Engineering Schemes

Classification Txe

The types of dividing heads used on milling machines are plain, universal, and helical. Dividing heads can also be classified as to size (for example, the Cincinnati Milacron Company dividing head is available in three different sizes).

Plain Dividing Head

The basic parts of a plain dividing head are shown in Figure 13-2 and Figure 13-3. In a plain dividing head, the spindle rotates about a horizontal axis.

The principal parts of the dividing head are spindle, worm wheel, worm, index plate, index pin, and sector.

The worm wheel is keyed to the spindle so that the spindle turns with the wheel. The worm wheel has 40 teeth on most dividing heads.

The worm meshes with the worm wheel. The ratio of the two gears is 40:1. Therefore, 40 revolutions of the worm are required to turn the worm wheel one complete revolution.

The index plate is one of a set that is provided with the dividing head. The index plate can be removed easily and another plate substituted as necessary for the desired spacing. Each index plate consists of several circular rows of holes—each circular row having a different number of holes.

The index pin is located on the end of the crank, which is attached to the worm shaft. The crank is used to rotate the spindle through the worm gearing. The arm length of the crank is adjustable so that the index pin can drop into any hole in any circular row of holes.

The sector consists of two radial arms constructed in such a manner that the angle between them can be changed and locked by a clamp screw in any included angular position. The sector can be used to save time and reduces the possibility of an error in counting the number of holes for each movement of the index pin.

In actual operation, the index crank should be adjusted for the correct circular row of holes. The index pin is dropped into the correct hole in the row. Then, rotate the sector arm A (see Figure 13-2) against the left-hand side of the index pin. Next, move the sector arm B in the same direction that the index pin is to turn until the correct number of holes is counted between the pin and the sector arm B. Now lock both sector arms in position. Never count the hole in which the index pin has been inserted; this hole is the "zero" hole.

Figure 13-2 Diagram showing basic parts of a plain dividing head (side view).

Universal Dividing Head

The universal dividing head differs from the plain dividing head in that the spindle can be tilted (that is, swiveled to any angular position in a vertical plane, within the angular range provided). As shown in Figure 13-4, the casting that carries the spindle is

PLAIN HEAD

Figure 13-3 Diagram showing interior parts of a plain dividing head (cross-sectional view).

Figure 13-3 Diagram showing interior parts of a plain dividing head (cross-sectional view).

mounted in a circular guide, forming part of the swivel or universal head. The spindle can be tilted to any angular position AB by rotating the spindle casting, which can be clamped in any angular position by tightening the angular clamp bolt. The circular guide is graduated in degrees and fractional degrees.

Figure 13-4 Diagram of the universal dividing head, showing the arrangement for swiveling the spindle (side view).

The universal dividing head is built for a greater range of work than the plain dividing head. For some kinds of tapered work, it is necessary to tilt the spindle at an angle to the table. The universal dividing head is designed for this kind of work. Angular graduations at the top of the housing serve as a guide to setting the spindle at any angle with reference to the horizontal. Figure 13-5 shows a milling operation in which the spindle is tilted at an angle.

Figure 13-5 Setup for the dividing head for milling cams.

(Courtesy Cincinnati Miiacron Co.)

Figure 13-5 Setup for the dividing head for milling cams.

(Courtesy Cincinnati Miiacron Co.)

Helical Dividing Head

The helical dividing head differs from the plain and universal types in that the spindle of the head can be connected to the table lead screw by gears so that the work can be rotated as it is moved longitudinally by the table. The two movements are in a definite ratio that is determined by the combination of gears used. These gears are similar to the feed gears on a lathe.

The combination of the rotary and longitudinal movements causes the tool to cut a helix AB, as shown in Figure 13-6. The cutting tool begins at A, cutting the helix from A to J3 as the tool rotates in the direction indicated by C, while the table is moving the work in the direction indicated by D. The helix cut is either a right-hand or left-hand helix, depending on the direction in which the work is rotated.

The pitch of the helix depends on the rate of rotation of the work with respect to the movement of the table. Milling the flutes of a cast-iron rotor is shown in Figure 13-7.

HELIX

(ERRONEOUSLY CALLED "SPIRAL"!

HELIX

(ERRONEOUSLY CALLED "SPIRAL"!

Figure 13-6 Diagram showing the production of a helix by a combination of rotary and longitudinal feeds.

Figure I 3-7 Milling the flutes of a cast-iron rotor. (Courtesy Cincinnati macron Co.)

Figure I 3-7 Milling the flutes of a cast-iron rotor. (Courtesy Cincinnati macron Co.)

The basic parts of a helical dividing head are shown in Figure 13-8. A plain dividing head is driven by a gear train from the table feed screw, which converts it into a helical dividing head. A bevel gear (shown in dotted lines in Figure 13-3 on the shaft to which the worm is attached) is usually free to rotate on the worm shaft. The index plate is fastened to the hub of the bevel gear and is kept from rotating on the shaft by a stop pin in the housing. The index crank is attached to the worm shaft (see Figure 13-3). If the stop pin is withdrawn and the index pin is inserted in one of the holes in the index plate, the worm shaft and the bevel gear are locked together. Hence, any motion transmitted to the bevel gear is transmitted to the spindle through the gear train, index plate, index pin, index crank, worm, and worm wheel (see Figure 13-3).

Figure 13-8 Diagram of a plain dividing head, showing the gear train used in cutting a helix.

The bevel gear (see Figure 13-3) meshes with another bevel-gear shaft (Figure 13-8). Spur gears of various sizes can be attached to the outer end of the bevel-gear shaft.

The bevel-gear shaft (which drives the spindle) is driven by the table feed screw through the gear train, which consists of the feed screw gear, two stud gears, and the worm gear (see Figure 13-8). The worm gear is actually a spur gear, but it is usually called the worm gear because it drives the worm in the driving head.

Various changes in the relative movements of the table feed screw and the spindle can be obtained by using gears of different sizes. Figure 13-8 shows only a single gear train. Sometimes an idler gear is interposed, as on an engine lathe when the selected gear diameters do not permit direct connection of the feed screw and the bevel-gear shaft.

+1 0

Average user rating: 5 stars out of 1 votes