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HIGH SPEED MACHINING
ABSTRACT
Over the past 60 years, high speed machining (HSM) has been applied to a wide range of metallic and non-metallic work piece materials including the production of components with specific topography requirements and machining of materials with hardness of 50 HRC and above.
With most steel components hardened to approximately 32-42 HRC, machining operations currently include:
Rough machining and semi-finishing of the material
Heat treatment to achieve the final required hardness
Machining of electrodes and electrical discharge machining of specific parts of the dies or moulds
Finishing and super finishing of surfaces
In leading industrial countries, in die and mould manufacturing, a significant portion of the lead-time is spent for machining and polishing operations .Therefore the machining and polishing portion of dies and moulds takes approximately two third of total manufacturing costs. If the quality level after machining is poor and if cannot meet the requirements there will be varying need of manual finishing work.
Machining with high speeds (HSM) is one of the modern technologies, which in comparison with conventional cutting enables to increase efficiency, accuracy and quality of work pieces and at the same time to decrease costs and machining time.
INTRODUCTION
Over the past 60 years, high speed machining (HSM) has been applied to a wide range of metallic and non-metallic work piece materials including the production of components with specific topography requirements and machining of materials with hardness of 50 HRC and above.
With most steel components hardened to approximately 32-42 HRC, machining operations currently include:
Rough machining and semi-finishing of the material
Heat treatment to achieve the final required hardness
Machining of electrodes and electrical discharge machining of specific parts of the dies or moulds
Finishing and super finishing of surfaces
In leading industrial countries, in die and mould manufacturing, a significant portion of the lead-time is spent for machining and polishing operations .Therefore the machining and polishing portion of dies and moulds takes approximately two third of total manufacturing costs. If the quality level after machining is poor and if cannot meet the requirements there will be varying need of manual finishing work.
Machining with high speeds (HSM) is one of the modern technologies, which in comparison with conventional cutting enables to increase efficiency, accuracy and quality of work pieces and at the same time to decrease costs and machining time.
DEFINITION OF HSM
The first definition of HSM was proposed by Carl Salomon in 1931.He assumed that At a certain cutting speed which is 5-10 times higher than in conventional machining, the chip removal temperature at the cutting edge will start to decrease.
The figure below illustrates his definition. There is a relative decrease of the temperature at the cutting edge that starts at certain cutting speeds for different materials.
Fig.1 chip removal temperature as a result of the cutting speed
Actually there are many different ways to define HSM, upon them HSM is said to be,
High cutting speed machining (Vc)
High rotational speed machining (n)
High feed machining (Vf)
High speed and feed machining
High productive machining
Finally, HSM is a powerful machining method that combines high feed rates with high spindle speeds, specific tools and specific tool motion.
Figure below shows the generally accepted cutting speeds in high speed machining of various materials.
Fig.2 High cutting speed ranges
ADVANTAGES
Major advantages of HSM are high material removal rates, the reduction in lead times, low cutting forces, dissipation of heat with chip removal resulting in decrease in work piece distortion and increase part precision and surface finish.
Low cutting force gives a small and consistent tool deflection. This in combination with a constant stock for each operation and tool is one of the prerequisites for a highly productive and safe process.
Cutting tool and work piece temperature are kept low which gives a prolonged tool life in many cases.
As the depths of cut are typically shallow in HSM, the radial forces on the tool and spindle are low. This saves spindle bearings, guide ways and ball screws.
The contact time between the cutting edge and work piece, must be extremely short to avoid vibrations and deflection of the wall. The feed is faster than the time for heat propagation.
Reduction of production process as hardening, electrode milling and EDM can be minimized. This gives lower investment costs and simplifies the logistics.
The figure below illustrates the above statement
Fig.3 Improvement of production process when using HSM
A) Traditional process. The steps are:-
1) Non-hardened (soft) blank
2) Roughing
3) Semi finishing
4) Hardening to the final service condition
5) EDM process-machining of electrodes and EDM of small radii and corners of big depths
6) Finishing of parts of the cavity with good accessibility
7) Manual finishing
B) Same process as A, where the EDM- process has been replaced by finish machining of the entire cavity with HSM thereby reduction of one process step
C) In this process,
1) Initially the blank is hardened to the final service condition
2) Roughing
3) Semi finishing
4) Finishing
5) Manual finishing.
Here the HSM most often applied in all operations and thereby reduction of two process steps. Normal time reduction compared to the process A is approximately equal to 30-50%
The other benefits include reduced material handling cost, lower residual stress, increased productivity, possibility of machining of very thin walls, enhanced damage tolerance, shortened delivery times, elimination of coolant and increased cutting efficiency etc.
