Graeme Smith, BE(mech), 3D Product Designer

Over the coming issues I am going to look at part design for the SLM additive manufacturing process and how designers can optimise parts to provide the best part at the best price.
Part 1. Understanding the cost of additive manufacture
As a design engineer I like nothing better than designing a part that functions perfectly and looks good for a reasonable cost. But designers are being pressured to create cheaper and cheaper parts. This is often achieved by compromising on function and forgetting about looks. We have all done it…a piece of RHS welded to a plate with a couple of holes drilled in it. RESULT…cheap, does the job but heavy and really ugly. Imagine if sleek, light and perfect for purpose was also cheap? Additive manufacturing could be your answer.

Costing for conventional subtractive manufacturing starts with the cost of the material in a form that the subtractive machine can hold. i.e., a bar with enough material for the lathe to hold onto. This is your material cost. Factors that influence this selection include final product material requirements, material availability and ease of machining. Now add on the cost to turn that material into the required part. Due to the nature of subtractive manufacture, every cubic millimetre of material you remove costs money and also the more complex the part the more it costs. SO what does the designer do? You access cheap easy to get materials, generally easy to machine and leave as much material on the part as possible. The RESULT…heavy, over-designed, simple parts. Fine for an industrial rock crusher but when the performance of a part is your focus, you can do a lot better!

Now let’s flip that costing model upside down. Imagine you only pay for the material in your finished part. There is no excess material needed to hold onto, no excess material to remove, and no cost for tools to remove that material. As a designer you can now design parts solely for their purpose. The parts are optimised to have the least amount of material and perform the task they are required for easily and often look a hell of a lot better than their over engineered counterparts. This is the additive manufacturing costing model. You pay for every cubic millimetre of material in your finished part so if that material doesn’t need to be there then get rid of it. Even if the amount of material doesn’t change you can add complexity to your part for free.

Looking at the two costing models above it is evident that it is designed for a method of manufacture that determines the cost. As designers, we have been designing for subtractive manufacture as there was no other option but now we have the option of additive manufacture. Design for additive is closer aligned to the desire of the designer to create an optimised part. If a part doesn’t need to be complex then perhaps a subtractive method might be better, however, with additive manufacturing, that complexity is free.

I have illustrated to point below…In subtractive manufacture the cost rises quickly with increased complexity, (irrelevant of part volume). For additive manufacture the cost remains constant for increasing complexity and the same part volume, or the cost can drop if the part volume also decreases.

In the next issue I will look at using high strength materials and their impact on part design and associated cost.