Machining vs. Grinding

Transcription

1 University of Connecticut Machining vs. Grinding -- Towards High Efficiency Machining Bi Zhang Mechanical Engineering

2 Presentation Sequence Introduction High Speed Machining High Speed Peel Grinding HSP Grinding vs. Hard Turning High Speed Spindle Component Linear Motor vs. Ballscrew Drives Future Topics Concluding Remarks

3 Introduction to Machining Machining is the material removal process that transforms raw material into a finished part. Raw material Finished part Source: T.F. McClure 2002

4 Machining vs. Grinding Machining is the material removal process that shears/damages the material Chip Tool Chip Abrasive grain Workpiece Workpiece Cutting with a positive Abrasive grinding with a negative

5 Introduction to Machining Chip removal temperature vs. machining speed Chip removal temperature, C Steel Cast iron Bronze Tungsten carbide: 900 C HSS: 650 C Carbon steel: 450 C Non-ferrous metals Traditional range Machining speed, m/min 3000 Source: Pasko et al., 2000

6 Introduction to Machining Source: Tlusty 1999

7 Introduction to Machining Source: Kalpakjian 2000

8 Introduction to Machining Comparison of specific energies between machining and grinding (Unit HP/in. 3 /min) Material Machining Grinding Aluminum Class 40 Gray Iron Low Carbon Steel Titanium Hardened Tool Steel (67 Rc) Source: Kalpakjian 2000

9 Introduction to Machining How to realize high efficiency machining? 1. Increasing machining/grinding speed 2. Increasing DOC, feedrate, etc. 3. Reducing machining downtime (tool change, wheel truing/ dressing, workpiece setup, machine setup, workpiece transfer, etc.) Source: Kalpakjian 2000

10 High Speed Hard Turning Source: Beard 2001, MS Online

11 High Speed Hard Turning Hard Turning vs. Conventional Turning Advantages Short cycle times; Low cost to machine investment; High accuracy if the green part is accurate; Good surface finishes; Machining heat mostly dissipated with the chips; High material removal rate (2-4 times); Multiple operations in a single chucking; Good for wet or dry cutting. Disadvantages Fast tool wear; High stiffness and high speed machine tools; Advanced machine controller; Good tool holder for high speed (e.g., dynamic balance and chucking); Advanced cutting tool materials and coatings.

12 High Speed Hard Turning Source: Fallbohmer et al. 2000

13 High Speed Hard Turning Centrifugal force 2 F c m r Clamping tight enough? Example: F=1,000 N at 3,000 rpm F=100,000 N at 30,000 rpm Source: Beard 2001, MS Online

14 High Speed Hard Turning Cutting Tools Tool strength vs. geometry Example: To soften a workpiece for an easy material removal, nickel alloys are better cut at about 1,200 C. Choosing right tool material (e.g. CBN and SiC whisker reinforced Al 2 O 3 ) and machining conditions (speed, feedrate, DOC) is critical. Source: Koepfer 2001, MS Online

15 High Speed Peel Grinding High-speed peel grinding with narrow superabrasive wheels Parallel axis Tilt axis Source: Koepfer 2001

16 High Speed Peel Grinding Advantages of HSP over Conventional Grinding High speed (up to 200 m/sec) for high efficiency; High material removal rate (up to 100 mm 3 /mm/s); Reduced grinding forces (less critical to machine stiffness); Reduced area of contact with the workpiece; Easier coolant delivery to the grinding zone; Larger G-ratio (up to 6000); Multiple operations in a single setup of the workpiece. Source: Luetjens 2004

17 HSP Grinding vs. Hard Turning For workpiece materials too hard or flexible to turn; For workpieces of different materials in different sections; Insensitive to breakage of individual cutting edges; Better surface finish and surface quality; Better geometric tolerance control due to spark-out capability to kiss workpiece surface. Source: Suzuki and Yamada 2001

