CNC Mistakes

Your masterclass in product design and development


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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: CNC Mistakes


Hello and welcome to this week’s Insight.

Today I’m going to help you save money on CNC machining. Over the years we’ve dealt with thousands of CADs for CNC and most, after a little bit of tweaking, are great.

We do however come across some common mistakes that if avoided will save time and therefore production cost.

Get the design right and CNC parts can be milled or turned in as little as a day. The technology is getting better all the time, but like other technologies it has its limitations.

So let’s run through the six most common mistakes that are made on a CAD design for CNC to help you get faster deliveries and lower production costs.

Number one on my list is to avoid features that require unnecessary machine cutting. If it’s not needed, then you are only adding to your parts run time and this will cost you more money.

If a design requires a circular geometry, then instead of cutting away the material surrounding it to reveal the finished part, a better design will have the machine simply cut the part straight from the block. This change cuts machine time in half.

So, point one is to keep your design simple to avoid extra run time and pointless machining.

My second suggestion is to avoid small or raised text.  It adds cost and the smaller the text the higher the cost. That’s because very small endmills for cutting the text operate more slowly which adds time and extra cost. Cutting larger text takes less time. 

Also use recessed text rather than raised text because the latter requires milling away the surrounding material to create the letters or number and that obviously takes far longer.

Next, avoid tall thin walls. Walls on part designs are tricky. Even though the cutting tools used in CNC machines are made from hard, rigid materials, they still deflect or bend slightly during machining, as does the material that they are cutting. This can cause rippled surfaces, difficulty meeting part tolerances or it can even chip, bend or break a wall.

The taller the wall, the thicker it needs to be to increase its rigidity. Check with your supplier what is the maximum height that they can achieve and how thin they can go down to – our limits are 51mm tall and 0.508mm thin.

Another good trick is to add some draft to a wall. An angle of 1, 2 or 3 degrees so that it tapers could make machining easier and leave less leftover material.

Okay now ask yourself whether your design needs to have perfectly square or small internal corner pockets. Sometimes the answer is yes of course, but if it doesn’t you could save yourself some money. Creating small pocket features means picking away at the corner material with smaller and smaller tools. Your supplier might need to use up to eight different cutting tools. 

The larger the corner radius you design, the bigger the cutting tool your supplier can use and the less run time it will take.

And so to number 5 on my list, don’t create holes that can’t be threaded. It’s best to check the details of this one with your supplier. For us, our quick turn process has a static set of threads available. When our software analyses your part, it looks for a hole diameter that corresponds to one of those threads. If that diameter isn’t within that range, then we can’t assign that thread to your part. 

There is a good rule to adhere to here. If you choose a diameter that is 75 percent of the hole drill size, then it will always work.

Okay and now for my final tip. If you are designing a prototype, then consider how you will manufacture that part. CNC machining is often used for prototyping, but different manufacturing technologies have different strengths and limitations so what may work for one might not work so well when it comes to another.

We often see designs for injection moulded parts uploaded to our machining service for prototyping. A thick machined part may suffer from sink, warp, porosity or other problems when moulded; while a well-designed moulded part with ribs, pockets and other features will take a long time to machine.

If your supplier provides both CNC and injection moulding services, then speak to them about how to modify your design to account for these differences. Your other option is to prototype your parts using the final production process – there are cost effective ways of doing this with injection moulding but that’s the subject of a future video.

I hope that has helped and more importantly will save you some time and cost in the future when it comes to CNC machining.

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





With special thanks to Natalie Constable.

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