## 3 Mechanical Vibration Tests
### Vacuum Clamping System  
Vacuum clamping technology is a popular and effective method used in CNC (computer numerical control) machining to securely hold workpieces in place during the machining process. This technology relies on creating a vacuum between the workpiece and a clamping surface, typically a perforated table, to generate a strong holding force.  
Vacuum clamping systems consist of a perforated worktable and a vacuum pump. The workpiece is placed on the perforated table, and a vacuum is created underneath the workpiece by the vacuum pump. As air is evacuated from the space between the workpiece and the table, atmospheric pressure forces the workpiece against the table, securely holding it in place. The schematic drawing of a vacuum clamp is illustrated in Figure 10.  
As the atmospheric pressure is 101,325 Pa (approximately 0.1 MPa or 10.1 N/cm\({}^{2}\)), the possible clamping force per 1 cm\({}^{2}\) in an ideal vacuum will be equal to 10.1 N. In practice, commercially available vacuum pumps allow 80 to 90% of this value (8.1...9.1 N/cm\({}^{2}\)). For example, a workpiece with an area of 100 \(\times\) 200 mm could be attached to the holder with a force of around 1600 N. Although this value is 8.4 times lower than the force exerted by a single M6 grade 10.9 bolt, it is often sufficient to hold a workpiece stationary during machining.  
The clamping force was also tested experimentally on a rig (Figure 11) using a steel plate measuring 100 \(\times\) 150 mm attached to a vacuum clamping system. The detachment force was set by an M12 screw, with the average break force at 936 N. The theoretical force was in a range of 947...1064 N, due to the smaller area (117 cm\({}^{2}\)) on which the vacuum was acting. Equation (1) is as follows:  
\[\mathrm{F}=(8.1\div\ 9.1)\frac{\mathrm{N}}{\mathrm{cm}^{2}}\cdot 117\mathrm{cm}^{2 }=(948\div 1064)\ [\mathrm{N}] \tag{1}\]  
The area being smaller than the resulting directly from the steel plate's dimensions was due to the grid size (12.5) and the slot size (4 mm) of the vacuum table. The actual surface area of vacuum interaction on the test plate was (6 \(\times\) 14.5 \(-\) 4 = 83 mm \(\times\) 10 \(\times\) 14.5 \(-\) 4 = 141 mm).  
Figure 11: Vacuum clamp during clamping force test: (**a**) vacuum clamp and (**b**) test stand.  
Figure 10: Schematic drawing of the cross section through the vacuum clamp (on the base [35]).  
The masses of vacuum clamping plates from various manufacturers range from 0.005 to 0.008 kg per 1 cm\({}^{2}\) for clamp heights of 20 or 30 mm, amounting to around 18 to 29 kg for a 600 \(\times\) 600 mm plate.  
The advantages of vacuum clamps include the following:  
* Versatility: ability to hold various materials, such as non-ferrous metals, wood, plastics, and composites;
* No point of mechanical contact: eliminates the risk of deformation or damage due to clamping;
* Reduced setup time: quick adaptation for different sizes and shapes of workpieces.  
The disadvantages of vacuum clamps include the following:  
* Limited holding force: lower compared to electromagnetic chucks or mechanical clamps;
* Dependence on seal quality: effectiveness relies on the quality of seals and the workpiece surface;
* Higher operating costs: continuous energy needed to maintain the vacuum;
* The DUT size is much bigger than the size of armature;
* Complex setup: vacuum clamps require more technical knowledge and initial investment in equipment.