A material removal process in which a sharp cutting tool is used to mechanically cut away material so that the desired part geometry remains. •Most common. PDF Drive is your search engine for PDF files. As of today we have 78,, eBooks for you to download for free. No annoying ads, no download limits, enjoy . Recommended Books. □ In English,. - Machine tools: design, reliability and safety. medical-site.infoon, New York: Nova Science Publishers, c -Handbook of .
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from book Handbook of Manufacturing Engineering and Technology The machine tools are discussed and categorized based on the. 50 Material Selection and Heat Treatment of Machine Tool Components. The book covers the technologies, machine tools, and operations of several. Researchers commencing their work on the machine tool and production sector may find this book useful. Finally, the authors would like to point out they have.
Move the tool bit to a safe distance from the chuck, collet, or face plate when inserting or removing your part. Place the tool post holder to the left of the compound slide. This will ensure that the compound slide will not run into the spindle or chuck attachments. When installing and removing chucks, face plates, and centers, always be sure all mating surfaces are clean and free from burrs.
Make sure the tool bit is sharp and has correct clearance angles. Clamp the tool bit as short as possible in the tool holder to prevent it from vibrating or breaking. Evenly apply and maintain cutting fluids. This will prevent morphing. Do not run a threaded spindle in reverse. Never run the machine faster than the recommended speed for the specific material. If a chuck or faceplate is jammed on the spindle nose, contact an instructor to remove it.
If any filing is done on work revolving in the lathe, file left handed to prevent slipping into the chuck. Always stop the machine before taking measurements. Stop the machine when removing long stringy chips. Remove them with a pair of pliers. Make sure that the tailstock is locked in place and that the proper adjustments are made if the work is being turned between centers. When turning between centers, avoid cutting completely through the piece.
Do not use rags while the machine is running. Remove tools from the tool post and tailstock before cleaning. Do not use compressed air to clean the lathe. Use care when cleaning the lathe. The cutting tools are sharp, the chips are sharp, and the workpiece may be sharp. Make sure the machine is turned off and clean before leaving the workspace.
Always remove the chuck wrench after use, avoid horseplay, keep floor area clean. Use care when cleaning the lathe, the cutting tools are sharp, the chips are sharp, and the workpiece may be sharp. When set too high the tool breaks down quickly, time is lost replacing or reconditioning the tool.
Too low of a CS results in low production. Notice the largest roughing cuts range from.
Cutting Tool Terminology There are many different tools that can be used for turning, facing, and parting operations on the lathe. Each tool is usually composed of carbide as a base material, but can include other compounds. Machine tool elements, mechanisms, tooling, accessories, and operations are also explained. Chapter 3 also presents abrasion machine tools, including grinding and surface finishing machines and processes. Chapter 4 describes the different types and applications of commonly used screw threads.
Thread machining by cutting and grinding methods are described, together with thread cutting machines and cutting tools. In Chapter 5, common types of gears are listed and their applications described. Gear production by machining methods that include cutting, grinding, and lapping are described, together with their corresponding machine tools and operations.
Machine components, features, and applications are described. Tool layouts for bar-type capstan lathes and chucking-type turret lathes are described and solved examples are given. Semiautomatic and automatic lathes are discussed in Chapter 7. Machine tool features, components, operation, tooling, and industrial applications are described. Solved examples for typical products that show process layout and cam design are given for turret-type and long-part automatics.
Chapter 8 presents computer numerical controlled machine tools, their merits, and their industrial applications. The basic features of such machines, tooling arrangements, and programming principles and examples are illustrated in case of machining and turning centers.
Hexapod mechanisms, design features, constructional elements, characteristics, control, and their applications in traditional and nontraditional machining, manufacturing, and robotics are covered in Chapter 9. Chapter 10 describes the fundamentals, instrumentation, and operation of machine tool dynamometers used for cutting force measurements.
Examples of turning, drilling, milling, and grinding dynamometers are explained. Chapter 11 presents modern machine tools and operations for mechanical nontraditional machining processes, such as ultrasonic and jet machining. Chemical milling, electrochemical machining, and electrochemical grinding machine tools are also described, along with the machine tools for thermal processes such as electrodischarge, laser beam, electron beam, and plasma arc machining.
