Below are details of how we made ball and socket joints for stop-motion armatures in the theatre metal workshop.
Although we have access to a milling machine, lathe and other specialist equipment I was keen to develop a manufacturing process that was transferable; using equipment that students would be able to access easily outside of the college. To that end, our ball and socket joints were manufactured almost entirely on a pillar drill with some simple extra tooling and the addition of hand tools.
The first thing to do is to establish the dimensions of the joints you wish to make. In this tutorial we are dealing with two joint plates made from 9mm x 20mm x 3.5mm steel with a stainless steel ball 6mm in diameter which can be used for armatures up to 350mm.
I should have really used a better quality steel for the joint plates than the commercial quality I used (a free cutting EN1A would have been nice) however, it doesn’t make any difference to the process.
Buy a length of 20mm x 3 or 3.5mm steel and chop it off to width. Above you can see how I fitted a bolt to the ‘stop’ on our bandsaw. This enabled us to saw each one acuratley to width. Other than the pillar drill, the bandsaw was the only other machine tool used.
You can of course use a hacksaw, but you need to cut each one accurately. The whole process will be undermined at this stage if you cut wonky, uneven, inaccurate joint plates.
For large armatures of around 300mm+ avoid using brass or aluminium for the joint plates, as the thread cannot withstand the force exerted on it when it’s tightenied up and can sheer clean away. This is my fear with the cheap steel I used, but so far it hasn’t happened.
Drilling the holes in the joint plates for the steel balls
To begin with I drilled 3mm holes right through each plate and then opened them out with a countersink drill. However, the double handling of each piece was time consuming so I used a 3mm ‘centre drill’ instead. See below
Centre drills are normally used to start holes off in the lathe where the standard ‘twist drill’ may wander if not pre-drilled. Here we use them as a drill and countersink combined. They are available from: Axminster tools and RS online components to name a few. I would buy High Speed Steel (HSS) rather than the cheaper Carbon Steel tools.
In the picture above, you can see how I have customised a standard drill vice in order to make it more accurate. A small step was machined into the top of the vice jaws so that I could sit each joint plate on it, thus avoiding having to use packing to raise the joint plate up. If you can’t machine a step like this, cut some 3mm plate and solder it to the front of the vice jaws with some lead solder, or drill tap and countersink it.
To the left and right of the rear vice jaw I drilled and tapped a 5mm threaded hole. On the right I screwed a static block of steel to act as a stop. On the left I did the same but drilled the stop block to take a small round rod that can be adjusted.
This way I can slide the joint plate from left to right and have each one drilled identically along the same centre line. It sounds a little pedantic, but I did try other methods which were unreliable. Either the plates never matched up when the balls were inserted causing the joint to rock around rather than fit snugly, or if they did match, the plates opposed each other at odd angles.
Drilling the thread and clearance holes
First mark one end of every joint plate with permanent market pen.
Adjust the drill so that you are drilling in the middle of each joint plate (centrally between the two holes previously drilled).
Make sure the same end of each joint plate locates against the stop (either the end marked with marker pen or the end NOT marked with marker pen.
Now with the start drill, drill a small ‘D’ dent into the surface of all of the joint plates. This will locate the thin twist drill which will be used later. Remeber to locate the same end against the stop everytime.
When drilled with the centre drill, divide you batch of joint plates in half.
Now drill half of them with a 2mm drill making sure that each one has the same end as before located against the stop.
Now drill the other half of the batch with a 3mm drill.
Tapping the 2mm hole with a 3mm tap
This is pretty straight forward, put the 3mm tap in the wrench and tap the 2mm hole. Obviously you can’t tap the 3mm hole with the 3mm tap as it falls straight through.
File the ends of each joint plate down
To ensure that there is plenty of movement around the ball joint, you will need to file down the ends of each joint plate. If you run the safe edge of the file against the vice jaw you should be able to keep the filing nice and straight.
As you can see, these were filed down before the centres were drilled, however it made it difficult to gauge where the middle was so I adjusted the order in which to do things.
The Cap screw
The 3mm cap screws were from RS online components. There are plenty of suppliers for these. I buy ones longer than necessary and cut them down, as there is little difference in price and it’s simple to do. Once again, buy good quality screws ‘unbako’ being the most expensive.
That’s it for the joint plates. unless you want to round them off with a file as I did here.
Sourcing the stainless steel balls
The stainless steel balls are avaiable from ebay at an what I think is an incredibly expensibe price. I sourced mine from a company called Spheric Trafalgar. The A302/304 grade stainless steel has proved itself fairly easy to machine. Follow this link if you intend to buy them: http://www.ballbiz.com/prod-ss-aisi302.html
The easiest way of drilling the ball is on the lathe. However as I have said, I wanted to develop a more achievable process outside of the college. This was done by mounting a self centering 3 jaw lathe chuck under the pillar drill. Cheap self centering chucks are being imported from China and fairly accurate. I got mine from a company called Chronos.
The chuck comes ‘un-mounted’ which means if you want to use it under the pillar drill you will have to make up a base plate to mount it on. I made mine from a piece of steel we had lying around. You might need to improvise with a couple of pieces of angle iron, or maybe a base plate from another machine.
Once you’ve found some material for the base plate, the chuck can be fitted with 3 cap screws as shown above.
Centre the ball under the drill press and pre drill with a centre drill.
Next; Change the drill, without moving the centred ball, and drill out the ball with a 3mm twist drill. We drilled ours clear through rather than stopping ‘blind’. This was the students choice.
Using silver solder to attach the ball to the steel rod.
File a little flat on the end of the steel rod. This will provide a little gap for the Silver Solder to flow into. If you are soldering into a blind hole, it also enables expanding air to escape.
Now follow the instructions for silver soldering and solder the ball to the rod. You can read how to silver solder by follwing this link.