Table.1 below shows the various effects due to the above features which positively impact the global manufacturing process chain with in a machine shop.
Features Effects
Reduced heat transfer in to the work piece Part accuracy
Reduction of cutting forces Part accuracy, surface quality
Increased cutting speed Stability of rotating cutting tool feed rate, increased material removal
Table.1
Graph below shows that the cutting force (Fc) decreases with increase in cutting speed (VC).
Fig.4 Fc vs. Vc for a constant cutting power of 10kw
APPLICATION AREAS OF HSM
High speed machining is being mainly used in three industry sectors due to their specific requirements.
The first category is industry which deals with machining of aluminum to produce automotive components, small computer parts or medical devices. This industry needs fast metal removal because the technological process involves many machining operations.
The second category which is aircraft industry involves machining of aluminum parts, often with thin walls
The third industry sector is the die mould industry which requires dealing with finishing of hard materials. In this category it is important to machine with high speed and to keep high accuracy. In this industry HSM is used to machine such parts as
Die casting dies: This is an area where HSM can be utilized in a productive way as most castings dies are made of demanding tool steels and have a moderate or small size.
Forging dies: Most forging dies are suitable for HSM due to their complex shape. The surface is very hard and often prone cracks.
Injection moulds and blow moulds: These are also suitable for HSM, because of their small sizes. This makes it economical to perform all operations in one step.
Milling of electrodes in graphite and copper: It is an excellent area for HSM. Graphite can be machined in a productive way with Ti(C,N) or diamond coated solid carbide end mills.
Modeling and prototyping of dies and moulds: This is one of the earliest areas for HSM. Easy to machine materials such as aluminum are used. The cutting speeds are often as high as 15000-50000 rpm and the feeds are also very high.
Using of the HSM in the above mentioned regions can cause the reduction of production process when electrode milling ECM and EDM. HSM ensures a dimensional tolerance of 0.02 mm, while the tolerance when using ECM is 0.1-0.2mm and EDM 0.01 0.02. Replacing ECM with machining causes the durability and tool life of the hardened die or mould is increased considerably.
HSM- MACHINING SYSTEM
MACHINE TOOLS
In HSM, various configurations of machine tools are being used. However 3-axis horizontal and vertical milling centers (HMC and VMC) are most configurations. Al though vertical machining centers have disadvantages concerning chip removal, they are the less expensive choice and therefore, are presently more widely used than horizontal machining centers. CNC 4-axis milling offers the option of tilting the milling cutter to improve the cutting conditions. Five axis machines with interchangeable spindle units allow to rough, semi finish and finish with a single set up. It also allows the machining of work piece having large diameter.
Below are some typical demands on the machine tool and the data transfer to HSM (ISO/BT40 or comparable size, 3-axis)
Spindle speed range <=40000 rpm
Spindle power >22 KW
Programmable feed rate 40-60 m/min
Rapid travels <90 m/min
Block processing speed 1-20 ms
High thermal stability and rigidity in spindle
Air blast/coolant through spindle
Advanced look ahead function in the CNC
CUTTING TOOLS
Among the cutting tools used for machining castings and alloy steels carbide is the most common tool material. Carbide tools have a high degree of toughness but poor hardness compared to advanced materials such as cubic boron nitrite (CBN) and ceramics. In order to improve the hardness and surface conditions carbide tools are coated with hard coatings such as titanium nitride (TiN), Titanium carbonitride (TiCN) and titanium aluminum nitride (TiALN) and recently with double/soft coatings such as MOVIC. Other cutting tool materials are Ceramics (AlO, SiN), cermet and poly crystalline diamond (PCD).
In general tools ranging from 0.5 to 1.5 inches inn diameter carbide insert tools with TiCN coatings are significant for the materials with less than 42 HRC, while titanium aluminum nitride coatings are used for materials with 42 HRC and over. However depending on application, materials and coatings for the best performance vary. High speed cutting application for such tool materials and coatings can be classified as
CBN and SiN for cast iron
TiN and TiCN coated carbide for alloy steel up to 42 HRC
TiAlN and AlTiN coated carbide for alloy steels having hardness 42 HRC and over.
For special applications especially for hard turning (HRC 60-65) PCBN inserts with appropriate edge preparation are also successfully used.
