Milling Operations and Milling Machines; Definitions, Terms, Types

Milling operations are very important machining operations in the manufacturing industry. There are various types of milling. And we produce lots of kinds of metal parts with them. Here, we explain the general things about milling machines. You can find this information about milling operations;

  • What is the milling process? How do we apply milling in manufacturing processes?
  • What are the types of milling machines?
  • What are the types of milling processes?
  • Differences between turning and milling centers

What is the Milling Process?

Universal milling
A universal milling machine(Image Source: Wikimedia.).

The most important difference between the milling process from others is the orientation of primary and secondary motions. The cutter performs the primary motion or cutting motion in milling processes. So, Cutter revolves around its central axis, and multiple cutting edges perform the cutting operation.

In milling operations, we obtain planar surfaces. And workpart performs the feed motion in the milling process. This feed motion can be parallel to the work part surface or direction perpendicular to the surface. But in general, there is no steady feed motion in milling operations. Work parts can move in different orientations in the milling process. It can start in perpendicular feed motion and goes on to another parallel motion. This is the most important difference between the milling process compared with other machining operations such as turning and drilling. 

What are the Types of Milling Operations?

In general, there are two types of milling operations; face milling and peripheral milling operations. The main difference between these two milling processes is the orientation of the cutter.

Face Milling Processes

Face milling operation
Image Source: Researchgate.com

In face milling operations, the axis of rotation of the cutting tool is perpendicular to the surface of the work part. And the cutting edges are at the outer periphery of the cutter. Also, there are different kinds of face milling processes.

  • Conventional Face Milling: In the conventional face milling process, the width of the cutter is greater than the work part surface that we machine. So, a conventional face milling machine all the surface of the work part.
  • End Milling: The end milling operation is the slot-making operation. Here, the surface of the work part is wider than the cutting tool. Operators can open slots with end milling operations.
  • Partial Face Milling: In partial face milling operation, one side of the work part is machined with a wide-face cutting tool.
  • Pocket Milling: In pocket milling operation, a narrow cutting tool creates shallow pockets on the surface of the work parts.
  • Profile Milling: The outside periphery of the work part is machined in profile milling operations.
  • Surface Contouring: We use the surface contouring process to obtain different shapes on the surfaces of work parts. For example, we produce negative molds in surface contouring operations.

These are the general types of face milling operations.

Peripheral Milling Processes

Types of peripheral milling.
Peripheral milling processes(Image Source: Mikell P. Groover; Fundamentals Of Modern Manufacturing, 4th edition, pg 525).

In the peripheral milling process, the rotation axis of the cutting tool is parallel to the workpiece. And the cutting edges are at the circumferences of this cutting tool. There are different types of peripheral milling operations.

  • Slotting: In slotting operation, a narrow cutting tool opens a slot on the work part which is wider than the cutting tool.
  • Straddle Milling Operation: In the straddle milling operation, the cutting tool cuts the outer sides of the surface of the work part in two directions.
  • Side Milling: Side milling is the same as straddle milling but there is only one side of the work part is machining.
  • Form Milling: The cutting edges of the cutting tools are shaping specially to obtain special slots on work parts.
  • Slab Milling: In slab milling processes, the cutting face of the cutting tool is wider than the workpiece. So, slab milling machines all the surfaces of the work part.

The other important classification of the peripheral milling operations is up milling and down milling operations. The orientations of the cutting edges on cutting tools are the defining factor in up milling and down milling classification.

What is Up Milling?

Up milling operation where the cutter edge orientation is the same as the rotation direction of the cutting tool. Up milling operations form longer chips and there is a lifting force on the work part. This lifting force is trying to lift the part from the worktable.

People call up milling also as conventional milling because of the orientation cutting edge.

What is Down Milling?

Down milling is the complete opposite of the up milling operation. In down milling, the orientation of the cutting edges is opposite to the direction of rotation. Also in a down milling operation, there is a thrust force acting on the workpiece. This thrust force pushes the work part to the worktable. And the chip formation is thicker if we compare it with the up milling operation.

Also, people call the down milling climb milling because of the orientation of the cutting edges.

What are the Types of Milling Machines?

In a typical milling machine, there is a spindle that holds the cutting tool. And there is a worktable that holds the workpiece. The spindle rotates to rotate the cutting tool. And worktable moves to make the feed motion. All the machinery that constitutes this contraption is the milling machines.

In general, there are two classes of milling machines. The first one is the vertical milling machines. And the second class is horizontal milling machines.

Horizontal vs. Vertical Milling –

Horizontal and vertical
Image Source: Pinterest.

