Milling Machine | Types | Parts | Processes

Milling Machine

Milling machines cut metal using a rotating cutting tool called a milling cutter. These machines cut flat surfaces, angles, slots, grooves, shoulders, inclined surface etc

Types of Milling machines

Milling machines are of two major types depending upon the orientation of the axis of rotation of the spindle. Another feature is its knee and column construction. These are general purpose machines.

     Thus we have two classes of machines:

1. Horizontal knee and column type milling machine: 

As the name implies, the spindle is situated horizontally. The spindle rotates horizontally. An Arbor is attached to the machine which holds the cylindrical disk shape cutter which cuts the metal workpiece.

2. Vertical knee and column type milling machine:

The spindle of this machine is in a vertical position. No Arbor is required in this machine. The cutter tool has the cylindrical shape and the cutting edges are situated at the circumference of the cylindrical face.

Component parts of Milling Machine:

1. Base: It is the foundation part of a machine and all other parts are joined on it. It carries the entire load so it should have high compressive strength and it is made up of cast iron.

2. Column: It is mounted vertically on the base. It supports the knee, table etc and work as housing for all the other driving member. The column is a hollow member which contains driving gears and sometimes motor for the spindle and the table.

3. Knee: It is a casting that supports the saddle and table. All gearing mechanism is enclosed within the knee. It is fastened to the column by dovetail ways. The knee is supported and adjusted by a vertical positioning screw(elevating screw). The elevating screw is used to adjust the knee up and down by raising or lowering the level either with the help of hand or power feed.

4. Saddle: This is placed between the table and the knee, and work as an intermediate part between them. This can moves transversally to the column face. This slides over the guideways provided situated on the knee which is perpendicular to the column face. The main function is to provide motion in a horizontal direction to the workpiece. This is also made by cast iron.

5. Table: This is a rectangular casting which is present on the top of the saddle. The table is situated over the knee. It is the part of a machine that holds the workpiece while machining. This is made by cast iron and has T-slot cut over it. This provides vertical motion by moving the knee up and down. It provides horizontal motion by the feed screw. This provides a horizontal (transverse) motion by moving the saddle.

6. Overhanging Arm: The overarm is used to fastened arbor support. It may consists of one or two cylindrical bars that slide through the holes in the column. It is made by cast iron.

7. Spindle: The spindle is the main part of the machine which holds the tool in the right place. The spindle provides the drive for arbors, cutters and attachments used on a machine.

8. Arbor: This is a mechanical part, which is used as an extension part of the spindle in a horizontal mill machine. It is fitted on the spindle whenever its required. This holds the tool and moves it in the correct direction.

9. Arbor supports: These are generally two types of arbor supports used in the mill machine. The first one has a small diameter bearing hole, 1 inch in maximum diameter, and the other one has a large diameter bearing hole, usually up to 23/4 inches. The arbor supports has an oil reservoir that lubricates the bearing surfaces. It can be clamped anywhere on the overarm. The arbor support is used only in the horizontal types of milling machines.

10. Milling head: It is the upper section of a vertical mill machine. It consists of a spindle, driving motor and other controlling mechanisms.

11. Ram: One end of the arm is attached to the column and other ends to the milling head. The ram can be moved transversally ( in and out ) on the column by a hand lever.

Milling processes:

1. Peripheral Milling: Peripheral milling is a machining process in which the milling cutter is placed parallel to the workpiece. In other words, the milling cutter is positioned so that the slides of the cutter grind away at the top of the workpiece.

2. Face Milling: Face milling is the milling of surfaces that are perpendicular to the cutter axis, as shown in fig. Face milling produces flat surfaces and machines work to the required length. In face milling, the feed can be either horizontal or vertical.

          In face milling, the teeth on the periphery of the cutter do practically all of the cutting. However, when the cutter os properly ground, the face teeth actually remove a small amount of stock which is left as a result of the springing of the workpiece or cutter, thereby producing a finer finish.

