In Issue 5 we looked at SLS printing, and in Issue 7 we looked at both SLA and FDM printing. All of these prototyping or modeling processes for 3D printing are classified as “additive manufacturing”. In this issue we look at the opposite “subtractive manufacturing” process of CNC machining.
Using 3D CAD data, a CNC machine shapes a product using a range of spindle cutting tools of different sizes and shapes to “subtract” material from a block of material (see Figure 1).
If you want to create a functional prototype, CNC is a great option.
The CNC process exceeds the accuracy of any 3D printing process in many ways.
- It is much more precise.
- CNC parts have a much better finish and can be polished in the harder plastics to achieve a high gloss finish.
- Many plastics can also be painted. This is also a process that can be used to create a prototype that looks or in some cases is better than a molded part.
- It can be machined in or near the material you want to shape it into. This is ideal for testing and getting a real feel for what the molded product would look like.
- In some cases, it can be just as cheap or cheaper than 3D printed parts.
- It is machined with cutters, which means that all cutters are round, so the inside corners have a radius because the cutter is round.
- Fine details in deep pockets of a part could be a problem. This is because if you need a 1 or 2 mm cutter to do this detail, the cutter may not be the length and it is in a 50 mm bag. You don’t get the cutter so small, there is always a relationship between the diameter of the cutter and the length of the cutter.
- Most of the time this is not a problem. As with 3D printing and SLS, you are limited to the tank size and many parts are made in segments and glued together. This problem does not usually occur with CNC machining, unless the material plate to be purchased is not large enough or thick enough, which is rare. However, it can be glued if certain parts are too difficult to machine and it makes more sense than machining parts and then gluing them together.
In summary, we recommend CNC machining because it is accurate, the material of choice is very cost-effective and what the molded part looks like. We would make more of these prototypes than anyone else.
Lead time of CNC processing faster than Vac-Casting of about 2 weeks, but sometimes not as fast as 3D printing.
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