Machine tools, basic elements, accessories, operations, removal rate, accuracy, and surface integrity are covered for each case. Environment-friendly machine tools and operations are described in Chapter 12; these tend to detect the source of hazards and minimize their effect on the operator, machine tools, and environment. The Programmable Logic Controller PLC which is included in the same hardware architecture today with the rest of the control cards, for enabling a high speed communication with the basic NC modules.
PLC controls the auxiliary machine functions generally coded as M functions in ISO , running a program implemented by the machine builder and taking into account the value of digital inputs collected from simple sensors placed in the machine. The PLC digital outputs are connected to switches or electrovalves to command the tool magazines, the coolant circuit, the pallets transfer system and other auxiliary devices.
Another important surveillance function is related to the end-of-stroke switches, placed at the ends of all guides to prevent moveable components from leaving the slideways: Nowadays, control manufacturers produce two or three basic CNC platforms which can include several options for different kinds of machines.
Therefore the machine tool manufacturer must adapt them to each specific machine model, following several steps: The definition of machine axes in relation to CNC ones, giving also the positive and negative senses and strokes of each axis. Control parameters of each axis must be adjusted to the machine dynamics.
PLC programming for each machine model with its user options. Customization of the user interface. General CNC control models are prepared for the milling machines, lathes or universal grinders most commonly sold. However, for specific applications special help utilities to prepare NC programs are absolutely necessary. A General View 33 5. Finally, tuning tests using idle movements and some machining tests are performed. These tests allow the fine adjustment of CNC parameters to machine to achieve the maximum precision and speed, as is explained in the next section.
To improve machining process reliability and facilitate the maintenance of machines, several sensors and monitoring techniques can be managed by the CNC. Spindle consumption, collision detection, thermal growth of spindle and the balance of the main spindle are monitored by the CNC in high end machines.
For this purpose some tests have been defined. An important result is the determination of the CNC compensation parameters for machine axes. Nonetheless, tests can also be used for several purposes, thus as acceptance tests by machine downloaders, for comparisons between similar machines, for periodical machine checking throughout machine life, etc.
Three kinds of tests can be mentioned. First, geometric tests for measuring straightness, parallelism or concentricity errors. ISO In the standards, methods for linear and rotary axes are suggested. When several rotation axes are simultaneously considered there are no standard tests, excepting those for bi-rotary heads in machines with a horizontal or vertical Z axis. Another important datum to achieve the best machine-CNC adaptation is the lag-error measurement using the internal utilities implemented in modern CNCs for monitoring the behaviour of the axis drives with respect to the axis positions.
Currently, milling tests are only focused on three-axis milling centres. But this test does not include complex surfaces, which is why different testparts have been designed in the last years, such as the so-called Mercedes or NCG parts available at NC-Gesellschaft association .
However, none of them are yet included in the ISO standard normative. Furthermore, there is no specific part for testing five axes machining centres, so customers have to define their own tests to check the machine they are interested in downloading, which is highly time consuming.
As was explained in Sect. In the following section some machines typical of important sectors are described; likewise other examples will be described in depth over the next chapters. The workpiece comes from forging, being disks or cases. Machining operations are turning, boring and milling. The maximum turning diameter can be up to 3 m and part weights of 30 tons are not uncommon. The plate rotation speed is in the range of rpm with a power of 50— kW. The ram stroke is long, more than 1, mm.
In the ram a turning tool or a milling headstock can be placed, in the latter case with a power ranging from 20 to 50 kW. Two machines of this type by P. Another example of a big machine with two rams mounted on the same screw is shown in Fig.
This is a drilling and milling machine focused on the drilling of circular rings for wind turbine support towers. Both rams are independent although they can work in parallel. A General View 35 Fig.
