Basic Elements and Mechanisms of Machine Tools

Introduction

Metal-cutting machines (machine tools) are characterized by higher production accuracy compared with metal-forming machines. They are used for the production of relatively smaller numbers of pieces; conversely, metal-forming machines are economical for producing larger lots. Machine tools constitute about 70% of the total operating production machines in industry. The percentages of the different types of operating machine tools are shown in Table 2.1.

The successful design of machine tools requires the following fundamental knowledge:

• 1. Mechanics of the machining processes to evaluate the magnitude and direction and to control the cutting forces
• 2. The machinability of the different materials to be processed
• 3. The properties of the materials used to manufacture the different parts of the machine tools
• 4. The manufacturing techniques that are used to produce each machine tool part economically
• 5. The durability and capability of the different tool materials
• 6. The principles of engineering economy

The productivity of a machine tool is measured either by the number of parts produced in a unit of time, by the volumetric removal rate, or by the specific removal rate per unit of power consumed. Productivity levels can be enhanced using the following methods:

• 1. Increasing the machine speeds and feed rates
• 2. Increasing the machine tool available power
• 3. Using several tools or several workpieces (WPs) machined simultaneously
• 4. Increasing the traverse speed of the operative units during the nonmachining parts of the production time
• 5. Increasing the level of automation for the machine tool operative units and their switching elements

TABLE 2.1

Percentages of Different Types of Operating Machine Tool

• 6. Adopting modern control techniques such as numerical control and computer numerical control
• 7. Selecting the machining processes properly based on the machined part material, shape complexity, accuracy, and surface integrity
• 8. Introducing jigs and fixtures that locate and clamp the work parts in the minimum possible time

Machine tools are designed to achieve the maximum possible productivity and to maintain the prescribed accuracy and the degree of surface finish over their entire service life. To satisfy these requirements, each machine tool element must be separately designed to be as rigid as possible and then checked for resonance and strength. Furthermore, the machine tool, as whole, must have an adequate stability and should possess the following general requirements:

• 1. High static stiffness of the different machine tool elements such as structure, joints, and spindles
• 2. Avoidance of unacceptable natural frequencies that cause resonance of the machine tool
• 3. Acceptable level of vibration
• 4. Adequate damping capacity
• 5. High speeds and feeds
• 6. Low rates of wear in the sliding parts
• 7. Low thermal distortion of the different machine tool elements
• 8. Low design, development, maintenance, repair, and manufacturing cost

Machine tools are divided according to their specialization into the following categories:

• • General-purpose (universal) machines, which are used to machine a wide range of products
• • Special-purpose machines, which are used for machining articles similar in shape but different in size
• • Limited-purpose machines, which perform a narrow range of operations on a wide variety of products

Machine tools are divided according to their level of accuracy into the following categories:

• 1. Normal-accuracy machine tools, which include the majority of general- purpose machines
• 2. Higher-accuracy machine tools, which are capable of producing finer tolerances and have more accurate assembly and adjustments
• 3. Machine tools of super-high accuracy, which are capable of producing very accurate parts

The main functions of a machine tool are holding the WPs to be machined, holding the tool, and achieving the required relative motion to generate the part geometry required.

Machine tools include the following elements:

• 1. A structure that is composed of bed, column, or frame
• 2. Slides and tool attachments
• 3. Spindles and spindle bearings
• 4. A drive system (power unit)
• 5. Work-holding and tool-holding elements
• 6. Control systems
• 7. A transmission linkage

Stresses produced during machining, which tend to deform the machine tool or a WP, are usually caused by one of the following factors:

• 1. Static loads that include the weight of the machine and its various parts
• 2. Dynamic loads that are induced by the rotating or reciprocating parts
• 3. Cutting forces generated by the material removal process

Both the static and the dynamic loads affect the machining performance in the finishing stage, while the final degree of accuracy is also affected by the deflection caused by the cutting forces.

Machine Tool Structures

The machine tool structure includes a body, which carries and accommodates all other machine parts. Figure 2.1 shows a typical machine tool bed for a lathe and a frame for a drilling machine. The main functions of the machine structure include the following:

• 1. Ability of the structure or the bed to resist distortion caused by static and dynamic loads
• 2. Stability and accuracy of the moving parts
• 3. Wear resistance of the guideway
• 4. Freedom from residual stresses
• 5. Damping of vibration

FIGURE 2.1 Typical bed of center lathe and frame of a drilling machine.

FIGURE 2.2 Examples of open frames (C-frames).