         It is important in face milling to have the cutter securely mounted and to see that all end play or sloppiness in the machine spindle is eliminated.

        During face milling operations, the workpiece should be fed against the milling cutter so that the pressure of the cut is downward, thereby holding the piece against the table. Whenever possible, the edge of the workpiece should be in line with the center of the cutter. This position of the workpiece in relation to the cutter will help eliminate slippage.

      When face milling, the workpiece may be clamped to the table or angle plate or supported in a vise, fixture, or jig.

     Large surfaces are generally face milled on a vertical milling machine with the workpiece clamped directly to the milling machine table to simplify handling and clamping operations.

3. End milling: A set of experiments designed to begin the characterization of surface quality for the end milling process have been performed. The objective of this study is to develop a better understanding of the effects of spindle speed, cutting feed rate and depth of cut on the surface roughness and to build a multiple regression model. Such an understanding can provide insight into the problems of controlling the finish of machined surfaces when the process parameters are adjusted to obtain a certain surface finish. The model, which includes the effect of spindle speed, cutting feed rate and depth of cut, and any two variable interactions, predicted the surface roughness values with an accuracy of about 12%.

4. Straddle Milling: When two or more parallel vertical surfaces are machined at a single cut, the operation is called straddle milling. Straddle milling is accomplished by mounting two side milling cutters on the same arbor, set apart at an exact spacing. Two sides of the workpiece are machined simultaneously and final width dimensions are exactly controlled.

          Straddle milling has many useful applications introduction machining. Parallel slots of equal depth can be milled by using straddle mills of equal diameters. Fig illustrates a typical example of straddle milling. In this case a hexagon is being cut, but the same operation may be applied to cutting squares or splines on the end of a cylindrical workpiece. The workpiece is usually mounted between centres in the indexing fixture or mounted vertically in a swiveled vise. The two side milling cutters are seperated by spacers, washers and shims so that the distance between the cutting teeth of each cutter is exactly equal to the width of the workpiece area required. When cutting a square by this method, two opposite sides of the square are cut, and then the spindle of the indexing fixture or the swiveled vise is rotated 90°, and the other two sides of the workpiece are straddle milled.

5. Form Milling: Form milling is the process of machining special contours composed of curves and straight lines, or entirely of curves, at a single cut. This is done with foemed milling cutters, shaped to the contour to be cut. The more common form milling operations involve milling half round recesses and beads and quarter round radii on workplaces, This operation is accomplished by using convex, concave and corner rounding milling cutters ground to the desired circle diameter. Other jobs for formed milling cutters include milling intricate patterns on workplaces and milling several complex surfaces in a single cut such as are produced by gang milling.

6. Slotting: Slotting with a versa mil (fig) covers a wide variety of operations from milling long wide slots in material to cutting curved or thin slots. Workpiece may be mounted in the lathe chuck or between centers for slotting operation.

7. Slitting: Sawing or slitting, thin stock into various widths for the production of flat gauges, templates, etc is a fairly common milling operation. It is performed with a slitting saw and is likely to give considerable trouble if extreme care is not exercised. A slitting saw of the smallest diameter permitted adequate clearance is used. It must be keyed to the arbor (the key should also pass into the spacers on, either side of the cutter). Best results can be obtained if the cutter is mounted for climb milling. That is the work and the cutter both move in the same direction of the point of contact. The cutting pressure is downward and will tend to press the work on to the table or holding device. Adjust the table gibs until there is heavy drag felt when the table is moved by hand. This will remove table "play" and pervert the cutter from jumping in the cut.

The peripheral or slab milling can be further subdivided into

1. Up Milling

2. Down Milling

1. Up Milling: It is known as conventional milling. In this workpiece is fed in direction opposite to the cutter. It requires rigid clamping due to tendency of lifting the job. Chip thickness is minimum at the beginning and maximum at the end.

2. Down Milling: It is also known as climb milling. In this workpiece is fed in the direction of cutter. Clamping is not so crucial. Chip thickness is maximum at beginning and minimum at end.

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Milan Tomic

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