Many industrial machinery manufacturers find themselves facing an increasing number of new requirements, such as the demand for more flexible machine tool designs that can be rapidly adapted to new products, as well as for machines that can be easily integrated into existing plants.
Increasing global competition, market demands and regulatory mandates require industrial machinery companies—who serve multiple industries—to continuously innovate and optimise their products. The market not only expects maximum uptime on a day-to-day basis, but also to remain productive for years. In addition, with the introduction, there now lies the challenge of manufacturing with decentralised, autonomous machines that communicate with one another and the products they are manufacturing, so production can be optimised. This adds a whole new level of technology integration requirements.
The realisation of machine tools and in particular, of machining CNC centres with high performance has become a requirement in recent times to enable a strong increase in productivity and a consequent decrease in product costs.
There are essential two strategies available to increase productivity. The first is a sharp increase in the speed of material removal, which can be made possible by cutting tools made using high performance materials such as cubic-boron-nitride (CBN) or polycrystalline diamond.
In order to reap the full benefits of these tools, it is necessary to perform the machining with high removal rates and high depth of cut. As a result, machine tools require a structure that is capable of withstanding the elevated dynamic stress that these working conditions induce.
Their design solutions are characterised by high stiffness and large capacity to dampen vibrations. In fact, the vibrations of the machine tool structure are, among other causes, the factor that restrict high speed operations or degrade cutting precision.
The second strategy is the reduction of the so-called ‘air cutting time’. This dead time represents the period during which the tool is not in contact with the workpiece as the machine tool is moved to the working position and the correct tool is introduced.
The impact of the dead time on the entire cycle is not negligible in the case of a modern work centre, especially when the production of complex parts is involved. According to some studies, dead time can take up as much as 70 percent of the overall cycle. In order to reduce the ‘air cutting time’, it is necessary to design very fast machines, up to 2 m/s in speed against 0.5 m/s, which is typical of a conventional machine.
Obtaining these high speed over a small distance (usually just a few metres) is possible only if the mobile parts of the machine can move under very high accelerations, over 10 m/s2. The related design solutions aim to obtain such high acceleration without compromising other aspects such as stability and precision.
The positioning accuracy is feasible only by using machine tools with moving parts that are characterised by high stiffness and low mass. In fact, one of the primary reasons for low productivity is the large mass of the moving parts of the machine tools, which cannot afford high acceleration and deceleration encountered during operation.
It has been estimated that a proper design solution can reduce the weight of the vertical and horizontal slides by 34 percent and 26 percent respectively, and increase damping by 1.5-5.7 times without sacrificing stiffness.
In general, both approaches highlight the need to design machine tools that are characterised by structures with high dynamic stiffness and reduced mobile masses. The realisation of this kind of machine is almost impossible using the structural materials that are traditionally employed, namely cast iron and steel.
It is, therefore, necessary to consider material innovations that can achieve these objectives without impacting machine cost. Typically, composite materials, such as carbon or glass fibre reinforced composites, offer a good compromise between alternative requirements (such as weight, manufacturability and costs). This is also evident in the case of substitution of traditional metals with light composite materials such as carbon fibre reinforced plastics (CFRP), for the realisation of machine structures and parts.
Earlier studies have shown that in fast moving components, such as the supporting frame or cover of the electro spindle, a reduction in weight can bring about improvements in speed and productivity thanks to the reduction of inertia masses. Other studies have described the effects of material substitutions, such as replacing traditional alloys with fibre reinforced composites. For example, the use of carbon fibre-epoxy composite as constitutive material for a spindle-bearing system in a machine tool.
Whichever the approach, the general aim of obtaining the highest precision in woodworking operations obliges the designer to follow several common considerations. In particular, it is noteworthy that the degree of accuracy of a machine tool, allowing for the production of high quality surfaces in wood pieces, depends on two basic factors:
1) Static and dynamic stiffness of the structure, particularly with respect to bending and torsion solicitations. The specific characteristics of the material that have a greater impact on these aspects are: the Young’s modulus and its internal damping.
2) Dimensional stability with respect to mechanical stress related to working and physical stresses related to the environment which the machine has to operate in.
The demands on the material are therefore:
• Very low internal tensions
• Very low coefficient of thermal expansion
• Large thermal conductivity so as to enable rapid heat transfer inside the whole structure by minimising the thermal gradients and thus, the thermal deformations that follow.
