Making a Cat5 o’ Nine Tails

A cat of nine tails made from cat 5 cableThe traditional naval ‘cat’ was made of a thick piece of rope which had been unwound into its nine constituent strands, and knotted. It’s pretty hard to knot Cat 5, which usually bends in a radius 4 times its width. However, we can add modular connectors. To make this a really painful piece of kit, we’ll leave the protective boots off the connectors, but if you’re going to want to soften the blows, then remember to put a pair of boots onto each length of wire before you crimp the ends.

You will need a scalpel or Stanley knife, side clippers, a Cat 5 crimp tool, cable, and modular connectors. Most crimp tools have a cutter on them, but we’ll need the side cutters to tidy up the last end of cable anyhow, and they are less unwieldy than the whole big tool. Optionally, you may want a flat screwdriver or awl to help tighten the big end knot. First, grab several lengths of cable. Four will be about the length of your full two-arm span, and the fifth should be at least three times that, depending on how long you want your handle to be. The crimp tool cutters are ideal for cutting these. I have chosen black, but there is no reason you shouldn’t use another colour, or a mix of colours. You will definitely want see-through modular connectors, as you need to see inside the block while you are building it. Start with the shorter pieces of wire, and add ends to each.

Cat 5 cable with stripped end.The outer coloured layer is softer than the inside. Strip off about half an inch of outer cable by scoring lightly around with a sharp knife. It should then pull off easily. If there is fibre inside the cable, trim that off. Untwist the wire pairs inside, and flatten them into the order in the photo. (This is not the only order, but it’s one standard to prevent cross-talk.) This order, T568A Wiring, is Green-and-White, Green, Orange-and-White, Blue, Blue-and-White, Orange, Brown-and-White, Brown. The wires are stiff, but you can flex them forward and back in the fan, and that will help to straighten them. There is no need to strip them, because the crimp tool will punch metal through to the inner wires.

Cutting Cat5 EndsAt this point, the wire lengths will be different. Once all of the wires are aligned and held easily in one hand, trim the ends. You’ll want to push the whole wire as far as it goes into the connector, so the ends need to be the same length. If the wires are still trying to escape you, bend them back and forth gently until they ease into their new positions, before clipping. Take off as little as you can in squaring up, because you’re going to need much of that half inch inside the connector.

Uncrimped Cat5Now take the connector and push it gently over the coloured wires. There are channels inside the connector and each wire will, with only a little wiggling, go into a single channel. I’ve found it’s best to angle them very slightly, so they go in one at a time, but as long as they are all straight you shouldn’t have a problem. Push in gently at first, but make sure the whole wire beds down to the end, for the best electrical connection. Then push the coloured wire sheath up into the plastic housing. This will also be crimped down, but needs to be shifted up a little around the inner wires.

Crimp ToolThe cable will bed down into the crimp tool. Here you can see the ridges on the tool are pushing down on the metal that is part of the connector. That forces the metal into the inner wires, right through their coloured sheathing. If you have a crimp tool with a ratchet on, this is much easier. Otherwise, you’ll need a little dexterity in with your strength. Keep pushing the outer soft sheath into the connector while you close the crimp tool as far as it will go. The connector below has not yet been crimped, but shows how far the sheath needs to be for a decent bite.

Once you have the crimps finished for the shorter pieces, take the longer piece of Cat 5 and put a decorative knot most of the way towards one end. Leave one length that we will crimp later. A monkey’s fist knot is ideal. Start this by winding the wire three times around your fingers. You want it to be loose. Take the winding off your fingers and, again loosely, run the wire around the winding three times, to make a cross shape. Now feed the end inside your first loop, but outside your second loop, three times. Image below is courtesy of Wikipedia.

Monkey's FistTighten it slowly and carefully, by feeding the wire through the whole knot, repeatedly. Don’t try to close it all at once, or the knot will become shapeless, and don’t close it all the way yet. Get it most of the way closed, and arrange it as a rounded ball. As a purely decorative knot it can take a centre to ensure a spherical shape, but we’re going to rely on the strength of the cable to hold the shape for us.