Machine tool structures are classified by layouts into open (C-frames) and closed frames. Open frames provide excellent accessibility to the tool and the WP. Typical examples of open frames are found in turning, drilling, milling, shaping, grinding, slotting, and boring machines (Figure 2.2). Closed frames find application in planers, jig-boring, and double-spindle milling machines (Figure 2.3). A machine tool structure mounts and guides the tool and the WP and maintains their specified relative position during the machining process. Machine tool structures must therefore be designed to withstand and transmit, without deflection, the cutting forces and weights of the moving parts of the machine onto the foundation. For a multiunit structure, the units must be designed to locate and guide each other in accordance with the required position between the tool and the WP.

FIGURE 2.3 Examples of closed frames.

The configuration of machine tool structure is governed by the arrangement of the necessary cutting and feed movements and their stroke lengths as well as the size and capacity of the machine. In this regard, chip disposal, transport, erection, and maintenance are also considered. The rate of material removal determines the power capacity of the machine tool and hence the magnitude of the cutting forces. The grade of production accuracy is affected by the deformation and deflections of the structure, which should be kept within specified limits. The assessment of the behavior of machine tool structure is obtained by evaluating its static and dynamic characteristics.

Static characteristics. These characteristics concern the steady deflection under steady operational cutting forces, the weight of the moving components, and the friction and inertia forces. They affect the accuracy of the machined parts and are usually measured by the static stiffness.

Dynamic characteristics. The dynamic characteristics are determined mainly by the dynamic deflection and natural frequencies. They affect the machine tool chatter and hence the stability of the machining operation.

The static and dynamic deflections of a machine tool structure depend on the manner by which the operational forces are transmitted and distributed and the behavior of each structural unit under operating conditions. A beam-like element, having a cross-section in the form of a hollow rectangle, is the preferred element. A typical application of this concept is given in the lathe bed shown in Figure 2.4; the adverse effect of cast holes on the stiffness of the closed-box cross-section is minimized by reducing their number and size. As can be seen in Figure 2.5, closed-frame structures, although deformed under load, keep the alignment of their centerline axes unchanged. This, in turn, results in an axial (not lateral) displacement of the tool relative to the WP, which does not affect the accuracy of machined parts. An open frame can, therefore, be supplemented with a supporting element to close its frame during the machining operation, as shown in the radial drilling machine in Figure 2.6.

FIGURE 2.4 Hollow box sections of the lathe bed.

FIGURE 2.5 Deformation in open and closed frames.

Machine-tool stiffness and damping of the machine-tool structure depend on the number and type of joints used to connect the different units of the structure. As a rule, the fewer the joints, the greater the stiffness of the structure and the smaller its damping capability. The ribbing system is an effective method for increasing the stiffness of the machine tool structures. In this regard, simple vertical stiffeners, seen in Figure 2.7a, increase the stiffness of the vertical bending but do not improve horizontal bending. The diagonal stiffness arrangement, shown in Figure 2.7b, gives higher stiffness in both bending and torsion. In some cases, to eliminate the tilting movement that usually acts on the tailstock of the lathe machine, raised rear guideways are introduced, as shown in Figure 2.8. Machine-tool frames can be produced using cast or welded construction. Welded structures ensure great savings of

FIGURE 2.6 Radial drilling machine with end support.

FIGURE 2.7 Arrangement of stiffeners in machine tool beds: (a) vertical and (b) diagonal stiffeners.

FIGURE 2.8 Lathe bed with raised rear suideways.

FIGURE 2.9 Cast and fabricated structures: (a) cast and (b) welded machine-tool bases.

material and pattern costs. Figure 2.9 shows typical cast and fabricated machine-tool structures. A cast iron (Cl) structure ensures the following advantages:

• • Better lubricating property (due to the presence of free graphite). Most suitable for beds in which rubbing is the main criterion
• • High compressive strength

• • Better damping capacity
• • Easily cast and machined

Light- and Heavy-weight Constructions

Machine tool structures are classified according to their natural frequency as light- or heavy-weight construction. The natural frequency 0 of a machine tool can be described by

where

к = structure static stiffness m = mass

where

F = force (N)

6 = deflection (mm)

To avoid resonance and thus reduce the dynamic deflection of the machine-tool structure, w₀ should be far below or far above the exciting frequencies, which are equal to multiples of the rotational speed of the machine.

If the natural frequency of the machine structure is kept far below' the speed working range of the machine tool, then

or

This requirement is achieved by the increase of the mass m, w hich, in turn, leads to a heavyweight construction. On the other hand, lightweight constructions are made when

or

Chip disposal, in the case of high-production machine tools, affects the construction of the machine tool frame as shown in Figure 2.10.

FIGURE 2.10 Chip disposal in a lathe bed.