Machinery's Handbook, 31st Edition
SCREW MACHINE CAM AND TOOL DESIGN 1239 20) When using a belt-shifting attachment for threading, clearance should be allowed, as it requires extra time to shift the belt. 21) When laying out a set of cams for operating on a piece that requires to be slotted, cross-drilled or burred, allowance should be made on the lead cam so that the transferring arm can descend and ascend to and from the work without coming in contact with any of the turret tools. 22) Always provide a vacant hole in the turret when it is necessary to use the transferring arm. 23) When designing special tools allow as much clearance as possible. Do not make them so that they will just clear each other, as a slight inaccuracy in the dimensions will often cause trouble. 24) When designing special tools having intricate movements, avoid springs as much as possible, and use positive actions. Stock for Screw Machine Products.— The amount of stock required for the production of 1000 pieces on the automatic screw machine can be obtained directly from Table 3 on page 1240. To use this table, add to the length of the work the width of the cut-off tool blade; then the number of feet of material required for 1000 pieces can be found opposite the figure thus obtained, in the column headed “Feet per 1000 Parts.” Screw machine stock usually comes in bars 10 feet (3.048 m) long, and in compiling this table an allow- ance was made for chucking on each bar. The table can be extended by using the following formula, in which F = number of feet required for 1000 pieces L = length of piece in inches W = width of cut-off tool blade in inches F L W 84 # = + ^ h The amount to add to the length of the work, or the width of the cut-off tool, is given in the following, which is standard in a number of machine shops: Diameter of Stock Width of Cut-off Tool Blade in mm in mm 0.000–0.250 0.000–6.35 0.045 1.44 0.251–0.375 6.38–9.53 0.062 1.57 0.376–0.625 9.55–15.88 0.093 2.36 0.626–1.000 15.90–25.40 0.125 3.18 1.001–1.500 25.43–38.10 0.156 3.96 It is sometimes convenient to know the weight of a certain number of pieces, when esti mating the price. The weight of round bar stock can be found by means of the following formulas, in which W = weight in pounds D = diameter of stock in inches F = length in feet
For brass stock: W = D 2 3 2.86 3 F For steel stock: W = D 2 3 2.675 3 F For iron stock: W = D 2 3 2.65 3 F
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