Machine tool structure are mostly designed from stiffness considerations and thus should possess high static and dynamic stiffness. As regards moving parts which travel along guide ways, the guiding and guided surfaces must have high wear resistance for high accuracy.
Materials for Machine Tool Structures:
Steel has higher strength under static and dynamic loading. Also the unit rigidity of steel under tensile, torsional and bending loads is higher. On the other hand, cast iron has higher inherent damping properties and has better sliding properties.
From manufacturing point of view, certain minimum wall thickness is required for cast iron, whereas welded structures of steel can have much thinner walls. Further the walls of differing thicknesses can be welded easily in comparison to casting where gradual fillet radii are to be provided. The machining allowances for cast structures are higher compared to weld steel structures.
Welded structure can be easily repaired and improved whereas such corrections on cast structure are difficult. In welded structures, the holes are obtained by welding but in casting, these are obtained by cores resulting in saving of metal in cast product.
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Thus steel is preferred for simple, heavily loaded structures required in small numbers. Cast iron is preferred for complex structures to be made in large numbers. Trend today is for combined welded and cast structures.
Types of Machine Tool Structures:
Most commonly used cross-sections are shown in Fig. 11.4. According to mathematical calculations, it is concluded that as far as rigidity in bending and especially in torsion is concerned, a section in the form of a hollow rectangle (i.e., Box Section) is the most rational.
The main requirement of bed, base or column is that it should maintain the proper relative positions over long period under all sorts of working conditions. Accordingly these should have sufficient strength and rigidity, use minimum metal and be easier to manufacture.
The configuration of bed depends on the arrangement of ways for various units, length of stroke of main units, necessity of housing various mechanisms inside the bed, various openings, apertures, etc. required in the walls of bed.
In practice box section is used with certain- modifications, as the strength and rigidity of the hollow frames is increased by incorporating ribs and partitions. This modification is specially necessary when the operating conditions of the components do not allow it to be completely closed so that it remains open on one or two sides e.g., lathe beds.
The effectiveness of partitions and ribs largely depends on their arrangement. A most commonly used cross-section of lathe bed is shown in Fig. 11.5. Ribbings are provided to offer maximum resistance to bending and torsion stresses and avoid massive sections in castings.
To basic types of ribbing are as follows:
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(a) Box ribbing.
(b) Diagonal ribbing.
The box formation (Box ribbing—refer Fig. 11.6) is convenient to produce, apertures in the walls permitting the positioning and extraction of cores. Diagonal ribbing (refer Fig. 11.6) provides greater torsional stiffness and permits swarf to fall between the sections.
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The strength and rigidity of lathe beds with parallel and diagonal ribs depends upon the following factors:
(i) Number of ribs.
(ii) Arrangement of ribs.
(iii) Thickness of main members of bed beam.
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(iv) Ratio of depth and length of bed.
(v) Ratio of width and length of the bed.
Fig. 11.7 (a) shows a cross-section of a cast iron bed for a heavy lathe.
The cutting forces are transmitted to the bed through the head-stock, saddle and the tail stock, and due to these the lathe bed is bent upwards at the head stock and the tail stock are bent downwards under the saddle.
Suitable section and proper material are the two factors mainly responsible for rigidity of any machine-tool.
As regards the material, usually bed, column and frame of machine tools are made of cast iron because it possesses certain advantages over other materials.
These are listed below:
(i) Cast iron possesses better lubricating property due to presence of free graphite in it and hence is most suitable for beds where rubbing is the main criterion. If required, hard surface can also be produced by induction hardening process.
(ii) It has high compressive strength and most of the beds and structures of machine tools are required to withstand compressive loads.
(iii) It possesses better shock absorption capacity.
(iv) In order to increase its hardness and reduce the effect of residual stresses, cast iron can be easily alloyed with nickel, chromium and molybdenum.
(v) It can be easily cast and machined.
However, mild steel and other alloys have the advantage over cast iron that they can be fabricated by welding, whereas cast iron cannot be welded easily. Moreover, intricate shapes cannot be cast from cast iron, these are prepared only by welding.
Modern welding developments have made it possible to produce fabricated steel structures as an alternative to cast iron and in many cases with advantage in regard to cost, lightness and time saving. Fig. 11.7 (b) shows a section of a fabricated lathe bed.
It is observed that box section is more suitable as it is strong in bending and torsion and can be easily produced. To avoid massive sections in castings carefully designed systems of ribbing are used to offer the maximum resistance to bending and torsional stresses.