used these splined metal servo discs in place of the plastic ones supplied with the servos, as they are cheap to buy and give a more accurate fit. They come with 3mm screws, so are easy to fit.
The bracket sets also come with 8mm solid brass bearings with a 3mm bore, I intend to replace these with flanged ball bearings, type F693ZZ, once again ordered from China for around 22p each. Anything that reduces the load on the servos is a worthwhile improvement. I am also awaiting delivery on some servo cable extension leads, so with luck, by the time these parts all arrive, I’ll have finished the other 5 legs and can make a start on connecting everything together and looking at some software design.