CNC machining is a common subtractive manufacturing technology. Unlike a 3D printing service, the process typically begins with a solid block of material (blank) and removes material to achieve the required final shape, using a variety of sharp rotating tools or cutters.
What affects the Cost of CNC parts?
The price of CNC machined parts depends on the following:
- Machining time: The longer it takes to machine a part, the more expensive it will be. Machining time is often the main cost driver in CNC.
- Start-up costs: These are related to CAD file preparation and process planning and are significant for smaller volumes. This cost is fixed and there is an opportunity to reduce the unit price by taking advantage of “economies of scale”.
- Material cost: The cost of the bulk material and the ease with which that material can be machined greatly affects the overall cost in CNC. Optimizing your design while having certain material considerations in mind, can greatly reduce the price.
- Other manufacturing costs: When you design parts with special requirements (for example, when you define tight tolerances or design thin walls), then special tooling, closer quality control, and more processing steps – at lower machining speeds – may be required. This, of course, has an impact on the total manufacturing time (and the price).
Now that it is clear where the cost of CNC comes from, let’s see how a design can be optimized to minimize it.
As a rule of thumb:
To minimize the cost of CNC machined parts, stick to designs with simple geometries and standardized features.
Tip #1 – Add a Radius in Internal Vertical Edges
All CNC milling tools have a cylindrical shape and will create a radius when cutting the edge of a pocket. Corner radius can be reduced by using a tool with a smaller diameter. This means that multiple passes at a lower speed will be required – a smaller tool cannot remove material in one pass as fast as a larger tool – increasing the machining time and cost.
To minimize cost:
- Add a radius of at least 1/3 of the depth of the cavity (the larger the better).
- Preferably, use the same radius in all internal edges.
- On the floor of the cavity, specify a small radius (.5 or 1 mm) or no radius at all.
Pro Tip #1: Ideally, the corner radius should be slightly larger than the radius of the tool that will be used to machine the cavity. This reduces the loads on the tool and will further reduce your manufacturing costs. For example, if your design has a 12 mm deep cavity, add a 5 mm (or larger) radius at the corners. This will allow a ø8 mm tool (that is a 4 mm radius) to cut them at a faster speed.
Tip #2 – Limit the Length of Threads
Specifying threads that are longer than necessary can increase the cost of CNC parts, as special tooling may be required.
Keep in mind that threads longer than 0.5 times the diameter of the hole do not actually add to the strength of the connection.
To minimize cost:
- Design threads with a maximum length of up to 3 times the hole diameter.
- For threads in blind holes, it is preferable to add at least 1/2 diameter of unthreaded length at the bottom of the hole.
In conclusion: Keep it standard…
To minimize cost, before you submit an order for quoting, consider the following questions:
- Is my part optimised using the Designing for Machinability guidelines?
- Are all features in my model necessary? Can I remove or simplify any of them and still retain full functionality of my part?
- Can my design be split into multiple parts that are easier to CNC machine and then assembled?
- Is there a way to modify my design to eliminate the need for multiple machine setups or special tooling?
- Is there a less expensive or easier to machine material that can fulfill my design requirements?
Complexity has a high cost in CNC: geometries that require special tooling or fixtures, multiple machine setups or specialty materials will have a higher cost. Do you have your design ready? Upload your parts for a free, instant CNC machining quote and use professional CNC services.
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