Cutting Tool Applications, Chapter 13: Milling Methods and Machines

Modern milling machines look much the same as they did 30 years ago. However, they now must cut superalloys, titanium, and high-tensile steels to closer tolerances and at faster rates than before. To handle these requirements, the new milling machines provide higher horsepower, greater stiffness and wider speed and feed ranges than before. In addition, more accurate lead screws closer alignment, numerical control (NC) and computer numerical control (CNC) all result in faster work with better finishes and greater accuracy than ever before attained.

Types of Milling Machines

The many types of milling machines used in manufacturing have been grouped into three general classes:
— Column and knee machines
— Bed-type milling machines
— Special purpose machines

Column and Knee Machines — Column and knee milling machines are made in both vertical and horizontal types. The schematic diagrams below show both types of machines. Versatility is a major feature of knee and column milling machines. On a basic machine of this type, the table, saddle and knee can be moved. Many accessories such as universal vises, rotary tables and dividing heads, further increase the versatility of this type of machine.

Regardless of whether the machine is of the vertical or horizontal type, several components on all column and knee milling machines are similar, except for size and minor variations because of manufacturer's preference. These similarities are described in terms of general shape, geometric relationship to the rest of the machine, function and the material from which the components are made.

Column — The column, which is usually combined with the base as a single casting, is cast gray iron or ductile iron. The column houses the spindle and bearings, as well as the necessary gears, clutches, shafts, pumps and shifting mechanisms, for transmitting power from the electric motor to the spindle at the selected speed. The gears usually run in oil and are made of carburized alloy steel for long life. Some of the necessary controls are usually mounted on the side of the column.

The base is usually hollow and, in many cases, serves as a sump for the cutting fluid. A pump and filtration system can be installed in the base. The hole in the center of the base houses the support for the screw that raises and lowers the knee.

The machined vertical slide on the front of the column may be of the square or dovetail type. The knee moves up and down on this slide. The slide must be machined at a 90-degree angle to the face of the column in both the lateral and vertical planes. The tolerances are very close and are usually expressed in minutes or seconds of arc. The large hole in the face of the column casting is for the spindle. The hole is very accurately bored perpendicular to the front slide in two planes and parallel to the upper slide.

Spindle — On a horizontal milling machine, the spindle is one of the most critical parts. It is usually machined from an alloy steel forging and is heat-treated to resist wear, vibration, thrust and bending loads. The spindle is usually supported by a combination of ball and straight roller bearings , or by tapered roller bearings that absorb both radial loads and end thrust loads. Spindles are hollow so that a drawbar can be used to hold arbors securely in place.

The front of the spindle is machined to accept standard arbors. The two keys that fit into corresponding slots in the arbor do the actual driving of the arbor. The internal taper, which is accurately ground so that it is concentric with the spindle, locates the arbor.

Knee — The knee is a casting that is moved up or down the slide on the front of the column by the elevating screw. Two dovetail or square slides are machined at 90 degrees to each other. The vertical slide mates with the slide on the front of the column, and the horizontal slide carries the saddle. It contains the necessary gears, screws and other mechanisms to provide power feeds in all directions. The operator can select various feedrates through the controls mounted on the knee.

Saddle — The saddle for a plain milling machine is a casting with two slides machined at an exact 90-degree angle to each other. The lower slide fits the slide on the top of the knee, and the upper slide accepts the slide on the bottom of the table. The surfaces of the slides that make contact with the knee and the table are parallel to each other. Locks for both the cross slide and the table are fitted to the saddle, along with the nuts that engage with the cross feed and table feed screws.

On a universal milling machine the saddle is made in two pieces and is more complex because it must allow the table to swivel through a limited arc.

Table — Milling machine tables vary greatly in size, but generally they have the same physical characteristics. The bottom of the table has a dovetail slide that fits in the slide on top of the saddle. It also has bearings at each end to carry the table feed screw. The top of the table is machined parallel with the slide on the bottom and has several full length T-slots for mounting vises or other workholding fixtures.

A dial graduated in thousandths of an inch is provided to allow for accurate table movement and placement. The table feed screw usually has an Acme thread.

Milling machines with vertical spindles are available in a large variety of types and sizes. The head, which houses the spindle, motor and feed controls, is fully universal and can be placed at a compound angle to the surface of the table. The ram, to which the head is attached, can be moved forward and back and locked in any position. A turret on top of the column allows the head and ram assembly to swing laterally, increasing the reach of the head of the machine.

Some ram-type milling machines can be used for both vertical and horizontal milling. On ram-type vertical mills that have the motor in the column, power is transmitted to the spindle by gears and splined shafts.

Bed-Type Milling Machines —High production calls for heavy cuts, and the rigidity of a knee and column type of milling machine may not be sufficient to take the high forces. A bed-type milling machine is often ideal for this kind of work. In this machine the table is supported directly on a heavy bed, while the column is placed behind the bed.

