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By Mike Worth
If you have been keeping up with recent developments in 3D printing, you will have heard predictions that all manufacturing will be replaced by printing objects at (or very close to) the point of use. Particularly fanatic believers expect this to happen in mere decades. In my opinion this is unlikely to happen any time soon. Clearly, techniques such as injection moulding will remain dominant where millions of identical parts are needed, in the same way as most bulk printing still uses a specific plate for each job.
Still, the belief that 3D printing will displace machining and other traditional processes, in the realms of prototyping and short-run production is even more widespread. I think these people are also wrong! While 3D printing is certainly very useful in many areas, there will always be a place for machining in product development.
The benefits of 3D printing are covered in many articles already and are too numerous to mention. There is undoubtedly a sense of satisfaction that having your component appear before your eyes without the swathes of material that are typically torn into shavings makes the whole process feel futuristic and efficient. The cost of this satisfaction can be huge however – the fact that the material must be added a small amount at a time puts restrictions both on what materials can be used and the final properties of that material.
Most 3D printing technologies require the build material to be a liquid at the point that it is added to the part; this liquid then either solidifies due to cooling or via a chemical reaction driven by light. This phase change does not progress smoothly and continuously through the part as in the case of moulding, casting or extrusion (the typical methods used to create stock from which parts are machined). As a result the material properties are not necessarily smoothly distributed throughout the part . This can lead a variety of unpredictable problems such as points of stress concentration that reduce overall part strength.
One method of 3D printing that doesn’t require a phase change is selective laser sintering (SLS), which is somewhat analogous to the way in which powder snow can be hardened and joined together into a snowball. But the main limitation of this process is that fusing together separate grains of material tends to leave gaps affecting the density, strength, surface finish, and many other physical properties . This might be convenient when the alternative is being pelted with fist sized lumps of ice; however less so when designing key structural components.
Another inherent limitation is that the part must be built up layer by layer; this leaves the material anisotropic – having different strengths in different directions. This is common in biological materials such as wood and fingernails, which break or tear much more easily if subjected to particular stresses. Some parts have a natural axis and cope with this well, although many others have no obvious direction in which weakness can be tolerated.
Machined parts, on the other hand, allow a vast choice of materials. These are not only generally more homogeneous than their 3D printed cousins , but because they are created independently of the machining operation can be stretched to align the molecules in a particular direction, heat treated for hours on end, or undergo any number of other processes that simply wouldn’t be feasible during a print cycle. This versatility allows practically any engineering material to be used, which has the critical benefit of enabling the properties of a final product to be closely matched during the prototyping stage.
It is for these reasons that, while at Cambridge Design Partnership we own two 3D printers, we are still expanding our machining capabilities. Only last year we added a new Hurco machining centre to our workshop, that already contained several lathes and mills as well as many smaller machine tools.
While I have argued the case for machining in this blog, many similar points can be made for other prototyping techniques in which we have in-house capabilities, such as vacuum casting, small part injection moulding and other fabrication methods.
So while it is important to keep up-to-date with the latest developments so that you are not left in the dust, it is equally critical to not get distracted by every shiny new toy, to the extent that we forget how effective the old methods still can be.
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