What is a milling machine?
Milling is the machining process of using rotary cutters to remove material from a workpiece by advancing (or feeding) in a direction at an angle with the axis of the tool. It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations.
Milling is the most common form of machining, a material removal process, which can create a variety of features on a part by cutting away the unwanted material. The milling process requires a milling machine, workpiece, fixture, and cutter. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to a platform inside the milling machine. The cutter is a cutting tool with sharp teeth that is also secured in the milling machine and rotates at high speeds. By feeding the workpiece into the rotating cutter, material is cut away from this workpiece in the form of small chips to create the desired shape.
Milling machines are tools designed to machine metal, wood, and other solid materials. Often automated, milling machines can be positioned in either vertical or horizontal orientation to carve out materials based on a pre-existing design. These designs are often CAD directed, and many milling machines are CNC-operated, although manually and traditionally-automated milling devices are also common. Milling machines are capable of dynamic movement, both of the tool and the workpiece, and many milling machines can perform multi-axis machining.
Because of variations in orientation, operation and application, milling machines have varying functions and different operating principles.
Milling machines can be outfitted with a number of tool heads to accomplish different machining needs. Some of these tool heads include cutters, rounding mills, fluted mills and ball end mills. Some milling machines have rotating tool ends that can change depending on the needed task computer programming communicates with the machine when to change its tooling.
The different tooling used in milling machines is based on material and desired shape. Because materials like wood and steel have different physical properties, different tool bits are needed to properly machine the materials. If a milling machine uses a tool bit that is not strong enough to machine steel, the tooling and even the machine itself can be damaged. Tooling that is too strong for softer materials can damage the workpiece.
The basic tooling bit on a milling machine is called the cutter. A cutter is a shaped bar that has saw teeth. The cutter rotates rapidly to cut down and shape materials. The cutter is attached to an arbor, which is sometimes called a mandrel or mandril, a shaped bar that varies in size, length and ending, and is used to hold the cutter firmly.
A milling cutter’s saw ending can be spaced, sized and oriented in many ways. Generally, the teeth are either positioned in a straight up-and-down orientation, or angled in a helical orientation. Straight teeth are preferable in operations on denser materials, while helical teeth can create very smooth cuts on softer materials. There are a variety of cutters within these categories, including dense end cutters, t-slot cutters, and angle cutters. Cutters are subject to different standardized sizes, with CAT sizes as the most commonly-used standardization category in the United States.
Milling Machine Part
1.Column & Base
Column including base is the main casting that supports all other parts of milling machine.
The column contains an oil reservoir and a pump which lubricates the spindle.
The column rests on the base and base contains coolant reservoir and a pump which is used during machining operation that requires coolant.
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 lever either with the help of hand or power feed.
3. Saddle and Swivel Table
Saddle is present on the knee and supports the table. It slides on a horizontal dovetail on the knee and dovetail is parallel to the axis of the spindle ( in horizontal milling m/c).
The swivel table (in universal machines only) is attached to the saddle that can be swiveled (revolved) horizontally in either direction.
4. Power Feed Mechanism
It is the knee which contains the power feed mechanism. It is used to control the longitudinal ( left and right), transverse ( in and out) and vertical (up and down) feeds.
To get the desired rate of feed on the machine, the feed selection lever is positioned as indicated on the feed selection plates.
On some universal knee and column milling machine, the feed is obtained by turning the speed selection handle until the desired rate of feed is indicated on the feed dial.
Most of the milling machines have a rapid traverse lever that can be engaged when a temporary increase in the speed of the longitudinal, transverse or vertical feeds is required. For example this lever would be engaged when the operator is positioning or aligning the work.
It is a rectangular casting which is present on the top of the saddle.
It is used to hold the work or work holding devices.
It contains several T-slots for holding the work and work holding devices (i.e. jigs and fixtures).
The table can be operated by hand or by power.To move the table by hand, engage and turn the longitudinal hand crank. To move it through power, engage the longitudinal direction feed control lever.
It is the shaft which is used to hold and drives the cutting tools of the milling machine.
Spindle is mounted on the bearings and supported by the column.
Spindle is driven by the electric motor through gear trains. The gear trains are present within the column.
The face of the spindle which lies near to the table has an internal taper machined on it. The internal taper at the front face of the spindle permits only tapered cutter holder or arbor. It has two keys at the front face which provides positive drive for the cutter holder or arbor.
The drawbolt and jamnut is used to secure the holder and arbor in the spindle.
7. Over Arm / Overhanging Arm
It is a horizontal beam present at the top face of the column. It may be a single casting which slides on the dovetail ways present on the top face of the column.
The overarm is used to fastened arbor support. It may consist of one or two cylindrical bars which slide through the holes in the column.
8. Arbor Support
It is a casting with bearing that supports the outer end of the arbor. It also helps in aligning the outer end of the arbor with the spindle.
It prevents the springing of outer end of the arbor during cutting operations.
There are generally two types of arbor supports used in the milling machine. The first one has small diameter bearing hole, 1-inch in maximum diameter. And the other one has large diameter bearing hole, usually upto 23/4 inches.
The arbor support 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 machine.
The overhanging arm in the vertical machine is called ram. One end of the ram is mounted on the top of the column and on the other end milling head is attached.
The ram can be a moved transversally ( in and out) on the column by a hand lever.
Types of Milling Machines
Milling machines are categorized by their orientation to their workpiece and their degree of motion
Knee-Type milling machinesemploy a vertical workspace supported by a knee, which is an adjustable vertical casting. The knee supports a saddle and can be adjusted to allow for a customizable workspace.
Plain Vertical and Horizontal
Milling machines with a standard work surface can either be oriented vertically or horizontally. The tooling assembly is generally affixed on a turret and swivel, typically positioned parallel to the workspace. The turret and swivel allow the tool to move freely around the workpiece to enforce tight tolerances.
Universal Horizontal Milling Machine
A universal horizontal milling machine differs from the plain horizontal type because it has a table swivel housing, which allows the table to move out 45 degrees from the standard horizontal position. This workpiece movement allows for easier angular or helical milling operations.
Ram-Type and Universal Ram-Type Milling Machines
A ram-type machine is used to allow the tooling to position itself on a greater range of space with regards to the workpiece. The ram-type machine has a spindle on a movable housing, which can move within a set horizontal plane. The universal ram-type milling machine includes a swivel housing that increases the range of cutting movements.
Swivel Cutter Head Ram-Type Milling Machine
With a swivel cutter, a milling machine can rotate from a completely vertical to a completely horizontal position. The worktable also moves, providing the user with a very liberal degree of motion and orientation. Many swivel cutters include both automatic or hand driven settings, increasing operation options.