Milling Machine and it's Operation-Part 1

Introduction:
           Milling machines were first invented and developed by Eli Whitney to mass produce interchangeable musket parts. Although crude, these machines assisted man in maintaining accuracy and uniformity while duplicating parts that could not be manufactured with the use of a file. Development and improvements of the milling machine and components continued, which resulted in the manufacturing of heavier arbors and high
speed steel and carbide cutters. These components allowed the operator to remove metal faster, and with more accuracy, than previous machines. Variations of milling machines were also developed to perform special milling operations. During this era, computerized machines have been developed to all eviate errors and provide better quality in the finished product.

Milling Machines:
      a. General. The milling machine removes metal with a revolving cutting tool called a milling cutter. With various attachments, milling machines can be used for boring, slotting, circular milling dividing, and drilling. This machine can also be used for cutting keyways, racks and gears and for fluting taps and reamers.
      b. Types. Milling machines are basically classified as being horizontal or vertical to indicate the axis of the milling machine spindle. These machines are also classified as knee-type, ram-type, manufacturing or bedtype, and planer-type milling machines. Most machines have self-contained electric drive motors, coolant systems, variable spindle speeds, and poweroperated table feeds.


Knee-type Milling Machines:
          Knee-type milling machines are supported by a knee. The knee is a massive casting that rides vertically on the milling machine column and can be clamped rigidly to the column in a position where the milling head and the milling machine spindle are properly adjusted vertically for operation.

    (a) Floor-mounted Plain Horizontal Milling Machine (figure 1 on the following page).
                  1 The floor-mounted plain horizontal milling machine's column contains the drive motor and, gearing and a fixed-position horizontal milling machine spindle. An adjustable overhead arm, containing one or more arbor supports, projects forward from the top of the column. The arm and arbor supports are used to stabilize long arbors, upon which the milling cutters are fixed. The arbor supports can be moved along the overhead arm
to support the arbor wherever support is desired. This support will depend on the location of the milling cutter or cutters on the arbor.

                    2 The knee of the machine rides up or down the column on a rigid track. A heavy, vertical positioned screw beneath the knee is used for raising and lowering. The saddle rests upon the knee and supports the worktable. The saddle moves in and out on a dovetail to control the crossfeedof the worktable. The worktable traverses to the right or left upon the saddle, feeding the workpiece past the milling cutter. The table may be manually controlled or power fed.

(b) Bench-type Plain Horizontal Milling Machine:
               The bench-type plain horizontal milling machine is a small version of the floor-mounted plain horizontal milling machine; it is mounted to a bench or a pedestal instead
of directly to the floor. The milling machine spindle is horizontal and fixed in position. An adjustable overhead arm and support are provided.


(c) Floor-mounted Universal Horizontal Milling Machine.
1. The basic difference between a universal horizontal milling machine and a plain horizontal milling machine is in the adjustment of the worktable, and in the number of attachments and accessories available forperforming various special milling operations. The universal horizontal milling machine has a worktable that can swivel on the saddle with respect to the axis of the milling machine spindle, permitting workpieces to be
adjusted in relation to the milling cutter.
2 The universal horizontal milling machine also differs from the plain horizontal milling machine in that it is of the ram type; i.e., the milling machine spindle is in a swivel cutter head mounted on a ram at the
top of the column. The ram can be moved in or out to provide different positions for milling operations.

(2) Ram-type Milling Machines:
(a) Description. The ram-type milling machine is characterized by a spindle mounted to a movable housing on the column, permitting positioning the milling cutter forward or rearward in a horizontal plane. Two widely
used ram-type milling machines are the floor-mounted universal milling machine and the swivel cutter head ram-type milling machine.

(b) Swivel Cutter Head Ram-type Milling Machine (figure 2 on the following page). A cutter head containing the milling machine spindle is attached to the ram. The cutter head can be swiveled from a vertical to a
horizontal spindle position, or can be fixed at any desired angular position between the vertical and horizontal. The saddle and knee are driven for vertical and crossfeed adjustment; the worktable can be either hand driven or power driven at the operator's choice.

c. Major Components. The machinist must know the name and purpose of each of the main parts of a milling machine to understand the operations discussed in this text. Keep in mind that although we are discussing a knee and a column milling machine, this information can be applied to other types. Use figure 1 on page 3 (which illustrates a plain knee and column milling machine) to help become familiar with the location of the various parts of these machines.

