CNC Machining vs Direct Metal Laser Sintering (DMLS)

The most obvious difference between these processes is that CNC machining is a subtractive method and Direct Metal Laser Sintering (DMLS) is additive. (For the sake of this article we will refer to DMLS from EOS, however the vast majority of the information applies to all the powder bed metal sintering options). 

Additive or Subtractive?

So, faced with additive or subtractive, how does one decide? Parameters such as time, material, geometry, and design, all impact that decision, as they all have implications on cost, which is usually the deciding factor if other issues are equal.

Size?

Starting with geometry, the first issue to tackle is size. The vast majority of DMLS machines that are available have a 250 x 250 mm bed size, whilst the build height (z) can vary from 215 to 325 depending on machine age. Newer machines can accommodate larger builds, but access and availability can be limited leading to longer lead-times and higher costs.

Size can eliminate additive (powder bed) options altogether, but even if it can be built bear in mind that cost rises with part size. The larger the part (especially in height), the greater the build time, and hence, the greater the cost.

Increasing the material volume has the same effect albeit to a smaller extent. Hence producing a part that has 10mm wall sections will take longer and cost more than a similar part with 2mm wall sections. Dropping the wall section further to say 0.5mm can make the geometry impossible for CNC whilst DMLS can still achieve it.

Complexity?

This leads us to complexity, where both systems can produce a 25mm cube; CNC will deliver it faster and more cost effectively. Now as we start adding complexity and the solid cube morphs towards a complicated manifold, with multiple set ups required to machine each face, DMLS comes into its own.

This can be driven to extremes by removing any thick sections and replacing them with lattice structures. This totally rules out a CNC approach, leaving DMLS as the only option.

Thus at this point a general rule would be smaller complex parts are likely to be more cost effective as DMLS whereas simpler or larger parts will be cheaper as CNC machined items.

Increasing complexity whilst suiting DMLS does have limitations. The process requires supports and these (usually) have to be removed post build. Support removal brings with it a cost and thus part orientation and part design are key to achieving a cost effective part.

Surface Finish?

Surface finish will be another difference between the two options. Features such as seal grooves are likely to require post machining, threads may require cleaning up, critical bores may require boring out if roundness is critical. So studying the interfaces and defining exactly what is required is important for a DMLS part. In most cases this is not an additional cost with CNC machining.

Material Selection?

Material selection is important on two counts, firstly it affects build speed, and secondly it can affect availability of machine capacity.

For DMLS different materials will build at different speeds and thus the situation arises whereby a Titanium part can be cheaper to produce than an aluminium part! Choosing ‘popular’ materials will also increase the probability of a quick turnaround, builds can stretch into days, so being the only person wanting a particular material can result in extended lead-times. DMLS can process some specialty materials that are otherwise challenging to machine, such as Inconels, Hastalloy and Cobalt Chrome.

Quantities?

The implications of quantities on relative costing are more difficult to summarise. With CNC, complexity brings programming time. A larger batch size allows this to be amortised more effectively, thus reducing the part cost. With DMLS the effect of quantity is really linked to the ability to pack the platform. A small thumb sized part can be built by the hundred, whereas a larger part might only be able to fit 4 to a platform. Thus as a rule of thumb, the smaller the part the more likely it is that DMLS can offer a cost effective solution, so long as any post machining is negligible.

Design for the Process

Whilst there is a general awareness of designing for CNC, to really extract the benefits of DMLS the component should be designed for the process. Making the design suitable is likely to have the biggest single effect on the cost competitiveness of an additive solution.

If you are unsure how to proceed, Plunkett Associates can offer an unbiased view of both options to find you the best solution, whether you are driven by cost or a performance factor. However please note the common misconception is that DMLS is just like Stereolithography (SLA), but in metal, could not be further from the truth, to make it work for you requires you to address its strengths and weaknesses.