Before you tighten the knot completely, feed the four shorter lengths of cable through it so they sit centrally. Arrange the shorter end of the uncrimped piece so that it is as long as the other tails, feeding the spare wire through the knot, and the reshaping it. The long bit, the working end, should still be considerably longer than the rest.

Take a hitch around the base of the whip with the working end, and then wind it loosely around the nine other strands, to make a handle. Take another hitch at the end then feed the working end back down as a tenth strand, inside the windings, and then tighten them until everything is held snug. As with the monkey fist knot, you may have to tighten and move the wires repeatedly, but the end result will be a tight, comfortable handle. Use the side clippers to take off any extra length from the working end once you’ve tightened it. Put on the last connector, and you’re done.

Cat CaseI made a presentation case for my Cat out of an ABS wall box and a laser cut front with hex bolts, but equally you could add a clip to the decorative knot, or have the whip unadorned. Just remember that if you do put pressure on the knot, you’ll be deforming it, as there was no room for an inner core once we passed the wire through it. If you do decide to add a core, let me know in the comments how that went, so I can update this post.

Happy Making!

My Little Pi-ny

Dev Machine
Here’s the development machine. I love hardware that folds down almost as much as I love free hardware that I picked up because someone else didn’t need it, and this is a blend of the two.

Everything I have in this photo fits into the official touch screen box, along with a chunky battery and a spare USB lead for power. It’s a Pi 3 with blinkt hat, touch screen, Rii-diculously small keyboard on which yes, I genuinely type, and two Fold Holds (by Jonathan Woolf) to hold it all up and protect the screen when it’s packed. You might say the touch screen is overkill – I don’t have a GUI on the machine.

I’m going to design a hat to go on top, which will be able to fly out the unused pins currently covered by the blinkt, giving me breadboarded GPIO access. Nothing could possibly go wrong with that plan.

Clinkt

Pi Larson ScanningI may have got a little carried away.

The Blinkt hat has a perfectly* good python library which does everything you might reasonably want to do with a set of 8 APA102 lights. My problem there is the word ‘reasonably’. So, I re-wrote it in C++, because my soul has never seen the light of day.

(* nearly perfectly. Some might say that flushing the visible buffer by writing white pixels off the end of it is a little much if you decide to define the number of LEDs as 6 for testing and OW MY EYES.)

It’ll take arbitrary pixel length, just like the python library, but unlike the python library it’s built with the assumption you’ll want to make and use multiple patterns, and manipulate those patterns with regard to each other. I can cross over patterns based on whatever input I like, as long as I painstakingly craft that input and sanitise it myself.

The reason I wanted to dig deeper than the python framework was that the Raspberry Pi is my lightweight development machine. I have a laptop that fits into a box. More on that in the next post.

I’m reliably informed that it’s against the law to have a blinkt and not use it to output something something pop culture Davis Hasselhoff – so here is the Larson Scanner in use: Video 19-04-2017, 14 00 28

The C++ library is available at https://github.com/dianaprobst/clinkt. You’ll need the bcm28somethingorother library, but the README.txt has that information. If you want to use cheerlights or APIs you’ll also need libcurl.

init.d/wtf

When using a shell script in /etc/init.d to start up a command, make sure you put the shell script there, and not the compiled program.

It didn’t work anyhow, so I’m calling it as a cron job on reboot, but I’m pretty sure the point still stands.

ETA: Aaah. So, there are things called runlevels, and Rasbian is based on Debian, which has them. I’m more a systemd person, so I was looking at the wrong sort of help file.

Coding project

I need a small coding project, that will let me practice more than just simple procedural problems, but is not so massively complicated I die of it. I also, let’s be clear here, need sleep, food, and to remember what the inside of a bathtub looks like. Nevertheless, I’m looking for things in about that order.

I don’t know enough to know what an ideal project would be, which is making this hard.

