What is Milling?

June 1, 2022

Milling is a machining process which is used to produce complex machine parts with precision. The shapes shown in Fig. 1 below are made by milling (1).

Fig. 1 Parts made by milling (1).

Milling is used to produce parts when they cannot be made through a conventional manufacturing process such as metal casting. It is a material removal process which uses a milling cutter (2) which can create a variety of features on a part by cutting away the unwanted material. The milling cutter is a rotary cutting tool (3) often with multiple cutting points. As the milling cutter enters the work piece, the cutting edges (flutes or teeth) of the tool repeatedly cut into and exit from the material, shaving off chips (swarf) (4)from the work piece with each pass. Fig. 2 below outlines the basic milling process (5).

Fig.2. The milling process (5)

There are two broad categories of milling operations namely, peripheral milling and face milling. In peripheral milling the teeth are located on the periphery of the cutter body and in face milling the teeth are on both the periphery and the face of the cutter as shown in Fig. 3(6).

In peripheral milling the surface of workpiece is parallel with the axis of rotation of the cutter but in face milling it is at right angles to the cutter axis. Peripheral milling can be used for both flat and formed surface depending on the axial contour of the cutter. In face milling, while most of the cutting is done by the peripheral portions of the teeth, the face portion provides the finishing action.

Peripheral milling is usually done on a horizontal spindle machine where the orientation of the spindle on which the cutting tool is fixed is horizontal. Face milling can be done on both horizontal and vertical spindle machines. Face milling produces a smoother surface than peripheral milling.

Fig.3. Peripheral milling and face milling (6)

Milling is typically used to produce parts that are not axially symmetric. Axial symmetry is symmetry around an axis; an object is axially symmetric if its appearance is unchanged if rotated around an axis. The milled products can have complex features as holes, slots, pockets, and even three-dimensional surface contours. Due to the high tolerances and surface finishes that milling can offer, it is ideal for adding precision features to a part whose basic shape has already been formed.

An advanced milling process is CNC milling (7) which is milling using computerised controls.  CNC milling offers higher precision, accuracy and production rates, but there are still some situations when manual milling is useful. CNC milling provides more precision than manual milling and can reduce the chance of human error.

Due to the high precision that CNC milling offers, it is used in a wide variety of industries including the aerospace, medical and electronics industries.

The full story of milling machine development may never be known because much early development took place in individual shops where few records were kept for posterity. A rotary file by Jacques de Vaucanson, circa 1760, (8) is the earliest milling equipment known.

Advantages of CNC Milling

  • Precision
  • Fast and efficient
  • Wide range of material options
  • Affordability

Disadvantages of CNC Milling

  • Material wastage
  • Lack of qualified CNC mill operators

The disadvantages of milling and CNC milling can be addressed through an additive technology known as 3D printing.

What is 3D printing?

3D printing is an additive technology which is used to manufacture parts (9). It is ‘additive’ in that it does not require a block of material or a mould to manufacture physical objects, it simply stacks and fuses layers of material. It is typically fast, simple and allows affordable in-house manufacturing with low fixed setup costs and can create more complex geometries with an ever-expanding list of materials.

SPEE3D (10), an Australian company has developed the world’s first metal 3D printer using supersonic 3D deposition technology to deliver manufacturing-grade printing at production speeds. The company’s cutting-edge technology allows application across several industries including automotive, marine, defence and aerospace.

SPEE3D’s metal printing technology differs from conventional metal 3D printing. Instead of melting the raw material to fuse parts, the process involves a rocket engine accelerating compressed air with an added metal powder to produce a part in solid state. The technology builds parts that are ready for use, eliminating the problems associated with melting such as solidification cracks, brittle intermetallics, gas porosities, shrinkage and harmful residual stresses.

It is the world’s first metal 3D printing production cell which has a metal 3D printer integrated with a heat treatment oven and a CNC 3 axis milling machine with support provided by SPEE3D’s experts in metal additive manufacturing.

References

  1. Kalpakjian and Schmid, Manufacturing Engineering and Technology, 7th edition, 2014.
  2. https://en.wikipedia.org/wiki/Milling_cutter
  3. https://en.wikipedia.org/wiki/Cutting_tool_(machining)
  4. https://en.wikipedia.org/wiki/Swarf
  5. https://engineering.myindialist.com/2009/principle-and-working-of-milling-machine/#.YiGadehBw2w
  6. https://multisourcemfg.com/2019/10/21/cnc-machining-milling/
  7. https://www.thomasnet.com/articles/custom-manufacturing-fabricating/understanding-cnc-milling/
  8. https://en.wikipedia.org/wiki/Jacques_de_Vaucanson
  9. https://www.twi-global.com/technical-knowledge/faqs/what-is-3d-printing