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11/09/2007 - Milling

Milling i san operation in which metal is removed by a rotating tool with multiple teeth: each tooth remove a small quantity of metal for each rotation. Because the piece and the cutter can be moved in more than one direction, surface of any orientations can be machined. By milling it’s possible to obtain many kind of surfaces as plain surfaces, seat for keys and tongues, and many kind of grooves.
In some cases (for limited productions) it’s also possible to cut gears. Milling is particularly indicated for those materials with hardness lower than 25 HRC (Rockwell Hardness) but is also used with materials until 35 HRC.
The cutting tool is a multiple cutter with teeth that can be arranged on various kind of surfaces (cylindrical, conical, etc.); each tooth can be regarded as a single cutting tool. Cutters can be straight or helicoidal, where this last are preferable, allowing to obtain a gradual cut. Adopted materials are usually steel or sintered carbides, fixed to tool body.
There are different kinds of milling cutters in the industrial reality. The choice of the particular kind of tool will be governed by considerations on the kind of component to be milled and on the particular kind of operation needed.
Just to make some examples there are (Figure 1) cylindrical cutters, used to realize plain surfaces, plain milling cutters, use to level surfaces, multiple cut tools, T shaped cutters, angular cutters and finally, a particular kind of cutters are the circular saws.
Apart from the kind of adopted tool, milling operations can be classified on the basis of the relative motion between cutter and machined piece. Then we can obtain the following classification:

- Peripheral milling: when the cutter rotation axis is parallel to the worked surface;

- Plain milling: when the cutter rotation axis is perpendicular to the worked surface;

Different kind of milling machines are nowadays used. The main families of machine are:

- Vertical milling machines (Figure 2): are used for general purpose applications and can mill plain and curve surfaces. They can also cut gears and threads, holes and seat for keys and tongues. This kind of machines has a vertical mandrel and the workplane can be moved perpendicularly to that axle.
- Horizontal milling machines (Figura 3): the main feature of this machines id the high stiffness of their structure that allow to obtain higher precisions and tighter dimensional tolerances. In this kind of machines, the mandrel is horizontal, and the workplane can be moved parallel to that axle.

Figure 2: A vertical milling machine.

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