Design for machining

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Myra Fisher
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1 Design for machining Machining processes are material removal processes which are a family of shaping operation in which excess or undesired material is removed from the work piece finally remaining with the desired geometry. The design of machining considers the conventional machining processes. In conventional machining a sharp cutting tool is used to mechanically cut the material to achieve the desired geometry. Three major machining processes are turning, drilling and milling. There are other machining operation include shaping, planning, broaching etc. Figure M3.1.1 shows the cross-sectional view of machining process. Figure M3.1.1: A cross sectional view of the machining process Advantages of machining processes Machining can be applied to a variety of work material. Virtually all solid metals can be machined. It can be used to create any regular geometry It can produce parts with dimensions to very close tolerances (±0.025mm) It is possible to achieve very good surface finishes (roughness value less than 0.4 microns) Disadvantage of machining process Wastage of material in the form of chips ( though chips can usually be recycled)

2 Very time consuming operation when compared to other alternative processes like casting or forging. Typical machined parts There are numerous applications of machined parts in industrial and consumer products. The materials used for machined components are both ferrous and nonferrous metals. Machined parts can vary from as small as miniature screw, shafts, gears, and other parts found in wrist watches and small precision instruments to as big as huge turbines, turbine housings. Recommended materials for machinability Parts made from both ferrous and non-ferrous material can be machined. However, materials like plastics (with or without reinforcement), hard rubber, carbon, graphite, wood, and ceramics are also often used. A summary of the machinability of common metals has been provided in Table M Table M3.1.2 summarizes the influence of certain materials properties on machinability. Table M3.1.1: Process for flat surface. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) Process Maximum size, flat Deviation Surface finish area from flatness Sand-mould casting 3 3 m 4.2 mm/m µm Die casting 0.6 m mm/m µm Planer and shaper Planer to 1 4 m 0.4 mm/m µm machining Milling-machine machining Planer type to 1 4 m 0.4 mm/m µm Surface grinding m 0.08 mm/m µm Sheet metal (cold rolled) m 1.3 mm/m µm Sheet metal (hot rolled) m 5.2 mm/m µm Magnesium tooling plate m 0.8 mm/m 0.8 µm

3 Table M3.1.2: Effects of Material Properties. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) Decrease in Machinability Finishability Tool life Material factor Strength/hardness Improves None Improves Ductility Improves Improves Improves Strain hardenability Improves Improves Improves Coefficient of friction Improves Improves Improves Heat conductivity None None Reduces Heat capacity None None Reduces Chemical reactivity None Improves Improves Grain size Improves Improves Reduces Abrasive insoluble Improves Improves Improves Free-machining additions Decreases Decreases Decreases Design recommendations Design recommendations for machined parts are listed below 1. It is always recommended to avoid machining operations if possible. It is always less costlier to produce a surface or feature by processes like casting or forming.(refer Figure M3.1.2) Figure M3.1.2: Avoid tolerances that involve machining without affecting the parts function.

4 2. The prime operations are to be simplified by considering the most liberal surface finish and dimensional tolerances consistent with the function of the surface, so that the costly secondary machining operation like grinding, reaming lapping etc. can be avoided. (Refer Figure M3.1.2) 3. Parts should be designed in such a way that it will be easy for fixturing and secure holding during machining operation. (Refer Figure M3.1.3) Figure M3.1.3: Design planer and shaper machined parts to withstand cutting-tool forces and to be solidly clamped. 4. Since sharp corners and sharp point in cutting tool are more prone to breakage, in the design these features should be avoided. 5. Use stock dimension whenever possible. This will eliminate machining operation or the need for machining additional surface (Refer Figure M3.1.4).

5 Not Figure M3.1.4: Use stock dimensions whenever possible to minimize machining. 6. In all single-point machining operations, it is recommended to avoid interrupted cuts, if possible. will shorten the tool life or will not allow the use of faster-cutting carbide or ceramic tools. 7. Parts should be rigid enough to withstand the forces of clamping and machining without distortion. (Refer Figure M3.1.5.) Not these Figure M3.1.5: Design the part to be rigid to withstand cutting and clamping forces 8. If possible rectangular shapes are preferred because of simple tooling and setup than the use of tapers and contours. 9. It is recommended to reduce the number and the size of the shoulders as these usually require extra operational steps and additional material.

10. Avoid undercuts to avoid separate operation of specially ground tools as shown in Figure M3.1.6. Not these Figure M3.1.6: Avoid undercuts as much as possible 11.

6 10. Avoid undercuts to avoid separate operation of specially ground tools as shown in Figure M Not these Figure M3.1.6: Avoid undercuts as much as possible 11. If possible, it is preferred to substitute a stamping operation for the machine component. Cost wise stamped parts are more economical. (Refer Figure M3.1.7) Figure M3.1.7: Stampings are often less costly than machining 12. Avoid machining of hardened or difficult-to-machine materials unless until their functional properties are essential for the part to be machined. 13. Sufficient allowance should be provided to the stock for both rough and finish machining. Often stress relieving between finish and rough machining is recommended. Recommended stock for finish machining is 0.4 mm. 14. Number of operations required are reduced by using the same plane for subsequent machining or same diameter if they are cylindrical.

15. It is required to provide access room for cutters bushing and fixture element. 16. Work piece is to be designed in such a way that standard cutters can be used. (Refer Figure M3.1.8) Not these Figure M3.

7 15. It is required to provide access room for cutters bushing and fixture element. 16. Work piece is to be designed in such a way that standard cutters can be used. (Refer Figure M3.1.8) Not these Figure M3.1.8: Design parts so that standard cutting tools can be used. 17. It is recommended to avoid parting lines or draft surfaces for clamping or locating surfaces. 18. It is required to provide relief space for burr removal.

Design for machining

Multiple choice questions Design for machining 1) Which one of the following process is not a machining process? A) Planing B) Boring C) Turning D) Forging 2) The angle made between the rake face of a