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: Milling

MAS 2.0 - Guides - F.A.Q. - Tutorials - Home

 

More Information

Milling includes a number of versatile machining operations, including: slotting, drilling, reaming, facing, and pocket removal. These operations are carried out by a cutting tool that revoloves around its central axis. A workpiece or stock is clamped on a bed, and the cutting tool moves into it, removing material as it goes. Either the tool itself is moved ( usually it moves along the axis of the tool) or the bed the stock is attaced to moves, bringing the workpiece into contact with the tool.

Another way that milling is versatile is that it is used to make parts from raw stock as well as adding high tolerance features to parts made with another process- such as sand casting.

Parts such as the ones shown in sample parts can be machined efficiently and repeatedly using various milling cutters.

Depending on the geometry of the cutter and the motion between the cutter and the workpiece, we distinguish the following milling categories: slab miling, face milling, end milling, straddle milling, and form milling.

Slab milling

In slab milling, also called peripheral milling, the axis of cutter rotation is parallel to the workpiece surface to be machined (Fig. 1a). The cutter, generally made of high-speed steel, has a number of teeth along its circumference, each tooth acting like a single-point cutting tool called a plain mill.

Cutters used in slab milling may have straight or helical teeth producing orthogonal or oblique cutting action.

 
Fig 1. (a) Schematic illustration of conventional milling and climb milling. (b) Slab milling operation, showing depth of cut d, feed per tooth f, chip depth of cut, t, and workpiece speed v. (c) Schematic illustration of cutter travel distance t, to reach full depth of cut.

The cutting action occurs either by conventional milling or climb milling.

Conventional Milling

In conventional milling, also called up milling, the maximum chip thickness is at the end of the cut. This is the dominant method of milling.

Pros:

  • Tooth engagement is not a function of workpiece surface characteristics.
  • Contamination or scale on the surface does not affect tool life.
  • The cutting process is smooth, provided that the cutter teeth are sharp.

Cons:

  • The tool has the tendency to chatter.
  • The workpiece has the tendency to be pulled up, thus proper clamping is important.

Climb Milling

In climb milling, also called down milling, cutting starts with the chip at its thickest location. A typical application is in finishing cuts on aluminum.

Pros:

  • The downward component of cutting forces holds the workplace in place, particularly for slender parts.

Cons:

  • Because of the resulting high impact forces when the teeth engage the workpiece, this operation must have a rigid setup, and backlash must be eliminated in the table feed mechanism.
  • Climb milling is not suitable for machining workpieces having surface scale, such as hot-worked metals, forgings, and castings.
  • The scale is hard and abrasive and causes excessive wear and damage to the cutter teeth, thus reducing tool life.

Face Milling

In face milling the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface and removes material in the manner shown in the figures below.

 
Face milling operation showing (a) action of an insert in face milling, and (b) climb milling. Note that the width of cut w is not necessarily the same as the cutter radius.

The cutter rotates at a rotational speed N and the workpiece moves along a straight path at a linear speed v.

When the cutter rotates such that its linear velocity is in the same direction as that of the workpiece, the operation is climb milling; when it rotates in the opposite direction, the operation is conventional milling.

The cutting tools are usually carbide or high-speed-steel inserts and are mounted on the cutter body.

Because of the relative motion between the cutting teeth and the workpiece, a face milling cutter leaves feed marks on the machined surface, much as in turning operations. Surface roughness depends on insert corner geometry and feed per tooth.

End Milling

The cutter in end milling has either straight or tapered shanks. The cutter usually rotates on an axis perpendicular to the workpiece, although it can be tilted to machine tapered surfaces.

Flat surfaces as well as various profiles can be produced by end milling. The end faces of some end mills have cutting teeth, and these can be used as a drill to start a cavity.

Ball nosed end mills have hemispherical ends for producing curved surfaces, such as cutting die cavities. Hollow end mills have internal cutting teeth and are used for machining the cylindrical surface of solid round workpieces, as in preparing stock with accurate diameters for automatic machines. End mills are made of high-speed steels or have carbide inserts.

A typical application is milling aluminum-alloy aerospace components and honeycomb structures, with spindle speeds on the order of 20,000 rpm. Chip collection and disposal can be a significant problem in these operations.

Straddle Milling

Two or more cutters are mounted on an arbor and are used to machine two parallel surfaces on the workpiece.

Form Milling

Cutters with very specially shaped teeth are used to produce curved profiles. Such cutters are also used for cutting gear teeth.

Source: Kalpakjian, Manufacturing Proceses for Engineering Materials.