https://geomiq.com/cnc-design-guide/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.
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Although the technology keeps changing, the practices mostly stay the same.
Vertical Radii
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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
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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
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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.
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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.
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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
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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.
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