So just what is a 3D printer?
There are two answers to this question: the individual printer, and the whole system that goes from idea to finished object. Let’s start with just the printer itself.
Think of an ink-jet printer. It has a print head that goes from side to side, and paper that goes from front to back, so we have movement in two dimensions. The print head lays down a very thin layer of ink, and then everything moves on. But just imagine, for a moment, that the ink was really thick, and that the paper could go backwards as well as forwards. We could lay down multiple layers of ink. After a bit we would either need to move the paper down, or the print head up, to allow for the gradual build up of thickness of what we are printing.
That’s exactly what a 3D printer is … or, at least, the sort of 3D printer that we are considering in this book. There are other sorts, and if you want to know more, there is a section on the website.
So there you have it; a machine that has bits that can move in three dimensions: side to side, backwards and forwards, and up and down. These dimensions are measured in what is called “Cartesian coordinates” after Rene Descartes. And this makes a 3D printer a member of a class of things called “Cartesian Robots”. Other tools in this class are Computer Numerically Controlled (CNC) milling machines, routers, laser cutters, plotters, etc. We will consider all these things in this book, because if you can build one, you can probably build others, and the electronics are all very similar, too.
Let’s look at the machine in a little more detail. We will consider:
The static frame
The moving bits
The drive mechanism for the moving bits
The motors that drive the drive mechanism
and we will go into much greater detail for all of these areas later in the book.
The Static Frame
First we have the physical structure of the machine. Some of them are built inside cases made of sheet acrylic, or MDF, or plywood, probably cut out with a laser cutter, or a CNC routing machine to get precise cuts. Others are a spidery structure of threaded rods with plastic joiners and nuts to hold them in place. Then there is the “grown up” Erector Set or Meccano version, and the aluminium extrusion or “T-slot” version … and many others.
We are going to consider a variety of ways of building the structure, and we’re going to focus as much as possible on using methods that don’t require you to already have some sort of sophisticated machinery, so we won’t be going for the “laser cut box” approach. And we are also going to focus on using construction methods that are Open Source and GPL-licensed, because that makes them more widely available, lowers the initial cost quite a bit, and the cost of building subsequent machines a lot, because you can make all the parts, too.In this book we will focus on Contraptor (http://www.Contraptor.org) a grown-up version of Erector Set, especially designed for building Cartesian Robots, “No Nuts Contraptor”, where the only tool you really need is a mallet, and MakerBeam, a miniature version of T-slot aluminium extrusion, designed for building small machines. We will also touch briefly on the commercially available T-slot systems, such as 80-20, but won’t spend much time there, because by the time you have built a working miniature machine in MakerBeam, it will be a trivial job to upscale to the bigger (and more expensive) 80-20 to build whatever size of machine that you want.
The Moving Bits
Next we have the carriages that move in three dimensions. Precisely what moves is up to the design of the individual machine. It might be that the “workpiece” moves from side to side and backwards and forwards, and all the print head does is to go up and down. Sometime the print head moves in two dimensions, and the workpiece just moves down. It doesn’t really matter; the decisions are fine design points that we’ll get into later int he book.
Probably more thought and design effort goes into these carriages than anything else. Sometimes they are wheels running on tracks, v-pulleys running on sharp edges, bearings running on rods, sometimes “linear bearings” (a sort of ball bearing inside a tube that runs on a cylindrical rod) … and just about every other sort of device you can imagine. The search is always on to find something that is:
Easy to build
Can be built from parts that are easily available, or
Can be 3D printed.
If you can design a carriage mechanism that fulfils all of these criteria, then you may well make your fortune, or at least have the world of Cartesian Robot builders very grateful to you!
To get an idea of some of the ideas that people are working on, check out these web references:
The Drive Mechanism for the Moving Bits
What we need here is something that can make the carriage move precisely and quickly, and also fulfils all of the other criteria for the static frame: easy to build, low cost, can be made from easily available parts, or can be printed.
To give yourself some idea of what’s involved, go find yourself a “dead” inkjet printer. My wife bought be an HP Officeject (scanner, printer, fax, etc) for just $5 (£3) from the local household waste recycling point. Take it to pieces, and examine what you’ve got, and how it works. Even if you break every single part inside, you’ll understand a lot about Cartesian Robots by the time you are done, and the lesson will have been very cheap! You may want to buy yourself a set of “star” (or “Torx”) screwdriver bits before you start; that’s what most of the screws will be.
The two main mechanisms are a long screw thread, held in some bearings, and attached to an electric motor. Riding on the thread is a long nut, with something to stop it going round. By spinning the threaded rod one way or the other, the nut can be made to move backwards and forwards. It’s a strong mechanism (I used it in my automatic hen-house door opener), and is as precise as the threaded rod that you use, but it may not be fast. There are special threaded rods, just for this purpose, called “lead screws” (Acme lead screws). These are very precise, and they have a long, open thread, so that the lead nut moves much faster than on an ordinary threaded rod. And they are very expensive! I just used ordinary M4 threaded rod from the UK version of Home Depot (B&Q) for my hen house door, but I needed to drive the rod pretty fast or it would have been time to put the chickens back to bed before the door was fully open!
The other mechanism is the toothed belt drive, and that’s what you will find driving the print head in your inkjet printer. It may not have quite the precision of an Acme lead screw, but most inkjet printers will print at 300 DPI, so they must be getting close to 1/300″. Which for extruded plastic thread isn’t bad!
And they have another advantage. If your carriage is not absolutely perfectly engineered, then it’s possible that it might skew slightly on it’s tracks. If you drive just one side of it, then you run the risk of it binding, so you usually want to drive both sides … which is easy to do with a belt on either side, running from two toothed wheels, one on each side of the machine, driven from one axle.
The disadvantages are that we can buy threaded rods in all sorts of sizes, and cut them to whatever length we want … in effect, we can make our own threaded rods. It’s highly unlikely that we will be able to make toothed belts, so we will be constrained in our design to what is commercially available.