The following points highlight the various types of milling machines used in industries. The types are: 1. Column and Knee Type Milling Machine 2. Vertical Milling Machine 3. Ram-Type Universal Machine 4. Universal Milling Machine 5. Bed-Type Milling Machine 6. Planer-Type Milling Machine.
Type # 1. Column and Knee Type Milling Machine:
This is a very versatile machine capable of performing a wide variety of operations.
In its simplest form, this is shown in Fig. 16.1. It consists of a column which is hollow structure housing the driving motor, speed change gears for spindle and mechanisms for controlling the table and knee movements. Base of the machine is made hollow and is used for storing the coolant.
On the front of the column, machined ways are provided, on which knee can slide up and down, with the help of elevating screw. Knee forms a base for the saddle to facilitate it in sliding towards and away from the column along with the work table fitted on the saddle. Work table can further slide lengthwise on the saddle and has the usual means provided on it for clamping of jobs or fixtures.
Thus a job fitted on the table can be given three motions by:
(а) Vertical (up and down) movement of knee by the mechanism that attaches the knee to the column.
(b) Cross (in and out) or transverse movements of table by moving the saddle in relation to the knee.
(c) Longitudinal (back and forth) movement of table by adjusting table position with relation to the saddle.
Cutters are mounted on the horizontal arbor which is rigidly supported by the over-arm, spindle and the braces. The spindle of the machine is hollow, with a tapering hole at its nose end. Arbors are held in place by a long bolt which screws into the tapered end and is fastened at the back of the machine.
In the hollow casing of the column, motor drives the counter shaft by means of a belt or chain. Whenever the clutch is engaged, power flows from the counter shaft to the spindle through a set of speed-change gears. Power can flow from the counter shaft to the feeds of the table, the saddle and knee through another set of gears.
Mechanical, electrical or hydraulic devices are used for the quick change of spindle speeds and power feeds for the job. By the use of stops and other control devices the machine can be adopted for automatic cycles.
Fig. 16.2 shows a horizontal spindle machine.
Type # 2. Vertical Milling Machine (Refer Fig. 16.3):
Vertical milling machine is so called, because of the vertical position of the cutter-spindle. The table movements are horizontal, transverse and vertical. In this case, overarm is small and provides a strong support for the spindle.
The spindle can be moved up and down to perform the operations like grooving, slotting, die-forming, facing, drilling, boring etc. Generally vertical milling machine is used to perform end milling and face milling operations.
Fig. 16.5, 16.6, 16.7 show table, saddle and knee of plain column and knee type machines.
Fig. 16.8 and 16.9 show horizontal and vertical knee type milling machines.
Type # 3. Ram-Type Universal Machine (Refer Fig. 16.10):
In place of ordinary type of over-arm, there is ram which can slide forward and backward for the adjustment of tool position or for shifting the cutter position quickly. The cutter head is pivoted to the face of the arm and is capable of any angle adjustment between vertical and horizontal positions.
This range of adjustment often makes it possible to complete jobs with one set up without having to change the job to some other machine. Thus the movable ram enables the throat distance to be adjusted in or out, bringing the spindle in each case as close to the column as is possible.
Type # 4. Universal Milling Machine (Refer Fig. 16.11):
It is similar to the plain milling machine, with the difference that the table is placed on the swivel and the swivel is placed on the saddle. Because of the swivel, the table and hence the job can be fixed at desired angle, so that inclined cuts can be taken and hence the helical gear and drilling flutes are easily cut by the machine. This type of machine is essentially a tool-room machine used for very accurate work.
Machines of this type are essentially production machines and are of rigid structure which yields high production of interchangeable jobs. The solid base provides inherent rigidity. In this machine, the table is directly mounted on the main body of the machine in place of knee so that it becomes more stout and robust.
The table moves longitudinally over bed ways. It thus can withstand heavy cutting loads for long time in production work. This type of machine can have more than one spindle, so that different operations can be performed simultaneously. Simplex, Duplex and Triplex type of bed machines have respectively single, double and triple spindle heads. The vertical motion is imparted to the spindle head instead of table which is fixed.
This machine is somewhat less flexible that keen and column type since it is meant for production jobs where changes in setup are infrequent. Automatic operation is essential on these machines for high production and as such these generally incorporate facilities like limit switches, spindle reverse, spindle stop, etc.
These machines are usually used with carbide cutters and require huge power for cutting. Generally their actions are automatic and hence need lesser attention of the operator. These are available in horizontal, vertical, planer, or boring and milling machine versions.
Type # 6. Planer-Type Milling Machine:
The usual features of this machine are that of planer. Actually it is a planer adopted for doing milling operation on very big jobs. Table movement is slightly slower than the actual planer. In place of tool head, milling heads are mounted on the cross-rails of planer.
Rotating cutters of the milling heads do milling operation on the jobs. The variable table feeding movement and the rotating cutter are the principle features that distinguish this machine from the planer. Transverse and vertical movements are provided on the cutter-spindle.