1.Column. The column, including the base, is the main casting which supports all other parts of the machine. An oil reservoir and a pump in the column keeps the spindle lubricated. The column rests on a base that
contains a coolant reservoir and a pump that can be used when performing any machining operation that requires a coolant.

(2) Knee. The knee is the casting that supports the table and the saddle. The feed change gearing is enclosed within the knee. It is supported and can be adjusted by the elevating screw. The knee is fastened
to the column by dovetail ways. The lever can be raised or lowered either by hand or power feed. The hand feed is usually used to take the depth of cut or to position the work, and the power feed to move the work during the machining operation.

(3) Saddle and Swivel Table. The saddle slides on a horizontal dovetail, parallel to the axis of the spindle, on the knee. The swivel table (on universal machines only) is attached to the saddle and can be swiveled
approximately 45° in either direction.

(4) Power Feed Mechanism. The power feed mechanism is contained in the knee and controls the longitudinal, transverse (in and out) and vertical feeds. The desired rate of feed can be obtained on the machine by positioning the feed selection levers as indicated on the feed selection
plates. On some universal knee and column milling machines the feed is obtained by turning the speed selection handle until the desired rate of feed is indicated on the feed dial. Most milling machines have a rapid
traverse lever that can be engaged when a temporary increase in speed of the longitudinal, transverse, or vertical feeds is required. For example, this lever would be engaged when positioning or aligning the work.


(5) Table. The table is the rectangular casting located on top of the saddle. It contains several T-slots for fastening the work or workholding devices. The table can be moved by hand or by power. To move the table by hand, engage and turn the longitudinal hand crank. To move it by power, engage the longitudinal directional feed control lever. The longitudinal directional control lever can be positioned to the left, to the right, or in the center. Place the end of the directional feed control lever to the left to feed the table to the left. Place it to the right to feed the table to the right. Place it in the center position to disengage the power feed, or to feed the table by hand.

(6) Spindle. The spindle holds and drives the various cutting tools. It is a shaft, mounted on bearings supported by the column. The spindle is driven by an electric motor through a train of gears, all mounted within the column. The front end of the spindle, which is near the table, has an internal taper machined on it. The internal taper (3 1/2 inches per foot) permits mounting tapered-shank cutter holders and cutter arbors. Two keys, located on the face of the spindle, provide a positive drive for the cutter holder, or arbor. The holder or arbor is secured in the spindle by a drawbolt and jamnut, as shown in figure 3 on the following page. Large face mills are sometimes mounted directly to the spindle nose.

(7) Overarm. The overarm is the horizontal beam to which the arbor support is fastened. The overarm, may be a single casting that slides in the dovetail ways on the top of the column. It may consist of one or two
cylindrical bars that slide through the holes in the column. On some machines to position the overarm, first unclamp the locknuts and then extend the overarm by turning a crank. On others, the overarm is moved by merely pushing on it. The overarm should only be extended far enough to so position the arbor support as to make the setup as rigid as possible. To place the arbor supports on an overarm, extend one of the bars approximately 1-inch farther than the other bar.

Always tighten the locknuts after the overarm is positioned. On some milling machines, the coolant supply nozzle is fastened to the overarm. The nozzle can be mounted with a split clamp to the overarm after the arbor support has been placed in position.

(8) Arbor Support. The arbor support is a casting containing a bearing which aligns the outer end of the arbor with the spindle. This helps to keep the arbor from springing during cutting operations. Two types of arbor supports are commonly used. One type has a small diameter bearing hole, usually 1-inch maximum in diameter. The other type has a large diameter bearing hole, usually up to 2 3/4 inches. An oil reservoir in the arbor support keeps the bearing surfaces lubricated. An arbor support can be clamped anywhere on the overarm. Small arbor supports give additional clearance below the arbor supports when small diameter cutters are being used. Small arbor supports can provide support only at the extreme end of the arbor, for
this reason they are not recommended for general use. Large arbor supports can be positioned at any point on the arbor. Therefore they can provide support near the cutter, if necessary. The large arbor support should be positioned as close to the cutter as possible, to provide a rigid tooling setup. Although arbor supports are not classified, a general rule of thumb can be used for arbor selection--the old reference type A is of a small bearing diameter, and the old reference type B is of a large bearing diameter.

Note:To prevent bending or springing of the arbor, you must install the arbor support before loosening or tightening the arbor nut.