Iterating Wobblebot (and a side project)

I’ve laid the cable track, planned the new screws and bought another identical to the second…

So, as it turns out, not all lead screws are the same, even when they are supposed to be. The first one I bought has a thread that’s at the right pitch, and the right height, but the trapezium shape is too narrow. This was leading to a huge backlash problem, which I thought I would have to deal with by making anti-backlash nuts. However, the other lead screw, which I had bought thinking it was a pair, doesn’t have that. There’s more metal, and it fits snugly in the nut I have. That means there’s hardly any backlash at all.

I’m still interested in making the anti-backlash nuts, and I have other uses for a tap for that shape of screw, so I’ve ordered a length that I’m going to try to tool into a tap, with the lathe and the mill. Clamping it is going to be a right beggar – I might enclose it in a thermoplastic and clamp that instead.

Itchy: Improving stuff

The current base of Itchy is a slice of plywood on some 40mm blocks with some nuts used as spacers. I’m calling that ‘non-ideal’. It’s going to have MDF on a raised base of some kind, probably made of lengths of 20×20 extrusion. It might be removable, and probably should be, but ‘removable’ just means having countersunk screws, and maybe some kind of cover for them. ETA:I just remembered I know where there’s some sheet steel. I must remember not to overmake this.

The entire gantry swivels about the lead screw. This is slightly more than non-ideal. It’s a significant error. Mat’s been helpful about that, after he stopped cringing. (He called it an abomination. I’m so proud.) Two lead screws worked from the same stepper would move that squarely. I’ll have to add anti-backlash nuts, but that isn’t a problem – I have two bearing blocks on either side, and I tapped them both. Adding in the second cross-piece hasn’t been a priority yet. When I do it, I’ll have to do it properly and arrange everything nicely. Right now crosspiece #1 is held on with two pairs of corner braces that are bolted to each other. That was only ever temporary, but I’m impressed by how well it’s been working.

The head needs building, rebuilding, or re-re-building, depending on how I’m keeping track. The belts are currently attached to a piece of acrylic, which is not actually terrible given the amount of pressure on it. However, I’ll be wanting to put more pressure on it. This is another non-ideal part. It’s not a critical part of this round of improvements. The motor at the end of the gantry is held on by a single piece of 5mm acrylic and the power of hope. That /is/ a critical part of the current round of improvements. I need to make that out of metal, and have a bottom brace to it as well. I’d have made it out of metal but that all went badly wrong when I put it onto the rotary table on the mill. We don’t have a single bit wide enough to get the raised part of a NEMA motor sit inside a hole. On the head I got around that by not having enough of a hole in the first place, but I knew the head was temporary, and I want to do this properly. The best way is probably to make a threaded rod for the mill that has a cone ending, so I can centre-find. I’m likely to eyeball it all and hope, though.

The biggest features now are the splorted wiring everywhere like someone has disembowelled a pinata full of multi-fruit bootlaces, and the fact that some of those wires are inherently unsafe. The limit switches are currently push-to-make, and should be push-to-break. That was a speed thing, but I can invert them easily enough now I know what the firmware is, and that it’ll support the pin inversion. I have cable track now, so I can run them all safely. The head/motor holder designs both need to take into account limit switches and wiring anchor points. I tried 3D printing track, but the joints were not good enough.

In order of importance, there are the safety items first – limit switches and wiring – and then the thing that’s arguably a safety item, in that I don’t want motors to snap off and should therefore get rid of the acrylic – and then the wobble along the bottom axis, which different people call the X or the Y. I’m thinking of calling it ‘the axis with the motor nearest to me’ which is my kind of naming scheme. (It’s a bit weird, because it’s not like a graph where you plot things. It’s a graph where the axes are used to move the other axes. So a movement in the X, from left to right, can be made by a stepper that is sitting along the Y axis. This has been a source of massive confusion to me, so the milling course at makespace just has ‘long axis’ and ‘short axis’ and then nobody has to think. And, more importantly, there’s less chance of fucking up through people not thinking.)

In terms of what I’m realistically going to do first, it’s got to be the wiring, then it’ll likely be the wobbly bottom, then the motor mounting, and then a big redesign of the head. I can probably deal with the wobble a bit by only having three blocks, which would let me position the rods to minimise the travel available – but that’s the wrong way of doing it, and a double lead screw is the right way.