The Following Summary was adapted from this guide

Basic Principles

• CNC (computer numerical control) is the automated control of machining tools (drills, boring tools, lathes) and 3D printers by means of a computer.

• A CNC machine processes pieces of materials to meet specifications by following a coded programmed instruction and without a manual operator.

• The CNC process involves the removal of material from a solid block or a pre-formed part using various cutting tools, it is a subtractive process.

  • The geometry of the part wanted is defined by a CAD model, a machinist uses CAM software to prepare the cutting paths with tool selections needed to achieve the part.

  • The cutting paths are then output as Gcode which instructs the machine how it will cut, supplying it with the orientation of the machine head, the part and speed of movement.

  • The machine is then set up and the part is jigged (secured) in position.

  • The machine then executes the Gcode to shape the part.

These are the various advantaged and Disadvantages:

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CNC Machining

• There are two categories of CNC machines, 3-Axis and Multi-Axis

• 3-Axis machines move the cutting tool relative to the part along the x,y,z axis.

• Multi-Axis machines add rotation to one or more axis allowing the parts to be cut from various angles.

The following picture illustrates the two:

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2-3 Axis CNC

Milling

• CNC Mills are quite common and can be used for many geometries

• The workpiece is held in place using a jig or vice and then the mill head moves in 3 axes to remove material using rotary tools or drills.

• Set-up costs are low since they’re easy to operate compared to other CNC processes

• Since they’re quite small, there is a limited range in movement which means that you can’t create some features

The Milling Process

• This process involves the use of specialized rotational cutting tools to remove material. This is often done in multiple stages, earlier stages using high speeds with a lower accuracy where later stages using higher accuracies to remove small amounts of material left over

• The parts are then de-burred and there are also post-processes like surface finishes and paint applied to the part

Cutting tools

• These processes often require various cutting tools for different cuts and features

• 1-3 are Flat, Bull, and Ball heads. These are the most common cutting tools for making slots, grooves, cavities and vertical walls. They are selected by the required form of the bottom cavity

• 4 is a simple drill, they are selected for standard holes

• 5 is a slot cutter, since their heads are larger than their shafts, it can create undercuts that remove material from the sides of vertical walls

• 6 is a tap, this is used for threaded holes

• 7 is a face mill cutter, it removes material from flat surfaces

CNC Turning

• This process is done using a lathe. These machines can produce parts at higher rates than milling, and turn lower cost, these machines are useful for large numbers of parts

• The work-piece is held on a spindle and rotated at high speeds. The cutter is normally a blade and slowly moves towards the part to describe its profile

• Lathes cannot produce many features since the machine can only produce “rotationally symmetrical” or “Revolved” parts. For these features, the part is usually transferred to a mill

The Turning Process

• This process is quite similar to the milling process, the difference is that parts in a lathe will be rotated when being cut instead of milled

• When additional features are needed, the part will be transferred to a mill before the post-processes are applied

Multi-Axis CNC

Indexed 3+2 Axis

• These machines were developed to reduce the time needed to machine a part

• Between operations, the tool/bed can rotate so you can work on the piece from various angles

• The ability to reorient the work-piece automatically allows for more complex parts which a conventional 3-axis CNC mill cannot

Continuous 5 Axis

• This process allows all 5 axes to move simultaneously during machining

• These parts are very accurate and complex but come at a much higher cost

Mill Turning

• These machines are hybrids of lathes and mills

• These machines are ideally suited for parts that need to have rotational symmetry along with other features. They come at a much lower cost than other 5-axis machines

• The part can either be rotated or precisely positioned

• Both tools used in Lathes and mills are used to remove material

The following table outlines the best process to select depending on the situation

CNC - Best Practice

• Although the technology keeps changing, the practices mostly stay the same.

Vertical Radii

• Creating a cavity requires an end mill tool

• End mill tools have limited cutting lengths

• If you increase the corner radii (e.g. +1mm), that will allow the tool to follow a circular path rather than a rectangular path. Making the circular path will, in turn, reduce the load on tools, will allow for a higher quality finish and will lower cycle times.

Cavities

• As stated previously, creating a cavity requires an end mill tool (pictured above) that has limited cutting lengths that are usually around 3-4 times their diameter

• Because of this, you should avoid making deep cavities as longer tools will flex under pressure which will reduce accuracy and possibly damage the part

• Creating deep cavities will dramatically increase the cost of the process as much more material must be removed and chips are harder to extract.

Minimum Wall Thickness

• When walls get thinner, vibrations will begin to increase as a result of reduced stiffness in the material which will end up reducing the accuracy when machining

• In this example, the reason why Plastic has a higher recommended wall thickness is that plastics, unlike metals are less stiff, prone to warping and soften as temperature increases

Holes

Use the following tap guide to get the various drill sizes needed with their specifications

• Hole Diameters

  • Holes are machined using an end mill ( pictures on the right) or standard drill bits ( pictured on the left) which will achieve the best accuracy under 20mm.

• Minimum Hole Diameters

  • Most standard CNC services offer drilling up to 2.5 mm in diameter. Once you go below, this is considered micro-machining which requires specialty tools and even the cutting physics change at this scale, this will in turn come at a much greater cost.

• Thread Depths

• Thread Diameters

Tolerances

• Tolerance defines the acceptable limits of a measurable or important dimension, what this means is pretty much the margin of error allowed during cutting.

• CNC machines have some of the tightest tolerance capabilities

• If no tolerance is specified, most machine shops will use +/- 0.025mm

Text and Lettering

• Applying text that will be accurate adds costs and time since you must use small tools for most of these processes

• When making text, especially embossed, reduces the use of profile cutters and roughing tools which will increase the CNC time

• It’s often challenging to get high-quality surface finishes at the foot of text

• Adding text should be avoided because of how much it increases the cost

• Embossed test if often preferred since less material must be removed, it also gives better results if the part is for injection mold tooling

• The recommended fonts are Arial, Verdana, or Helvetica since they have fewer sharp features and are often pre-programmed into CNC routines

• Remember to always check that your text will apply to CNC rules being thin wall, cavity depth, cavity width etc.