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 Material Properties
Hi and welcome to another Insight video. This week and next we are going to delve deeper into what you need to consider when selecting materials for CNC machining.
One problem is that you have too much choice. There are dozens of metals and resins to choose from. How do you begin to sift through all the datasheets that are in front of you? And it’s important to get it right if you want to produce a functional part at a reasonable price.
This week I’m going to explore what some of the headings on those datasheets mean - things like tensile strength, elasticity and elongation to break. Then next week I’m going to look at some of the actual materials and make some comparisons.
But before you even open up the material datasheet pdfs, it’s a good idea to prioritise your part’s requirements. Start with what properties it must have and then work your way down to the identifiable nice to haves. This will allow you to sift through the datasheets quickly and shortlist the alternatives down to a manageable number.
To help you do this let’s take a closer look at some of the material properties.
First on the list is strength.
Strength is the capacity to withstand force or stress, just like pushing weights at the gym if you like.
There are lots of different types of strength with one of the most commonly quoted being tensile strength. The ultimate tensile strength or UTS is the breaking point under tension. UTS is measured in megapascals or MPa for short or in newtons per square metre. A low strength material would have an Mpa of less than 100, while a high strength material would be greater than 400. What do I mean by strong? Well, a 10 millimetre diameter of 400Mpa material could hang 3200 kilograms of weight at break point – that’s more than 3 tons!
Impact resistance or strength could be another important factor and you might also want your part to be hard and to resist wear. Even friction might be important with materials ranging from a high friction to one that has low friction and is self-lubricating.
Right, I’m going to go off on a tangent at this point. I’ve just mentioned wear and at first glance you might think that you need this for your part. Fair enough but a hard-wearing material is going to be hard to machine. It will need more time and more tools to produce the part you need, and that means it will cost you more. When you hear the term machinability this is what we mean.
Right back to my list. Now strength is great but in modern manufacturing you often need something with a high strength to weight ratio. So, you may need to consider material density or weight as well. Quite simply this, it’s weight to volume ratio, it is often measured in kilograms per metre cubed.
Another property that you are likely to see on a datasheet is elasticity. This is the ability of a material to resume its normal shape after stretching or compression. I won’t get into the measurement of elasticity because I could be here all day. You will often find common terms in a material description relating to elasticity with low elasticity referred to as plastic, medium as malleable or tough and high elasticity as stiff or brittle.
Elongation on the other hand refers to the change in length of a part compared to its original size, or its stretchability. You will often see a figure quoting the elongation at break and you’ve guessed it, this is how long it is in relation to its original length when it breaks.
Okay that’s strength, elasticity and elongation. I think it’s time to move on to some non-mechanical properties.
Temperature can also be important. The operating temperature is key. With plastics this might be broken down further to its short-term service temperature or what it can withstand over a short period of time measured in minutes or sometimes hours and its long-term service temperature.
You may also need to think about the material’s melting temperature for obvious reasons.
Talking about temperature, the thermal conductivity refers to a material’s ability to conduct or transfer heat. For some applications you want thermal conductivity for others you might want to prevent or insulate against it.
That brings me to electrical conductivity which easy enough to understand. To state the obvious copper is a great conductor whereas most plastics are excellent insulators.
I could go on as you can imagine there is a huge list of properties that might or might not be relevant. I haven’t even touched upon flame retardancy, UV resistance, anticorrosion, or resistance to chemicals. And then there are material approvals you need to consider for the food and medical industries.
Remember my advice at the beginning - think through what properties your part must have, prioritise them, make a shortlist and then speak to your supplier.
Okay that’s it until next week when I’ll take a look at some of the common material selections for CNC machining.
Until then have a great weekend.
With special thanks to Natalie Constable.