Therefore, machines must provide high metal removal rates, fast feedrates, greater accuracy and stability, increased overall productivity and finally, the flexibility to meet changing production volumes. Commonly produced parts are blocks and cylinder heads in iron or high silicon casting, component cases, gears, crankshafts, rods, or small pieces for the transmission and suspension systems. In Chap. Automotive components are usually machined in horizontal three-axis machining centres, provided with a B axis for part rotation or in some cases with a simple rotary indexing table Fig.
Horizontal centres bring benefits to an application which vertical ones are usually unable to, including faster power spindles and more power spindles, larger tool magazines, a smaller footprint and chip disposal.
Parts are held and fixed on universal pallets, which can be transferred by automated systems. In some cases the inclusion of a swivel head or rotation table enables the production of more complex shapes.
Very rapid idle movements between operations or tool positioning are also required. Spindles are in the range of 6—12, rpm powered with 60—90 kW motors to be able to move large diameter multi-insert rotary tools. Versions with a geared drive or a direct connection of main motor to spindle are possible, the torque being higher in the geared versions in the range of —1, Nm. Some machines include as an option two twin spindles and tables or drums to fix several parts in one setup; see Fig.
When this is done, the borderline between individual machines and transfer machines see Sect. On the other hand, vertical centres are applied to wheel, steering, brakes or suspension components, for small and medium production. A General View 37 1. For small moulds the C-frame is typical, such as the model shown in Fig. For medium size moulds the gantry frame becomes more interesting due to its improved stiffness, with a sliding table where the mould is placed with bolts or fixtures.
For big moulds the gantry type with a travelling beam and with a two rotary head allows machining complex shapes. However, for medium and small moulds the two-axis rotary bed is stiffer and therefore more widely used As to the main motion, an electro spindle with 20,—25, rpm and 17—25 kW is a typical picture but a direct drive by a flexible coupling is also very widely used. This latter solution is cheaper, and more robust against collision and allows 12, rpm and even more.
Currently, electrospindles are able to give enough torque even at a low rotational speed allowing roughing to be performed in the high speed milling centre starting from the hardened steel material.
Ball screws are the economical and high performance solution for drive trains, allowing an acceleration of 0. In high end machines linear motors are used. Machines are not usually provided with pallet transferring, because machining on moulds is a long operation not requiring too many tools and, therefore, the setup time is not critical.
The heavy die is fixed on the table, 38 L.
Usually a two rotary head is used for sculptured surface milling, as shown in Fig. There, to obtain the maximum possible accuracy, the B and C axes are both driven by brushless motors and the transducers are mounted directly on the axes. In addition, the axis kinematics chains have a crown gear-worm screw system with a variable pitch for the compensation of the backlash. New models of rotary heads based on torque motors for the orientation axes have been launched in the last four years.
This is going to be the main design solution in the near future, at least for medium and high level machines. Torque motors are ring-shaped and can be placed directly on the rotary joints, eliminating gear transmissions. Some machines for large dies can change the spindle head automatically, selecting a head with a geared spindle for roughing and another with two axes and a high speed spindle for finishing.
Rapid-change connectors must be mounted at both the ram end and base of the head for the electricity, oil and coolant supplies. This group includes the universal milling machine, the press drill and the engine lathe which are always present in workshops, with only slight modifications in relation to the inventions of the second half of the 19th century. An easy and inexpensive option is the inclusion of measurement rules in each axis for the digital display of axis positions, as in the case shown in Fig.
A General View 39 Fig. Options for this model are a universal milling head, a universal milling head with quill feed, a universal dividing head, a precision circular table and attachments for slotting, rack milling and gear hobbing.
This model reaches 2, rpm with a 5. Here the compromise of technology-price-reliability and functionality must be balanced, with parts for multiple applications in mind. A lot of small and medium subcontractors do not know what kinds of parts will be produced in the medium term, and therefore they prefer versatile machines. A pocket tool changer is standard. The heavy cast-iron bed and column damp vibration and provide rigidity.
On the other hand, in some areas of the world, machine size and footprint are important due to the expensive industrial ground. For this reason compact machines are produced by several manufacturers. Usually, these ultra-compact machines are small enough to fit through a doorway, can be easily moved using a pallet jack or equipment dolly, can be provided with a caster kit to be rolled from one location to another and even fit into most freight elevators.
