Everything you always wanted to know about CNC milling

I was recently asked by a friend what exactly this CNC milling is. And why do you need it at all. Now that I have dealt a lot with CNC milling, I was able to tell him something about it. And then I thought to myself, I’ll just write a blog article about important facts that almost no one knows about CNC milling.

Milling is like 3D printing – only the other way around!

With a CNC milling machine, the possibilities are almost unlimited. Milling is a very versatile manufacturing process even without numerical control, with few restrictions in terms of the workpiece geometries that can be produced. In this respect, it is similar to 3D printing, except that when milling, the shape is created by removing material while it is gradually built up in the 3D printer. The coordinate-based computer control during milling not only increases the flexibility of the process but also increases its manufacturing accuracy. With computer-aided CNC milling, much more complex parts can be produced than is possible with conventional milling.

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Milling is like drilling – but in 2 additional spatial directions!

From the point of view of manufacturing technology, milling is very closely related to drilling. In both processes, the tool performs the main movement, namely in the form of a rotation. When drilling, however, the feed movement takes place exclusively in the direction of the axis of rotation, whereas when milling it is predominantly perpendicular to it. Because of the great similarity of the machining processes you can also drill with milling machines. Provided, of course, that you use the right tools. Milling with a drilling machine should not be done, by the way, because the drilling spindle is only designed for forces in the axial direction: On the one hand, the drilling spindle is not stiff enough for transverse forces, on the other hand, the drive will wear out much faster. So it’s better to use a CNC milling machine. But why is CNC milling so universally applicable?

CNC milling – How to use the right material and mill it precisely

These workpiece contours can be CNC milled

Due to the high degree of freedom of multi-axis CNC milling machines, almost any part geometry can be produced:

  • Milling of rotationally symmetrical workpieces
  • Machining of cuboid workpieces with plane-parallel surfaces
  • 3D freeform surfaces (such as turbine blades or aircraft wings)
  • Milling of gears
  • Milling of camshafts and crankshafts

When milling, you determine the workpiece contour not only through the feed movement but also through the shape of the milling tool:

  • Shell and face milling cutters are primarily used to produce flat surfaces.
  • Form cutters allow the production of grooves with a given cross-section, such as a dovetail shape.
  • The end mill is suitable for milling undercut-free, but otherwise largely arbitrary three-dimensional shapes.

Positioning the axes on a CNC milling machine

In milling machines, the positioning and the feed of the main spindle are always carried out using coordinates in a right-angled, Cartesian coordinate system. Drilling with a milling machine is therefore comparable to machining on a jig boring machine.

With CNC milling, a computer determines the exact movements of the main spindle in the coordinate system based on the machining steps specified by the NC programmer. The use of a computer also allows, for example, the consideration of complex relationships between the tool geometry and the contour to be produced or even the Correction of elastic deformations of workpiece and tool during machining as well as tool wear compensation.

Which materials are usually machined?

The following materials can usually be machined on a milling machine without restriction:

  • Steel, cast steel
  • Aluminum, cast aluminum
  • Plastics
  • titanium
  • Wood
  • Precious metals (gold, silver)
  • bronze
  • Brass

This accuracy is achieved with CNC milling

The accuracy during milling is dependent on the quality of the machine, the adjusted cutting parameters, and the material used. Under ideal conditions, the tolerance grade IT6 and surface roughness of Rz1.6 can be achieved for metals. Note: IT6 corresponds to a tolerance band of 8 µm for diameter 6, for example. Hobby machines are usually not that stiff and therefore move within a tolerance range of +/- 0.1 mm. However, this accuracy is often sufficient for modeling tasks. Precision mechanical assemblies, however, require higher levels of accuracy (e.g. +/- 0.02 mm).

This is how path control works in CNC milling

In order to guarantee the low manufacturing tolerances in CNC milling, an exact calculation of the path points to be controlled, i.e. the setpoints is necessary. In addition, the position actually reached by the tool must be constantly monitored. These actual values can be determined directly and indirectly.

The indirect measuring method converts the feed movement of the tool slide into a rotary movement that is relatively easy to detect. All that is required is a compact rotary encoder as the measured value pick-up, which can, for example, be coupled directly to the drive spindle of the tool slide.

For a direct measurement, however, a sufficiently precise scale along the entire path of the tool slide and a corresponding sensor on the slide itself is necessary. A major advantage of the direct measurement method, however, is the higher accuracy, since in the case of an indirect measurement by converting the rotary movement into the longitudinal movement of the slide, additional measurement inaccuracies come into play, for example, due to wear, due to elastic deformation of the spindle due to the transmitted forces or if the Spindle does not work completely free of backlash. For details, please visit: https://www.abdultraders.com/