When you see these objects printed out, do you ask yourself what is a 3d printer and how does it work ? Do you ever wonder what is under the covers? if you do then you are in the right place to find out what’s inside and how most of these printers work.
3d cad drawing of jet engine by catia5ftw (thingiverse)
Imagine drawing a picture on a piece of paper, you can, with a little skill give the illusion of having depth to your drawing.
With a lot of skill you can really fool the eye into thinking that the object on the sheet is real. You could almost pick it up off the sheet and hold it in your hand.
3d printable jet engine by catia5ftw
Now, think of really picking something up off the build platform of a printer and being able to rotate it to see all of the sides. How it feels in the hand. How it looks with other objects. This is 3d printing.
It is amazing that in the last 20 years we have now started to accept the fact that we can print off objects which are in a three dimensions in the home.
So now with a CAD program you can draw on the screen in 3d, you can save the finished object, you then pass it through another program which slices it into layers, then you send it to a printer, which, after heating up will extrude a thin string of plastic filament. Creating, layer by layer, the object which you drew on the screen in CAD.
Think of a pad of paper. What you have is layers of paper building up a thickness by having multiple sheets laid on top of each other. This is the way a 3d printer works, the slicer program defines the path of the print head (extruder) on each page , laying the thin string of plastic filament down in such a way as to build up the model from the build platform up.
How does it work?
The 3d printer is made up of a few systems, all of which need to operate together to produce accurate prints. The main systems are detailed below, they consist of :
The printer uses a controller, which interprets the data passed to it from the computer. The data forms the object you drew in 3d space. This means giving every point defined a X,Y and Z co-ordinate. The controller has the job of feeding these co-ordinates to the three axis of the printer. The three axis of the printer are moved by stepper motors, using either drive belts or lead screws. The stepper motors will rotate a certain angle for a ‘step’ input. The rotation of the stepper motor is fed onto a belt or lead screw, translating it into linear movement. This linear movement either moves the print head or the build platform to the desired position.
The controller also heats up the build platform heater and the hotend of the extruder to a precise temperature.
The controller accepts signals from the end stops, this defines where the virtual 0,0,0 or home point is on the printer. It is from here all of the points are defined for the model.
The stepper motors are as the name implies motors which rotate in a series of steps. Moving the shafts by hand you will feel a ‘notchy’ feeling as you force the shaft round. This is the magnets within the motor repelling the windings. Dependent on the number of magnets, and windings you can have a number of steps per revolution. In other words how many steps it takes to complete one rotation of the shaft. If the motors have 200 steps per revolution and one revolution has 360 degrees, every step of the motor is 360/200 = 1.8 degrees.
The motor has two coils internally fitted at 90 degrees to each other. When you feed pulses to each of these coils in quadrature phase you can move the motor in full steps as shown in the diagram.
The rotation of the shaft is translated either by a gear wheel or a lead screw into movement within the printer.
The heaters
There are two heaters within most 3d printers.
You have the build platform heater, this heats the build platform to prevent stress build up in the plastic. As you have the plastic cooling to ambient temperature the plastic will shrink. This shrinkage with the hot plastic being placed on the top of it can lead to warping of the model. The model may detach itself from the build platform or just bend at the edges. How this is reduced is by heating the build platform. This reduces the difference in temperature of the model and the newly deposited plastic. Thereby reducing the stresses acting on the model and the model stays flat on the build platform.
The heater is controlled from the controller. The controller knows the temperature of the build platform from a thermocouple fitted to the build platform. The thermocouple produces a voltage related to the temperature it is exposed to. This is read by the controller and it then knows when to stop the heating input. This temperature is controlled with one degree centigrade.
The other heater is in the hot end. This is the part of the printer which produces the thin string of plastic. The heater is typically a cartridge heater ( cylindrical heater) with two wires coming out of it. The heater is usually made from nichrome wire, this is insulated from the case to prevent short circuits. The outer case is normally stainless steel.
If the a voltage was applied across the heater without control, it would continue heating until it destroyed itself. So like with the build platform a feedback voltage from a thermocouple is fed back to the controller to tell it what the temperature of the hot end is. This can be controlled with 1 degrees centigrade.
The different types of plastic which can be used fro building the objects on the 3d printer require different temperatures. This is the reason you set the hot end temperature in the slicer program, or it is from a preset for the material. You may set the first layer slightly hotter than the rest of the layers to help with build layer adhesion. As the first layer is going onto a different material, glass or metal and you want it to stick to it you heat it up more to allow the plastic to adhere better.
To know where the print head is you need a reference point. Even inkjets or laser printers need to know where the start of the line is. Without this the print head would deposit ink wherever it wanted and you would be sorely disappointed with the end result.
