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)


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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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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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

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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

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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

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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.

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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.

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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

m/min.

High thermal stability and rigidity in spindle-

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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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

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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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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 03.11 300 HB < 100 > 200 <400

Steel 03.11 39 –48 HRC < 80 > 150 <350

Steel 04 48 – 58 HRC < 40 > 100 <250

GCI 08.1 180 HB < 300 > 500 <3000

Aluminum 60 –75 HB < 1000 >2000 <5000

Non- ferrous 100 HB < 300 > 1000 <2000

(R- roughing, F-finishing)

4. CONCLUSION

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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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Date post:	13-Nov-2014
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