The key advantages and disadvantages of CNC technology are summarised below:

• CNC machining offers excellent accuracy and repeatability and can produce parts with very tight tolerances, making it ideal for high-end applications.
• CNC materials have excellent and fully-isotropic physical properties and are suitable for most engineering applications.
• CNC is the most cost-effective manufacturing process for producing low-to-medium numbers of metal parts (from one-off prototypes, up to 50000 units)
• Due to the subtractive nature of CNC machining, specific geometries are either very costly or impossible to manufacture.
• The start-up cost of CNC machining is high compared to 3D printing, so CNC is less suited for low-cost prototyping (for example, comparing metal and plastic). 

The lead times of CNC machining (10 days) are longer than the lead times of 3D printing (2-5 days), as CNC machines are not as widely available since they require expert knowledge to operate.

CNC Design Rules and Tips

When designing 3D CAD parts that are going to be CNC programmed using CAM software, a few rules or pointers can be followed to allow the part to be machined on a CNC machine. 

For example, one needs to remember that the cutting tool used is round, so you cannot have square corners in some places, so one needs to be aware of the cutting tools used.

Here are some rules to follow when designing for CNC machining to prevent the CNC programmer from redesigning your part when they are doing the programming. 

1. Maximum part size

• CNC Milling 1500x800x800mm 
• CNC Turning Ø800x1000mm

Maximum part size depends on your machine size; giant machines can produce parts bigger than 2mx0.8mx1m, five-axis CNC machines have a smaller volume.

2. Take note of wall thickness.

• Metals: Minimum 0.8mm Plastics: Minimum 1.5mm
• Absolute minimum 0.6mm

Thin walls on a part allow for more opportunity to break, no matter what the material. Wall thickness is proportional to the stiffness of the material, so thinner wall thickness leads to vibrations when machining. As a guideline, keep this in mind: for metals, try not to go below 0.8mm, and plastics not less than 1.5mm. Try to design your part from sheet metal instead or use another manufacturing method if required.

3. Height of tall features

• Typically Maximum: 4 x part length

Vibration becomes a problem when machining tall features. Try to work by the ratio of max height is 4 x the length to minimize machine vibrations.

4. Try not to use too many tolerances in your design.

• Standard: ±0.125mm
• Minimum: ±0.001mm

Tolerances are necessary for a design; excess tolerancing only increases machining time and cost. Tight tolerances should only be specified where essential and should keep with the CNC machines tolerance ability. Some standard tolerance to stick by for most CNC machines is ±0.125mm, whereas down to ±0.001mm is feasible. Tolerances should be defined on all critical features.

5. Holes should have accurate length to width ratios.

• Maximum: Hole length = 4x Hole diameter

Try to work with the ratio hole diameter is less than 4 x the length of the hole. If the hole is too deep, this results in the tool deflecting and makes it difficult to remove the chips, leading to tool fracture. An example is a hole 10mm wide that should not be more than 40 mm deep. Deeper holes need to be machined with larger diameter cutting tools; bear this in mind.

6. Limit thread length

• Maximum thread length=3 x hole diameter
• Recommended: M6 or higher
• Smallest: around M2

Strong threads are formed in the first few turns, so long threads are pretty unnecessary; as a rule of thumb, try to design your thread three times longer than the hole diameter. Choose larger threads if possible; they are easier to machine.

7. Avoid designing features that can’t be machined

Not all features can be machined on a CNC. If one sticks to some simple guidelines when designing, the part will be redesigned before machining.

One needs to understand the machine’s capability; for example, A 3 axis machine cannot machine negative angles. Another example is one cannot machine curved holes on a CNC; to do this, one would have to split the part into two and reattach the two halves. EDM is a better process for this.

8. Try to design functional parts that can be machined.

When designing, try to think about the machining processes involved in creating the part; one needs to think practically about how the machine will move and if the part is to be machined on a three-axis or five-axis machine. Three-axis machines can only work with simple geometry and curves, whereas five-axis machines can manage more complex parts.

9. Size of small features

• Minimum: 2.5mm
• Smallest: 0.100 mm

Most CNC machines use a minimum of a 2.5mm tool diameter; this can be considered when designing. It is feasible to develop features smaller than 0.1mm, although this should be avoided.

10. Design restrictions

One needs to think of the surfaces that can be machined with the CNC tool, internal geometries such as a curved hole are impossible to machine, and such a part would need to be split into two separate parts machined separately and then combined to produce that geometry.

The vertical parts of a machined part will always have a radius due to the tool geometry – drills are always round and can only cut radii.

The geometry of the part decides how it is held in the CNC machine; this impacts the cost; if it can’t be bolted down, it would need to be clamped in place, which could add to a positional error when completing subsequent runs.

Workpiece stiffness can change when thin walls are machined into the parts leading to the vibration of the part. Take this into account when designing.

Read Geomiq’s CNC Design Guide > 

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