high speed machining principles and applications

high speed machining principles and applications, process parameters

1. High Speed machining Mrudula Prashanth

2. History of HSM Initial attempts were made in early twenties of the past century The first definition of HSM was proposed by Carl Salomon in 1931 He has assumed that at a certain cutting speed which is 5 –10 times higher than in conventional machining, the chip tool interface temperature will start to decrease

3. Solomon’s curve

4. Definitions High cutting speed machining ( Vc ) High spindle speed machining (N) High feed machining (f) 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

5. What is HSM? High speed machining or High speed cutting (HSC) or High velocity cutting (HVC) High-speed machining (HSM) is usually associated with high-speed spindles (10K to 100K rpm), higher feed rates and lower depth of cut.

6. HSM Cutting speed achieved depends upon the Workmaterial , The type of cutting operation, The cutting tool used The range of cutting speed for high speed milling is summarized in fig. 1. Defining high-speed machining is difficult because it can be one of many operations, or a combination of them. It can be defined as: Machining at a high cutting speed ( v c ). Machining with a high spindle speed (n). Machining with a high feed rate ( v f ). Machining with a high removal rate (q ). Fig. 1: Cutting speeds for HSM

7. Objectives of HSM The major objectives of high-speed machining research are classified into 3 categories To machine materials such as titanium and steel alloys which are difficult to machine under conventional machining process To achieve improvement in surface quality and machining accuracy To achieve higher productivity or throughput in machining of various materials The demands for high speed machining of materials, such as aluminum, titanium, steels are increasing not only in the aircraft industries , but also in electric and electronics industries, automobile, bio-medical and aerospace industries. The interests in recent research and development are directed to high speed and high throughput machining of difficult-to-machine materials, such as titanium alloys, nickel-based alloys for aerospace, automotive and bio-medical applications and ferrous alloys for tool and die making

8. Benefits of High Speed machining

9. Improvement of production when using HSM

10. FEATURES EFFECTS Reduced heat transfer in to the work piece Part accuracy is increased Reduction of cutting forces Part accuracy increases Surface quality increases Increased cutting speed Stability of rotating cutting tool & feed rate Increased material removal rate (MRR)

11. COMPARISION BETWEEN CONVENTIONAL AND HIGH SPEED MACHINING

12. Comparision between Conventional Machining and High Speed Machining

13. Sl. No. Conventional Machining HSM 1 The contact time between tool and work is large Contact time is short 2 Less accurate work piece More accurate work piece 3 Cutting force is large Cutting force is low 4 Poor surface finish Good surface finish 5 Material removal rate is low Material removal rate is high 6 Cutting fluid is required Cutting fluid is not required

14. Sl.No . HSM EDM 1 Material removal rate is high Material removal rate is low 2 Dimensional tolerance 0.02 mm Dim tolerance 0.1- 0.2 mm 3 There is no need of making cutting tool according to the contour to be machined Cutting tool has to be made according to the contour to be machined Comparision between EDM and HSM

15. HSM Cutting temperature increases with an increase in the cutting speed The cutting force decreases with an increase in the cutting speed due to softening of the work material It is reported in some cases that the cutting force reaches to minimum and increase with a further increase in the cutting speed Fig. 2: General HSM characteristics General characteristics of HSM

16. Limitations of HSM The cutting speed is limited by the tool wear in machining of the difficult-to-machine materials , such as superalloys, titanium alloys, hardened alloy steels etc. (Ref. Fig. 1) slide no 6. One of the possible limiting factors for future high speed cutting is the momentum energy

17. Limitations contd.. Cost of machine , requirement of special tooling , issues related to vibration are some of the disadvantages The higher acceleration and deceleration rates, spindle start and stop give a relatively faster wear of guide ways, ball screws and spindle bearings . This often leads to higher maintenance costs Specific process knowledge , programming equipment and interface for fast data transfer needed.

18. Effect of cutting speed on cutting force

19. Effect of cutting speed on specific cutting force for aluminium 7050

20. Important requirements of HSM Spindle speed range 22 kW Programmable feed rate 40-60 m/min Rapid traverse < 90 m/min Axis dec./acceleration > 1 g (faster linear motors) Block processing speed 1-20 ms Increments (linear) 5-20 microns Or circular interpolation via NURBS (Non-uniform rational Basis spline ( NURBS) (no linear increments) Data flow via RS232 19,2 Kbit/s (20 ms ) ( RS232:Simple analog communication over the telephone wires to the typical USB cables for data exchange/ Serial data base communication )

21. Important requirements of HSM Data flow via ethernet 250 kbit /s (1 ms ) High thermal stability and rigidity in spindle - higher pretension and cooling of spindle bearings Air blast/coolant through spindle Rigid machine frame with high vibration absorbing capacity Different error compensations - temperature, quadrant, ball screw are the most important Advanced look ahead function in the CNC

22. Tools for HSM For high speed cutting tools four criteria are decisive: A) Cutting alloy, B) Cutting edge geometry, C) Design and the interface between tool and work D) Machine spindle.

