The cutter may also be held at an angle relative to the axis of the tool. Milling covers a hydraulic milling machine circuit pdf variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes for machining custom parts to precise tolerances. CNC capability, coolant systems, and enclosures.
The milling process removes material by performing many separate, small cuts. In this case the blades of the cutter can be seen as scooping out material from the work piece. Peripheral milling is well suited to the cutting of deep slots, threads, and gear teeth. Many different types of cutting tools are used in the milling process. Milling cutters may also have extended cutting surfaces on their sides to allow for peripheral milling. Tools optimized for face milling tend to have only small cutters at their end corners. Thin film coatings may be applied to decrease friction or further increase hardness.
As material passes through the cutting area of a milling machine, the blades of the cutter take swarfs of material at regular intervals. The distance between ridges and the height of the ridges depend on the feed rate, number of cutting surfaces, the cutter diameter. Trochoidal marks, characteristic of face milling. The face milling process can in principle produce very flat surfaces.
Revolution marks can have significant roughness depending on factors such as flatness of the cutter’s end face and the degree of perpendicularity between the cutter’s rotation axis and feed direction. Often a final pass with a slow feed rate is used to improve the surface finish after the bulk of the material has been removed. In a precise face milling operation, the revolution marks will only be microscopic scratches due to imperfections in the cutting edge. All the completed workpieces would be the same, and milling time per piece would be minimized.
4- or 5-axis control obviate gang-milling practice to a large extent. Mill orientation is the primary classification for milling machines. However, there are alternative classifications according to method of control, size, purpose and power source. There are two subcategories of vertical mills: the bed mill and the turret mill.
The most common example of this type is the Bridgeport, described below. Turret mills often have a quill which allows the milling cutter to be raised and lowered in a manner similar to a drill press. Turret mills are generally considered by some to be more versatile of the two designs. However, turret mills are only practical as long as the machine remains relatively small.
Therefore, larger milling machines are usually of the bed type. A third type also exists, a lighter machine, called a mill-drill, which is a close relative of the vertical mill and quite popular with hobbyists. A mill-drill is similar in basic configuration to a small drill press, but equipped with an X-Y table. They also typically use more powerful motors than a comparably sized drill press, with potentiometer-controlled speed and generally have more heavy-duty spindle bearings than a drill press to deal with the lateral loading on the spindle that is created by a milling operation. A mill drill also typically raises and lowers the entire head, including motor, often on a dovetailed vertical, where a drill press motor remains stationary, while the arbor raises and lowers within a driving collar. Other differences that separate a mill-drill from a drill press may be a fine tuning adjustment for the Z-axis, a more precise depth stop, the capability to lock the X, Y or Z axis, and often a system of tilting the head or the entire vertical column and powerhead assembly to allow angled cutting.
Z-axis is controlled in basically the same fashion as drill press, where a larger vertical or knee mill has a vertically fixed milling head, and changes the X-Y table elevation. These are frequently of lower quality than other types of machines, but still fill the hobby role well because they tend to be benchtop machines with small footprints and modest price tags. While endmills and the other types of tools available to a vertical mill may be used in a horizontal mill, their real advantage lies in arbor-mounted cutters, called side and face mills, which have a cross section rather like a circular saw, but are generally wider and smaller in diameter. Because the cutters have good support from the arbor and have a larger cross-sectional area than an end mill, quite heavy cuts can be taken enabling rapid material removal rates. These are used to mill grooves and slots. Plain mills are used to shape flat surfaces. Several cutters may be ganged together on the arbor to mill a complex shape of slots and planes.
Special cutters can also cut grooves, bevels, radii, or indeed any section desired. These specialty cutters tend to be expensive. Simplex mills have one spindle, and duplex mills have two. Some horizontal milling machines are equipped with a power-take-off provision on the table. The choice between vertical and horizontal spindle orientation in milling machine design usually hinges on the shape and size of a workpiece and the number of sides of the workpiece that require machining. Heavier and longer workpieces lend themselves to placement on the table of a horizontal mill.