Recent Trends In Manufacturing

Recent Trends in Manufacturing.

RECENT TRENDS IN MANUFACTURING – HIGH SPEED MACHINING

AUTHOR AMBARISH A. WALIMBE (B.E. MECH) (PGD TOOL DESIGN & CAD/CAM)

Recent Trends in Manufacturing.

TABLE OF CONTENTS ABSTRACT 1. INTRODUCTION 1.1 Why High Speed Machining? 1.2 Need for HSM Development

2. HIGH SPEED MACHINING 2.1 Machine Tool for HSM 2.2 Cutting Tools for HSM 2.3 NC Program for HSM 3. HSM APPLICATIONS 3.1 Die and Mould Making 4. CONCLUSION

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Recent Trends in Manufacturing. 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.

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Recent Trends in Manufacturing. 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 workpiece 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.

Figure 1. Effect of Cutting Speed on Cutting Temperature 4

Recent Trends in Manufacturing.

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.

Figure 2. The Variation of Cutting force with respect to cutting speed 5

Recent Trends in Manufacturing. 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 6

Recent Trends in Manufacturing. 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. 7

Recent Trends in Manufacturing. 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 nonproductive 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.

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Recent Trends in Manufacturing. TABLE. 1 HSM Machine Tool Parameters (Pasko, et al, 2000). Spindle speed range < 40,000 rpm. Increments (linear) 5-20 m Spindle power > 22 kW. Circular interpolation via NURBS. Programmable feed rate 40 – 60 High thermal stability and rigidity in spindlem/min.

higher pretension and cooling of spindle

bearings. Rapid traverse < 90 m/min Air blast/ coolant through spindle. Axis dec /acceleration > 1g Different error compensations. Block processing speed 1-20 ms. Advanced look ahead function in CNC. Data flow via Ethernet 250 kbit/s (1ms)

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Recent Trends in Manufacturing. 3. HSM APPLICATIONS HSM are performed with very specific methods and production equipments and not merely on the cutting speeds, spindle speeds and high feed rates. It is possible for machining of hardened steels with high speeds and feeds for finishing operations. A usual problem in the end milling of Aluminum structures for aerospace applications is that of maintaining good surface finish on either sides of thin ribs, which tend to deflect under cutter pressure. If machining is done at conventional speeds, the cutting force tends to deflect these ribs so that it is not possible to achieve smaller thickness with high dimensional accuracy. This problem can be overcome by using HSM. HSM is being mainly used in three industrial sectors due to their specific requirements. The first category deals with machining Aluminum to produce automotive components, small computer parts or medical devices. This industry needs fast metal removal, because a technological process involves many machining operations. The second category, which is the aircraft industry, involves long Aluminum parts, often with thin walls. Usually the work piece deflection and heat or stress induced deformation limits the machining with conventional speeds. At high speeds and feeds the heat generated in the cutter- work piece interface is carried away quickly with chips and less heat is transferred to the uncut workpiece. Now many aerospace die cast components are replaced by components machined by HSM. The HSM has ability to cut thin walls, which makes lighter components and minimizes the number of parts required in an assembly. The third category is the die and mould industry, which requires dealing with finishing of hard materials. Here it is important to machine with high speed and to keep high accuracy. 3.1 Die and Mould Making Die and mould making is one of the most significant areas of production technology as it plays an important part in the economics of producing large number of discrete parts. In the case of dies and moulds, conventional methods of machining needs more number of setups, use of costly machines like EDM and apart from manual finishing of components to achieve the required quality level. Shorter lead-time and production of better quality parts are the main goals in die/mould manufacturing. The use of HSM in die and mould industry can reduce the machining time produce an improved workpiece quality and also provide longer tool life. The design and engineering of modern dies and moulds are 10

Recent Trends in Manufacturing. increasingly growing sophisticated as these dies and moulds become complex and require tighter tolerances. Intricate geometry of die and mould surfaces and relatively high hardness of die and mould materials necessitates use of HSM. The die and mould makers are relying on HSM for more reasons, such as a reduction in machining time as well as less time needed for hand polishing and preservation of computer generated geometry. The HSM allows a trade-offs between time on the milling machine and that on the polishing bench to have better advantage. The key factor is making passes with very small stop over at very high feed rates with high spindle speeds to achieve adequate chip load on the cutter in roughing operations. A smaller depth of cut using positive rake cutters often achieves higher overall metal removal rates than attainable conventionally, even though the cutting tool is of a smaller diameter, compared to typical roughing operations involving fewer, slower and heavier cuts. In many cases after high speed roughing, stock remaining in the workpiece is close enough to the amount allowed for finishing, so that semi-finishing operation can be eliminated. Due to limited time of engagement of tool cutting edge, the chip produced was short, completely segmented and having variable thickness with ball end mills. The optimized tool path and cutting conditions result in high metal removal efficiency, improved tool life and process stability. The HSM employs new NC tool path generation methods and using CNC milling machines equipped with proper controlling sensors. Nowadays PCBN ball end mills have been used to machine dies and moulds. A cutting speed of 500-1000 m/min and feed rates upto 10 m/min. can be employed for machining alloy steels with hardness 30-45 HRC (Rigby, 1993). The cutting tool manufacturers recommend some typical cutting data for machining of dies (Table 2). The use of HSM technology could reduce machining time by 30-40%. HSM ensures a dimensional tolerance of 0.02 mm, which is comparable with 0.1- 0.2 mm for ECM and 0.01- 0.02 mm for EDM. The replacing of ECM by HSM increases the life and durability of the hardened dies.

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Recent Trends in Manufacturing. TABLE3. HSM cutting data by experience Cutting speeds Vc (m/min) Material

Hardness

Conventional

HSM-R

HSM- F

Steel 01.2

150 HB

< 300

> 400

< 900

Steel 02.1/2

330 HB

< 200

> 250

<600

Steel 03.11 Steel 03.11 Steel 04

300 HB 39 –48 HRC 48 – 58 HRC

< 100 < 80 < 40

> 200 > 150 > 100

<400 <350 <250

GCI 08.1 Aluminum Non- ferrous

180 HB 60 –75 HB 100 HB

< 300 < 1000 < 300

> 500 >2000 > 1000

<3000 <5000 <2000

(R- roughing, F-finishing)

4. CONCLUSION 12

Recent Trends in Manufacturing. HSM is regarded as a process where the operations are performed with specific methods and production equipment. Many HSM applications are performed with moderate spindle speeds and large sized cutter. HSM is performed in finishing in hardened steel with high speeds and feeds, often with 4-6 times of conventional cutting speeds. HSM is a high productive machining for small sized components in roughing to finishing and finishing to super finishing. In the case of some sized components, the various operations like roughing, semi-finishing and finishing can be performed in a single step because of low material allowances for machining, hence the number of set-ups and material handling are reduced. Productivity in finishing and possibility to achieve extremely good surface finish as low as Ra-0.2 microns and dimensional tolerance of 0.02 mm is ensured. Machining of very thin walls is possible with HSM. The negative aspects of HSM is attributed to high maintenance cost of machine tools due to higher acceleration and deceleration rates, spindle starts and stops leading to faster wear of guide ways, ball screws and spindle bearings. HSM requires knowledge in advanced processing and programming techniques and also an interface for fast date transfer. The development of a new machine tool architecture will allow the performance of high productivity roughing and semi-finishing combined with five axis high quality finishing.

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