But I need metal…

Firstly, that is not a problem!

Secondly, it would help to know why? This is because in most instances a metal prototype is going to cost more than the equivalent plastic part, and we need to be confident that the application warrants the additional cost. This is not to say that I don’t trust your specification, more that there could be some interesting RP applications/materials out there that could deliver an alternative! (eg metal coated Stereolithography)

The other reason to know why is to understand whether it is a specific material being sought, or whether any generic metallic will meet the requirement.

Thirdly we need to talk quantities and shipment schedules.

And fourthly I need to see the data. This is usually a good indication of the sophistication of the requirement. For example an stl file is usually fine to quote from but contains no surface finish, threads or tolerance information. As such the quote will not specify this information either, or at best give a blanket overall accuracy of form. In most instances going to metal will warrant a toleranced drawing with all the associated call outs and it is imperative that this is available at the time of quotation as it impacts both process selection and cost.

Now we are off and running!

From a production perspective

Most parts are designed as either CNC, Castings or Die castings in production, although as the capability of DMLS continues to improve we are starting to see some items designed especially to take advantage of layer based manufacture.

Using CNC or DMLS to prototype a part designed to be made this way in production is clearly the obvious starting point. Quantities and timescales are likely to have a dramatic effect on price, but this remains the starting point.

Castings will frequently involve some post machining and therefore if the geometry permits machining, this is the best starting point as both operations can be performed together.

Die castings really fall into the same category as sand or investment castings, although typically these parts are smaller making them more amenable to a potential DMLS solution.

From a process perspective

DMLS is best suited to highly complex, small parts. That said it is capable of producing some stunning parts up to the limit of the bed size (250mm x 250mm) although at this size the geometry is usually the driver as the premium over CNC will be significant. DMLS can really score where multiple processes are required to tackle the geometry from another angle. For example a die casting that would need EDM and CNC to produce.

When considering DMLS we must also look at what will happen with the supports, as these have to be removed post build, and it is not a simple exercise with some wet and dry as it is with Stereolithography!

CNC offers the greatest range of material options and the best potential accuracy but is limited by cutter reach and geometry. Two recent examples illustrate how this can be ‘pushed’! The first involved fabrication and welding to create a geometry that was otherwise impossible to machine. The part was finally dressed/polished such that the join ‘vanished’. The second example employed five axis machining to progressively chase out material until the desired form was achieved. Whilst this can prove costly in time, the result was a stunning piece of precision machining.

Casting a part either via sand or investment can be an excellent solution. However, post machining can further increase cost, raising the question as to whether the geometry permits a straight CNC approach.

The impact of quantity

Looking at the impact of quantity, introduces another perspective. When considering CNC there is the upfront programming time that needs to be amortised into the number of parts. Consequently unit prices tend to fall from 1 to 10 off, after which they generally stabalise. Alternatively if the quantities are heading towards 100 plus parts, a change of supplier can bring a low volume production philosophy into play as opposed to a prototype philosophy, helping to further reduce costs.

With DMLS quantity is all about optimisation of platform area and part geometry. Hence just because 100 parts are required does not rule out DMLS as a solution, assuming they can all fit in a compact area and that minimal supports are required.

The higher the quantity the more applicable a cast solution becomes, even if post machining is required. The CNC cost levels out at higher quantities as the material and machine time become fixed. Casting also has some fixed upfront costs that need to be addressed, but once these are taken care of the unit price can be attractive alongside a full CNC version.

In how long?!

Time is always a factor in assessing which route to take. The important factor is that this is time from availability of the purchase order to delivery, and not from quote to delivery!

In deciding which route to take, available capacity is very pertinent. For example, DMLS may be the quickest solution, but if the job is queued behind an 80 hour build this can destroy any advantage. Hence complying with a delivery is frequently more about locating a supplier who can initiate the work immediately, than it is about the process.

Material choice

When it comes to material, the level of flexibility is linked to the application. Most prototypes are in aluminium or stainless steel, however DMLS offers options such as Cobalt Chrome, Inconel and Titanium. (See Materials) If the purpose is to emulate the mechanical properties of the production part then this will require that the process be matched as well. For example the grain flow in a casting will not be present in a part CNC machined from wrought billet.( Allowances may still need to be made for different casting techniques, ie plaster as opposed to investment.)

Constraining the material to a particular grade or standard is likely to have a significant impact on the process options and hence time to produce the prototypes.

And finally geometry

This affects the selection process through features such as thin walls, internal sharp edges, aspect ratio of the features, part size, surface finish, accuracy etc etc, in other words its pretty important! That said when prototyping many of these attributes are open for discussion and in comparing quotations it is vital to compare like with like. Internal sharp edges are the classic that can add time and cost when the only reason they are on the data is that no one has thought to add a radius.

In short this…

…is just some of the work that Plunkett Associates does on your behalf when you ask for a quotation! Helping us to help you entails considering whether:

  • Different options can apply to different shipments, for example the first 10 are produced with CNC and the balance are cast and post machined?
  • How critical is the material call out and how flexible can this be?
  • Have you got the correct control drawings in place to control what you purchase?
  • Only calling up tight tolerances where really required and allowing some fluidity of form where not important. (Note a 0.5 mm radius can make a big difference form a machining perspective!)
  • Allowing us early access to your requirement and the opportunity to develop the prototyping solution with you.

With no vested interest in any one technology over any other, our goal is to supply you the best quality part in your allocated leadtime. If you would like to know more please contact Plunkett Associates.