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RECENT TRENDS IN MANUFACTURING – HIGH SPEED MACHINING
ABSTRACT
In recent years large amount of research has taken place to improve productivity in machining. One such research area developed to increase the metal removal rate is High Speed Machining (HSM). Machining of materials at four to six times the cutting speed used in conventional machining is called as High Speed Machining. The high speed machining technique has great economic potential due to high metal removal rate, better surface finish and ability to machine thin walls. The newer materials such as composite materials, heat resistant and stainless steel alloys, bimetals, compact graphite iron, hardened tool steels, aluminum alloys etc., needs this new machining (HSM). High speed machining offers a means to shorten delivery times boost productivity and increase profitability.
The aim of this paper is to give an overview of HSM and related technologies used in production systems for obtaining increased efficiency, accuracy and quality of finishing. A high speed machining center can reduce the need for polishing the surfaces of dies and moulds. It can produce EDM electrodes more efficiently. The high speed machining center also produces complex tooling competitively in a single setup. The HSM requirements, such as machine tool, cutting tools etc. are discussed in this paper. The application of high speed machining to die and mould machining is also presented.
1. INTRODUCTION
Machining of materials at four to six times the cutting speed used in conventional machining is called as High Speed Machining (HSM). HSM is one of the modern technologies, which in comparison with conventional cutting enable to increase the efficiency; accuracy and quality of the work piece and at the same time decrease the cost and machining time . The HSM technology allows the manufacturing of products with excellent surface finish with relatively little increase in total machining time. Carl Salomon conceived the concept of HSM after conducting a series of experiments in 1924-31. His research showed that the cutting temperature reached a peak value when the cutting speed is increased and the temperature decreases for a further increase of cutting speed (Figure 1). The increase in cutting speed demands a new type of machining system like the machine tool, cutting tool, CNC program etc. The use of high feed rate with high speed increases the metal removal rate, but the machine in turn requires lighter inertia tables, powerful motor drives and more responsive control systems. One definition of HSM states that, it is an end milling operation at high rotational speeds and high surface feeds. HSM normally uses a high speed in excess of 1000 m/min, feed rates above 1m/min and spindle speeds greater than 10,000 rpm.
1.1 Why High Speed Machining?
High material removal rates can be achieved by using high cutting speed, high rotational speed, high feed machining or high speed and feed machining. Practically it can be noted that HSM is not simply high cutting speed. It should be considered as a process where the operations are performed with very specific methods and production equipments. In many applications, HSM is used for machining the components with high spindle speeds and feeds for roughing to finishing and also for finishing to super finishing. In HSM the cutting tool and workpiece temperatures are kept low due to short engagement time. This normally increases the tool life. The increase of cutting speed decreases the cutting forces (Figure 2). The deflection of tool is kept less during cutting, which results in good surface finish (Ra 0.2 micron). The shallow depth of cut in HSM reduces the radial forces on tool and spindle. This increases the life of the spindle bearings, guide ways and ball screws.
1.2 Need for HSM Development
1. To survive in the competitive market, it is necessary to use HSM in order to reduce machining time and hence cost of production.
2. The newer materials such as composite materials, heat resistant and stainless steel alloys, bimetals, compact graphite iron, hardened tool steels, aluminum alloys etc., needs this new machining (HSM).
3. HSM offers high quality of products by avoiding manual finishing of dies or moulds with a complex 3-D geometry, aluminum thin walled component machining etc.
4. HSM eliminates the number of setups and simplifies the flow of material, which can reduce considerably the manufacturing throughput time.
5. HSM technique is one of the main methods in rapid product development.
2. HIGH SPEED MACHINING
The machining activity is an important component in the overall manufacturing. The HSM processes are increasingly used in modern manufacturing. However, such processes can lead to discontinuous chip formation that is strongly correlated with increased tool wear, degradation of the work piece surface finish, and less accuracy in the machined part. The variations of cutting force components are functions of chip load and cutting speed. The variations in cutting force produces severe self excited and forced vibrations which are detrimental to the tool life, work piece geometry, finish and finally machine tool itself .
