Meccano Robotics Prototyping

I have a lot of meccano. I loved it as a kid, used it a few times in early teen, and now, with some ingenuity (and a few custom parts), I am enjoying it as a tool for robotics prototyping.

What is meccano?
Meccano is nuts and bolts, perhaps the original nuts and bolts set. It has parts from two-hole plates, to 26cm diameter gears. It builds easily with an allen key for new sets, or a flat screwdriver for the older ones. It is intended as a toy aged for low teens, but can be enjoyed at any age.

Why is meccano good for prototyping?
- First off it's cheap-ish. In a set it's not, but spare parts are available cheap enough
- There are a large variety of shapes and sizes, angles, straights, corners, plates, wheels etc.
- Can build virtually anything with it
- Toy shops have it. There is a better chance of getting some locally than with other systems like Vex
- Nearly anything can be mounted to it
- It is sturdy enough for a full range of testing
- There is a fair chance that you already have some (or your kid, or brother)
- Can be made and remade a million times a day
- Easy to make custom parts with nothing more than aluminium and a drill

The disadvantages?
- No pre-made parts for robotics, except for DC motors
- Metal, so it will conduct electricity if you don't insulate properly

This ible covers the techniques I used to build this 4wd mobile vehicle, but many of them can be used for other things.

While I haven't managed to kill or injure anyone with meccano yet, if you are stupid enough (or maybe smart enough), it could potentially hurt. Overall, I rate this as a 0.5/10 for danger.

Over the course of meccanos manufacture, it went through several different companies, materials and styles. Here I list a few pro's and con's of the different types.

Old:
I have an old meccano 7 set, from my grandfather, who used it as a boy. I also some additional old meccano from my father, but I don't know what set they are part of.
These older parts are, in my opinion, nearly completely superior to the newer ones. The parts are high quality metal (there is very little rust) and you also get these big-ish plates (5X11 with flanged edges), and some other parts, some of which I haven't seen in newer sets.
The disadvantage is that they use flat screws, and myself, I prefer the allen keys of the newer sets. Also, there are virtually no pre-built parts suitable for robotics.

New:
New is a little broad-er term. There are various sets ranging from plastic (horrible) to metal. I have a new 20-model set (with 3V motor!) and, from a friend, parts of a 60-model set and some random bits of the 10 set. Much of these sets are useful, and they present some parts not seen in the older set. Most of these I don't find useful, but a few are great.
The gears in the new sets are plastic, which isn't great, but I haven't actually used any of the meccano gearing in any of my robotics projects yet.

I use both. I use the plates of the old set, the screws of the new one, and the girders and angles of both.

This is undoubtedly the most important thing to know, as most robotics projects use at least a few servos. I know two ways that work well. This first way is eacy, but requires some long bolts, and can takes more space than the second.

Requires:
- 1 x 5 hole plate (for standard servo. Others are different)
- 2 x Long Bolts (longer than the servo is thick)
- 2 x Plastic Spacers
- 6 x nuts
- Something to mount the servo to

Assembly:
Thread the two bolts through the ends of the 5-hole plate, put spacers on them, and tighten nuts up to them (image 1).
Wind on two nuts, but leave them near the end of the bolts.
Place this around the servo, pushing it back until the screw-plates on the serve are pressed against the spacers.
Thread this into the place you want it mounted, and tighten some nuts around it.
Finally, bring those two nuts that are drifting around on the bolts, and tighten them up (see the final image)

This method requires you to bend a strip of metal around the servo. I just eyeballed it, and it worked fine, but if you are pedantic you can measure it.

Make sure you leave a gap on the bottom bends where you can tighten the servo down. The image is a lot more clear than anything I can say.

I have two favourite servo's.

Futaba S3001 indirect drive
This servo is quite cheap and has plastic gearing, but it's main advantage is that it can be converted from continuous rotation to normal back-forth motion really easily. Instead of removing the potentiometer, you can just remove the plastic tag that keeps it in contact with the gearing. Then cut out the mechanical blocks and it's continuous. To convert it back, put the plastic tag back in, so the gears once again turn the potentiometer.
Because it is a plastic main gear, you can drill it out to accept a meccano bolt. Then you can mount anything to it.

