Knowledge base
Moving at micron/µm level with air bearings
In the manufacturing industry there is a growing demand for (production) machines with a high precision. At STT Products we speak of high accuracy or high precision when we work at µm or sub-µm level (factor 10-6 meter). For your clarity, the diameter of a human hair is approximately 70 µm.
Building machines for µm level production
The more accurate a machine has to operate, the more external factors there are to take into account. Temperature fluctuations in the environment, vibrations, dust, airflow, geometric dimensioning and tolerancing, all affect the functioning of the machine. This high level of sensitivity makes it even more important to look closely at the materials and techniques that are used, because it is all interrelated. The machines is only as strong as its weakest part.
Granite is a widely used material in high-precision machines. The material has a number of advantages that allow it to limit the influence of external factors. Firstly, granite is very heavy and can therefore have a vibration absorbing effect.
Besides, granite is relatively insensitive to temperature fluctuations. When the environment get warmer or colder, granite will not expand or contract as quickly as other materials (especially metals). Steel expands 0.012 mm per meter per degree Celsius. With granite this is only 0.006 mm per meter per degree Celsius.
At last, a high dimensional accuracy can be achieved when processing granite. Granite can be made very flat by lapping (a type of grinding). This dimensional accuracy is essential is high-precision machines.
Moving at µm level
Linear or rotational moving parts in a highly accurate machine require a good approach. The drive, sensors and guidance all have to be tuned to the external factors.
First of all, it is important to take into account as many external factors as possible in the mechanical design. The aim here is to control as many degrees of freedom as possible. Thermal symmetry (see image) is a design is an example of this. The machine can grow or shrink under temperature fluctuations but the heart of the process is centred as much as possible. In addition, it is possible to work smartly with available margin. By pushing the margin into one direction, there is more stability on the other side. Avoiding an overdetermined construction is also a way of doing this.
A statically determinate construction is a construction in which exactly enough has been done to limit the degrees of freedom where this is desired. There is a balance between accuracy and manufacturability. Any extra addition to the construction makes it statically overdetermined.
A good example is a tripod stool. This stool cannot wobble and is sturdy, independent on the surface it is placed on, which is a statically determined construction. A stool with four legs is statically overdetermined, the fourth leg is not necessary for the stool to stand firmly. Even more, a four legged stool will make the stool unstable depending on the surface and legs. An overdetermined construction becomes unpredictable due to too much or too little tension.
A statically determinate construction is a construction in which exactly enough has been done to limit the degrees of freedom where this is desired. There is a balance between accuracy and manufacturability. Any extra addition to the construction makes it statically overdetermined.
A good example is a tripod stool. This stool cannot wobble and is sturdy, independent on the surface it is placed on, which is a statically determined construction. A stool with four legs is statically overdetermined, the fourth leg is not necessary for the stool to stand firmly. Even more, a four legged stool will make the stool unstable depending on the surface and legs. An overdetermined construction becomes unpredictable due to too much or too little tension.
High tech machine building with air bearings
In a recently developed high-precision machine, we applied two linear movements along an X and a Y axis.
Drive
Both movements are driven by a linear motor with an electromagnetic field. These motors consist of a magnet and a coil. By sending power through the coil, it will move away from the magnet. When the voltage changes, it will be attracted again, thus creating the linear movements. We used a round motor with single coil and a flat motor with multiple coils.
Positioning
In addition to movement, feedback is also needed about the current position. The sensors used for this purpose are preferably contactless. We opted for an optical sensor in combination with a glass scale. The glass scale has a resolution at nano level (10-9 meter). This is because a measurement is more accurate when the measuring instrument can measure one decimal more accurate than the desired size. The sensor can thus determine the exact position, after which the software can pass on any needed adjustment to the drive.
Air bearings
The guide reduces the friction and stress of the movement. In addition, the guide is important for accurate positioning of the shaft. All degrees of freedom, besides the desired movements, must be restricted. Hydrostatic, air or magnetic bearings can be used for non-contact guidance. We used air bearings for this machine.
In an air bearing, a constant air flow is applied between the bearing housing and the shaft, causing the shaft to float in the bearing. The most common are round air bearings, for rotational and/or linear movements along the shaft, and flat air bearing, only for linear movements.
The surface of the bearings is often made of porous graphite. Graphite is air permeable and can be manufactures with high dimensional tolerances, by lapping or grinding the surface. The surface is therefore always purely round or purely flat. Air bearings have great rigidity and load-carrying capacity and are therefore very suitable for high-precision movements.
Flat surface on air bearings
Why is having a purely flat surface so important? For the stiffness of the air bearing it is important to keep the distance from the bearing to the surface at <5µm. To avoid friction, any unevenness on both surfaces must be <2 µm.
Producing our own air bearings
At STT Products we develop and produce flat air bearings in all shapes and sizes for our machines ourselves. The advantage in this, is that we always have the perfect shape and size for the machine. Customisation is an added value, especially in high-precision machines, because all parts are in balance. Finally, it saves costs and lead time when we produce the air bearings in-house.