We classify the milling machines as horizontal and vertical according to the orientation of the spindle. If the orientation of the spindle is parallel to the worktable, it is called a horizontal milling machine. And if the spindle is perpendicular to the worktable, this is the vertical milling machine.

In the vertical milling machines, operators perform face milling operations. Also in horizontal milling machines, operators perform the peripheral milling operations.

Also, there is another classification of milling machines.

Types of Milling Machines

In general, we classify the milling machines into 5 groups other than the spindle orientation.

Knee and Column Milling Machines

Knee and column
Universal milling machine(Image Source: Flickr.)

Knee and column milling machines are the most classical milling machines. Knee and column milling machines have both vertical and horizontal milling types. The Knee term refers to the construction that holds the worktable. Column means the total construction that holds the spindle and cutting tool.

In horizontal knee and column milling machines, there is an arbor that holds the cutting tool. But in the vertical knee and column milling machines, there is no arbor construction.

The most important advantage of the knee and column milling machines is the worktable can move in three dimensions. The worktable can move left and right. The saddle that holds the worktable can move forward and backward. And the knee can move up and down. This provides very big versatility to the column milling machines.

Universal milling machines and ram milling machines are the two important types of knee and column milling machines.

Bed Type Milling

Bed-type milling machines are the milling machines where high production rates are required. The general construction of the bed-type milling machines is more rigid than the knee-type milling machines.

The general difference between the bed-type milling machines is the worktable directly mounted on a bed. The bed provides a more rigid structure compared to the knee structure. The spindle is mounted on the spindle heat which is also more rigid compared with the column type.

The spindle head makes the up and down movement and the worktable makes the left and right movement. Bed-type milling machines obtain deeper cuts and rapid feeds thanks to their more rigid structure.

Also, we can mount multiple cutting tools on bed-type milling machines. If we mount two cutters, we call duplex milling machines. If we mount three cutters, we call them triplex milling machines.

Tracer Milling

The working principle of tracer milling machines is very different. There is a probe that traces the pattern part, and the machine performs the cutting operation according to this trace.

Tracer milling machines can be controlled by operators or they can be automatically controlled according to the tracer probes.

Parts that have complex contours such as dies or molds can not be produced in conventional milling machines. Operators use tracer milling machines or CNC milling machines to produce these complex contours.

Planer Type

The construction of the planer-type milling machines is very same as the planer machines. The only difference between the planer-type milling machine, milling cutting tools are used instead of single-point planning cutting tools.

Manufacturers produce the biggest parts in planer-type milling machines. Also, the construction of a planer-type milling machine is the biggest one among the other types. The cutting tools are mounted on a bridge structure. And these cutting tools perform the feed motion in three axes. The worktable is very big that we can mount big parts on them.

CNC Type

CNC
CNC Milling Machine(Image Source: Wikimedia.).

Computer numerical control milling machines are working with an alphanumerical code instead of physical orientations of movements. CNC machines are doing all the required movements for the milling operations automatically. Operators only load and unload parts and change the cutting tool of the machine.

Conclusion

As you see above, there are various kinds of milling processes. According to the requirements, these different types of processes are used by the manufacturing companies. To implement these different types of processes, also there are different types of milling machines. Manufacturing and machining businesses are using these different types of machines to implement different types of processes.

Finally, do not forget to leave your comments and questions below about the milling operations.

Your precious feedbacks are very important to us.

FAQs About Milling

What are the steps of the milling process? 

The first step of the milling operation is attaching the work part to the worktable. And attaching the proper cutting tool to the spindle. After that, the operator starts the milling machine and creates the required shapes in the technical drawing of the part.

How does a milling machine work?

The working principle of these machines is very simple. There is an electrical motor that turns the spindle that is controlled by software. And there is a little step motor to obtain sliding motions of the worktable and spindle. These motors are also controlled by the operator’s instructions to the machine. Gear systems control the spindle speed and belt mechanisms control the worktable and spindle feed motions.

What is the difference between grinding and milling?

In grinding operation, the purpose is to obtain a smooth and finished surface by not changing the dimensions of the part. But in milling, we change dimensions to obtain the final shape. So, grinding is a shaping operation and milling is not.

What is the advantage of milling operations?

The main advantage of the milling operation is; that milling operations can create very complex shapes from very hard materials such as metal alloys. Also, very tight tolerances of machining are another advantage of milling machines.

What is the disadvantage of milling processes?

The main disadvantage of the milling processes is that they are not suitable for the serial production of parts. The production rates are low compared with other manufacturing processes such as casting or forging.

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