The Frame
In This Chapter
- Cartesian-layout 3D printers
- The importance of frame rigidity
- Choosing the right-size printer for you
The frame of a 3D printer is what everything else is built on, making it a very important factor in the quality of your prints. Printers are built in a variety of different ways, and all the different layouts and construction materials make a difference. To understand why, it’s best to start by learning how 3D printer frames are generally constructed.
In this chapter, I go over the Cartesian layout of 3D printers, construction techniques for them, and the importance of size when it comes to your 3D printer.
When it comes to 3D printers, layout refers to the way the printer is designed to move in three-dimensional space. That movement can be achieved in different ways, but the most common of those is the Cartesian style. These printers are named after the Cartesian coordinate system, which is the most basic way to define a point in 3D space. This coordinate system is what you probably remember using in your math classes in school: a point is defined by its X coordinate and Y coordinate on a 2D plane. With 3D Cartesian coordinate systems, you’re simply adding another axis (Z) to define the vertical position of the point.
DEFINITION
Cartesian is an adjective used to describe things related to René Descartes, who was a French mathematician and philosopher. His many contributions to mathematics were the reason for the Cartesian coordinate system being named for him (though he wasn’t solely responsible for its development).
Cartesian 3D printers use the same basic principle to move the tip of the hot end to a specific point. They have mechanisms to move in the X axis, Y axis, and Z axis, allowing them to position the hot end anywhere in 3D space. For example, if the hot end needs to move from the point [10, 10, 10] (X, Y, Z) to [20, 5, 10], it’s a simple matter of moving the X axis 10 units in the positive direction and the Y axis 5 units in the negative direction.
3D printers with Cartesian layouts are by far the most common, both in the consumer market and the professional market, most likely due to the simplicity of the math involved in controlling their movement. Other types of layouts, like Delta printers, require the use of trigonometry to adjust for a simple move on a single axis. Cartesian printers also seem to be easier for people to understand because they mirror the Cartesian coordinate systems used in CAD programs (see Chapter 15).
Ways Cartesian 3D Printers Are Constructed
Despite the simplicity of the way Cartesian 3D printers handle positioning, their actual construction can be quite varied. The X, Y, and Z movement is only how the hot end moves relative to the print bed. The parts that are actually physically moving can be completely different between printers. One 3D printer model might have the print bed move in the X and Y directions, and the hot end move in the Z direction. Another printer might have the print bed move in the Y direction, and the hot end move in the X and Z directions.
This 3D printer has a bed that moves in the Y axis and an extruder carriage that moves in the X and Z axes.
The most common setup right now seems to be to have the print bed moving in one direction (either X or Y) and the hot end moving in Z and the other direction (either X or Y). But this is by no means a rule, and it’s not even necessarily the best method. That’s mostly because what’s best depends on your priorities and what you’re trying to achieve.
Cartesian Layout Considerations
So why do 3D printer manufacturers use different Cartesian layouts? Should it even matter to you? The short answer to the first question is that one 3D printer manufacturer might be trying to achieve something different than another manufacturer. The particular layout they choose to use can affect the cost of the printer, the quality of the prints, the speed at which it can print, the overall size of the printer, and the difficulty of building it.
Cost is usually the most obvious of the factors, and it’s certainly noticeable to both the manufacturer and the customer. The cost varies between different setups because of the materials needed and how powerful the motors need to be. If all of your movement is done by the hot end while the print bed stays stationary, the Z axis motors will need to be very powerful in order to lift the weight of all the components of the X and Y axes. And of course, if a particular layout uses more material in its construction, it will cost more.
The quality of the prints and the speed at which the printer can print are also inherently interrelated. Generally speaking, print speed is limited by print quality. Most printers are capable of physically moving much faster than they actually print. It’s a matter of how fast a printer can move while maintaining acceptable print quality. These two things are largely determined by the amount of mass being moved and how well the frame of the printer can handle the stress of that mass.
Print quality is affected by moving mass because of simple inertia. If you remember your physics lessons, you’ll recall that inertia increases proportionally with mass. Inertia is the resistance of a mass to change in its current state of motion. The reason this affects print quality is pretty straightforward: if you’re printing in one direction and need to change direction (for example, when you reach a corner), inertia will resist that change. And because inertia is related to mass, that change in direction will be more difficult as more mass is added. So the printer will have a tendency to overshoot the corner, resulting in poor print quality on that corner.
