WHAT IS MILLING?

Milling is the process of cutting away material by feeding a workpiece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat,angular, or curved. The surface may also be milled to any combination of shapes. The machine for holding the workpiece, rotating the cutter, and feeding

CLASSIFICATION OF MILLING

Peripheral Milling

In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body. The axis of cutter rotation is generally in a plane parallel to the workpiece surface to be machined.

(Kalpakjian S., Introduction to Manufacturing Processes)

Face Milling

In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter.

End Milling

The cutter in end milling generally rotates on an axis vertical to the workpiece. It can be tilted to machine tapered surfaces. Cutting teeth are located on both the end face of the cutter and the periphery of the cutter body.

METHODS OF MILLING

Up Milling

Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the workpiece is fed to the right in up milling.

(Boothroyd G. & Knight W., Fundamentals of Machining and Machine Tools)

Down Milling

Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling.

(Boothroyd G. & Knight W., Fundamentals of Machining and Machine Tools)

The chip formation in down milling is opposite to the chip formation in up milling. The figure for down milling shows that the cutter tooth is almost parallel to the top surface of the workpiece. The cutter tooth begins to mill the full chip thickness. Then the chip thickness gradually decreases.

Other milling operations are shown in the figure.

(Kalpakjian S., Introduction to Manufacturing Processes)

MILLING EQUIPMENT

The milling machine is one of the most versatile machine tools in existence. In addition to straight milling of flat and irregularly shaped surfaces, it can perform gear and thread cutting, drilling, boring and slotting operations which are normally handled on machine tools designed specifically for these specific operations.

The above is a Bridgeport CNC Milling Machine

Column & Knee type Milling Machines

Used for general purpose milling operations, column and knee type milling machines are the most common milling machines. The spindle to which the milling cutter is may be horizontal (slab milling) or vertical (face and end milling). The basic components are:

Work table, on which the workpiece is clamped using the T-slots. The table moves longitudinally with respect to the saddle.

Saddle, which supports the table and can move transversely.

Knee, which supports the saddle and gives the table vertical movements for adjusting the depth of cut.

Overarm in horizontal machines, which is adjustable to accomadate different arbor lengths.

Head, which contains the spindle and cutter holders. In vertical machines the head may be fixed or vertically adjustable.


Bed type Machines

In bed type machines, the work table is mounted directly on the bed, which replaces the knee, and can move only longitudinally. These machines have high stiffness and are used for high production work.

Planer Machines

Planer machines are similar to bed type machines but are equipped with several cutters and heads to mill various surfaces.

Rotary Table Machines

Rotary table machines are similar to vertical milling machines and are equipped with one or more heads to do face milling operations.

Tracer Controlled Machines

Tracer controlled machines reproduce parts from a master model. They are used in the automotive and aerospace industries fro machining complex parts and dies.

Computer Numerical Control(CNC) Machines

Various milling machine components are being replaced rapidly with computer numerical control(CNC) machines. These machine tools are versatile and are capable of milling, drilling, boring and tapping with repetitive accuracy.

Milling Cutters

A milling cutter is a cutting tool that is used on a milling machine.Milling cutters are available in many standard and special types, forms, diameters, and widths.The teeth maybe straight (parallel to the axis of rotation) or at a helix angle. The helix angle helps a slow engagement of the tool distributing the forces .The cutter may be right-hand (to turn clockwise) or left-hand (to turn counterclockwise).The figure shows a typical end milling cutter.

Features of Milling Cutters

Some of the terms used to identify the major features

of a milling cutter are given in the figure.

(Olivo C.T., Machine Tool Technology and ManufacturingProcesses, C Thomas Olivo and Associates)

Types of Milling Cutters

STEWART PLATFORM as a Machine tool

VARIAX

Researchers have been looking at a device called a parallel kinematic link mechanism to replace the conventional base and tower milling machine tool. The analysis showed that the type of motion and forces needed could be provided in large part by a mechanism called Stewart platform- a type of parallel kinematic link mechanism. A number of research labs and a few companies are working at aspects of this design. Giddings and Lewis have designed the VARIAX (shown above) and Ingersoll Milling Co. have designed the octahedral hexapod. These machine tools consist of a lower platform, an upper platform and six legs that connect the two. The top platform (head) contains the machine spindle and the bottom platform (bed) holds the workpiece. The six legs perform the task of positioning the head woth respect to the bed. Since the machine has no prescribed axis, no linear bearings and no ways in which to travel it offers extraordinary machine stiffness. For more on this machine tool look up the following links:

A 3 DOF parallel link mechanism is being developed in the SMARTCUTS project at the University of illinois, Urbana-Champaign.

Machine Tool Structures page.

Miscellaneous Links

MIT - NMIS Machine Shop Tutorial

Society of Mechanical Engineers

Manufacturer's Information Network

Thomas Registry

TradeWave Galaxy

METAL Machining and Fabrication

CNC Manufacturing Systems

WWW Virtual Library: Mechanical Engineering

Precision Machined Products Association Home Page