What do I need to do to ensure successful CNC machined parts?

Benefits of CNC machining

CNC (Computer Numerically Controlled) machining is simply, the use of computers to control machine tools. There are many benefits of using this method to manufacture or prototype parts:

• High precision.
• Reliable repeatability.
• Vast range of material choice – CNC machines can work in cheap, commodity materials, anything from wax, transparent plastics, PCBs, wood, plaster, aluminum, to steel.
• Attractive cost for low volume numbers.
• Suitable for larger part sizes than most additive manufacturing techniques.
• Fast with limited post processing time to factor in. After machining, there are no intricate supports to remove, and there is no need to wash, sandblast, polish, seal, or post-cure.
• A great option for metal prototyping.           

How can we design a great part for CNC machining?

10 Top Tips

1. Top of the list is communication! It is vital that we know what the part is and understand what it is going to be used for. Be as specific as you can. Define the characteristics needed along with the required finish. Discuss whether text is required. The more information we have, the more we can use our experience to help optimise the design.
2. Be considerate of the material to be used. If you are looking for metal parts, the most easily machined are aluminium, brass and other soft metals. Harder metals, such as steels, titanium and more exotic alloys are harder to machine and subsequently take longer. By comparison, if you are looking for plastic parts, most plastics are easy to machine, although adding fiberglass or carbon fibre can make the process more difficult.
3. Drawings are key. 2D drawings are important references for dimensions and tolerances and 3D CAD data is required for exact geometry alongside making the machine programming easier and quicker. Accuracy in both are critical – simple things, like parts being allocated unique names and part numbers, are paramount to ensuring the data is interpreted correctly.
4. Careful dimensioning and tolerances are required.  Tolerances of around  +/- 0.1mm are generally achieved however, the tighter the tolerance required, the more expensive the part will be to machine. Tolerances must be specified on a feature by feature basis, and the cad data supplied to mid tolerance.
5. Avoid sharp, square inside corners with small radii and deep pockets. Inside corners should be rounded to take into account endmill diameters, as the endmill will have a large portion of its surface area in contact with the work during the finishing pass. The result could be part chattering and the cutters breaking, plus the surface finish will be poor.
6. Holes should not be tapped too deep and high walls checked so there is sufficient clearance for the toolholder and spindle.
7. Time. As CNC machining is a subtractive process, it is worth remembering that it is the time taken to remove the material that will have a bearing on machine cost. Therefore volume and shape will play their role and have financial implications.
8. Drawings. Requesting a quotation against a cad file without a supporting drawing is usually a pointless exercise. The tolerances called out on the drawing will influence how the part is machined and hence the cost. 
9. Avoid multiple setups when possible and be careful about adding chamfers and radii to external sharp corners, as these will usually need to be machined on and can add considerably to the cost. If all that is required is ‘break sharp edges’, this can be done by rumbling at minimal cost.
10. Quantities. Be aware that the machine programming is amortised into the batch size quoted, so reducing the quantity will usually increase the price. Further as the programming is the initial operation, changing the data after starting can have severe consequences.

We have successfully delivered CNC machined metal, plastic and clear parts at competitive price points in lead times from around 5 days.

There are examples of high cosmetic CNC machined parts on our gallery page or please contact us for further information or a quote.