An example is shown in Fig. In these machines particular attention must be made to the work envelope, typically a cube with sides ranging from — mm. A General View 41 Fig. Workpieces weighing over 45 tons can also be machined on.
Of course, the size and weight of parts requires the use of big steady rests. The main motion is a powerful motor in the range of — kW. An operator can travel on it, as shown in Fig. The size and weight of components requires the motorised movement of the tailstock and its automatic locking to the bed. Moreover, it is an essential operation for steel structure constructors.
A typical band saw Fig. Machine mitre positioning is carried out manually or NC controlled, depending on customer needs. The tensioning device of the saw blade is hydraulic, manual or automatic. A coolant pump provides a constant flow of liquid coolant over the blade when metals are cut. Companies specialised in transfer machines work only as per customer order. The main requirement is high productivity, so hard automation is recommended here.
Machine modularity makes it possible to obtain different solutions without a lot of changes in the design and production. Regarding the machine frame, a ring bed is a classic solution for rotary indexing table models, made in cast iron or weld steel.
These are called dial-index machines. Other machines are in-line systems, with a U-shape layout. Drum or crown type tool turrets, as shown in Fig. All tools are driven by a common motor, using a gear transmission. These modules can be used in different machines with little changes. On the other hand, the use of parallel spindles when performing the same operation leads to high productivity. A General View 43 1. The 7. This spindle also has a dedicated 40 position tool magazine.
This ram spindle is in addition to the standard B-axis spindle with its 10, rpm and 37 kW motor and the turning table with rpm and 37 kW power. Milling head Ram head Fig. Chapter 2 is about new structural concepts currently being developed in some research projects.
Chapter 3 is focused on the basics of spindles. Chapter 4 explains the classic and recent advances on guides and drive trains. The current CNCs are described in Chap.
Chapter 6 describes the most important aspects in design and assemble precise machines. Lamikiz Chapters 7, 8 and 9 aim to present either some particular machine types or those machines specialised in the most important industrial sectors. Thus the grinding machines, lathes and electrodischarge machines are updated.
Chapters 10 and 11 describe two small machine groups but at high technological level: Here special concepts are used, complementing all others presented in previous chapters. Finally, a chapter devoted to aeronautical machines followed by one more on automotive machines complete this book, hoping the reader has encountered updated knowledge on machine tool construction over these pages. The basic machine tool theoretical bases are explained, but unfortunately recent advances are not included in them.
More recently, books by Altintas , Tlusty , Kibbe et al. Acknowledgements Our thanks to Mr. Pedro Ortuondo of the Elgoibar Machine Tool Museum for the information provided; thanks to all the companies cited for their pictures and information. Special thanks to Prof.
Metal Cutting Mechanics. Marcel Dekker, Inc. Int J of Mach Tool Manufacture Int J of Mach Tool Manufacture doi: Industrial automation systems and integration — Numerical control of machines — Coordinate system and motion nomenclature International Organization for Standardization  Kibbe RR et al. JA, Salgado MA Toolpath selection based on the minimum deflection cutting forces in the programming of complex surfaces milling. Verlag Dr. Int J Mach Tool Manufacture Campa Abstract This chapter is focused on analysing new concepts and trends related to the structure of machine tools.
In fact, the structure of the machine has a decisive influence on the three main parameters that define the capabilities of a machine, which are: In this respect, this study on structural components will add a new basic parameter, eco-efficiency, because the structure of the machine also has a decisive influence on the whole life cycle of the machine and especially on the materials and energy resources consumed: Within this view, the basic challenge for machine tool builders is to conceive machine tool structures that are capable of withstanding with minimum possible deflections the effects of the foreseen forces and of heating foci and at the same time consume the minimum possible in terms of materials and energy resources.
