Computer numerical control (CNC) machining is a subtractive manufacturing method using a number of high-speed, precise machines to achieve desired structures. Basically, parts of a block of material are gradually stripped away using a variety of methods, including cutting, drilling, beveling, and slotting, among others.
As with all manufacturing processes, better results can be obtained by accommodating the specifics of CNC machining into the design stage. Following a brief review of the kinds of CNC machines, this article will go into detail on a number of design aspects.
For all the above machines, material is removed from a solid block of metal or plastic, with various high-speed cutting tools performing the different processes required. The majority of tools used for CNC processes have a cylindrical shank with a specific tip shape and a limited overall cutting length.
As the material is removed from the workpiece, the geometry of the tool is transferred onto the part. This means internal features of a CNC-machined part will always have corner radii, no matter how small the cutting tool selected for the process.
Material selection is critical for determining the cost of the part. Characteristics to be considered when selecting them for machining include hardness, rigidity, and chemical resistance, among other mechanical and aesthetic qualities.
Tolerances define the acceptable range for any given dimension. If no overall tolerances are defined, most machine shops will use a standard tolerance of ±0.125 mm, but it is usually important to define your own standard as the application may require a tighter tolerance than this.
Fit can be referred to as the proper application of tolerances, and are either specified as shaft-basis or hole-basis fits. Fits can be split into three categories:
Specific tolerances should only be used when the fit needs to be controlled for two or more mating parts. The following example takes in all three fits:
But say the housing is one of three housings set up across a larger frame, with a long shaft (or axle) running through all of them, how do you control clearances of the mounting holes but also the concentricity of the bearings for the shaft?
This is where geometric dimensioning and tolerancing (GD&T) comes in, using a symbolic language to explicitly describe nominal geometry and the permissible variation. For our example above, you could control fit using either concentricity or position characteristics.
Although standard tolerancing generally suffices for part design, for more complex parts where features have bearing on each other, certain geometric characteristics used in GD&T (flatness, straightness, cylindricity, concentricity, etc.) are necessary. Make sure to avoid any unnecessary tight tolerancing as this may increase part cost by slowing machines down, requiring additional jigging, or using special measurement tools.
Undercuts are features that cannot be machined using standard cutting tools, as some of their surfaces are not accessible directly from above. There are two main types of undercut features:
When machining, it is best to avoid undercuts as they are difficult to machine and require specialist tooling or multiple setups. If completely necessary, keep the undercut amount as small as possible.
For undercuts on internal walls, add enough clearance for the tool. Add space equal to a minimum of four times the depth of the undercut between a machined wall and any other internal wall.
Other specialist finishes such as dry lubricants (reduces friction) and other performance-enhancing finishes are available, so it is important you do due diligence for your application.
License: The text of "CNC Design – How to Design Parts for CNC" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
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