Insight

DMLS & CNC Machining

Your masterclass in product design and development

 

Protolabs’ Insight video series

Our Insight video series will help you master digital manufacturing.

Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.

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Insight: DMLS & CNC Machining - Working Together

Transcript

Hello and welcome to this week’s Insight. 

Today we’re going to talk about balancing CNC machining and 3D printing techniques for producing metal parts.

If you’ve been following this series you’ve probably heard me talk about these a lot already, and individually, they are both incredibly useful design processes. By incorporating both into your toolbox, they allow you to embrace even greater flexibility in part design and can even save time and money. In order to take full advantage of this, however, you must understand the shared strengths and inherent differences of each process, and how to best use them to your benefit.

A great example of this relationship comes between CNC machining and direct metal laser sintering, known as DMLS. Machining offers incredibly fast production speeds but is comparatively limited in terms of geometry, while DMLS can produce virtually any part shape using nothing more than a laser beam and a pile of metal powder, but it can be a slow process.

With this in mind, you usually want to choose a process based on two questions:

  1. A) Can the part or parts be machined, and
  2. B) How many parts need to be made?

Once you have the answers to those, you just pick one or the other and go with it, right? Well, in many cases, the two manufacturing processes can actually work together – covering the weaknesses of the other.

Indeed, 3D printing quite often relies on its more traditional cousin to finish the job. Holes must be bored or reamed, threads tapped or thread-milled, critical surfaces milled, turned, or ground to size. At the very least, 3D-printed parts usually need a bit of care in the form of cleaning, blasting, and support removal, all of which require a visit to the machine shop.

For example, during the DMLS process significant heating and cooling of the metal takes place – which makes sense when you realise the part is being blasted with a laser – creating internal stresses that must be removed with some post-build heat-treating. This stress relief is important to us because it always results in some amount of part movement and therefore some loss of accuracy.  This is why even a well-designed DMLS-produced part requires a touch-up for any feature where tolerances tighter than plus or minus 0.1mm are required, and that means machining.

Another major reason for combining DMLS and machining is sorting out the surface finish. On a vertical or horizontal surface, the DMLS process tends to produce parts with a roughness about equal to a sand casting, while all other surfaces will see some amount of stair-stepping.

Therefore, if your part design requires a smooth finish, it will need to be blasted, sanded, or quite possibly machined. For this reason, it’s important to plan DMLS builds so that any part that does need to be touched-up can actually be reached by a mill, drill, or turning tool.

Another part of the build that you might need to think about accessing are the support structures. The scaffold-like supports are needed to keep the semi-molten metal from drooping, curling, or otherwise misbehaving. And while they can usually be removed with a Dremel, machining may be the preferred method when you’re dealing with larger part volumes, or when the workpiece is headed to the machine shop anyway.

And when it does get there, if you haven’t properly planned, you might find yourself running into another unexpected issue – finding something to hold onto. Unlike DMLS, machined parts must be clamped, bolted, or otherwise securely fixtured to the machine, so if your 3D-printed workpiece is composed entirely of curved, organic shapes - one of the process’ greatest appeals - how will the machinist hang on to it for turning or milling?

The final thing you need to think about before bringing these two techniques together is the metal itself. The lasers used by DMLS don’t really have a problem with how hard or tough a metal is, but cutting tools certainly do. Before choosing to make a printed part out of titanium or cobalt chrome, you need to think about whether it’s worth consuming more cutting tools and machining time when it comes to making those adjustments.

The overall point is this: You can in fact leverage the best of both worlds—3D printing and machining—together for metal parts, but you need to carefully consider the design options before doing so. Ask questions, embrace each process, and understand that both are close-knit partners in manufacturing.

Right that’s it for this week. I look forward to seeing you again next Friday.


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