Campa As these two aims are opposing, the process of designing a machine tool is a trade-off between these two targets, so that the specific characteristics of a machine tool will define that balanced position between these two approaches. In this respect, the final characteristics of a machine tool are defined by means of the following parameters: The productivity of a machine is measured in terms of its metal removal rate MRR , which largely depends on its kinematic and dynamic capabilities and especially on its static stiffness, which in fact is the primary reason to define the dimensions and shapes of the structural components.
To this end, higher stiffness involves bigger masses, which in combination with faster motions aimed at achieving higher productivities, leads to motors of higher power that generate higher inertial forces. This in turn demands stiffer structures, which again demand a larger amount of material, so that stiff machines become productive and at the same time very energy and resource consuming.
The accuracy of a machine is defined according to the deviations of the tool with respect to a desired profile while it is being moved and positioned see Chap. These deviations are largely associated to thermal effects and to the mechanical deflections that the machine components bear when inertial and process forces act on them. Therefore, the basis necessary in order to achieve accurate machines will lie in conceiving both thermally-stable and stiff structures.
The eco-efficiency of a machine is measured in terms of energy and material resources used and the waste and pollution created in the process. As sustainability is an issue of increasing concern in the manufacturing sector, this book will add a new aim for machine tool builders: The structural components of machines have a twofold impact on the global eco-efficiency of a machine: On the other hand, the energy that a machine consumes during its use phase is largely associated with the motion of movable structural components both in positioning motions and in machining motions.
Figure 2. Accepting thus that the largest portion of the environmental impact of a machine tool is associated with the motions of its movable components, the reduction of each gram of material in these movable structural components will have a decisive role in the final ecological impact of the machine tool. As a conclusion, the structure of a machine plays a key role on the final functionalities of the productivity, accuracy and eco-efficiency of that machine.
The following sections will explain different strategies that can be employed to con- 2 New Concepts for Structural Components 1 2 Raw material Manufacturing extraction 3 Transport 49 4 Use forest hectare CO2 Tons With regard to these motions, milling machines have three Cartesian motions, with at least two of the axes mounted in serial.
In some cases, the three motions are applied to the tool really to the spindle onto which the tool is fitted ; in other cases the motions are divided into the tool and the part, and only on a few occasions are the three motions applied to the part, because it is quite unusual to vertically move the table on which the part lies.
Additionally, it is increasingly common for machine builders to add two additional rotational degrees of freedom to the machine, either to the headstock, or to the table or to both. They thus become machines with five degrees of freedom, i. The structure of a machine covers the components that allow the achievement of the degrees of freedom.
These elements are mainly of two types: They are the static part of the machine. The frame constitutes the main body of the structure, in which the bed is the solid base of the machine. The frame is usually composed of several bolted or welded components. They are the movable parts of the machine structure, and are linked in different frame configurations.
The interfaces among elements with relative motion must be as stiff as possible and highly damped along the perpendicular direction to the sliding one. For an analysis of the advantages and drawbacks of different machine structures, it is useful to divide the structural design approaches into two groups: The main conceptual difference between them is that in the open-loop case, process and inertial forces are conducted to the ground through in just one structural way.
In the closed-loop concept, forces are conducted to the ground in several ways. For the same machine strokes, the main advantage of the open-loop concept with respect to the closed-loop one is the easy access to the workzone and the lower cost. Otherwise, the closed-loop concept presents a higher stiffness at the tool tip and symmetrical behaviour with respect to thermal and mechanical loads. Finally, the main structural components in an open-loop concept are the bed, table, column and ram, linked with sliding guideways.
In the closed-loop concept, the main structural components are the table, bridge and guideways. Any of the involved components in any of the concepts can be either static or movable, i.
Indeed, the different relative motions among these structural components will define the different machine architectures and configurations. There are some general rules detailed here that can help the designer to select the optimal machine configuration: Indeed, cantilever components are the most critical components from a mechanical point of view, since they generate Abbe type errors see Sect. Several machines include a ram element, which is always a source of flexibility machines in Chap.
The overhang of this moveable component depends on the point to be machined, so this is a variable stiffness element. The less material that separates the parts of the structural machine components that hold the tool and the part the quicker the machine will reach a stable equilibrium.