18 High Speed Spindle Component Motorized spindle with hybrid ceramic bearings Motorized spindle Hybrid ceramic bearing

19 High Speed Spindle Component Comparison among the high speed spindles Bearing type Rolling element Magnetic Aerostatic High static and High speed due to large bearing High dynamic stiffness dynamic stiffness gap Low frictional loss Pros Simple structure Low cost Very low frictional loss Very high static stiffness Bearing characteristics highly High rotational accuracy controllable On-line monitoring of bearing conditions from bearing sensors High frictional loss Low dynamic stiffness Low static stiffness Cons Periodic maintenance needed Complicated structure High cost Low load capacity Contamination due to lubrication Source: Suzuki and Yamada 2001

20 High Speed Spindle Component Motorized spindle with magnetic/aerostatic hybrid bearings Air inlet Axial air bearing Water jacket Radial air bearing Built-in motor Cutter Tool holder Radial air bearing Sensor Axial air bearing Unclamping unit Spindle: Permalloy-Nickel alloy containing about 20 to 60% Fe, having low thermal expansion coefficient, high magnetic permeability and electrical resistivity Source: Suzuki and Yamada 2001

21 High Speed Spindle Component Motorized spindle with magnetic/aerostatic hybrid bearings Maximum speed Spindle diameter Spindle material Spindle coating Load capacity Static stiffness Dynamic stiffness Motor power Supply air pressure 60,000 rpm 40 mm Permalloy (Nickel alloy containing 20-60% Fe, having low thermal expansion coefficient, high magnetic permeability & electrical resistivity) Spray coated ceramics Axial: 600 N; Radial: 400 N Axial: 500 N/ m; Radial: 100 N/ m 20 N/ m at 1 khz Nominal: 4.4 kw; Maximum: 7.8 kw 0.98 MPa Source: Suzuki and Yamada 2001

22 High Speed Spindle Component Load, Aerostatic only Air+Magnetic Workpiece surface profile, m Cutter: 6 endmill Speed: rpm Feedrate:3 m/min Axial DOC: 5 mm Radial DOC: 0.5 mm MB on Uncut MB off Axial displacement, m Axial stiffness Workpiece feed direction, mm Machining performance Source: Suzuki and Yamada 2001

23 Linear Motor vs. Ballscrew Drives Source: Hyatt

24 Linear Motor vs. Ballscrew Drives Source: Copley Controls

25 Linear Motor vs. Ballscrew Drives Drive type Pros Cons Linear Motor Easy to operate at high speed and high acceleration; Less maintenance even at high speed and high acceleration operations; Easy to achieve fast and accurate positioning because of direct drive; Higher dynamic stiffness and servo bandwidth; Unlimited stroke. Lower thrust under a given limitation of stage size; Higher cost; Difficult application (pulling force is 3 times of pushing force); Non stop in case of power loss, crushing problem. Ballscrew Larger thrust under a given limitation of stage size; Lower cost due to mass production; Optimal deceleration during loss of power; Better damping effect to minimize chatter. Limited Speed and acceleration; Frequent maintenance if operated at high speed and acceleration due to wearing Lower dynamic stiffness due to backlash; Difficult positioning due to backlash; Limited stroke.

26 Future Topics Engineered machining tools; Modern machines (e.g., fewer components, light and stiff structure); Intelligent machines (nanostructured sensors; sensor fusion, etc.); New cutting tool materials and coatings.

27 Future Topics Milling/ Turning??? Grinding

28 Future Topics Hard fiber/ whiskers Hard fiber/whisker wheel Quick change mechanism to allow for 1 m radial runout

29 Concluding Remarks HEM/HSM is an everlasting topic. Grinding and Milling will merge into a single whole. Today s HEM/HSM will be regarded as tomorrow s conventional machining. HEM/HSM always represents the fastest, most efficient and inexpensive machining technology of its era.

30 Thank You Questions?