Machinery's Handbook, 31st Edition
1238 SCREW MACHINE CAM AND TOOL DESIGN Threading cams are often cut on a circular milling attachment. When this method is employed, the number of minutes the attachment should be revolved for each 0.001 inch rise, is first determined. As 15 spindle revolutions are required for threading and 400 for completing one piece, that part of the cam surface required for the actual threading opera tion equals 15 ÷ 400 = 0.0375, which is equivalent to 810 minutes of the circumference. The total rise, through an arc of 810 minutes is 0.413 inch, so the number of minutes for each 0.001 inch rise equals 810 ÷ 413 = 1.96 or, approximately, two minutes. If the attach ment is graduated to read to five minutes, the cam will be fed laterally 0.0025 inch each time it is turned through five minutes of arc. Practical Points on Cam and Tool Design.— The following general rules are given to aid in designing cams and special tools for automatic screw machines, and apply particularly to Brown and Sharpe machines: 1) Use the highest speeds recommended for the material used that the various tools will stand. 2) Use the arrangement of circular tools best suited for the class of work. 3) Decide on the quickest and best method of arranging the operations before designing the cams. 4) Do not use turret tools for forming when the cross-slide tools can be used to better advantage. 5) Make the shoulder on the circular cutoff tool large enough so that the clamping screw will grip firmly. 6) Do not use too narrow a cutoff blade. 7) Allow 0.005 to 0.010 inch (0.127–0.254 mm) for the circular tools to approach the work and 0.003 to 0.005 inch (0.076–0.127 mm) for the cutoff tool to pass the center. 8) When cutting off work, the feed of the cutoff tool should be decreased near the end of the cut where the piece breaks off. 9) When a thread is cut up to a shoulder, the piece should be grooved or necked to make allowance for the lead on the die. An extra projection on the forming tool and an extra amount of rise on the cam will be needed. 10) Allow sufficient clearance for tools to pass one another. 11) Always make a diagram of the cross-slide tools in position on the work when difficult operations are to be performed; do the same for the tools held in the turret. 12) Do not drill a hole the depth of which is more than 3 times the diameter of the drill, but rather use two or more drills as required. If there are not enough turret positions for the extra drills needed, make provision for withdrawing the drill clear of the hole and then advancing it into the hole again. 13) Do not run drills at low speeds. Feeds and speeds recommended in Table 1 on page 1235 should be followed as far as is practicable. 14) When the turret tools operate farther in than the face of the chuck, see that they will clear the chuck when the turret is revolved. 15) See that the bodies of all turret tools will clear the side of the chute when the turret is revolved. 16) Use a balance turning tool or a hollow mill for roughing cuts. 17) The rise on the thread lobe should be reduced so that the spindle will reverse when the tap or die holder is drawn out. 18) When bringing another tool into position after a threading operation, allow clearance before revolving the turret. 19) Make provision to revolve the turret rapidly, especially when pieces are being made in from three to five seconds and when only a few tools are used in the turret. It is some times desirable to use two sets of tools.
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