This is why speed is a significant factor as well. If the moving mass is high on that axis, the printer may not be able to turn that corner quickly at high speed. But at lower speeds, the effect will be reduced, resulting in better print quality. The lesson here is that you can print at faster speeds as moving mass is lowered while maintaining the same print quality.
FASCINATING FACT
Backlash (an undesirable delay in movement during direction changes) produces somewhat similar effects to those caused by poor rigidity, but is a separate phenomenon with a different cause. Backlash is caused by looseness in the interfacing parts of the linear movement systems used on 3D printers. When that looseness is present, there is a small delay before the system is engaged when the direction of an axis’s movement is changed.
But what does that have to do with the particular layout used for a given 3D printer? It means that printer manufacturers try to reduce the moving mass on the X and Y axes, which is why that movement is often divided between the hot end and the print bed. That way, the one axis only has the mass and inertia of the extruder to deal with, while the other axis only has to handle the mass and inertia of the print bed. This allows you print at higher speeds while maintaining an acceptable print quality.
The size of the printer is also directly affected by the layout chosen, especially when you consider the total area needed by the printer when the print bed moves. If the print bed were to move in both the X and Y directions, the printer would need four times the area of a completely stationary bed. This is simply because each axis would need to be twice as long as the bed in order for the nozzle to reach every point on the bed. So if the goal is to produce a very compact 3D printer that takes up very little desk space, a stationary bed with all movement done by the hot end would be ideal. Of course, that would lead to a lot of moving mass, and potential print speeds would be lower.
The size difference between a Printrbot Simple (left) and a LulzBot TAZ 4 (right). Note, however, that the Printrbot has a 6×6-inch bed, while the LulzBot has a 12×12-inch bed.
How all of this affects the difficulty of actually building the printer should be readily apparent. The more complex the design of a 3D printer, the longer it will it take the manufacturer to assemble it (or you, if you buy a kit). Between this and all of the factors involved, it’s obvious that 3D printer manufacturers have a lot to consider when designing a printer.
But should you be concerned with what layout a particular model uses? It does matter, for the reasons I’ve gone over in this section. But it’s difficult to determine real-world results based on the layout alone. That’s because things like the inertia and momentum can be counteracted, and one of the best ways to do that is with a high-quality frame.
The Importance of Frame Construction
How the 3D printer is actually constructed is one of the most important factors when it comes to print quality and reliability. Every other part of the printer could be perfect, but if the frame is poorly constructed, the results will be very poor.
A 3D printer frame doesn’t just need to hold the other parts together; it also has to keep them stable and aligned. It has to hold up to the forces of momentum and inertia caused by the mass being moved around the printer’s axes, as well as keep the axes properly aligned at all times and maintain the calibration of the printer under the stress of constant movement and vibration.
Rigidity and How It Affects Quality and Reliability
The single most important characteristic of a 3D printer frame is rigidity. Flexibility in the frame is the biggest enemy of print quality. If there is any flex in the frame, the momentum of the moving parts will result in poor print quality, unless you print at slow speeds.
Reliability is also affected by the rigidity of the frame. 3D printers require calibration in order to produce high-quality prints. Calibration involves, among other things, setting a Z height in a “Goldilocks zone.” If the Z height is too high, the first layer won’t adhere properly to the print bed; if it’s too low, you won’t be able to extrude a clean and solid line.
The acceptable range for Z height is very small; usually, it needs to be within 1/20 of a millimeter for good results. A Z height outside of that range will result in failed or poor-quality prints. To keep from having to frequently calibrate the Z height, a 3D printer should be capable of maintaining your calibration for a long time. A flexible or loose frame will cause your Z stop, Z axis components, or hot end to move slightly over time. That slight movement means that you will have to constantly recalibrate your printer to continue to get good results.
What Makes a Good Frame
So you now know why it’s important for the frame to be rigid, but what makes a high-quality and solid frame? Rigid frames have a few characteristics in common:
The design of the frame structure: 3D printer frame structure design is a fairly complex subject. Because of the complexity involved in designing a frame structure optimized for rigidity, a lot of 3D printer manufacturers take the overkill approach by using heavy-duty materials for the frame. The best way to be sure you have the most rigid frame possible is to make it fully boxed, with each frame piece connected at both ends to another part of the frame. Other more unconventional designs can still yield good results, but difficulty of engineering them makes them less common.