As the print head moves across the printer, on a 3d printer, it will activated a switch at some point. This is defined as it’s end stop. When you select the auto zero function within the menu structure or a G code (the data the printer requires to form the 3d object) is seen by the controller the printer knows to go in certain directions until it reaches the switches at the end. A lot of the time the printers will activate the switch and change direction. This will activate and then deactivate the switch. As the motor is accelerating towards the end stop it will over shoot. If you then stop the motion and reverse then slowly come back out of the end stop you react the deactivation point more reliably and therefore have more accuracy with the zero or home position.
The signals from the end stops on the X,Y and Z axis are sent to the controller board.
The extruder
The plastic filament is produced in either 1.75mm or 3mm diameter. If you used this in its original diameter you would struggle to make intricate details on models. You also need a method of pushing the filament out of the hotend in a continuous length. If you melted the filament and allowed gravity to pull it down then you would not have an even string of plastic.
What has been developed, and is continuing to be developed, is a method of using the filament itself as a plunger to force the molten plastic out of the hotend.
The hot end will have a small aperture, 0.3mm to 0.6mm, dependant on the detail you want. This nozzle is fitted to the hot end. A threaded tube with a PTFE inner sleeve is in contact with the nozzle.
The nozzle is heated via the cartridge heater and the filament melts. The filament goes through several stages while melting from the solid to a flexible then to molten.
This flexible stage occurs within the PTFE tube. This effectively seals the filament within the PTFE. As the PTFE is very slippery this forms a plunger forcing anything in front of it out of the nozzle with te force being applied via a stepper motor. This stepper motor rotates enough to extrude the plastic filament at a steady rate. This is what defines the maximum speed of the printer. If you cannot melt the filament and extrude it at a steady stream you will get a blobby finish and you will need to lower the mm/second speed of the printer to get better results.
The belts
There are normally two belts on the 3d printer. One controlling the movement of the X axis, the other controlling the movement of the Y axis.
There is a gear wheel connected to the shaft of the stepper motor. Around this is fitted the belt. This extends across the axis, around a pulley and back to form an almost continuous belt. The ends of the belt are tied to the movable part of the axis, in the case of the X axis the extruder, and in the case of the Y axis the build platform.
The belts have teeth on the surface which is in contact with the gear. With the correct tension the belt moves correctly.
If, however, the belt is a little loose or the motor moves too quickly the belt can ‘slip’ a tooth. This causes an error in the axis leading to the layer being mis-registered. The model looks like it has been shifted in that axis.
To correct this you will either need to tighten the belt or slow down the printer speed.
These belts are used where the axis need to move at speed, using a lead screw here would limit the speed of the axis and therefore the print time.
The lead screw
On the Z axis you will normally find threaded rod, or lead screw. Some models use allthread bar and two nuts tensioned away from each other, but more commonly now are one or two lead screws controlled by stepper motors.
If there are two stepper motors these need to be matched both in rotation and screw pitch. This can lead to errors in layer height over time and needs to be checked periodically with a test print.
A lot of the newer printers are only using a single lead screw on one side, this gets rid of this problem.
The lead screw is able to position the build platform with increased resolution, so in a 3d printers specification you will see that the Z axis resolution is higher than the X axis or Y axis. The layer resolution is limited by the angular rotation of the stepper motor and the pitch of the lead screw. To improve the resolution of the layers you could reduce the thread pitch of the lead screw.
The frame
The frame is what holds all of the mechanics together. It holds the ends of the X axis along with its motor , the Y axis along with its motor and the Z axis motors. The controller is normally screwed to the frame, in certain printers to try to reduce its footprint this is separate. The mains power supply is fitted to the frame to make a compact unit.
The frame must be rigid with the movement of the print head moving and stopping and moving again. Flexing in the frame will cause errors to be seen within the object being printed. Placing the printer on a level surface will help with accuracy of the print.
Putting it all together
These are the main parts of a 3d printer, all of them have to work together to get accurate prints from your printer. Most of the time your printer will work without any problems. An occasional adjustment for the Z axis or cleaning of the nozzle. But if you get larger problems you may find it useful to know how the printer systems work.
Common problems will be detailed in another post, along with solutions.
Note that all of the above is in relation to a FFF (Fused Filament Fabrication) printer, the stereo lithography, and digital light processing models work slightly differently. They are detailed in this post.
I hope that this has helped you understand some of the technology which goes into a 3d printer, the complexity of the systems which all have to work together to produce the amazing print which you see at the end. If you have any comments on what is a 3d printer and how does it work leave them in the box below and I will answer them.