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NEED OF COATED TOOLS
Hard coating applied to cutting tools can significantly alter the properties of the tools. Low coefficients of frictions and low tendency to adhesion of the coating to the work piece material will result in less heat generation during the cutting operations. The low thermal conductivities of the coating will generate a thermal barrier on the tool surface. Less heat will be transferred to the tool and thus a major part of the generated heat will be transported away from the cutting areas with the chips. In addition high thermal stability, hot hardness and oxidation resistance resistances will reduce the wear of the tool.
SOME RECOMMENDED PARAMETERS FOR HSM
1. TRUE CUTTING SPEED
As cutting speed is dependent on both spindle speed and diameter of tools, HSM should be defined as true cutting speed above a certain level. The linear dependence between the cutting speed and the feed rate result in high feeds with high speeds.
Fig.5 Cutting speed
Where ap = axial distance from the tool tip to the reference point
n = spindle speed
De = effective diameter
2. MATERIAL REMOVAL RATE
The material removal rate, Q is consequently and considerably smaller than in conventional machining with the exception when machining in aluminum other non ferrous materials and in finishing and super finishing operations in all types of materials.
Where Vf = feed speed
ae = step over distance
3. SURFACE FINISH
Like in conventional machining the surface finish in HSM is determined by conditions like the cutting tool geometry, coating of the cutting tool, wear status of the cutting tool, lubrication, cutting strategy determined on the CAM system, cutting tool extension, work piece material etc. Assuming all these parameters are controlled, the surface finish to be expected may be calculated through the following approach.
Where Rth = theoretical surface roughness
D = diameter of cutting tool
ae = step over distance
Since the maximum cutter diameter is often limited by the part geometry, Rth only can be minimized by decreasing the step over distance
4. TOOL COST AND MACHINING COST
When introducing HSM, cost of cutting tools will increase significantly. The benefit of HSM is given by a reduction of processing time and cost, better surface finish, reduction of manual finishing work, better accuracy etc. For this reason cutting tool cost should only be seen as an integral part of the overall cost accounting. Machining cost will be less than the conventional machining process due to elimination of process steps, reduction of process time etc.
COMPARISON
Table.2 gives the comparison between conventional and high speed machine
CONVENTIONAL MACHINE HIGH SPEED MACHINE
Maximum speed 600 m/min
Maximum speed 40 ipm
Require high levels of coolant Speed starts at 600 m/min
Feed starts at 100 ipm
With coolant, feed rate can go more than 2000 ipm
No feed for coolant for low feed rate.
Table.2
Table.3 comparison between speeds used in conventional machining and HSM using some selected materials
Table.3
Table.4 gives the comparison between conventional machining and HSM process
CONVENTIONAL HSM
The contact time between the cutting edge and work piece is large Contact time between the cutting edge and work piece is short
Less accurate work piece More accurate work piece
Cutting force is large Low cutting force
Cutting fluid is required Cutting fluid is not required
Low surface finish High surface finish
Material removal rate is low Material removal rate is high
Table.4
Table.5 gives the comparison between high speed machining (HSM) and electronic discharge machining (EDM) process
HSM EDM
Material removal by interference between tool and work contact process Non contact process
Dimensional tolerance 0.02mm Dimensional tolerance 0.1-0.2
Material removal rate high Material removal rate low
Table.5
DISADVANTAGES
Need for expensive and special machine tools with advanced spindles and controllers
Excessive tool wear.
The higher acceleration and deceleration rates, spindle start and stop give a relatively faster wear of guide ways, ball screws and spindle bearings which leads to higher maintenance cost.
Emergency stop is practically unnecessary! Human mistakes, hard ware or software errors give big consequences.
Good work and process planning necessary.
It can be difficult to find and recruit advanced staff.
CONCLUSION
HSM is not simply high cutting speed. It should be regarded as a process where the operations are performed with very specific methods and production equipment
HSM is not necessarily high spindle speed machining. Many HSM applications are performed with moderate spindle speeds and large sized cutters
HSM is performed in finishing in hardened steel with high speeds and feeds often with 4-6 times conventional cutting data
HSM is high productive machining in small sized components in roughing to finishing and in finishing to super finishing in components of all sizes
Even though HSM has been known for a long time, the research is still being developed for further improvement of quality and optimization of cost.
REFERENCES
A.O.Stevenson, High speed machining, SEARCH the industrial source book, vol.11.No.2,pp 49-52, 2003
Ashley.S, High speed machining goes main stream, mechanical engineering, pp 56-61, May 1995
Plaza.M, The prons and cons of high speed machining, Canadian machinery and metal working,pp8-10, September 1995
Fall bohmer.P, H.K.Nakagawa, Survey of the die and mould manufacturing industry, Journal of materials processing technology,pp59,158-168,1996