Designing for CNC machining when prototyping.
CNC machining is one of the most utilized manufacturing processes for prototypes and low volume production runs. It offers great advantages in terms of speed and range of materials available and is also fairly cost effective when compared to other processes. CNC is highly versatile and especially with 3,4 and 5 axis machines the type of part and geometries able to be machined are varied. That said it also pays to be aware of certain design elements which could reduce the machining time, complexity and cost of your job.
First and most simply the type of material has a big impact upon cost. CNC machines can cut everything from plastics to metals but there are some materials which are easier to work with than others.
If we take plastics first of all there are a number of common and easy to machine grades of plastic, ABS for example is cheap, versatile and very machinable. On the other hand PP or POM is more difficult to cut and prone to warping, which means the manufacturing process is quite a bit more complicated increasing the costs.
Metal materials also offer different ranges of machinability, aluminium is a fairly soft metal and so easy to cut and quick to machine. Steel is a much harder material and so much tougher on the machines, this is often reflected in the prices of the parts.
The next thing to consider is the part geometry, here there are so many different aspects to consider it would be impossible to list them all but here is a quick summary of a few.
Corner radii, CNC machine cutter tools are circular, so even the smallest diameter cutter is going to leave a radius in the corner. If you need square corners this will require post process finishing either by hand on plastic parts or spark erosion on metals. If you can get away with radii, and the bigger the better, you can bring the costs of machining down a little.
Deep pockets, these are also tricky to machine and often result in tool chatter leaving you with imperfect surface finishes. If you can do away with these kinds of features all the better, if not the best ratio between length to depth is 4:1.
Thin walled features are also tricky to machine and easy to become damaged so machinists generally don’t like them.
For some parts it may also make sense to split them into several separate parts if they are to be machined. Where machining in one piece might have advantages in terms of strength and cosmetic benefits often if you can accept the part being split you can save some money. Compare areas 1 and 2 in the image below. In this instance it may make sense to split the part as area 2 makes the part more difficult to machine, increases the initial material costs and would require an additional machine setup.
Other issues which drive up costs are the tolerances required on the parts, obviously the higher the desired tolerances the more care and attention needed when machining the part. Very high tolerances on certain features may also require the manufacture of special jigs and fixtures to hold the work piece or even the use of specialized cutting tools which will all increase machining time and costs.
This is also the case for threads and tapped parts, if you do need threaded holes try as far as possible to stick to standard threads.
One way of driving individual part costs down a little is to order multiples. Once the tool paths for the part have been programmed the CNC machine can run the same parts over again allowing economies of scale.
CNC machined parts offer versatility of material, high precision, great surface finish and speed. If anyone can think of any other ways to drive CNC machine part costs down pelase do leave a comment below.
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