General producibility requirements meant for design of machine parts and machining techniques may be formulated as follows:
1. The amount of machining should be reduced, as far as possible, by assigning size tolerances only for fits between mating surfaces; all other elements should have free dimensions. Parts may be
obtained without any machining if precise methods of blank manufacture are applied (see Fig. 6.18).
2. Convenient and reliable locating surfaces should be provided to set up workpiece for machining. Whenever possible, the measurement datum should be made to coincide with the set-up datum surface by proper dimensioning of the part drawing.
3. There should be sufficient rigidity of workpiece so as to eliminate significant deformation in the process of machining.
4. Provisions should be made for conveniently advancing rigid, high-production cutting tools to the surface being machined. Difference in height between adjacent, rough and machined surfaces should be sufficient, making adjustment for the machining allowances, to enable the cutting tools to clear the rough surface in its overtravel (when errors in rough blank size are within the permissible limits). 5. Clearance recesses: Dimension A should be provided to allow overtravel of the cutting tool whenever it is necessary, see Figs. 6.19 and 6.20.
Fig. 6.19 Provision of allowance for tool Fig. 6.20 overtravel.
Provision of allowance for tool overtravel in milling a slot.
6. Parts should be designed so that several workpieces can be set up to be machined simultaneously, as shown in Fig. 6.21. The following considerations are important for elementary surfaces of machine parts:
Fig. 6.21 Increasing production through mounting two gears
on one shaft during gear cutting.
7. External surfaces of revolution, upset heads, flanges and shoulders should be extensively applied to reduce machining and to save metal.
8. It is advisable to retain the centre holes on the finished components (shafts and similar parts) that were machined between centres.
9. The elements of shank design should be unified, whenever possible, so that the same multiple-tool set-up can be employed in machining them, as illustrated in Fig. 6.22.
Fig. 6.22 Unified shank design.
10. It is a good practice to provide a spherical convex surface with a flat end surfaced (see Fig. 6.23).
Fig. 6.23 Provision of flat end surface on a sphere.
11. Holes:
(a) Through holes are to be used, wherever possible, because such holes are much more simple to machine than blind holes. The form of blind holes should correspond to the design of the tool to be employed in machining for example, with the reamer (Fig. 6.24(a)) or counterbore (Fig. 6.24(b)). (b) Holes should not be located closer to a certain minimum distance from an adjacent wall of the part:
Fig. 6.24 Design recommendation for reamed holes.
A ³ D/2 + R (Fig. 6.25). This distance for holes accommodating fastening bolts should be A ³ Dn/2 +
R, where Dn is the diameter of a circle circumscribing the nut.
(c) Centre distances of holes should be specified, by considering the possibility of using multispindle drilling heads. For this purpose, the location and sizes of the holes in flanges have to be unified. The number of holes and their location in a flange should be designed so that the holes can be drilled by standard three or four spindle heads with subsequent indexing.
(d) Holes to be drilled should have their top and bottom surface square to the hole axis to prevent drill breakage.
(e) When several holes are located along the same axis, it is a good practice to reduce the diameter of each consequent hole by an amount exceeding the machining allowance for the preceding hole. This will enable a set-up to be used in which all the holes are bored simultaneously.
(f) In drilling holes at the bottom of a slot, their diameter should be less by 0.5–1 mm than the slot width.
(g) In stepped holes, maximum accuracy should be specified for the through step.
(h) Either a blind hole or a through hole should be provided on the axis in the design of concave spherical surfaces to avoid zero cutting speeds at the axis, as shown in (Fig. 6.26), thus preventing damage to the tool point.
Fig. 6.26 Design recommendation for concave spherical surfaces.
(i) It is advisable to avoid recesses in holes that are to be machined on single or multiple-spindle drilling machines since they complicate machining operations. Machined recesses should also be avoided by using cored recesses (see Fig. 6.27).
Fig. 6.27 Provision of cored holes.
12. Threads:
(a) It is advisable to use an entering chamfer on threaded holes.
(b) The number of incomplete threads cut with a tap in a blind hole with no recess should be equal to three for grey iron casting and five for steel parts.
(c) A neck at the end of a thread is not required for milled threads.
(d) Preferred thread standards should pertain, not only to the machine under consideration,
but to all the threads used in the plant or branch of industry. Small diameter threads (6 mm and less) should be avoided if they are cut.
(a) The outline of a machined flat surface should ensure, as far as possible, uniform and impactless chip removal.
(b) The size of a machined flat surface should be in accordance with the sizes of standard milling cutters, i.e., the width of the surfaces should be unified to suit the standard services of face mill diameters or the widths of plain milling cutters.
(c) If no elements are provided for cutting tool overtravel, the transition surfaces should correspond in size and form to the cutting tool (as illustrated in Figs. 6.28 and 6.29).
Fig. 6.28 Assigning proper radius R on component. Fig. 6.29
14. Formed surfaces: The radii of concave and convex surfaces should correspond to those of standard convex and concave milling cutters.
15. Slots and recesses: Whenever possible, through slots should be employed. If through machining is possible, the end of the slot should correspond to the radius of the cutter, as shown in Fig. 6.30.
The width and depth of slots should be specified according to the sizes of standard side milling, viz. end mills. The corner radii at the bottom of recess should be the constraint for all around recess (Fig. 6.31), and should correspond to the size.
Fig. 6.30 Assigning proper slot depth and radius.
Fig. 6.31 Assigning correct radii R.