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OTHER WORK-HOLDING ACCESSORIES FOR THE HORIZONTAL MILL

A rotary table or circular milling attachment (Figure K-37) provides rotary movement to a workpiece. The rotary table shown in this  illustration is used to machine a spur gear. The cutting movement is accomplished by raising and lowering the knee. The rotary table can be used for angular indexing, milling circular grooves, or cutting radii.

FOR THE HORIZONTAL MILL K-37 FOR THE HORIZONTAL MILL K-38
The dividing head (Figure K-38) is shown cutting a small spur gear. Work is held between centers, in collets, or in a chuck. The indexing head, also called the dividing head, is a useful milling machine accessory. Figure K-39 identifies the major parts of an indexing head. It is used to rotate the workpiece a full or partial turn to machine a specific number of divisions, an angle, or a circular feature. The indexing
head gets its rotational movement from a worm and worm gear assembly (Figure K-40). The index crank is geared to the spindle worm gear, most commonly in a 40:1 ratio. This means that 40 complete turns of the index crank will result in 1 complete revolution of the spindle. A lock is provided so that the spindle may be secured at a particular setting to increase the rigidity of a setup.

FOR THE HORIZONTAL MILL K-39 FOR THE HORIZONTAL MILL K-40
The index crank contains a pin on a spring-loaded plunger that engages the holes in the index plate. Hole circles in the index plate provide the indexing function. The pattern on the plate consists of equally spaced hole circles, and the crank pin is adjusted in and out radially so that the desired hole circle may be used. Two sector arms rotate about the crank hub and can be adjusted to indicate the correct number
of holes for a partial turn of the crank. For example, if you are indexing one turn plus 11 holes for each division, the sector arms can be set so that you will not have to count the 11 extra holes each time around (Figure K-41). The sector arms rotate about the index crank hub and are locked in place by a lock screw. The facing edges of the arms are beveled, and the number of holes required for fractional turns is established
between the beveled edges.

FOR THE HORIZONTAL MILL K-41 FOR THE HORIZONTAL MILL K-42
Remember that the number of holes required for a fractional turn will be new holes.
The hole in which the crank pin is engaged is not counted. The index head is secured to the mill table and aligned with the alignment keys that fit the table T-slots. The end of a workpiece is supported by a footstock (Figure K-42).
Common machining operations that involve indexing include cutting splines, keyseats, hexagons, octagons, squares, and other geometric shapes (Figure K-43). The indexing calculation to machine a square, when using an indexing head with a 40:1 gear ratio, is The result 10 means that 10 full revolutions (turns) of the index crank are required from one division to the next. The calculation for a hexagon would be
This fraction shows that 6 complete index crank turns plus turn are needed. To achieve this partial turn of , reduce this fraction to its lowest common denominator, which is . The available index plate has 8 circles of 24, 25, 28, 30, 34, 37, 38, and 39 holes. Three of these hole circles
(24, 30, and 39) can be divided by this lowest common denominator of 3.

FOR THE HORIZONTAL MILL K-43
To operate the index head, unlock the spindle and pull back the crank pin plunger. Rotate the crank the required number of turns plus the fraction of a turn. Set the sector arms to keep track of the additional holes beyond the full turn of the crank. Bring the crank pin around until it just drops into the required hole. If you overshoot the position, back off well past the required hole and come up to it once again. This
will eliminate any backlash in the worm and worm gear assembly. Always index in only one direction, and always lock the spindle before beginning the machining operation. After each indexing move, the sector arms must be rotated so that they will be properly positioned for the next move.

 

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