Extruded Holes

If you've ever taken apart pretty much anything modern with screws in it you will find you get dozens of screws and no nuts.

There are two types of screw used in this situation (well, rather more than that, but this is a very general point). Self tapping screws, and machine screws.

Self tapping screws are usually intended to screw into soft materials like wood or plastic, or into a single turn thread formed in a piece of metal.

Machine screws are designed to be used with a nut.

So what you usually find is that plastic cases are usually held together with self tapping screws, and machine screws are often screwed into, not nuts, but EXTRUDED HOLES!

(of course there are exceptions, such as plastic cases with brass inserts to take machine screws, and rivet-nuts set into the metalwork. They're just less common)

So I started building my most recent project and found that I had bought some screws that are a tiny bit too short to take a nut, and my mind slowly revolved until it clicked into place on the subject of this method of assembly, eventually resulting in the little punch and die shown here.

There's plenty of room for improvement, but it's a start.

This section is usually called tools and materials. The title gets a bit repetitive...

You will need:

A small piece of steel at least 4mm thick. It only needs to be about 20mm across, though bigger is better.

A small piece of flat wood, chipboard, whatever you have, at least 15mm thick. Needs to be big enough to screw the piece of steel down onto.

A small spring, size depends on the holes you are making

Something you can use as a centre. It needs to closely fit in the hole you will drill in the steel, and have a thin hole down the middle. I used a little roller out of an old printer.

Small steel bar you can make into a punch.

A wood screw

Drill

Drill bits - size depends on the holes you are making

Screwdriver to fit your wood screw.

The theory applies no matter what size of screw you have, but I made this for M3 screws, so everything below is dimensioned accordingly.

A screw has a minor (inner) and a major (outer) diameter. For metric screws, the major diameter is given by the size of the screw. So for an M3 screw, the outer diameter is slightly less than 3mm. I believe those UNF/UNC screws you use across the pond are dimensioned according to the same principle. For other types you will have to find a chart somewhere.

The inner diameter is determined by the mathematics of the screw thread. Fortunately this is standard data and widely published. For an M3 screw it is (approximately) 2.4mm, and the drill size is given as 2.5mm.

The diagram I copied from http://www.roymech.co.uk/Useful_Tables/Screws/Thr... shows how the different parts of the thread relate to each other.

So the hole in the die which you are going to form the extrusion into needs to be the minor diameter of the screw, or the drill size, at most, plus twice the thickness of the steel.

I made extruded holes in steel which is 0.6mm thick, so twice that is 1.2mm. Add this to the minor diameter of 2.4mm, and you get 3.6mm. The nearest metric sized drill is 3.5mm, although the imperial size of 9/64 is closer, so use that if you have one.

I drilled my hole at 3.5mm and found that because my drill is a bit wobbly it was slightly large, and works perfectly well. You may find you need to file the inside a little with a needle file.

Next, you need something which will fit in the hole and also has a much smaller central hole. I used a little rubber covered roller out of a printer, and removed half the rubber from it (so the other half could form part of the guide in the base, and also retain the piece), but there are all sorts of things you could use. Just look at the plastic stuff you regularly throw out. There's bound to be something. You also need a strong little spring which can push this item up.

Ideally this part would be made of metal, but that's a bit more than I'm able to do at the moment.

Finally, you need a punch. I used a little steel bar that came from something (probably a printer), put it in the drill and ran it whilst filing the end, which gave me a point. The bar is 2.4mm diameter, just right for this project. The point has to be long enough to go through the steel you are punching, and into the hole in the centre.

Measure the length of your centre and relaxed spring combined together, and subtract the thickness of the steel die. The number you get is the depth of hole you need to drill in the piece of wood. it has to hold the centre closely enough that it doesn't flop about, but not so close it can't move easily.

Put the spring in the hole, fit the die over the top and drop in the centre.

If you happen to have a little roller like I used, trim the rubber back so the bare plastic fits right through the hole in the steel, there is also a ridge around the end you need to file down a little bit. Drill a slightly larger hole in the base just deep enough so the rubber goes all the way in, and make sure it can move up and down freely.

You also need to make a screw hole in the die.

Get everything lined up and screw the die down.

Drill a small hole in the piece where you want to make your extruded hole. I actually drilled 1.5mm, although smaller would be better, but a 1mm bit won't stay secure in my power drill.

Place the piece over the die, so the hole lines up with the hole in the centre, fit the punch, and give it a few whacks with a hammer. Use a big hammer to minimise the amount of bouncing you get. You have to kind of hold back a bit with the blows or you will jam the centre in the die.

When it goes through, pull the piece out of the die, and knock or pull out the punch. Try not to wiggle it.

So there you are. You can tap the hole, or screw in a thread forming screw, which is a better option because it doesn't remove any metal. You can form a quite few threads with one screw before it starts looking worn. If you have taken apart any amount of old stuff you probably have a few thread-forming screws. They feel sort of triangular if you turn them between your fingers.

This project would be improved by making it possible to screw up the punch rather than hitting it, as hitting the steel makes it bounce and become mis-aligned, and could damage the die. It would also be much better if the steel were held down by some kind of ring shaped clamp that could go around the hole.

If anyone manages to implement these things, it would be nice to see!