There are several advantages of the bed-type machine, particularly for production runs. Hydraulic table feeds are possible; the hydraulic components are housed in the bed casting. This allows for very high feed forces; variable feedrates during any given cut, and automatic table cycling. The spindle may be raised or lowered by a cam and template arrangement to produce special contours.

The basically heavier construction allows more power to be supplied to the spindle, which gives higher productivity through faster metal removal. Duplex bed-type milling machines have two columns and spindles for milling two surfaces on a part simultaneously.

The chief disadvantage of a bed-type milling machine compared with one of the knee and column type is that it is less versatile for machining small parts. Its advantages lie in its higher productivity, its adaptability to large sized machines and its ease of modification to special applications.

Special Purpose Milling Machines — As industrial products have become more complex, new and unusual variations of the more common milling machines have been developed. The objectives are to accommodate larger work, make many duplicate parts, locate holes and surfaces precisely, or to do other unusual machining jobs.

Planer-Type Milling Machines — The general arrangement of these types of machines is similar to that for planers, except that in place of individual tool bits, milling heads are installed. Planer-type machines are used mostly for machining parts like the bedways for large machine tools, and other long workpieces that require accurate flat and angular surfaces or grooves.

Profile Milling Machines— Two-dimensional profiling can be accomplished by using a template, or with a numerically controlled vertical milling machine. Some profilers have several spindles, and a number of duplicate parts can be produced in each cycle. Hydraulic-type profilers have a stylus that is brought into contact with the template to start the operation. The operator then moves the stylus along the template, causing hydraulic fluid under pressure to flow to the proper actuating cylinders. The table moves the work past the cutter, duplicating the shape of the template.

Die sinking and other processes involving the machining of cavities can be done on 3D profilers. An accurate pattern of the cavity is made of wood, plaster or soft metal. The stylus follows the contour of the pattern guiding the cutter as it machines out the cavity. Numerically controlled milling machines are also used for this type of work.

Computer-Controlled Machining Systems Several of the standard machines discussed in previous chapters of this text are capable of performing multiple operations. A lathe for example is capable of turning, facing, drilling, threading, etc. A drilling machine is capable of drilling, reaming, countersinking, tapping, and so on. However, when increased production rates require the purchase of additional machining capability, it is almost always more economical and feasible to purchase multifunctional machines capable of quick changes, simultaneous machining, and automatic processing.

Machining Centers — Machining centers are designed and built to provide for flexible manufacturing. They can be used to machine just a few parts or large production runs. Programming can be relatively simple and the use of "canned" cycles provides a great deal of versatility. An NC machining center by definition is able to perform milling, drilling and boring cuts and has either indexing turret toolholders or provides for automatic tool change.

Machining centers are built in either horizontal or vertical configuration. The relative merits of each will be discussed briefly.

Horizontal Milling Machines — Horizontal machines tend to be advantageous for heavy box-shaped parts, such as gear housings, which have many features that need to be machined on the side faces. The horizontal machine easily supports heavy workpieces of this type. If a rotary indexing worktable is added, four sides of the workpiece can be machined without re-fixturing.

Pallet systems used to shuttle pieces in and out of the workstation tend to be easier to design for horizontal machines, where everything in front of the main column is open and accessible. A horizontal machining center with a pallet shuttling system is shown below.

Vertical Milling Machines —  Vertical machining centers are often preferred for flat parts that must have through holes. Fixtures for these parts are more easily designed and built for a vertical spindle. Also, the thrust of the cut developed in drilling or in milling pockets can be absorbed directly by the bed of the machine.

The vertical machine is preferred where 3-axis work is done on a single face as in mold and die work. The weight of the head of a vertical machine as it extends away from the column, particularly on large machines, can be a factor in maintaining accuracy, as there may be some tendency for it to drop and lose accuracy and cause chatter.

Flexible Machining Systems — Flexible machining systems employ one or more machining centers, usually along with other equipment, to produce medium-volume workpieces. A workpiece-handling system is required, and a central computer typically controls the entire arrangement.

Material handling — Parts are moved from storage and between machine elements by means of one of several different types of systems. The material-handling system selected must be capable of routing any part to any machine in any order and also to provide a bank of parts ahead of each machine to realize maximum productivity. Parts are normally loaded and unloaded manually. The various types of material-handling systems used include: automated guided vehicles, towline systems, roller conveyer systems, overhead conveyer systems, monorails, cranes and robots.

Control Systems— The computer controls of flexible machining systems have three functional levels:

Master Control. The master control monitors and controls the entire system, including routing workpieces to appropriate machines, scheduling work and monitoring machine functions.

Direct Numerical Control. A DNC computer distributes appropriate programs to individual CNC machines and supervises and monitors their operations.

Element Control. The third and lowest level of control is computer control of the machining cycles of individual machines.

Milling Machine Attachments and Accessories — Many accessories have been developed for milling machines. Some are specialized and can be used for only a few operations. Others, such as vises, arbors and collets, are used in almost all milling operations.

