6 Considerations for CNC Part Design

Posted On 26 September 2018 Saleem Shariff

Is it possible to get perfection plus fast and cost-efficient prototypes and production parts from a CNC machining supplier?

In truth there is a balance to be made when it comes to manufacturing parts using on demand CNC machining.  After all you wouldn’t expect a Bentley for the price of a Mini, yet both do the same basic job of getting you from A to B.

Just as there are a number of important points that would lead you to a decision on your next car purchase, there are a number of considerations that you can make when designing parts for milling and turning processes.  These decisions can accelerate production and reduce your costs.

In this post we will look at some decisions that will help you make the best design for CNC machining.  We will cover hole depths and diameters, sizes and types of thread, text on parts, wall heights and feature widths, live tool lathes and multi-axis milling.

1. The hole truth

There is a lot of debate between using a drill bit vs an end mill.

For rapid production and to help lower cost, end mills provide greater flexibility in the hole sizes possible with a given tool and they offer a better surface finish.  They also allow us to use the same tool for machining slots and pockets and thus cut down on the cycle time and cost of your part.

As a designer you do need to be aware that holes that are more than six diameters deep become a challenge due to an end mill’s limited length.  Any holes longer than this may need machining from both sides.

2. Threading decision

Instead of using a tap to cut internal threads, check that your supplier uses a thread mill to create the thread profile.  It creates an accurate thread and a single milling tool can be used to cut any size that shares that pitch to save production and set up time.

To illustrate the point UNC and UNF threads from #2 up to 1/2in and metric threads from M2 to M12 are all possible from a single toolset. Find out more about threading here.

3. Less Text

Less really is more if you want to save time and money as far as text is concerned. Ask yourself do you really need a part number, description or logo milled onto your parts?

If the answer is still yes, then the spacing between characters and the stroke used to ‘write’ them should be at least 0.5mm.  Also make the text recessed, instead of raised, use 20 point or larger fonts such as Arial, Verdana or other sans serif options.

4. Sizing up walls and features

Wall heights and feature sizes are very dependent on the individual part geometry as well as the toolset being used. 

Remember even the strongest tools deflect, as do the metals and plastics being machined. So, avoid very small features and tall thin walls.  At Protolabs for example the minimum feature thickness we can achieve is 0.5mm and the maximum feature depth is 51mm, but that doesn’t mean you can design a ribbed heatsink using those dimensions

5. Be aware of your supplier’s capabilities

It pays to do your research when outsourcing a supplier as capabilities do vary considerably and this may affect what it is possible

6. Live tool lathes

For instance use similar toolsets to the ones on other machining centres.  It means that off centre holes, slots, flats and other features can be machined parallel or perpendicular to the long axis of the turned workpiece.  It will typically follow the same design rules as those applied to the parts made on machining centres.

The main difference when using live lathes is the shape of the raw material used.  Turned parts such as shafts and pistons start out as round stock, where as milled parts such as manifolds, instrument cases and valve covers use square or rectangular blocks.

I started this post with a car buying analogy.  I think it is worth pointing out that technology is driving progress in both CNC machining capabilities and the manufacture of cars.  For the former it means that you can get better prototypes and production parts more cost effectively and faster than you could a few years ago – as long as you consider both the design of your part and also the capabilities of your supplier and the technology that they have available.