Size Designation. All milling machines are identified by four basic factors: size, horsepower, model, and type. The size of a milling machine is based on the longitudinal (from left to right) table travel, in inches.
Vertical, cross, and longitudinal travel are all closely related as far as the overall capacity. However, for size designation, only the longitudinal travel is used. There are six sizes of knee-type milling machines, with each number representing the number of inches of travel.

STANDARD SIZE                                     LONGITUDINAL TABLE TRAVEL
No. 1                                                                                        22 inches
   No. 2                                                                                     28 inches
   No. 3                                                                                     34 inches
   No. 4                                                                                     42 inches
   No. 5                                                                                     50 inches
   No. 6                                                                                     60 inches

If the milling machine in the shop is labeled No. 2HL, it has a table travel of 28 inches; if it is labeled No. 5LD, it has a travel of 50 inches. The horsepower designation refers to the rating of the motor which is used to power the machine. The model designation is determined by the manufacturer and features vary with different brands. The type of milling machine is designated as plain or universal, horizontal or vertical, and
knee and column, or bed. In addition, machines may have other special type designations and, therefore, may not fit any standard classification.

 Milling Machines Accessors and Attachments:
         a. Arbors. Milling machine cutters can be mounted on several types of holding device. The machinist must know the devices, and the purpose of each to make the most suitable tooling setup for the operation to be performed. Technically, an arbor is a shaft on which a cutter is mounted. For convenience, since there are so few types of cutter holders that are not arbors, we will refer to all types of cutter holding devices as arbors.


(1) Description.
(a) Milling machine arbors are made in various lengths and in standard diameters of 7/8, 1, 1 1/4, and 1 1/2 inch. The shank is made to fit the tapered hole in the spindle, the other end is threaded.

Note: The threaded end may have left-handed or right-handed threads.

(b) Arbors are supplied with one of three tapers to fit the milling machine spindle (figure 4 on the following page), the milling machines Standard taper, the Brown and Sharpe taper, and the Brown and Sharpe taper with tang.
(c) The milling machine Standard taper is used on most machines of recent manufacture. It was originated and designed by the milling machine manufacturers to make removal of the arbor from the spindle much easier than will those of earlier design.

(d) The Brown and Sharpe taper is found mostly on older machines. Adapters or collets are used to adapt these tapers to fit the machines whose spindles have milling machine Standard tapers.
(e) The Brown and Sharpe taper with tang also is used on some of the older machines. The tang engages a slot in the spindle to assist in driving the arbor.

(2) Standard Milling Machine Arbor (figure 4 below, and figure 5 on page 13):

(a) The Standard milling machine arbor has a straight, cylindrical shape, with a Standard milling taper on the driving end and a threaded portion on the opposite end to receive the arbor nut. One or more milling
cutters may be placed on the straight cylindrical shaft of the arbor and held in position by means of sleeves and an arbor nut. The Standard milling machine arbor is usually splined and has keys, used to lock each cutter to the arbor shaft. Arbors are supplied in various lengths and standard
diameters.

(b) The end of the arbor opposite the taper is supported by the arbor supports of the milling machine. One or more supports are used, depending on the length of the arbor and the degree of rigidity required. The end may lathe center, bearing against the arbor nut (figure 4 on the previous page) or by a bearing surface of the arbor fitting inside a bushing of the arbor support. Journal bearings are placed over the arbor in place of sleeves where an intermediate arbor support is positioned.

(c) The most common means of fastening the arbor in the milling machine spindle is by use of a draw-in bolt (figure 4). The bolt threads into the taper shank of the arbor to draw the taper into the spindle and hold it in place. Arbors secured in this manner are removed by backing out the draw-in bolt and tapping the end of the bolt to loosen the taper.

(3) Screw Arbor (figure 5 on the following page). Screw arbors are used to hold small cutters that have threaded holes. These arbors have a taper next to the threaded portion to provide alignment and support for tools that require a nut to hold them against a tapered surface. A right-hand threaded arbor must be used for right-hand cutters; a left-hand threaded arbor is used to mount left-hand cutters.

(4) Slitting Saw Milling Cutter Arbor (figure 5). The slitting saw milling cutter arbor is a short arbor having two flanges between which the milling cutter is secured by tightening a clamping nut. This arbor is used
to hold the metal slitting saw milling cutters that are used for slotting, slitting, and sawing operations.

(5) End Milling Cutter Arbor. The end milling cutter arbor has a bore in the end in which the straight shank end milling cutters fit. The end milling cutters are locked in place by means of a setscrew.