Special Milling Heads —Several types of special heads have been developed for use on horizontal or vertical milling machines. These accessories increase the versatility of the machine. For example, a vertical head can be attached to a conventional horizontal column and knee-milling machine, greatly increasing its usefulness, especially in small shops with a limited number of machines.

Vises and Fixtures —In all milling operations, the work is held by fixtures, vises or clamping arrangements. In most cases the work is held stationary in relation to the table while it is being machined, but work held in indexing heads and rotary tables can be moved in two planes while machining operations are in progress.

Arbors, Collets and Toolholders —Several basic types of arbors and collets are used to hold milling cutters and to transmit power from the spindle to the cutter. Regardless of type, they are usually precisely made of alloy steel and heat treated for wear resistance and strength.

Arbors—Arbors for horizontal milling machines are available in three basic types: style A, style B and style C. A draw bolt, that goes through the spindle of the machine, screws into the small end of the taper and draws the arbor tightly into the tapered hole in the milling machine spindle. Power is transmitted from the spindle to the arbor by two short keys that engage with the slots on the flange of the arbor.

Collets— On some vertical milling machines the spindle is bored to accept a collet that has a partly straight and partly tapered shank. The collet is secured by a drawbar that is screwed into a tapped hole in the back of the collet and tightened from the top of the spindle. Some milling machine manufacturers offer collet arrangements that do not need a drawbar. Collets of this type can be closed with a lever-operated cam or with a large locking nut.

Toolholders— Standard tool holders are available for end mills and shell mills. For some operations that require the use of tools with non-standard shank sizes, chucks can be used to hold the tool. These chucks are available with Morse taper or straight shanks. Either type can be used in milling machines when the proper adapters or collets are available.

Types of Milling Operations

Milling cutters are used either individually or in combinations to machine various surfaces as described below and shown above.

Plain milling— Plain milling is the process of milling a surface that is parallel to the axis of the cutter and basically flat. It is done on plain or universal horizontal milling machines with cutters of varying widths that have teeth only on the periphery.

Side milling— For side milling, a cutter that has teeth on the periphery, and on one or both sides, is used. When a single cutter is being used, the teeth on both the periphery and sides may be cutting. The machined surfaces are usually either perpendicular or parallel to the spindle. Angle cutters can be used to produce surfaces that are at an angle to the spindle for such operations as making external dovetails or flutes in reamers.

Straddle milling— In a typical straddle milling set-up, two-side milling cutters are used. The cutters are half-side or plain side milling cutters, and have straight or helical teeth. Stagger-tooth side milling cutters can also be used.

Gang milling— In gang milling, three or more cutters are mounted on the arbor, and several horizontal, vertical or angular surfaces are machined in one pass. When making a gang milling set-up, several different types of cutters can be used, depending on the job to be done. Cutters used for producing vertical or angular surfaces must be of the side-cutting type; plain milling cutters of the proper width can be used for horizontal surfaces. In some cases face mills with the teeth facing inward can be used at one or both ends of the gang milling set-up.

Form milling— The number of parallel surfaces and angular relationships that can be machined by peripheral milling is limited almost only by cutter design. Form cutters are expensive, but often there is no other satisfactory means of producing complex contours.

Slotting and slitting operations— Milling cutters of either the plain or side-cutting type are used for slotting and slitting operations. Slotting and slitting are usually done on horizontal milling machines, but can also be done on vertical mills by using the proper adaptors and accessories.

Face milling— Face milling can be done on vertical and horizontal milling machines. It produces a flat surface that is perpendicular to the spindle on which the cutter is mounted. The cutter ranges in size and complexity from a simple, single-tool flycutter to an inserted-tooth cutter with many cutting edges. Large face mills are usually mounted rigidly to the nose of the spindle. They are very effective for removing large amounts of metal, and the workpiece must be securely held on the milling table.

End milling— End milling is probably the most versatile milling operation. Many types of end mills can be used on both vertical and horizontal milling machines. End mills are available in sizes ranging from 1/32 inch. to 6 inches (for shell end mills) and in almost any shape needed.

Turn Milling

Turn milling consists of a number of different machining methods where a milling cutter machines a rotating workpiece. These methods are primarily used for machining various eccentrically shaped parts; planes, tapered and cylindrical surfaces; grooves and inside holes.

Turn milling requires a machine tool with certain functions and a number of axes. Machining centers, turning centers, specially adapted lathes, milling machines, boring mills and special-purpose machines are used. When other operations of turning and milling are combined in the machines, single set-up machining leads to advantages of fast through-put times and flexibility of production.

Advantages associated with turn milling are: capability of machining large and unbalanced parts which cannot be rotated at high speeds; complex surface shapes, eccentric parts and components with additional elements that protrude; log, unstable shafts or thin-walled parts.

George Schneider, Jr., is the author of Cutting Tool Applications, a handbook to machine tool materials, principles, and designs. He is the Professor Emeritus of Engineering Technology at Lawrence Technological University, and former Chairman of the Detroit Chapter of the Society of Manufacturing Engineers.