With these basic rules, and taking into account that there is not a unique optimal architecture, the design task will focus on making the components of the selected configurations as stiff and light as possible, as well as other considerations such as the cost, the ease of assembly, the workzone accessibility, the footprint and the total space occupied by the machine.
The company studied six architectures defining seven indicators to allow an even weighting of the drawbacks and advantages of each architecture. The seven indicators considered were the following: The main characteristics of the six considered architectures were: C Structure, with a fixed column, and crossed slides on which a table moves in X and Y, and vertical ram in Z 2. Fixed bridge structure, with a movable table in X and crossed slides in Y and Z axes of the tool 3.
Fixed bridge structure, with a movable table in X, a movable slide in Y and a vertical embedded ram in Z 4. C Structure with a fixed column, a movable table in X, and a movable embedded slide in Y and vertical ram in Z 5. Travelling bridge structure, with fixed table, travelling bridge in X, embedded movable slide in Y and vertically movable crossbeam in Z 6.
Fixed bridge structure, with movable table in X, movable embedded slide in Y and vertical ram in Z The main conclusion of this comparison study was that number 5 produced the stiffest architecture. The most economical solution was produced by number 1, which also provided the most workspace accessible solution. Furthermore, the most homogeneous behaviour was seen in solutions 5 and 6, both of which also provide the safest solution.
Finally, there are no notable differences as regards the room that each solution occupies. Table 2. The drawing in Fig. The drawing on the right depicts a Bridge type, symmetrical, a closed-loop machine, with a fixed bridge, a movable table in X, a movable outer slide in Y and a vertical ram in Z.
Despite their remarkable differences in dimensions and occupied room, their stiffness values at the three axes are somehow similar. Therefore the information of interest consists in knowing the most compliant elements of the machine, because as structural components are mostly in serial, the total compliance of the machine will be larger than the most compliant component.
Moreover, when reinforcing the machine, the maximum effect is achieved when the most compliant component is acted upon. Campa Table 2. The data in this table confirms that concerning the static compliance of machines, the ram is the most critical component in almost all directions and architectures. There is also a notable influence from the column with respect to the bending directions of C-type machines.
Therefore, designing optimised rams and columns for the case of C-frame machines is an issue of special relevance.
The following section will analyse the main rules for designing robust, stiff and lightweight rams and columns. As the length of these components is defined by the strokes of the machine, there are two important aspects in designing rams and columns: Thickness and mass are directly related. With respect to ribs, they can be either longitudinal or transversal. Longitudinal ribs are not an optimal solution for rams and columns from the mechanical point of view, because for the same amount of mass, an appropriate increase of the thickness of the outer walls allows for achieving a higher stiffness both for bending and torsion loads.
Therefore, longitudinal ribs are not an optimised solution according to the stiffness-to-mass ratio. Campa Unlike longitudinal ribs, transversal ribs do improve optimally the mechanical behaviour of rams and columns concerning their bending and torsion resistance. Therefore, an important issue when designing columns and rams is to select the appropriate thickness of these transversal walls as well as the distance between these walls.
As a reference, for an average machine ram with a square section, an optimal thickness for transversal ribs is 10 mm. There is also an optimal space between transversal ribs, which is of the order of — mm. Similar to the case of longitudinal ribs, either an excess in the amount of transversal ribs or an excessive thickness of theirs will increase the total stiffness of the ram or column at a lower pace than the involved mass, as shown in the charts of Fig.
The optimisation of the machine constitutive structures is an important task for designers. As explained in the chapter introduction, the reduction of moving mass has an extraordinary impact on the environmental impact of a machine. With the aim of contributing to the definition of those threshold values for stiffness, Table 2. Conventional tools: In addition to the requirement for static stiffness, dynamic stiffness is also an issue of special concern.
Indeed, it is the primary reason to define the dimensions and shapes of the machine tool structural components. For an accurate definition of the threshold values for dynamic stiffness, the best utility available is the stability lobes diagram.
