MACHINING Copyright Goodheart-Willcox Co., Inc. Chapter 26 Milling Machines 459 required number of equal spaces. For a dividing head with a 40:1 ratio, for example, cutting 10 teeth in a gear would require 1/10 of 40 turns, or four full turns of the index crank for each tooth. For 28 teeth, the number of crank turns would be 1/28 of 40 turns for each tooth. This equates to 1 12/28 (or, in lowest terms, 1 3/7) turns of the index crank for each tooth. This is where the holes in the index plate come into use: they allow fractional turns to be made accurately. To machine 28 teeth, select an index plate with a series of holes divisible by seven. On one such plate, the circles have 46, 47, 49, 51, 53, 54, and 57 holes. In this situation, 49 is divisible by 7. Thus, 3/7 of 49 equals 21 holes on the 49-hole circle. It is not neces- sary to count 21 holes each time the work is reposi- tioned after a tooth is cut. The two arms, called index fingers, are loosened. One is positioned touching the pin in the index crank the other is moved clockwise until the arms are 21 holes apart, not including the hole the pin is in. To index, rotate the crank one complete revolution, plus the section taken up by the index fingers. Drop the pin into the hole at this position and lock the divid- ing head. Move the index fingers in the same direction as crank rotation to catch up with the plunger. Repeat the operation after each cut. CNC milling machines and machining centers have taken over many of the manually operated divid- ing head operations. Servo motors are used to drive the dividing heads on these machines. A dividing head can be fully integrated into a CNC milling machine as a separate axis. This eliminates the need to inter- change indexing plates and calculate hole positions. 26.7 Horizontal Milling Operations The versatility of the milling machine permits many different machining operations to be performed. Only those operations commonly performed in the school metalworking laboratories with plain hori- zontal and vertical milling machines are described in this chapter. Automated machining operations performed on CNC milling machines and machin- ing centers will be explained in Chapter 28, CNC Machining. SAFETY NOTE SAFETY NOTE Please read through the safety precautions at the end of this chapter. Have your instructor review the safety pre- cautions with you and provide a demonstration of milling operations before attempting any of these procedures. Do not attempt these procedures unless you have been properly instructed. Be certain about what must be done and how to do it. Get help from your instructor when you are unsure of a procedure. 26.7.1 Milling Flat Surfaces Flat surfaces may be milled with a plain or slab mill- ing cutter mounted on an arbor (peripheral milling), an inserted-tooth milling cutter, or a shell face mill- ing cutter (face milling). The method employed is determined by the size and shape of the work. After you have decided on the method, use the following sequence of operations: Index crank Lock Lock Index plate Index plate Foot stock Chuck can be removed and work mounted between centers Number of holes in each circle Goodheart-Willcox Publisher Figure 26-40. Dividing head and foot stock.