Vacuum in Woodworking – Part 1
Vacuum – what is it, really? In woodworking, vacuum is mainly used for:
In wood processing, vacuum is used for:
- Drying sawn and construction timbers, and
- Impregnating wood.
Holding and lifting of wooden piecesVacuum is most often used for vacuum clamping on CNC routers and other machining equipment. Here, the holding force of vacuum is used to quickly and securely fasten various panel materials or solid wood parts during woodworking, without leaving pressure marks. Handling equipment and assembly robots also often get their holding force from vacuum.
Fig. 1: Atmospheric pressure depending on altitude. Source: Busch Vacuum Solutions.
Fig. 2: Standard pressure units used for vacuum. Source: Busch Vacuum Solutions.
Forces when clamping with vacuumWill the clamped work piece hold? Is the vacuum sufficient or is the vacuum pump's performance strong enough? These are questions that everyone has asked – at the latest when the spinning tool is approaching the clamped work piece. From a purely theoretical perspective, the pressure between the work piece and the machining table, supporting surface or suction cup is reduced during clamping by extracting air and generating a vacuum. The atmospheric pressure is now higher and presses the work piece onto the machining table, thus 'clamping' it. Physical pressure is defined as a force that affects a specific area. Pressure is thus a function of force and area (Fig. 3).
Fig. 3: Pressure is the force that acts on a defined area. Source: Busch Vacuum Solutions.
A solid wood board is placed on the CNC router grid table for processing, and a seal is created with a rubber seal that fits the dimensions of the board (Fig. 4). A vacuum pump extracts the air between the wooden board and the grid table within the sealed area.
Fig. 4: The pumping speed curve shows the relation between pumping speed and ultimate pressure. Source: Busch Vacuum Solutions.
40,000 N = 4,000 kg
Now the question is how powerful the vertical holding force of the vacuum has to be to securely clamp a work piece. The example calculations show that the holding force for panel materials with large surfaces can be evaluated less critically than smaller wooden pieces or materials that are air-permeable or have uneven surfaces that lead to leaks. In practice, the necessary holding force is difficult to determine because different factors like operational or clamping factors, cutting and feed speeds and directions, materials and frictional resistance have to be taken into account, so rules of thumb are normally used.
Various possibilities for vacuum clampingAn essential distinction is made between three possible vacuum clamping procedures:
- Grid table
- Nesting method (suction technology)
- Console system with vacuum blocks
Vacuum technology parametersIn principle, there are three parameters that need to be considered when selecting the most suitable vacuum technology.
Ultimate pressureThe ultimate pressure indicates the vacuum pump's highest possible achievable vacuum level, or the lowest underpressure.
Pumping speedThis is understood to be the volume of air that a vacuum pump can extract within a certain period of time. The pumping speed is influenced by the geometry (volume) of the vacuum pump and its speed. The higher the pumping speed, the faster the air is extracted, and the ultimate pressure is achieved. High pumping speeds help compensate for leakage, for example, when using the nesting method.
Power consumptionMechanical vacuum pumps are usually driven by an electric motor. Depending on the construction principle and efficiency factor, vacuum pumps consume different amounts of energy.
All three parameters are dependent on each other. Firstly, the pumping speed decreases as the vacuum pump gets closer to achieving the ultimate pressure (Fig. 4). Secondly, a vacuum pump's energy requirement is lowest at a certain operating point. Designing vacuum generation perfectly is difficult for laypeople, which is why we always recommend consulting a vacuum specialist.
In the second part of our "Vacuum in Woodworking" series, we will take a closer look at the different types of vacuum generation.