23. Cutting alloys Various cutting tool materials in HSM The choice of the right cutting tool material is based on the wear processes . Conventional machining- wear by abrasion High speed machining- diffusion wear and wear processes between work piece and cutting edge are dominant in HSM. Selection of cutting tool is based on  toughness and hardness

24. Diamond Possesses the highest hardness , Good conductivity and Excellent in chemical stability The four types of diamonds being used as cutting tools are Industrial Grade Natural Diamond (ND), Synthetic Single Crystal Diamond, Polycrystalline Diamond (PCD) , and Chemical Vapor Deposition (CVD) diamond coating .

25. Types of diamonds Synthetic single crystal diamond It has good dimension stability , Shape and Function stability polycrystalline diamond (PCD) PCD is obtained with some binders such as cobalt powders by sintering under ultra-high pressure and high temperature The sintered diamond body shows excellent abrasion resistance with the cutting of the nonferrous metals as well as non-metallic material

26. Types of Diamonds chemical vapor deposition (CVD) diamond coating tools are Well suited for the high speed machining of aluminum and other non- ferrous alloy s such as copper , brass , bronze , and abrasive advanced composites such as graphite , carbon-carbon , glass-fiber reinforced plastics , and carbon filled phenolics CVD thin film diamond can be applied to such complex tool geometries as inserts with chip breakers , solid end mills, routers, and drills . Thick-film diamond coating tools are used in high speed machining of high-eutectic aluminum alloys.

27. Cubic Boron Nitride (CBN) Cubic boron nitride (CBN) is synthesized artificially under the condition of the high pressure and high temperature similar to that of diamond It has polycrystalline structure similar to that of diamond CBN has small reactivity with the iron system materials It is more chemically and thermally stable than those of the diamond Hence it is useful for machining of iron system materials for which the diamond is unusable

28. Polycrystalline type CBN (PCBN): PCBN has an excellent characteristic for cutting tools to machine the cast iron and heat resisting alloy , and the iron system hard materials with hardness > HRC 45 Eg ., Automobile parts, the engine blocks and gears , shafts , bearings . PCBN machining operations should run dry It can be run at cutting speeds above 2000 m / min in machining of gray cast iron materials Used in high speed hard machining in particular , the finish machining of steel alloy automotive engine components , such as gears, shafts and bearings , which have hardness between HRC 60 and HRC 65.

29. Ceramic cutting tool Ceramics are nonmetallic materials Ceramic tools are extremely high resistant to abrasive wear , cratering and extremely high temperature hardness Available Ceramic tool materials: Alumina (A1 2 O 3 ) based ceramics and Silicon nitride (Si 3 N 4 ) 2 based ceramics. Si 3 N 4 based ceramics is not susceptible to adhesion with iron hence used in the high speed machining of cast iron

30. Ceramic cutting tool Alumina based ceramics They have good chemical stability and low affinity with ferrous metals It is therefore not susceptible to adhesion. A1 2 O 3 is used for machining steel . Si 3 N 4 -Al 2 O 3 (sialon) ceramic tools have relatively high strength, fracture toughness, anti-oxidization, thermal conductivity, thermal shock resistance and creep resistance at high temperatures . It is used to machine Cast Iron and high temperature Ni based alloys.

31. TiC (N) based cemented carbides They are compose of Titanium Carbon Nitride ( TiCN ) with a nickel or cobalt binder They are hard and chemically stable , which leads to higher wear resistance Cermets work on materials such as S teels and Ductile Irons that produce a ductile chip Their increased speed capability enables them to machine carbon steels, stainless steels, ductile irons, and cast irons at high cutting speeds and to produce better surface finishes

32. Coated tools To achieve performance at higher temperatures coated tools are used Coating is added to the tool tip to improve cutting capabilities Coating include a blend of materials that resists heat and fracture, maintains the cutting edge and strengthens the bond between coating components and the base materials Two widely used coating methods are Chemical vapor deposition (CVD) and ( coating applied through vacuum sputtering or arc evaporation)( TiN , TICN and Al 2 O 3 ) Most are multilayers Physical vapor deposition (PVD) (CVD coated , cobalt enriched inserts will be used in 40 %—50 % of general machining operations)(carried out at 500 C similar to CVD) Both mono and multilayer coating process are feasible

33. Coated tools The three coatings most widely used today are Titanium Nitride ( T i N ) , Titanium Carbo -nitride ( T i CN ), and Titanium Aluminum Nitride ( T i A l N ).

