Picking the best material for CNC

If the sheer amount of choice between the different materials for CNC machining leaves you confused, then find out how you can hone the number down to a more manageable shortlist in this blog.

Selecting the right material for CNC machining can make all the difference if you want to produce a functional part or prototype at a reasonable price.

How to develop a shortlist

When you are faced with dozens of different datasheets 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.

Material properties

The first step is to understand what some of the most common material properties are.

Strength. There are lots of different types of strength. 

One of the most common is 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. 

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.

Wear resistance vs Machinability

At first glance you might think that you need a material that resists wear and/or abrasion, but you need to consider that 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.

Strength to weight ratio

Strength is important 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.

Elasticity

This is the ability of a material to resume its normal shape after stretching or compression. You will 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

Elongation 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, which is how long it is in relation to its original length when it breaks.

Temperature

The operating temperature is the temperature at which a device operates. With plastics this might be broken down further to their short-term service temperature or what they can withstand over a short period of time measured in minutes or sometimes hours and their long-term service temperature.

You may also need to consider your chosen material’s melting temperature.

Thermal conductivity

This 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.

Electrical conductivity

The ability of the material to conduct electricity.  Good conductors include copper while most plastics are bad conductors or excellent insulators.

Some of the more popular materials

While you might be faced with dozens of different metals and plastics to choose from you will find that some tend to be more popular than others.  Here we run through some of the more common options.

Titanium

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.

Aluminium

Aluminium alloys 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.

Stainless Steel

Stainless steel has dozens of different grades and classes to consider.  They are all known for being corrosion proof and having excellent mechanical properties. 

Alloy 17-4PH.The PH stands for Precipitation hardenable.As well as being a stainless-steelalloy,it is also hardwearing making it great for harsh environments where your part might experience wear and corrosion. It gets used in mould tooling, aerospace, rocketry, valves, chemical, dairy, food processing, and, oil and gas rigs. These alloys are hardand so they are expensive to machine.Two common grades of stainless steelare 304(known as A2) and 316(A4 or marine grade). 304 is stronger and more cost effective, while 316 has higher corrosion resistance. Different stainless steels have a wide range of machinability, but they all take longer to machine than aluminium.

Other metals

Other metals worth considering are mild and carbon steel alloys, copper and brass, all of which can be CNC machined – although some are harder and more expensive to machine than others.

ABS Plastic

ABS plastic is both mouldable and machinable and offers excellent toughness and impact resistance.

Acetal (or Delrin®)

An engineering grade plastic with a high mechanical strength, good dimensional stability, and a relatively low cost; a combination that makes it most commonly used for applications such as gears and sporting goods.

Nylon

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.

Polyether ketone (PEEK)

This material is often used to replace metal.  It 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. 

Polycarbonate

This is one of the most widely used materials in modern manufacturing. 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.

Selecting the right material

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. At Protolabs we have done a lot of that research, hand selecting 30+ of the most popular for you. Choosing a plastic or metal material for CNC machining can be difficult. Keep in mind functionality and any potential secondary operations.

When you upload a CAD model to our website, our quoting tool calculates what can be machined within our capabilities, and what has risk. Its conclusions are clearly spelled out in the quote, giving you a chance to adjust the part design if so desired and spin the quoting wheel again. If you're ever in doubt, however, contact our applications engineers at or [email protected]