The Maker's Manual: A Practical Guide to the New Industrial Revolution 1st Edition (2015)
Part III. From Bits to Atoms
Chapter 13. Milling
Michelangelo, artist and master of subtractive techniques, used to say that what he did was to free his works from the marble which kept them trapped. Today, thanks to machines, we can also free “objects” trapped in foam blocks, resin or wood, without necessarily becoming artists of the chisel. These machines, which make our objects rise from an ocean of burrs, are called CNC (Computer Numerical Control) machines--machines that can control other tools (such as mills, lathes, grinders, and drills) via computer.
CNC Machines
Just like 3D printers, CNC machines are nothing new: they have been used for decades to manufacture precision mechanical parts. These machines are often very large and expensive, made of cast iron and filled with gravel to reduce vibrations, and carefully mounted in industrial plants not accessible to everyone, for obvious safety reasons.
These CNC machines are essentially robots that can move tools in space to create objects. In subtractive technologies, the tool being moved may be a laser beam, a blade, a supersonic water jet, a rotating instrument... the important thing is for it to be suitable to remove material. The digital revolution couldn’t help taking CNC machines into consideration, so some years ago a series of open source projects appeared on the net to manufacture CNC machines in one’s own garage.
Obviously, these home-brewed machines aren’t as precise as industrial models (such as the ShopBot 5-axis CNC shown in Figure 13-1), but to meet a maker’s needs they are more than sufficient. Some manufacturing brands have created low-cost “desktop” models, taking up just about 4 square feet.
Figure 13-1. An industrial-grade 5-axis CNC (ShopBot Tools).
The machines we are going to speak about in this chapter are CNC milling machines. They use a drill-type tool equipped with a cutting head to dig out the material. Milling machines can work on many kinds of material: wood, Plexiglas, plastic, cardboard, printed circuits, foams, and metals. Of course, milling machines that work on metals have a very different structure, weight, and features from home-based milling machines used for balsa wood.
The simplest machine moves in three directions, namely along the x, y, and z axes: for this reason it is called 3-axis machine. More complex and expensive machines use four, five, or more axes; some even have the ability to rotate around the main axes. The rotation axis around the x axis is called A axis; then there are the B and C axes for the rotations around the y and z axes. Some machines use robotic arms able to make very complex movements and to reach any part of the piece that the machine is carving. Fortunately, it will not be you, but the software, that manages the whole process.
What’s The Use?
CNC milling machines carry out repetitive operations quickly and with great precision. They are used in all cases in which subtractive technology is more suitable than additive (for example engravings, bas-reliefs, molds, cuts, holes) and to work with materials different from plastic, like wood and metals, which you have to use when you need objects characterized by a high mechanical resistance.
Let’s have a look at some examples of what you can create:
§ customized guitars by engraving logos or names on existing guitars or by building all necessary parts from scratch;
§ artistic joints, for example the 3D version of the works made by M.C. Escher, engraver and graphic designer;
§ molds for chocolate bars, as seen at the Fab Lab of Turin;
§ wooden boxes and containers of different shapes and dimensions, engraved and decorated;
§ molds for model building, e.g. airplane models;
§ different gearwheels and mechanisms, in metal;
§ milling machines can also be used to make printed circuits. Instead of photoengraving and forming them with acids, a milling machine head can engrave a plastic or copper sandwich;
§ artistic objects.
The space within which the working tool can be moved defines the working area of the machine. For most purposes, unless you want to manufacture customized furniture pieces or a prefabricated building, a small working area is enough. For example, the Mebotics Microfactory, shown inFigure 13-2, has a working area of 12x12x6 inches.
Figure 13-2. The Mebotics Microfactory at work
Even if the working area may appear limited, consider that, just as in 3D printing, the biggest projects can usually be split into smaller parts. You can and should think of ways to make the project modular, which will enable you to create the object by joining or wedging in the different components. If you search online, you can find many examples of how to create wonderful snap-fit joints. For our own “garage projects”, a small working area is more than sufficient.
