After getting an introduction to Direct Metal Laser Sintering (DMLS) parts from Protolabs few months ago, I had the opportunity to use their DMLS 3D printing service recently on couple of aluminium alloy parts for a prototype (Figure 1).
As demonstrated by Airbus recently, DMLS and other additive manufacturing techniques have advanced and reached a stage where it is now used to produce critical production parts. However, due to fundamental limitations of laser based processes for metal additive manufacturing, there is a significant ongoing need for these parts to be subject to additional machining operations such as threading holes, mating interface machining, polishing etc. before it can be used in an end product.
As the figure 1 shows, both parts had challenging features to machine for a one off without spending on tooling charges and set up costs. Because the parts were manufactured to test a concept by building a one-off prototype, 3D printing was by far the cheapest and quickest option.
It was fairly straight forward to open an account with Protolabs where you can get instant online quotations for both plastic and metal 3D printing (DMLS) parts. Once you select the metal 3D printing option, it allows you to select the material and resolution for the print as shown in figure 2. Material options include stainless steel (1.4404), titanium, aluminium alloy (AlSi10Mg) and maraging steel (1.2709).
Then 3D CAD files can be uploaded along with any special instructions, drawings, post operations etc directly into their online ordering portal. Getting quotation was instant as online tool calculates the cost by taking the material selection, build volume and the quantity into consideration.
Overall cost for the parts were about 60% cheaper than the CNC machining option and got delivered in 4 working days compared to 3 weeks for CNC machining.
Some of the other companies who offer metal 3D printing are;
The dimensions that could be measured with a vernier caliper were all within +/- 0.1mm as per the print specification. For this reason for one-off prototype, especially if the final production part is a casting, DMLS is definitely a viable option. Though the build area of these new printers needs to be considered as the build volume is still small compared to what you can achieve with a casting.
The most prominent issue you would notice is the surface finish, especially if you are used to get the parts made using conventional CNC machining. As you can see from figure 3, the parts look like it’s been manufactured using Sand casting.
So depending on the application, the parts might need post processing such as polishing, aqua blasting etc to achieve a suitable finish. Surfaces such as bearing bores, threaded holes, interfacing faces, reamed holes, mating edges might need machining depending on its tolerances and functions.
3D model had a pilot hole for M3 threaded hole, so it can be threaded. Unfortunately as the figure 4 shows, the hole was not circular but more oval. But I did manage to tap the hole using a hand tap and it had enough thread engagement for a M3 grub screw to grip (Figure 5).
It is very difficult to predict the reason for the oval shape hole without knowing what orientation the part was printed. But generally holes on the vertical walls do not print well compared to horizontal plane.
Parts did function correctly within the assembly as intended but overall cost for a larger batch would be still cheaper using CNC machining than the 3D printing.
Direct metal laser sintering is not going to replace the CNC machining anytime soon because of the high cost and poor surface finish of the parts, but like any other manufacturing process it has its benefits and a place in the manufacturing field. So rather than looking at additive manufacturing as replacement to other manufacturing processes, we should look at how we could use it to overcome some of the short comings of conventional manufacturing processes.
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