The designer of any product, other than the software, must get involved with material selection. The designer must understand the materials and their properties as deeply as possible so as to be able to design a competitive product. This consideration is also valid in the case of machine tool design.
There is an incredibly large range of materials that are available in the market. Contrary to popular perception, a lot of information on these materials is available and can easily be found. As the first step, it is necessary for the designer to focus on the product’s function before preliminarily defining the class of materials to be selected.
The material selected must possess these important technological characteristics:
1) Wear, impact and fatigue resistance
2) Low coefficient of friction
3) Great resistance to chemical attack by aggressive liquids, in particular, lubricants and refrigerants that are used during mechanical machining.
The important economic factors to consider include:
2) Price per unit mass
3) Cost of processing
4) Ability to obtain complex structures by casting and welding
5) Possible rates of curing and/or thermal treatment
Correct material design has to include these machine tool considerations: strength, brittleness, hardness, weight, machinability, weldability, cost, corrosion resistance, conductivity, wear resistance and thermal conductivity. All of them assume a specific relevance in design and manufacturing of machine tools for precise woodworking.
Steel and cast iron
Strong, stiff, heavy, cheap
Weaker, lighter, more expensive than steel
Strong, stiff, very light, but expensive
Main machine tool materials.
Cast iron is still one of the most used materials in many industrial sectors. In particular, in the design of woodworking machines, the material is used for the realisation of parts and frames for the following characteristics:
• High dimensional stability
• High vibration damping
• High thermal conductivity
• Low cost
• Good heat transfer
• Well-established design solution
These positive features that characterise cast alloys can be further reinforced through thermal relaxation, vibration or an appropriate period of natural ageing. These benefits are demonstrated by the large variety of components realised in cast iron and used as parts for machine tools, from small inserts to large bases.
However, cast iron is just a generic term representing a large gamma of iron-carbon alloys. From the microstructure perspective, cast alloys are conventionally classified into the following:
• White cast iron
• Grey cast iron
• Malleable cast iron
• Ductile cast iron
Grey and ductile cast irons are the most commonly used cast alloys. Most of the production of white cast iron is reprocessed for obtaining malleable or ductile cast irons. The use of malleable cast iron has been on a decline since the higher complexity in processing cannot be justified by the little improvements in properties.
Another possibility is vermicular graphite iron, sometimes also called compacted graphite iron. While grey cast iron is characterised by randomly oriented graphite flakes and ductile iron, graphite exist as individual spheres, in vermicular iron graphite, flakes are randomly orientated and elongated as in grey iron, but they are shorter, thicker and with rounded edges which are in some aspects, more similar to ductile cast iron.
In the case of composites, the correct use of these light and reinforced materials cannot ignore the fact of being in presence of anisotropic microstructures and alternative manufacturing processes.
As a result, with the aim of obtaining industrially sustainable results by a change in materials, mechanical designers should also abandon the traditional way to design machine tools (mainly by metal sheets and castings), including the necessity of acquiring specific competencies regarding these new materials and advanced processes.
The use of polymer matrix composite materials reinforced with fibres can permit the construction of sandwich structures with filling materials in the shape of foam or honeycomb that can optimised the response to specific stresses.
Considering the benefits offered by these advanced materials, their introduction has not been limited to the design of machine tools, but also involves other categories of production such as packaging.
A novel and interesting application of uncommon materials in design of machine is offered by magneto-rheological fluids. The growing interest for magneto-rheological fluids mainly derives from their ability to provide a rapid and simple interface between the electronic and the mechanical control system.
Potentially, these materials have the ability to radically change the electromechanical design of the machines. For instance, the use of magneto-rheological dampers in semi-active tuned vibration absorbers to control structural vibrations. However, the utilisation of theses advanced materials is only feasible if the devices containing magneto-rheological fluids demonstrate the capability of ensuring the implementation of precise and rapid movement.
For example, a commercial magneto0rheolotical clutch is able to provide resistive torque against a rotating shaft by changing the viscosity of the fluid material through modification of the magnetic field. The clutch was able to provide a specific resistive torque that is proportional to the current passing through the electric winding inside the device. In this way, it is possible to control a mechanical propriety (the torque) by modulating an electrical current. This solution can be considered very interesting where a high accuracy in torque control is required.
However, at the same time, it is recognised that magneto-rheological devices can be affected by a significant delay between activation of signal to the application of the torque.