The stability lobes diagram is a plot that separates stable and unstable machining operations for different spindle speeds. Stable cuts occur in the region below the stability boundary, while unstable cuts chatter occur above the stability boundary . An example of the stable and unstable regions is shown in Fig. Stability lobes are functions of the dynamic stiffness at the tool center point TCP , the tool geometry, the radial immersion of tool into material, as well as of the material to be machined.
In the former case, recognisable for a low-pitched sound, the chatter is associated with the structural modes of the machine, shown in Fig. In the latter case, recognisable for a high-pitched sound, the chatter is associated with the modes of the tool or spindle. The type of chatter depends on the cutting frequencies; low cutting frequencies excite structural modes and high cutting frequencies excite spindletoolholder-tool modes.
Therefore, the aim of an optimised machine is to ensure an acceptable critical depth of cut in every cutting direction, using the minimum movable mass. Campa Fig. Inertial forces Feed drive 0 10 Hz old values to consider in the machine design. That assures that an eco-efficient and lightweight machine is, at the same time, productive and accurate enough. The machine mechanical modelling is a key to estimate the static and dynamic behaviour of a machine.
Finite element method FEM based models are widely used, and will be explained over the course of the next section. The mechanical analyses that are conducted on a machine tool by means of FEM analyses commonly cover three stages: The analysis of the static deformations and strain in a structure that is bearing static forces.
The analysis of natural frequencies and modes of the machine structure. The analysis of the dynamic stability of cutting processes by means of analytical stability diagram lobes. The FEM can be used here as a calculation tool for the assessment of the frequency response function FRF , instead of using the experimental modal analysis.
With FEM, the most complex aspect is that related to contacts between structural components along the degrees of freedom, in which stiffness, backlash and friction have a decisive influence on the damping of the machine. In this respect, experimental modal analysis allows for the measuring of the natural frequencies and modes of an already-built machine, and above all allows for the measuring of their associated damping coefficients, which is the most difficult mechanical parameter to estimate.
The experimental 2 New Concepts for Structural Components 59 dynamic parameters allow the calibrating of the FEM models that were developed in the design phase. The updated FEM models can be used to calculate the analytical stability lobes of the machine. Campa to the machine axes. The experimental data shown in the left part of the chart have been achieved for a specific tool and a specific material to be machined.
The right part shows the same stability lobe diagrams, with the only difference being that in this case data has been achieved from a FEM-modelled machine, in which the damping associated with each mode has been obtained from an experimental modal analysis on an already built machine. Otherwise, both the modelled-FRF and stability lobes diagrams show an acceptable level of coincidence with experimental-FRF and stability lobes.
The advantage associated with modelled diagrams is that the machine models enables an evaluation of the effect of mechanical and architecture changes on the stability of cutting processes. Thus, it is possible to test several design approaches to lighten a specific machine and at the same time to validate that the aimed values of productivity are also achieved. In fact, a reduction of masses that at the same time allows surpassing the threshold values defined for those parameters will allow an optimisation for that machine tool from the point of view of the eco-efficiency.
Focusing on reducing the movable masses, the topological optimisation of structural components is a critical issue, because this optimised mass-to-stiffness ratio will be the key to tackle effective strategies to reduce the total masses. With the aim of supporting this objective, there are commercial programs with optimisation algorithms that starting with a given structural component remove material on that component until no further removal is possible without deteriorating the static and dynamic properties, achieving thus a topologically optimised component.
What is more, an important additional objective of topological optimisation programs is to assure that the topologically and dynamically optimised structure can be manufactured in an economic way. As an example, Fig. This typical structure, with a fixed support on one end and forces on the other end, has been used to analyse the performance of optimisation tools.
This example shows that the topologically optimised structure Fig. With regard to the manufacturability and the economical feasibility of these optimised structures, the truss-like structures, which are the most frequent results of these automated topological optimisations, present some difficulties. As an example, cavities inside structures are difficult or impossible to manufacture.
Structural materials have a decisive influence on movable masses, moments of inertia, static and dynamic stiffness and both the modal and thermal properties of the machine. In the machine tool sector, the most commonly used materials are steel and cast iron, both of which offer an excellent stiffness-to-mass ratio as well as a good quality-to-price ratio.