The milling tool (the endmill) is so similar to a drill head that an inexperienced maker might not be able to tell the difference. However, a drill head is used to make holes, and works vertically, with its tip making the cuts. In contrast, most milling machines cut mainly on the edges, move on a horizontal plane, and remove material layer after layer.
Milling machines have molded heads with different shapes that are chosen according to the working needs:
§ Pointed-tip heads are used to carve or engrave;
§ Ball nose endmills are suitable to create rounded or “organic” surfaces;
§ Square heads with one or more grooves (“flutes”) are a compromise solution, able to work both flat and curved surfaces.
Figure 13-3. From left to right, representation of a multi-flute, ball nose, and single-flute endmill.
In general, a milling head is composed of a tapered stem with fixed dimensions, between 6 and 10 mm. The stem is tightened in the spindle, a retention system similar to that of conventional drills. The best spindles allow the head to be precisely inserted and always aligned: this is extremely important because milling cutters can work on fractions of millimeters, but if they are not perfectly assembled, the actual precision can be distorted and the tool life expectancy may drop by 50%.
Many industrial machines are able to automatically change the tool on the spindle without an operator’s intervention, thus guaranteeing the best precision in the replacement. The different working tools are kept in an automatized rack.
The CNC’s control unit supervises the movement of the motors through circuits called drivers. To obtain higher precision, movements are measured with specific sensors called encoders.
Designing with a CNC
Knowing what a CNC can make will help you understand better what kind of drawing or design you are able to create.
A 3-axis milling machine can carry out three types of carving:
1. 2D (Figure 13-4): the tool digs precise shapes with straight borders at a fixed depth, effectively working only on the plane of the x and y axes;
2. 2.5D (Figure 13-5): the machine works again on areas parallel to in the xy plane again, but at different depths. The milling machine can cut profiles or dig. The surfaces created are only horizontal or vertical.
3. 3D: the machine works simultaneously on three axes, so surfaces can have any kind of orientation.
Figure 13-4. A ShopBot Handibot resting after some 2D carving
Figure 13-5. 2.5D carving on a ShopBot
In the object designing and drawing step, it is important to bear well in mind the features of the machine which will do the manufacturing.
The finished object resolution depends on both the features of the material and the dimensions of the tool being used. A 6-mm head may have some problems in reproducing very tiny features; moreover, there may be the risk that, within the cut, some details get lost. The same problem occurs when the head hasn’t got enough space to move ( Figure 13-7‚ , because moving there could ruin the part you’re cutting.
Figure 13-6. The toolpath planning software (CAM) excludes all parts in red because they are not workable on a 3-axis machine.
A 3-axis machine is not able to rotate the material bring worked, and so cannot carry out operations on the side that’s resting against the working surface: for this reason, control software eliminates all impossible work steps, thus avoiding damage to the workpiece.
Should you want to operate even on unreachable areas that cannot be reached in one step, you have to create systems of reference that allow you to turn the piece manually and realign the machine so that the process can continue. For this purpose, you can use holes and joints provided by pivots and alignment pegs, creating additional ones outside the “real” workpiece if necessary.
In addition, you have to consider the fact that the milling machine has a certain length (Figure 13-7) and that the spindle covers a quite large amount of space, so you can’t dig wherever we want, for example close to borders and walls.
Figure 13-7. The gap is too deep and you won’t be able to dig it.
Software
Just as with 3D printing, milling requires three types of software, too:
§ Computer Aided Design (CAD) software to create a model you will export in STL, EPS, or DXF format;
§ Computer Aided Manufacturing (CAM) software to make G-codes;
§ Software to control the machine.
CAD software
As with 3D printing, there is no particular software we recommend: you can choose the one you like the best; you could look at some of the 3D CAD packages from Chapter 11, but these CAD/Drawing packages are worth checking out: Inkscape is free and open source, and works well for 2D CAD. Autodesk Fusion 360 is free for some users (students, educators, startups) and you can use it for everything from 2D to 3D.