2.1 Machine Tool for HSM
HSM has grown in popularity tool making industry. After an initial period of skepticism, high speed machining offers a means to shorten delivery times, boost productivity and increase profitability. The spindle is the most fundamental component of the HSM processes. In some cases retrofitting a faster spindle to a conventional machining center can realize some of the HSM benefits. The increased cutting speed, introduce dynamic stability problems into the machine tool components. This leads undesirable resonance in the machine parts, which require additional damping considerations in the design of machine tool components. A more accurate representation of high speed machining from a spindle design point of view is the DN number. DN is the spindle diameter in mm multiplied by the spindle speed in rpm. The commercial high-speed machines are available with DN number in the range of 1.5 million. The stability of the machines used for HSM become important to reduce the vibrations and chatter produced during machining. It was shown, that a substantial productivity gain as well as reduced vibration could be achieved by utilizing stability lobs in HSM machine tool design. One of the main objectives of HSM is high metal removal rate, which is achieved by using higher speed and depth of cut, particularly in roughing operation. Machining at surface speed higher than 915 m/min is more common in HSM and the chatter produced at that speed can be suppressed or avoided by either using an analytical model or an experimental technique or more desirably by a combination of both. The spindle dynamic characteristics at high speed were analyzed and observed that a spindle with angular contact ball bearings exhibits some change in dynamic stiffness as the speed increases. With the aid of computer aided modeling, the machine builders are able to analyze the machine dynamics and dynamic stiffness. The machine’s servo drives, spindle design and torque power curves are different for each application of HSM. The major development in HSM is correcting unstable machine conditions by a Chatter Recognition and Control System (CRAC). It is an on-line system for stabilizing the cutting conditions automatically by adjusting the cutting speed and feed. It uses the sound of the cutting operation, measured spindle speed and number of teeth on the tool to determine when chatter occurs and to automatically choose a new spindle speed. Winfough and Smith (1995) reported a new CRAC system as a tool in an NC program to use spindle speed and axial depth of cut combinations to obtain maximum metal removal rates.
2.2 Cutting Tools for HSM
The cutting tools are specially designed to suit HSM for high metal removal rate. All the cutting and holding tools used in HSM are to be designed for the specific purpose machining. The tools are normally provided with reinforced cutting edges by using either zero or negative rake angles. One typical and important design feature of the cutting tool is having thick core for withstanding maximum bending. The increased run-out error in the tool or tool holder reduces the life of the tool to a great extent. A method is described for changing the length of tool, so that the most stable region (machining condition) falls at the top speed of the spindle. Many different designs of tool–tool holder interface are developed to reduce the instability. Stability of the interface can be improved by shortening of the overhang portion and also using shrink fit tooling. The increased spindle speed limits the use of conventional taper interface provided with cutting tools. A modification has reported in traditional taper design to achieve more stiffness through face contact. The strong development of cutting tool materials and holding devices has increased the applications of HSM. Also the development of super hard cutting materials such as Cubic Boron Nitride (CBN), Poly Crystalline Cubic Boron Nitride for machining hard steel has created many new applications for HSM. Another development of tooling with exotic coating technologies is able to withstand the high temperature produced in HSM. In HSM the super hard materials as well as cutting edges resistant to high temperatures are the solutions for providing maximum performance for different category of materials.
2.3 NC Program for HSM
The productivity of a machine is always a concern for the machine developers and users. In conventional machining the increase of feed rate increases the productivity. But in most cases of HSM, the increase of the feed rate does not significantly improve the productivity. The productivity can be evaluated by calculating the productive and non-productive times. The productivity of the high speed machining centre depends directly on the quality of NC programs. A NC program was developed with a simulator to evaluate the productivity of the NC programs by considering an effective feed rate factor and a productivity factor. The effective feed rate depends on: (1) the command feed rate (2) the average per block travel of the tool (3) moving vectorial variation of the tool and (4) acceleration/deceleration or time constants. NC programmers must alter their overall machining strategy to construct tool paths to anticipate the cutting tool for its engagement with the work piece. Sharp turns and slow execution create jerky tool movements. This alters the load on the cutter, which causes tool deflection. This leads to reduced accuracy, surface finish and tool life. The servo controllers used in HSM many times failed to position the drives accurately. “Remaining stock analysis”–ability of the CAM system to know precisely where the stock is available after each cut, is used for predicting the constant cutter load. Experience has shown that tooling manufacturers and CAM software developers need to work closely together to ensure that the customers are able to get the major benefit from deploying new tooling technologies with optimized machining strategies. Using slower CAM software or a less powerful computer will lead to frustrating delays, with a new machine tool lying idle while NC programs are being generated (delcam). To perform HSM it is necessary to use rigid and dedicated machine tools and controls with specific design features and options. The machine should use advanced programming techniques with a more favorable tool path. The program should ensure constant stock for each operation. To achieve the above requirements the machine tool designers and engineers have been developing the machines for HSM with parameters specified below.