Toward pro mg996r
This is a metal-geared, ballraced servo that (when converted to continuous rotation) I use as motive force for my mobile vehicles. It takes a standard M3 screw, so you can really mount anything to it. I really must get around to buying a few more of these servos, they really are worth it.
With some wooden disks I've mounted these servo's to turn some pretty decent tires. At the edges of each tire they're producing over a kilo of torque each, but as with any servo, they don't go anywhere fast.

The newer sets can come with a motor, and while I haven't bothered to make the motor suitable for use with robotics, the meccano-mounting battery pack it rather useful.

All that I did was remove the reverse switch, and wire it instead to a servo connector. I don't have any high-current parts, so I didn't need thick wires. 

It was done a while back, so I don't have pictures of the construction, but here's the process:
- Grab the bit with the switch and wires
- Remove the screw and open it up
- Pull out the switch parts (metal bits included)
- Solder on the new wire (make sure you've threaded it through the plastic bits
- Tie a knot in the new wire to stop it putting strain on the solder joints
- Close it up, put the screw back in and re-build the battery pack.

I use a RC receiver in a few projects. The RX itself can be secured in the same way as the servos (cover it in foam to protect it first), but the aerial is often a problem.

The solution is a section of RC plane pushrod inner (or any other thin plastic tube).
For short segments you can just slot this into one of the rod holders, but if the aerial is long enough, it will be too floppy.
To fix that get a long girder and put it through a 90 degree angle to hold it at the top

Parts:
2x short screws with nuts
1x longest girder you have
1x 90 degree angle
1x rod holder

Assembly:
Put the angle at one end, and the rod holder at the other. Put the aerial through as shown in the images. Screw onto whatever it is you're building. Don't worry if it isn't a tight fit, gravity holds it in place fine for me. If you really want to tighten it, modify the rod holder with a pair of pliers.

I haven't really had a problem with vibration, but that may be because I'm dealing with pretty hard components. I'm mounting a raspberry pi on my next bot, and wanted something to cut down on vibration from the wheels.

For this you compress a rubber disc between two plates:

Parts:
2x 1cm+ bolts
2x nuts
2x rubber holder-things
Plates or girders to damp between

Assembly:
Well, it's about as simple are you can get. Look at the pic

Bike inner tubes are better!
Doesn't matter which you use, it's nice to mount them for use in suspension systems instead of springs.

Simply a short bolt and washer works well.
To avoid twisting the band so it's facing the wrong way, hold the bolt and tighten the nut.

You can mount strips by putting a hole in the strip, and threading the bolt through that.
You can mount bands by looping them over the bolt.

Assembly:
Um, put the band over a bolt that has a washer on it and tighten it. It isn't that hard really.

I don't use castors very often, but here are a few pics of a large castor I made.
I haven't put any instructions up because the images are clearer than anything I could say, and it is quite simple. Chances are you'll need to modify it slightly anyway!

Buttons can be mounted with tape to a bent girder
Micro Controllers need to be insulated. (although in older bots I just used a meccano bolt and spacers through two of the PCB holes).

In my latest project, I'm using a rasperry pi to talk to a picaxe, which drives the servo's. I'm planning to build a web-interface and use wifi to control it.

I have a standard plug: the servo lead. Most PCB's will fit pins on at the right spacing, and anything you plug in needs Positive, Ground and Signal. If you have a device that needs two signal wires (ie serial coms) then glue two servo leads together with cyno or epoxy.

Meccano poles/axles can be made from straight #8 wire
M3's work in meccano holes, but are a bit loose. 

A quick search informs me that:
- Spacing between holes is 12.7mm (1/2 inch)
- Nuts and bolts use 5/32 inch BSW thread

If your vehicle is mobile, be prepared to lose lots of nuts and bolts, So tighten them up really well with a spanner and allen-key.

I hope someone finds this useful.