(6) Shell End Milling Cutter Arbor (figure 5). Shell end milling arbors are used to hold and drive shell end milling cutters. The shell end milling cutter is fitted over the short boss on the arbor shaft and is held against
the face of the arbor by a bolt, or a retaining screw. The two lugs on the arbor fit slots in the cutter to prevent the cutter from rotating on the arbor during the machining operation. A special wrench is used to tighten and loosen a retaining screw/bolt in the end of the arbor.



(7) Fly Cutter Arbor (figure 5). The fly cutter arbor is used to support a single-edge lathe, shaper, or planer cutter bit, for boring and gear cutting operations on the milling machine. These cutters, which can be ground to any desired shape, are held in the arbor by a locknut. Fly cutter arbor shanks may have a Standard milling machine spindle taper, a Brown and Sharpe taper, or a Morse taper.

b. Collets and Spindles.
(1) Description. Milling cutters that contain their own straight or tapered shanks are mounted to the milling machine spindle with collets or spindle adapters which adapt the cutter shank to the spindle.
(2) Collets. Collets for milling machines serve to step up or increase the taper sizes so that small-shank tools can be fitted into large spindle recesses. They are similar to drilling machine sockets and sleeves except
that their tapers are not alike. Spring collets are used to hold and drive straight-shanked tools. The spring collet chuck consists of a collet adapter, spring collets, and a cup nut. Spring collets are similar to lathe
collets. The cup forces the collet into the mating taper, causing the collet to close on the straight shank of the tool. Collets are available in several fractional sizes.
(3) Spindle Adapters. Spindle adapters are used to adapt arbors and milling cutters to the standard tapers used for milling machine spindles. With the proper spindle adapters, any tapered or straight shank cutter or
arbor can be fitted to any milling machine, if the sizes and tapers are standard.

c. Indexing Fixture:

(1) The indexing fixture is an indispensable accessory for the milling machine. Basically, it is a device for mounting workpieces and rotating them a specified amount around the workpiece's axis, as from one tooth space to another on a gear or cutter.
(2) The index fixture consists of an index head, also called a dividing head, and a footstock, similar to the tailstock of a lathe. The index head and the footstock are attached to the worktable of the milling machine by Tslot bolts. An index plate containing graduations is used to control the rotation of the index head spindle. The plate is fixed to the index head, and an index crank, connected to the index head spindle by a worm gear and shaft, is moved about the index plate. Workpieces are held between centers by the index head spindle and footstock. Workpieces may also be held in a chuck mounted to the index headspindle, or may be fitted directly into the taper spindle recess of some indexing fixtures.
(3) There are many variations of the indexing fixture. The name universal index head is applied to an index head designed to permit power drive of the spindle so that helixes may be cut on the milling machine.
"Gear cutting attachment" is another name for an indexing fixture; in this case, one primarily intended for cutting gears on the milling machine.
d. High-Speed Milling Attachment. The rate of spindle speed of the milling machine may be increased from 1 1/2 to 6 times by the use of the high-speed milling attachment. This attachment is essential when using cutters and twist drills which must be driven at a high rate of speed in order to obtain an efficient surface speed. The attachment is clamped to the column of the machine and is driven by a set of gears from the milling machine spindle.

e. Vertical Spindle Attachment. This attachment converts the horizontal spindle of a horizontal milling machine to a vertical spindle. It is clamped to the column and driven from the horizontal spindle. It incorporates provisions for setting the bead at any angle, from the vertical to the horizontal, in a plane at right angles to the machine spindle. End milling and face milling operations are more easily accomplished with this attachment, due to the fact that the cutter and the surface being cut are in plain view.

f. Universal Milling Attachment. This device is similar to the vertical spindle attachment but is more versatile. The cutter head can be swiveled to any angle in any plane, whereas the vertical spindle attachment only
rotates in one plane from the horizontal to the vertical.

g. Circular Milling Attachment. This attachment consists of a circular worktable containing T-slots for mounting workpieces. The circular table revolves on a base attached to the milling machine worktable. The
attachment can be either hand or power driven, being connected to the table drive shaft if power driven. It may be used for milling circles, arcs, segments, and circular slots, as well as for slotting internal and external
gears. The table of the attachment is divided in degrees.

h. Offset Boring Head. The offset boring head is an attachment that fits to the milling machine spindle and permits a single-edge cutting tool, such as a lathe cutter bit, to be mounted off-center on the milling machine. Workpieces can be mounted in a vise attached to the worktable and can be bored with this attachment.

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