HOT TIP
Your intuition might be your best tool when it comes to determining the quality of the frame. Ideally, you want to actually get your hands on the printer and see how it feels. If you can flex it with your hands, it’s probably not a good choice. But even just looking at it should give you a good idea: if it looks nice and sturdy, it probably is.
The connections between frame components: Even the most ingenious frame design is going to fail if the frame is held together with duct tape. Frame pieces should be connected together with a strong material that resists flex, while the connections themselves should be designed to resist movement in all directions. A simple flat sheet metal 90-degree connector will do a great job of resisting flex and movement parallel to its plane but will do a poor job when a perpendicular force is applied to that plane. To resist forces in all directions, connectors should be 3D instead of flat. Or, if flat connectors are used, two should be used together in a perpendicular orientation.
The frame material: In essence: strong materials are good. Metal is better than wood. Steel is stronger (but heavier) than aluminum. Plastic could be okay if it’s a high-quality plastic and the frame is well designed.
The best possible performance would be achieved with a big, heavy, cast-iron frame. These are the kinds of frames used in heavy-duty machine tools, because they’re incredibly rigid and strong. However, the weight and cost make them pretty impractical for use in 3D printers (especially for consumer desktop printers).
A safe and common frame material is standard t-slot aluminum extrusion. This is a high-quality structural material used in a variety of industries. It’s popular in 3D printer construction because it’s fairly inexpensive, easy to find in all kinds of sizes, easy to work with and to connect parts to, and pretty strong for its weight.
T-slot aluminum extrusion is a very versatile construction material used in a wide variety of applications, including 3D printer frames like this one.
Wood is also used pretty frequently, mostly because it’s cheap and easy to laser cut and work with. However, wood probably isn’t the best material to use in 3D printing. It’s hard to achieve and maintain dimensional accuracy in wood parts, especially because they can expand or contract in the presence of moisture. Wood also tends to be at least somewhat flexible, which, as you know by now, is a bad thing.
Plastic and sheet metal frames can both be acceptable as long as they’re well designed. They should be fully boxed and preferably reinforced. Plastic frames should be made from a sturdy and rigid plastic, making acrylic a popular choice.
Size Matters
Now that I have all of that dense engineering stuff out of the way, I can move on to a more straightforward topic: the size of the printer itself.
There is no denying the benefit of a large 3D printer. The bigger the printer, the bigger the parts you can print. Having the ability to print large objects is certainly useful. However, the size of a 3D printer influences more than just the size of the objects you can print.
The larger a printer gets, the more it’s going to cost. This is partially because of the obvious material increase, but that’s not the only reason. Bigger printers mean more mass, which means more powerful motors are needed for movement. That additional mass also means the frame needs to be stronger to resist flex. The longer smooth rods (see Chapter 6 for more on smooth rods) also need to be thicker so they don’t sag.
Those powerful motors and a larger heated bed are also going to need more power. Not only does that mean you need a bigger power supply to feed them, but it also means you may need special control electronics that can handle the load.
Put that all together, and there are a lot of costs associated with increasing the size of a 3D printer. Subsequently, size usually ends up being the single biggest factor in the price of consumer FFF 3D printers.
Of course, price might not be the only downside to large 3D printers. Depending on how much space you have available, you just might not have enough room for a big 3D printer. A small printer that sits comfortably on the corner of your desk might be more suitable.
In order to figure out what size printer you need, you should ask yourself what size objects you’re likely to print. Big parts can take a very long time to print, use a lot of material, and increase your chances of some kind of print failure during the print—there is nothing worse than a 24-hour print getting ruined 1 hour before completion. But the usefulness of a large print area is hard to deny. The ability to print large parts when needed is very handy, and it will be equally capable of printing small parts the rest of the time.
HOT TIP
No matter what size printer you’re looking at, make sure the frame is nice and sturdy. Pay attention to the layout and look for any potential flaws, like the print bed or hot end moving on both axes. Look at the printer as whole, not just the individual components, to determine its quality.
The Least You Need to Know
- Cartesian 3D printers have mechanisms to move in the X axis, Y axis, and Z axis, allowing them to position the hot end anywhere in 3D space.
- Rigidity is very important in 3D printer design. Frames with flex will have poor print quality or will only be able to print slowly.
- The larger the printer, the bigger the parts you can print. However, larger printers tend to cost more.