Nevertheless, there are materials whose properties can fit better to the specific needs of a concrete machine, as is explained over the course of the next section.
High values of E have a positive influence on the static and dynamic stiffness of the machine. High values for both are a positive influence on the machine torsional stiffness. Low values of density within movable structures have a positive influence on the dynamic properties of the machine as well as on the bandwidth 62 d e f g J.
Campa of control loops, and at the same time high value density have a positive influence on the static elements of the structure, i. Specific heat capacity c: Indeed, high values of c make machines thermally stable to changing environmental temperature.
At the same time, high values of c mean that machines take a long time to reach a steady state after they have been turned on, so that a trade-off is required between these two opposite effects. In this respect, the machine users usually prefer thermally robust machines when faced with changing environmental conditions, though in order to achieve stable conditions a longer period will be necessary.
In such cases, a high value of c is desired. Thermal conductivity k: Similar to the previous case, the low or high value of k is neither positive nor negative per se. Indeed, high values of k make machine temperatures become quickly homogeneous throughout the machine, thus avoiding partial and asymmetrical elongations in the machine. At the same time, high values of k make machines heat up in the presence of non-desired sources of heat such as motors, bearings etc.
Machine users usually prefer thermally robust machines though it will lead to heat concentrations in the machine, so that in that case, a low value of k will be desired. One possibility to reach a trade-off between these two opposite effects is to have materials with low thermal conductivity k and in parallel to isolate heat sources or to evacuate heat by means of cooling systems. Material and structural damping: High values of damping have a positive influence on the dynamic properties of the machine as well as on productivity, because high values of damping implies that stability lobes rise for a given cutting speed.
These properties are analysed for several materials and classified into two groups, the currently common structural materials and the innovative materials.
Steel is commonly used in welded structures; whilst for cast iron, the most common solutions are the sand casts obtained from grey cast iron and spheroidal graphite ductile cast iron . Some parts such as headstock housings are made of cast steel. The main advantages of these conventional materials are their low cost in comparison with other materials and their very good machinability, with possibility to 2 New Concepts for Structural Components 63 Table 2.
Grey cast iron 5 2. Moreover, steel excels in terms of its high value of elasticity modulus and excellent mass-to-stiffness ratio, and the cast iron has a more than acceptable material-damping ratio, especially when compared to steel. The main disadvantages are their relatively high thermal expansion coefficients, and in the case of steel, its very low material-damping ratio.
The combination is bonded together using a resin system. Polymer castings are appropriate for precise machines due to their extremely low thermal diffusivity, which makes this material very stable and robust from the thermal point of view.
Moreover, mineral-cast elements are resistant against oils, coolants and other aggressive liquids. This material includes different types of compounds and in the near future others will be developed, which will increase its application for machine tools. Nevertheless, wider use of composites in machine tools is complicated due to some remarkable limiting factors such as their high price, their complicated joining and their complicated recycling.
Indeed, the technical community is not familiar with common commercial machine tools with applied carbon fibre composites CFCs , and in fact only some experimental research prototypes have been designed.
Their mechanical properties in the fibre direction are collected in Table 2. Therefore, it is important to know first the exact functionalities of the part that is to be developed, and then to find proper topological shapes as well as a macroscopic combination of involved materials . A practical strategy is to use low cost materials such as steel or cast iron for the majority part, and then to use a minimal amount of high-cost materials in order to tune the properties of the part by means of computational analyses.
In the field of machine tool structures, the following hybrid structures are already known: This ram displays higher damping, that has been 1 A technique of draping a cloth over a mould with epoxi preimpregnated into the fibres 2 New Concepts for Structural Components 65 noticed in the stability lobe diagrams.
Nevertheless, this increase of damping has been anyway lower than the increase achieved when using polymer concrete. The table summarises the values of minimum and maximum costs found on the European market. This is an interesting piece of information significant to the design of eco-efficient machines, since the high life cycle cost associated with the machine production time, highly recommends the use of lightweight materials.