Cutting Tool Design Machineability, Tooling Cost

1. Cutting Tool DesignMachineability, Tooling Cost Tool Design, 3331 Dr Simin Nasseri Southern Polytechnic State University

2. Economic and Production Design Consideration in Machining Machinability Tolerances and Surface Finish Selection of Cutting Conditions Product Design Considerations in Machining

3. Machinability Relative ease with which a material (usually a metal) can be machined using appropriate tooling and cutting conditions Depends on work material and Type of machining operation, tooling, and cutting conditions

4. Machinability Criteria in Production Tool life � longer tool life for the given work material means better machinability Forces and power � lower forces and power mean better machinability Surface finish � better finish means better machinability Ease of chip disposal � easier chip disposal means better machinability

5. Machinability Testing Most tests involve comparison of work materials Performance of a test material is measured relative to a base material Relative performance is expressed as a machinability rating (MR) MR of base material = 1.00 (100%) MR of test material > 1.00 (100%) means better machinability

6. Example A series of tool life tests are conducted on two work materials under identical cutting conditions, varying only speed in the test procedure. The first material (test material) yields a Taylor equation VT0.28=350, and the other material (test material) yields a Taylor equation VT0.27=440, where speed is in m/min and tool life is in min. Determine the machinability rating of the test material using the cutting speed that provides a 60-min tool life as the basis of comparison. The speed is denoted by V60.

7. Machinability Tests Tool life (most common test) Tool wear Cutting force Power required in the operation Cutting temperature Material removal rate under standard test conditions

8. Mechanical Properties of the Workpiece and Machinability Hardness High hardness of the part means abrasive wear increases so tool life is reduced Strength High strength means higher cutting forces, specific energy, and cutting temperature Ductility High ductility means tearing of metal as chip is formed, causing chip disposal problems and poor surface finish

9. Tolerances Tolerances Machining provides high accuracy relative to most other shape-making processes Closer tolerances usually mean higher costs

10. Surface Finish Surface roughness in machining determined by: Geometric factors of the operation (Type of machining operation (e.g., milling vs. turning), Tool geometry (especially nose radius) and Feed Work material factors (Built-up edge effects, Damage to surface caused by chip, Tearing of surface when machining ductile materials, Cracks in surface when machining brittle materials, Friction between tool flank and new work surface) Vibration and machine tool factors (Chatter (vibration) in machine tool or cutting tool, Deflections of fixtures, Backlash in feed mechanism)

11. Effect of Work Material Factors

12. Selection of Cutting Conditions

13. Selection of Cutting Conditions One of the tasks in process planning For each operation, decisions must be made about machine tool, cutting tool(s), and cutting conditions Cutting conditions: depth of cut, feed, speed, and cutting fluid These decisions must give due consideration to workpart machinability, part geometry, surface finish, and so forth

14. Selecting Depth of Cut Depth of cut is often predetermined by workpiece geometry and operation sequence In roughing, depth is made as large as possible to maximize material removal rate, subject to limitations of horsepower, machine tool and setup rigidity, and strength of cutting tool In finishing, depth is set to achieve final part dimensions

15. Determining Feed Select feed first, speed second Determining feed rate depends on: Tooling � harder tool materials require lower feeds Is the operations roughing or finishing? Constraints on feed in roughing Limits imposed by forces, setup rigidity, and sometimes horsepower Surface finish requirements in finishing Select feed to produce desired finish

16. Optimizing Cutting Speed Select speed to achieve a balance between high metal removal rate and suitably long tool life Mathematical formulas available to determine optimal speed Two alternative objectives in these formulas: Maximum production rate Minimum unit cost

17. 1- Maximum Production Rate In turning, total production cycle time for one part consists of: Part handling time per part = th Machining time per part = tm Tool change time per part = tt /np, where np = number of pieces cut in one tool life

18. 1- Maximum Production Rate Total time per unit product for operation: tc = th + tm + tt / np Cycle time tc is a function of cutting speed

19. Cycle Time vs. Cutting Speed

20. 2- Minimizing Cost per Unit In turning, total production cycle cost for one part consists of: Cost of part handling time = Coth , where Co = cost rate for operator and machine Cost of machining time = Cotm Cost of tool change time = Cott / np Tooling cost = Ct / np , where Ct = cost per tool life or cost per cutting edge

21. 2- Minimizing Unit Cost Total cost per unit product for operation: Cc = Coth + Cotm + Cott / np + Ct / np Again, unit cost is a function of cutting speed, just as tc is a function of V

22. Unit Cost vs. Cutting Speed

23. Comments on Machining Economics As C and n increase in Taylor tool life equation, optimum cutting speed increases Cemented carbides and ceramic tools should be used at speeds significantly higher than for HSS

24. Comments on Machining Economics

25. We can now put the two optimums in perspective: The Economics of Metal Cutting

26. Product Design Guidelines

27. Product Design Guidelines Design parts that need no machining Use net shape processes such as precision casting, closed die forging, or plastic molding If not possible, then minimize amount of machining required Use near net shape processes such as impression die forging

28. Product Design Guidelines Tolerances should be specified to satisfy functional requirements, but process capabilities should also be considered Very close tolerances add cost but may not add value to part

29. Product Design Guidelines Machined features such as sharp corners, edges, and points should be avoided They are difficult to machine Sharp internal corners require pointed cutting tools that tend to break during machining Sharp corners and edges tend to create burrs and are dangerous to handle

30. Product Design Guidelines Select materials with good machinability As a rough guide, allowable cutting speed and production rate correlates with machinability rating of a material Thus, parts made of materials with low machinability take longer and cost more to produce

31. Design parts with features that can be produced in a minimum number of setups Example: Design part with geometric features that can be accessed from one side of part Product Design Guidelines

32. Product Design Guidelines Machined parts should be designed with features that can be achieved with standard cutting tools Avoid unusual hole sizes, threads, and features requiring special form tools Design parts so that number of individual cutting tools needed is minimized

33. The Economics of Metal CuttingSummary

34. The Economics of Metal Cutting As with most engineering problems we want to get the highest return, with the minimum investment. In this case we want to minimize costs, while increasing cutting speeds. EFFICIENCY will be the key term - it suggests that good quality parts are produced at reasonable cost. Cost is a primarily affected by, tool life power consumed

35. The production output is primarily affected by, accuracy including dimensions and surface finish RMR (metal removal rate) The factors that can be modified to optimize the process are, cutting velocity (biggest effect) feed and depth work material tool material tool shape cutting fluid The Economics of Metal Cutting

36. The Economics of Metal Cutting We previously considered the log-log scale graph of Taylor's tool life equation, but we may also graph it normally to emphasize the effects.

37. The Economics of Metal Cutting Low cost - exemplified by low speeds, low MRR, longer tool life