In order to get the print head to move in all sorts of different directions there needs to be a method to transmit the power from the stepper motor to the print head. For the vast majority of printers on the market this is achieved through grooved belts. Belts are lightweight, simple to use, and are cheap.
There is another way to drive the 3D printer, that is with lead screws. Lead screws are designed to take rotational motion and transfer it into linear motion. On many 3D printer build a lead screw drives at least one axis, typically the Z-Axis. Lead screws are viewed to be slow and cumbersome when compared to belts. However they have one big advantage, they can be accurate.
Speed vs. Accuracy
1/2-10 single start lead screw will advance .100in per full revolution. Per full step of a 1.8deg 200 step/revolution motor, that is .0005in. A 1/2-10 two start lead screw with advance .200 per revolution, giving a per step revolution of .001in. With microstepping these steps can be divided by the degree of microstepping to provide even higher resolution at the cost of reduced torque.
In terms of speed, a 1/2-10 single start lead screw can move 1 inch per second at 600 revolutions per minute this is equal to 25.4mm/sec. With NEMA 17 Stepper Motors, it seems the max useful RPM is 600. In the 3D printing world this is fairly slow, but if we move to a 1/2-10 double start lead screw we can double the travel in the same RPM. At 600 RPM the head would move 2in/sec or 50.8mm/sec. This speed seems to be on par with many 3D printers on the market.
We can compare this to a belt and pulley set up. If a 16 tooth wheel is used on a 2mm pitch belt. Each full revolution of the pulley will move the print head by 32mm (1.25in). A typical stepper motor with 200 steps/rev will have a resolution of .006in or about .15mm. That is a decrease in resolution by a factor of 6 over a 2 start 1/2-10 lead screw. The potential speed is increase as it only takes .8 revolutions to move 1in or 25.7mm. To move at 2in/sec the stepper motor would only need to turn 96 revolutions per minute. Well within the capability of the motor.
There are other things that limit the speed of a 3D Printer, for example the print head extruder can only heat and extrude the plastic filament so fast. So even if everything else is blazing fast you end up being limited by the extruder. Even still, it is clear as to why belts are the choice for fast printers.
The same holds true for the limitations on accuracy. How accurate is a 3D printer part? Typically an average printer can hold +/-.019in or .5mm tolerances. This is due to many factors, the positional accuracy of the print head not typically being a major factor. Extruding molten plastic can be unpredictable, couple with the fact that plastic contracts when it cools, and uneven cooling can cause warping. So even though the lead screw is pretty darn good about getting things positioned correctly, there are other factors that will hinder the potential accuracy of the printer as a whole.
Power Transmission
Lead screws are designed to transmit torque, they move more slowly but they do so with more authority. With light 3D printer heads, torque is not as big of a deal, however when moving CNC routers or light duty milling heads, the torque transmitted by the lead screw not only moves the milling head but also provides the force needed for cutting the material. They tend to be better options when you need to move a high load to an accurate position.
Belts can do everything a lead screw can do, faster, and tend to be more efficient, but they have their drawbacks. Belts can be susceptible to changes in temperature causing them to contract or expand, if not properly compensated for this can mess with timing. Also belts can skip a tooth under high loads to break if subjected to sudden shock. For this reason belts are typically relegated to lighter duty applications.
What will I be doing on this build?
You may have sensed it with the tone of the article, but I am leaning heavily towards using lead screws for all of the Axis for this build. For the follow reasons;
- High Positional accuracy potential
- Will handle high loads associated with a light duty mill head better, large printer head
- Low maintenance
Part of the design of this printer is being able to have something that will also serve as a platform to do some light duty milling. As with most multipurpose machines, you might be able to do a lot with them, bu they may not be able to excel at any one thing. For this project I will be sacrificing speed, for rigidity and accuracy.
Design Changes
I’ve had to put a lot of thought into how I want to design the drive train. Everything from where to position the linear rods to maximize print space, to how to mount everything. This has lead to a lot of changes in the frame, the gussets, and how everything is mounted.
I really wanted to keep the form factor of the printer to a 26x26x26. It is close, but not exactly. In all likelihood once the panels are installed it will be closer to 27x27x30 inches.
I know the X-Axis carriage is going to be heavy, and I am concerned that a single Nema 17 Stepper rated for 82in/oz torque will have issues. I decided to double up on the motors. Besides added cost, and a little complexity, I cannot find a downside to doing so. I kept the dual motors on the Z-Axis which is typical for 3D Printers.
Where it made sense to do so, I put the linear rods within the frame. I figure this will help add rigidity to the frame.
Since I do not have a mill, or a 3D Printer, I’ll be using alot of Baltic Birch. It is a stable and strong plywood that is popular in the marker world. It’s also pretty easy to work with using basic tools, and it’s light.
80/20 is expensive and the 80/20 specific hardware is also quite expensive, so wherever I can I’ll be making due with hardware I make or source. This is too try and keep the costs down.