Digital Manufacturing

Protolabs Insight: CNC Material Selection

Friday 27 August 2021, 2:25 PM

9 Minute Read

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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 Selection


Hi and welcome to this week’s Insight video.

Today I’m going to tackle material selection for CNC machining.  Last week I looked at what some of the properties that you find on product datasheets mean, so things like tensile strength, elongation to break and elasticity.  This week I’m going to look at some of the materials themselves.

Getting the material right is just as important as how parts are machined, because a wrong decision can cost you a lot of money.

Let me give you an example, titanium is the darling of the aerospace world it is strong, hard wearing and light.  The problem is, it is really hard to machine and will cost way more than parts made out of aluminium or stainless steel.  So, if you don’t need it, then choose a less expensive metal.

And it’s the same for plastics. Polyether ketone or PEEK is sometimes known as the superman of polymers and is strong enough to replace metal for some applications, but it’s also about five times more expensive than other high-performance thermoplastics. Specialist plastics like PSU, PPSU and ESD enhanced Acetal can cost even more again.

As I mentioned last week, a better approach is to decide what properties your part must have and what properties are nice to haves, but not essential, and then use this to make a shortlist of potential materials.

It’s also worth thinking about where in the production process you are and remember that this can affect your material choice.  If you are using CNC to develop an early prototype for look and fit, then it might not need all of the properties of the final part. But unlike 3D printed parts, CNC components can be made from exactly the same material as its enduse equivalent.

There are dozens of different metals and plastics that you can choose from and we simply don’t have the time to go through all of them, so let’s take a look at some common options.

Let’s begin with some metals.

We’ll start with alloy 17-4PH.  The PH stands for Precipitation hardenable, so as well as being a stainless alloy it is also hardwearing making it great for harsh environments where your part might experience wear and corrosion.  This alloy gets used in mould tooling, aerospace, rocketry, valves, chemical, dairy, food processing, and, oil and gas rigs.  These alloys are hard, and so expensive to machine, so unless you really need it, I’d advise you to find an alternative.

Aluminium alloys on the other hand are easy to machine, have a high strength–to-weight ratio and are corrosion resistant (particularly when anodised).  It’s often used for aircraft parts but also for general engineering, computer components, cookware and much more.  It’s worth checking out the specific properties of different alloys for this metal – 6000 series are the workhorse, 7000 are stronger, and 2000 deliberately softer to increase fatigue resistance.

This brings me neatly onto stainless steel, which has dozens of different grades and classes to consider.  They are all known for being corrosion proof and having excellent mechanical properties.  We stock the two most common grades – 304 (known as A2) and 316 (A4 or marine grade), 304 is stronger and more cost effective, 316 has higher corrosion resistance. The machinability for stainless steels range, but they all take longer to machine than aluminium.

And let’s not forget other metals including mild and carbon steel alloys, copper, brass, and we’ve already mentioned titanium, all of which can be CNC machined – although as stated some are harder and more expensive to machine than others.

While many of us think of metals when it comes to CNC machining, there are also a huge number of polymers or plastics that you can use. In fact, many applications are replacing metal with plastic where the material properties allow – they tend to be less dense or lighter, which for many designs is a real plus.

So now let’s run through some of plastics commonly used in CNC.

We’ll start with ABS, which is both mouldable and machinable and offers excellent toughness and impact resistance. Another engineering grade plastic acetal (often referred to as Delrin®) has a high mechanical strength, good dimensional stability, and relatively low cost; a combination that makes it one of the most commonly used engineering grade plastics – often used for gears and sporting goods.

Nylon is another great material, combining strength with flexibility. It is used for all sorts of engineering applications in the automotive, heavy engineering and other industries.

But if you really want a tough plastic that often replaces metal, then as I’ve already said Polyether ketone or PEEK is the go-to do it all plastic for high end aerospace, formula 1 and medical components.  It offers a high abrasion and wear resistance, low moisture absorption and a low coefficient of friction.

Okay I could go on about different polymers all day, so I’ll finish off with one of the most widely used materials in modern manufacturing, polycarbonate.  Did you know that about 60 percent of all the plastic used in a modern car is polycarbonate?  There’s a good reason for this, it has an excellent impact strength, good temperature resistance and is a good alternative to glass. Polycarbonate and acrylic are naturally transparent, but machining leaves a matt finish, which can be polished to return its clarity.

One thing that I have only really touched upon here is the cost of using different materials for CNC.  On that note it’s not just the cost of the material that you must consider. The cost of your CNC part will also depend on how long it will take to machine and the number of tools needed to machine it.  That comes down to design, machinability and any secondary processes that might be needed.

Clearly material selection can be complex, but with a bit of research you can increase the number of materials you are familiar with. Once you have a longer “go to list” you could save yourself money or find a material that better meets the needs of your part. We have done a lot of that research, hand selecting 30+ of the most popular.

If you talk to your supplier about your part’s functionality, then they can help and advise. Sometimes there are no absolute right and wrong answers, but some material choices will be better than others.

Okay that’s enough for this week.

Have a great weekend and I hope to see you next Friday.

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