CAM Software
To turn a two- or three-dimensional model into a series of instructions for a CNC machine, you need CAM software that generates the toolpaths that the machine must follow to create your object. The best software packages improve paths and have elaborate strategies to reduce working times and achieve better results. CAM software for milling machines are mostly of a commercial kind; they are extremely expensive and use complex and old user interfaces. Thanks to the spread of designs for the self-manufacturing of milling machines, a series of software programs have appeared that are more suitable for a maker in terms of costs and complexity. We recommend FreeMill (Figure 13-8) by MechSoft. FreeMill is a high quality CAM for CNC and it is available for free; it uses quite simple algorithms and doesn’t have complex optimization strategies. However, it is a perfectly functioning software derived from VisualMill, a more full-featured product.
Figure 13-8. The CAM FreeMill software by MechSoft
FreeMill
Here’s an overview of how you’d work with FreeMill.
Once the drawing is uploaded, you need to set the working parameters. On the left-hand side of the window there is a wizard organized in cards on which you can set the main information. In the first step, include the axes orientation of your machine.
In the second step, indicate the dimensions and the margins of the block you intend to work on (Figure 13-9 ).
Figure 13-9. Setting the working area and margins
The CNC works on three axes and needs a point of reference or an axes origin. Here each machine is different; you can provide machine-specific information in this step (Figure 13-10 ).
Figure 13-10. Setting the point of reference
In the next step, you will provide the type of tool (flat, rounded, or blunted head) and the corresponding dimensions, as well as the length and features of the spindle. (Figure 13-11 ).
Figure 13-11. Defining the tool dimensions
Then, you have to specify the movement (feed) speed of the machine and the rotation speed of the tool (Figure 13-12 ).
Figure 13-12. Setting the speeds of the tool movement and rotation
Finally, you’re ready to set the final parameters to generate the G-code and visualize the work path. The panel allows you to indicate on which axis the main shifts will take place (x or y axis) (Figure 13-13 ).
Figure 13-13. Configuring the work paths
The last step, called Post-Process Operation, is used to generate the file: you will find a long list of names among which you have to choose our control software; if you can’t find it, you can use the General postprocessor option.
Control Software
When you buy a professional milling machine, you usually receive matching software to operate it, called control software. If you build the machine by yourself, you will have to find or write your control software; we recommend a couple of options:
§ MACH3 for Windows (Figure 13-14) , created by ArtSoft USA; the free-of-charge version of the software has a 500-line G-code limit, while the complete version costs less than $200;
§ EMC2 LinuxCNC, an open source solution.
Figure 13-14. The MACH3 control software
Regardless of the software you choose, after installing it, you will have to configure it and set many parameters that vary for each machine. For example, the dimensions of the work area, as well as the working and shifting speeds. You will then have to verify the signals received and transmitted and run some tests to fine tune the system functioning and performance. Such an operation can take many hours of work and a lot of patience.
Does it sound complicated? It actually is!
Where Shall We Turn?
Milling machines have not generated the same level of buzz as 3D printing, partly because it is more complicated. Moreover, the machines take up much more room, and they are harder to manage. However, milling machines offer a great deal of possibilities and are indispensable for some types of work.
If you are mainly interested in working on small objects, and in milling electronic circuits, it may be convenient to buy a small desktop milling machine (there are machines with build areas smaller than 10x10 cm).
If you need to work bigger objects, you can turn to a Fab Lab, makerspace, or hackerspace, which are usually well equipped for this kind of activity. There, you can also find someone who will give you a hand with your creation or even make the object for you.
If you want to build a milling machine all for yourself (we have done it!), we can find a lot of information on the Internet; a good starting point is the site Build Your CNC, which also sells ready-made kits.
For objects that need very precise dimensions, or for metals, you will have to turn to a professional prototyping studio or a mechanic’s workshop, because very big and heavy machines are needed to avoid vibrations that would jeopardize the working precision. The same is valid if you need machines with more than three axes. As all these machines take up a lot of room and are expensive, they can’t be found everywhere.
Finally, the working costs mainly depend on the materials used.