34. TiN (Titanium Nitride) TiN is considered a good general purpose coating and is easily recognized by its gold color. The advantage of TiN Coating are Increased surface hardness, Increased tool life, Better wear resistance, and Higher lubricity, which provide less friction and reduces edge build-up. TiN coating is mostly recommended for machining low alloy steel and stainless steel.

35. TiCN TiCN coating is gray colored, harder compared to TiN . Its advantages are increased cutting speed and feeds (40 % to 60 % higher compared to TiN ) , Greater metal removal rates , and superior wear resistance. TiCN coatings are recommended for machining all material types.

36. TiAlN TiAlN coating appears gray or black and is primarily used to coat carbide. It can work at very high temperatures (800 ℃ ) , Ideal for high speed machining without coolant . Pressurized air is recommended to remove chips from the cutting zone. It works well on hardened steels, titanium and nickel alloys , as well as abrasive materials like cast iron.

37. ii) Cutting Edge Geometry In order to achieve sufficient tool life and low forces the cutting edge geometry must be optimized. Cutter speed  creats heat at the cutting edge of the tool Maintaining a high chip load or feed  heat is dissipated. Right cutter rake angle for the material being machined, produces a chip of sufficient density to carry heat from the cutting zone so work hardening can be avoided. Positive rake angles  sharper edge , pull w/p towards them during cut, chips are pushed up and away from cutting

38. ii) Cutting Edge Geometry Negative rake tools have a much stronger leading edge and tend to push against the work piece in the direction of the cutter feed. This geometry is less free cutting than positive rakes and so consumes more horsepower to cut. High speed tooling geometry, in general, mirrors the geometry of conventional machining Preferred for HSM  positive rake angle

39. iii) Spindle – tool interface HSM dependent on static and dynamic rigidity among many components Highly rigid connection between the tool, tool holder and the machine tool spindle . The results of high speed cutting depends on the interface spindle - tool and on the clamping system The interface is situated directly in the force flow between work piece and machine. The optimum design must guarantee the rapid automatic tool change and high performance functions In addition to the general requirements on cutting, (for example the transmission of torque and cutting forces), there are additional demands on HSM : small balance error, high concentricity, high run-out tolerance and position accuracy, reduced centrifugal force influenced by small radial dimensions and masses.

40. iii) Spindle – tool interface Because of the high speeds there are centrifugal effects both on the spindle cone and the tool taper Frictionally engaged transmission of torque occurs After stopping the spindle rotation  a press fit develops. To avoid axial off set  straight shank tool s can be clamped by a chuck or a hydraulic expansion chuck. A hollow taper shank enhances the secure system of locking and a fourfold strengthening of force takes place

41. Hallow taper sank for HSM (HSK) Draw Bar Gripper return mechanism Thread attachment for length adjustment Clamping cone Gripper finger Locknut for length adjustment

42. iv) Machine tools for HSM a) Machine base To obtain good dynamic performance the bases should be made out of polymer concrete . The advantages specific to material forming with appropriate materials leads to improvements and cost reduction

43. b) High frequency Main spindles The main spindle is designed as a motor spindle with an integrated motor The frequency regulated motor is always situated between the bearings A short and rigid construction results in higher critical frequencies . Special roller bearings and air bearings utilized in respect to maximum motor performances. The largest diameter is limited by the allowed circumferential speed For continuous operations a speed of approx. 125 m/s is used, for operation of short duration, 150 m/s is possible. Oil cooling lubrication not only cools the bearings but also removes the heat which flows from the rotor to the bearings. The stator is cooled by oil or water in order to limit the temperatures in the interior of the motor and therefore the bearing temperatures

44. High speed spindle

45. c) Carriages All moving machine parts  produced in lightweight construction using  aluminum titanium or fiber reinforced plastics. Mass reduction can be achieved by  reducing the linear and rotating masses a circulatory system is opened that results in the reduction of inertia of mass in adjoining machine components. Reduction of weight can be achieved by: selection of appropriate construction material, Application-specific  light weight construction, Optimize the geometric dimensioning by using finite-element analysis and investigation of the effects on adjoining machine components.

46. Finishing operaTION

47. Cemented carbide tools

48. Advantages High surface finish Increased productivity Reduced lead time Less Heat developed Low cutting forces High MRR Need of coolant can be eliminated Accuracy is more and less tolerance Production process can be reduced Disadvantages Expensive equipment Special tooling required Higher maintenance cost Tool wear is very high More complexity in tooling Difficult to find skilled labour

49. HSM Machine