Influence of foundation foundation on machine tool precision
Editorial department



The large components of most machine tools, such as beds, columns, etc., whose working accuracy and longevity are directly affected by the foundation of the foundation.

The machine tool's working accuracy is affected by the foundation system. The basic system mainly includes: basic structure, installation location and environmental conditions (both the location of the building and the weight inside the factory and the vibration isolation effect), as well as various components of the machine tool ( Installation conditions and installation methods for bed, base, column, etc. The design and installation methods of the machine tool foundation must pay attention to the corresponding measures.

The effect of the foundation on the accuracy of the machine tool is explained below. The basis of most machine tools is the concrete block structure, which must have:

1) have sufficient strength and rigidity to avoid strong external vibrations affecting its accuracy, performance and life;

2) It has stability and durability, prevents oil and water erosion, and ensures that the machine tool and foundation are not partially sunk;

3) Separating the machine from the surrounding source;

4) The cast iron plate foundation of large machine tools needs to be pre-pressed before installation.

The preload weight is 1.25 times the sum of the dead weight and the maximum load. The pre-pressed material is evenly pressed against the foundation to ensure that the foundation is uniformly sunk. The preloading work should be carried out until the foundation does not continue to sink.

The main factors affecting the accuracy of the foundation of the machine are as follows:

1) The quality of the foundation (geology).

Foundations of different natures have different effects on the machine tool foundation. The mass of the foundation refers to its strength, elasticity and stiffness. The strength of the foundation is related to its structure and the depth of the foundation. The tilt of the machine tool base has little effect on the accuracy of the machine tool. However, if the foundation sinks locally, the impact on the accuracy of the machine will be great. Such as a factory 7A28-14 type planer. The bed foundation foundation has a total length of 30m, local sinking occurs at the end 3m, and three bed foot fastening screws are loosened, resulting in the vertical straightness of the tail of the bed from the original 0.02/1000mm to 0.05. /1000mm. This is due to insufficient geological strength. The general geological strength requirement is 5t/m2 or above. If there are deficiencies, it is necessary to use piling and other methods to strengthen.

2) Checking the strength of the foundation.

The maximum static pressure P0 applied to the unit area of the foundation foundation is less than the calculated strength P of the foundation below the foundation, which can be calculated by the following formula: P0=Q/F, where the Q-foundation is subjected to the maximum static load, including the weight of the machine tool, and the load. And the basis weight (t); F-ground bearing load area (m2). The calculated strength P is related to the soil type (foundation structure) and the base thickness.

3) Basic calculation.

In the refinement of machine tools, a large amount of rework is generally not performed on the original foundation. It is necessary to check the basis of the new installation of the machine tool. For building materials with a large machine base of more than 10 tons, 20% of the 200 stones are allowed to be added to the concrete in consideration of cost savings. The influence of ground vibration should be avoided as much as possible, so it must be installed on a separate block concrete foundation and an anti-vibration groove should be added as much as possible around it. The anti-vibration layer is generally filled with coarse sand or with a certain amount of protective slag.

4) Determination of the base thickness.

The thicker the foundation, the better. However, considering the economic effect, the foundation thickness is based on the principle of preventing vibration and deformation of the base body. For large machine tools with improved accuracy, the thickness of the cast iron platform is generally between 1000 and 2500 mm.

5) Ground stiffness problem.

The base thickness has a decisive influence on the stiffness. For most portal milling machines, the main request mid section has such stiffness to ensure the machine's working accuracy. However, for a gantry type moving milling machine, since the gantry moves over the entire length of the bed, the foundation foundation is required to have such a stiffness value over the entire length.

6) The influence of the basic load and surrounding weight on the static accuracy.

Large objects such as large workpieces and blanks are often placed and unloaded around the foundation of large machine tools, often causing local deformation of the foundation, thus causing changes in machine tool accuracy. Machine tools with better rigidity, such as vertical lathes, have a lesser degree of influence. For the foundation of the new pouring, due to the large change in concrete strength and unstable performance, do not install the machine in the last month. Check or adjust at least monthly during the year after the machine is installed. After installation of large machine tools, it is recommended to place railings around the machine to avoid stacking heavy objects.

7) The influence of basic vibration on machine tool accuracy.

The vibration source of the working environment of the machine tool comes from the influence of various conditions. Therefore, it is impossible to make a unified standard. Usually, the natural vibration frequency of the machine tool is about 20~25Hz, and the amplitude is in the range of 0.2~0.1μm. When the machine resonates or there is a source of vibration from outside, the amplitude will increase. Irregular vibrations will have an impact on machining accuracy. Therefore, the finishing machine should be as far away as possible from the beam or column, or take measures to isolate it from the beam and column (plus guard grooves).

The quality of the basic system of the machine tool has a great relationship with the accuracy of the machine tool. To improve the accuracy of work and its life, it is necessary to determine and verify the basis of the machine tool, and make necessary improvements according to the characteristics of the machine tool and the surrounding conditions.

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