Milling Machine Operations
Based on the working principle, several operations are performed on milling machines in various industries where they are employed. Below are a few discussed in brief.
Up-Milling Operation
The up-milling is also known as conventional milling. In this, a cutter revolving in the opposite direction of the workpiece removes the metal in the form of tiny chips as shown in the figure below. The chip thickness varies from minimum at the beginning and maximum at the end as the cutter advances.
The primary drawbacks of the up-milling technique are the tendency of the cutting force to lift the work from the fixtures and the poor surface finish.
Fig 3: Up-milling or Conventional Milling
Down Milling Operation
A down-milling operation is also known as ‘climb milling’. In this operation, the cutter rotates in the same direction as the feed. The chip thickness varies from maximum at the beginning to minimum at the end.
There is always less friction in climb milling which produces less heat. Thin slots, lengthy cuts, and sharpening of the pieces can be easily achieved in this operation.
Fig 4: Down Milling Operation
Plain Milling Operation
The most commonly used milling machine operation is plain milling. It is also referred to as slab milling (Hence, the labelling in the image below). The workpiece is firmly mounted on the machine before this operation. After choosing the proper speed and feed, the machine is then turned on.
This operation creates a smooth and horizontal surface that is parallel to the axis of rotation of the cutter as shown in the figure below.
Fig 5: Plain or Slab Milling
Face Milling Operation
The most simple operation on a milling machine is face milling. This procedure is carried out with a face milling cutter that is rotated about an axis perpendicular to the work surface as shown in the figure below.
By turning the crossfeed screw of the table, the depth of the cut can be changed. This operation creates a flat surface with the cutter positioned on a stub arbour.
Fig 6: Face Milling
Gang Milling Operation
The operation of simultaneously milling multiple surfaces of a workpiece by feeding the table against numerous cutters with the same or various diameters mounted to the arbour is called gang milling operation.
The speed of this group of cutters is determined by the cutter with the greatest diameter. The approach reduces machining time significantly and is commonly utilised for repeated tasks.
Fig 7: Gang Milling Operation
Straddle Milling Operation
The milling operation in which two surfaces are milled simultaneously is a straddle milling operation. With two side milling cutters mounted to the same arbour, straddle milling creates flat and vertical surfaces on both sides of the workpiece.
Using adequate spacing collars, the distance between the two cutters is regulated. T-slot milling is an exceptional illustration of straddle milling. The operation is used to create hexagonal or square surfaces.
Fig 8: Straddle Milling Operation
Angular Milling Operation
The process of generating an angular surface on a workpiece which is not at right angles to the spindle axis of the milling machine is known as angular milling.
The angular groove may have a single or double included angle which depends on the type and geometry of the angular cutter employed. V-block manufacturing is an example of angular milling.
Fig 9: Angular Milling
Form Milling Operation
The process of creating an uneven shape like convex, concave, or any other form employing form cutters is called form milling operation. The selection of form cutters depends on the shape needed. This operation has a 20% - 30% slower cutting rate than plain milling.
Fig 10: Form Milling
Side Milling Operation
The process of side milling involves using a side milling cutter to create a flat, vertical surface on the side of a workpiece. The depth of cut is provided by adjusting the vertical feed with the help of the screw on the table.
Fig 11: Side Milling
Keyway Milling Operation
The operation of making keyways, grooves, and slots of various shapes and sizes is called keyway milling. It can be done with an end mill, a side milling cutter, a plain milling cutter, or a metal slitting saw.
Keyways generally have minute dimensions of width or depth. Hence, a special tool is necessary for keyway milling like standard helical or staggered teeth cutters.
Fig 12: Keyway Milling
Profile Milling Operation
The process of replicating an intricate shape of a master die on a workpiece is known as profile milling. For milling profiles, various cutters like helical plain cutters are utilised. One of the milling cutters that is frequently used in profile milling is the end mill cutter.
Fig 13: Profile Milling
Thread Milling Operation
The precise threads are produced in small or big quantities using single or multiple thread milling cutters. The thread is finished by feeding the rotating cutter longitudinally over a distance equal to the pitch length of the thread. Thread milling operations are carried out on specialised thread milling machines.
Fig 14: Thread Milling
Gear Cutting Operation
A form-relieved cutter on a milling machine executes the gear-cutting operation. The cutter type may be either cylindrical or end mill. The cutter profile is made to precisely match the gear tooth spacing. A universal diving head is used to hold the workpiece while a process called indexing creates evenly spaced gear teeth on a gear blank.
Fig 15: Gear Cutting or Milling Operation
As we just discussed, this operation involves a process called Indexing.
Indexing is the process of splitting the circumference of the workpiece into an arbitrary number of equal sections. The perimeter of the gear blank is divided into 'n' equal sections, and each tooth is milled individually.
Almost all gear teeth divisions can be covered by index plates through crank rotation. The index crank is always next to the spindle making it easier to index the divisions to fractions of a turn. This is done to accurately cut the gear teeth spacing.
Fig 16: Indexing Plates
The movement of the index crank is calculated by a formula which is given below,
Index Crank Movement = \(\frac{40}{N}\), where N \(\to\) is the required number of divisions.
