This will be the research of various types of motion platforms that would best satisfy the criteria set out in the Cartesian 3D Printer Brief. A broad view of most types of motion platforms will be looked at in this blog post. A motion platform is how some of the components are put into motion. There are a large number of ways to do this all which depend on the design of the printer.
General Motion Criteria
There are aspirations to what the maximum performance specifications of the system should be. Here are what some of the ultimate goals of the motion platform are:
- High Speed
- High Acceleration
- High Reliability
- High Structural Integrity
- High Ease of sourcing parts
- Low Gantry Mass
- Low Part Count
- Low Cost
- Low Maintenance
Getting close to these ultimate goals is a serious engineering challenge. Generally there are sacrifices in some areas to improve others. It is about balancing the design.
There are a number of base kinematic motion platforms such as:
These are the main ones. From my own experience of building a few printers I have found Cartesian to be the most reliable and more likely to fulfill the criteria required in the brief so therefore will skip the research into Delta and Scara to speed up the research process.
The Cartesian coordinate system is based on the x,y,z whose linear range of motion are based on these axis.
Benefits of Cartesian in comparison to other motion platforms:
- Less computationally intensive on the control processor
- Mechanics can be simple
- Industry standard
- Tend to be more reliable of a system if machine is built well
- Simple in concept
Standard Cartesian Types
There are an exhaustive amount of combinations of Cartesian setups that can be done. Ranging from simple to complicated. All have advantages and disadvantages. Finding the right one for the GMT criteria is key. Firstly there are a number of ways the x,y,z axis are used and what part has linear motion on that axis.
X,Y,Z Axis Setup
- XY – Hotend, Z Heatbed
- XZ – Hotend, Y Heatbed
- X – Hotend, YZ Heatbed
- Z – Hotend, XY Heatbed
The two standard setups are (XY – Hotend, Z Heatbed) and (XZ – Hotend, Y Heatbed). Having built these two types of printer. I can say from experience that the (XY – Hotend, Z Heatbed) is the most suitable for the GMT printer. This is why:
- Tall Narrow Prints are a requirement. A moving heat-bed in the xy direction would be detrimental to the print as the print gets higher the more mass there is away from the heat-bed, this will create a leveraging effect between the print and build plate. A heatbed that travels in the z direction is essential to prevent this.
- Heatbeds are generally heavy, having it on the z axis were there is relatively little motion is suitable. Having the hotend on the xy which is where most of the motion happens allows for lower gantry mass. Which allows higher acceleration since force = mass times acceleration.
Standard (XY-Hotend, Z-Heatbed) Cartesian types
These types can achieve relatively high speed with right configuration. Here are the most common high speed, low weight gantry types:
- Ultimaker System
- Miscellaneous Cartesian System
Diagram of typical Hbot configuration:
Hbot uses one continuous length of belt to transfer force to the gantry and carriage. The direction and the rate of rotation of the stepper motors sets the direction along the x and y axis. Benefit of using this system is the potential low mass of the moving gantry due to the stepper motors being part of the chassis. The force vector arrows show a major flaw with the HBot system, that it has a racking problem. A moment is created which can be solved with a very rigid sturdy frame.
Diagram of typical CoreXY configuration:
This system is very similar to the HBot system, but the racking moment problem is solved by using two crossing belts.
Diagram of the CoreXY configuration:
This Cartesian system has been used in a number of successful printers such as the Ultimaker and Zortrax. Difficult system to convey in a two dimensional diagram. The gantry is set in motion by a linear shaft in the x and y orientation. These linear shafts are mounted at their ends on to perpendicular linear shafts. These perpendicular shafts are mounted on to bearings at either end which allows the shaft to rotate. Pulleys are mounted on to these perpendicular shafts on the far right and far left of the shaft. Stepper motors give rotation to these perpendicular shafts. As the pulley rotates they apply motion to the linear shafts which are connected to the gantry by means of two belts, one on each side. The benefit of this system is that it allows a very low gantry mass and there is no racking effect due to 2 belts being used for each axis. It is commercially proven system. A caveat to this system is that some of the bearings must be able to handle a rotational and linear motion, only bushings can handle this.
Miscellaneous Cartesian Systems
Here are two more diagram’s of ideas for Cartesian motion platforms:
A further exploration into what other configurations exist
Final Choice of Cartesian Motion System
I have decided on the Ultimaker style. Due to:
- Low gantry mass
- Proven with commercial/consumer printers such as the Zortrax and Ultimaker
- No racking problem due to belts on far sides of the rotating shafts
- Use of linear shaft’s which are low cost for even the precision variants
- Stepper motor not in motion, lowering moving mass
- Small lengths of belt required in comparison to a Hbot or CoreXY
- Less play inherent in the design
Problems that are inherent with the Ultimaker style that must be solved or minimized to ensure criteria is met.
Problems such as:
- Equally tensioned belts
- Linear bearings that have to be able to handle linear and rotational motion
- Rotational linear shafts
- Bending moment of the linear shaft
The ultimaker style gantry system will be the basis for motion platform for the GMTECH 3D Printer.