Daniel Hall

Technical Support

13 years engineering experience, specializing in CNC & manual machining, milling and horizontal bores.

Composite materials are some of the most difficult to machine. Their unique properties, confounded by the vast number of different combinations of materials possible, means only specialist tooling should be used in its machining. However, composites also offer massive potential in advanced technologies and emerging industries.

This blog will outline key issues associated with composite materials, as well as some tips and tricks you'll need to effectively machine the trickiest of composite substrates.

What are composite materials?

A composite material, as suggested by its name, is a material created from combining at least two ‘base’ materials. This new material will feature properties from both base materials, with the distinct purpose of creating a material chemically and physically better than its original counterparts. More technically, the process involves the embedding of fibres or particles from one material in the other base (commonly called the matrix material). Due to the huge variety of combinations possible in creating composites, characteristics such as hardness, tensile strength and rust resistance can be manipulated to suit each individual job or application.

Whilst improved strength is a key driver in composite creation, for instance in steel composites designed to increase the Rockwell (HRc) value of tool steel or carbon steel, innovations in modern manufacturing techniques have increased the variety and range of properties sought in composites. Heat conduction or insulation, for instance, has tremendous benefit in electrical goods including lasers, transistors and sensors. Thermal conductivity can be altered through material characteristics like fibre orientation, volume fraction and resin material. This shows just how intricate composite material composition can get.

What are the different types of Composites?

As stated, composites are made up of two main components:

Matrix

The base of the composite, giving the new material its shape. Usually the more flexible material with less resistance, so more malleable.

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Reinforcement

Provides strength, resistance and extra rigidity to the matrix, creating a ‘superior’ new material.

Due to the numerous combinations of matrix and reinforcement materials, there is no set ‘grade’ or ‘types’ of composite material. However, we can classify them based on the general category of base materials. For instance, there are a whole host of plastic composite materials including epoxy resins, polyactel, phenolics and plastics reinforced with glass fibre for increased toughness.

Other common composites include:

Carbon fibre-reinforced polymers (also referred to as CFRP or carbon fibre).

Mud bricks, which are one of the oldest known composites (dating back thousands of years!)

Metal matrix composites (made from e.g. aluminium, copper and steels).

Fibreglass.

Reinforced concrete.

Wood (made from cellulose and lingnin).

Honeycomb, named after a bee hive’s hexagonal structure, are specialised composite materials sandwiched between two layers, often thin and used for strengthening the honeycomb core. Such honeycomb materials can range from low strength, low thickness (e.g. paper, card, thermoplastics) up to high strength, high stiffness materials like aluminium, PVC and polymers like Polymthacrylimide. Honeycomb materials care very common in aerospace, where low weight and high rigidity is important.

Composite Machining Tips & What to look out for?

We have compiled a comprehensive list of common questions and concerns engineers have when approaching the machining of composite materials, as well as solutions backed up by Cutwel’s extensive range of composite tools and the expertise of our Technical Team.

Cost Efficiency

Probably the most pressing concern on the mind of machinists is the cost implications of being forced to redo or scrap a job which includes composite materials. Due to the intricate and cost-intensive process involved in creating or purchasing composite materials, there is a large capital investment made to a composite job before it even makes it to the machine bed. Therefore, it is useful to ensure tooling is designed to tackle the specific composite, which can be found within the cutting data; if there’s no cutting data, it’s not suitable! Not only this, but the actual machine program needs to consider the below issues.

Unpredictability

Depending on the makeup of the composite, any machined features like holes or slots may be smaller than anticipated, as the composite relaxes when cut. Unpredictability of the material fibres mean it’s very hard to compensate for these potential discrepancies. Such unpredictability can be reduced through extensive knowledge and inspection of the material’s composition, as known properties of the matrix and reinforcement materials can help reduce the margins of error.

Heat Dissipation

Many composites feature low thermal conductivity, due to composites ‘shattering’ as opposed to chipping during milling processes. The cutting tool fractures the fibres in the matrix material causing high heat levels, whereas traditional metals will create chips. The main issue with this is that the composite becomes distorted, ultimately melting the material. In some situations, depending on the composite material used, it may be beneficial to use coolant (or water) when machining composite materials, which will cool the workpiece and prevent any distortion in the workpiece or damage to the cutting tool. Again, suitable CFRP tooling will help with heat dissipation.

Health & Safety

A side effect of composite machining (partly a consequence of no chips being formed in the machining process), is that there is a large quantity of dust generated by even a single process on composite machining. For instance, when machining carbon fibre, the generated dust can be dangerous to the lungs if inhaled, posing significant health and safety concerns in a machining shop. Protective headgear, such as a mask or face shield, will help in preventing dust from entering the lungs, but in the long-term proper dust extraction techniques (such as open windows or extractor fan) will prove most useful.

Surface Finish

Finally, surface finish plays a big role in composite machining. In traditional machining, a common sequential method is to rough off large quantities of metal, before using a finish cutter to smooth off the workpiece and ensure it fits a tighter tolerance or more accurate size. However, roughing in composite machining can damage the fibres within the base materials, which has the adverse effect of decreasing material tensile strength. Meanwhile, finishing applications increased tensile strength considerably. Again, tooling will largely affect the achieved surface finish, with diamond cutters being the most effective at ensuring a high surface finish. Alternatively, waterjet cutting is becoming more popular in precision machining of composites, with the added benefit of reduced heat generation by traditional cutting tools.

Should you use coolant when machining Composites?

This is a question which has no simple answer, due to the wide variety of materials defined as composites. Each material will require its own inspection and assessment based on its matrix and reinforcement mix, and its associated properties. As stated above, discharging heat from a composite workpiece can help prevent disfiguration, and one of the easiest ways to do this is through the application of lubricant, or coolant. On some composites, this will be your best and most cost-effective option. However, in more complex composites like carbon fibre, this may not be sufficient.

A counter argument that many machinists make against using coolant on composite materials is that the material could absorb the coolant, causing the fibres to swell and damaging the integrity of the material. This will have adverse effects on the strength, resistance and other characteristics of the composite. Furthermore, mixing the lubricant and fibre within a composite can make a workpiece more difficult to machine. Again, getting the fullest picture of a material’s composition will help find out whether coolant is right for a job.

Looking at real world applications, an experiment by a group of researchers in 2015 tested various cutting and machining fluids in the machining of six different CFRP’s (carbon fibre reinforced polymers, or plastics). In its results, they found that out of all the cutting fluids, including water, only one fluid did not damage the materials tested. From this test, we can conclude that although it is possible to apply cutting fluid to composites, in a lot of cases the fragile makeup of these materials mean that caution must be taken. Always check the coolant being used and ensure that it will not cause any adverse effects or damage to the workpiece.

Are Composites hard or tough?

Again, this depends on the makeup of the composite being worked with. It’s certainly true that one of the main reasons for creating a composite material is to increase its strength or hardness. This is especially true in metal matrix composites like steel. A benefit of hardening steels through creating a composite material is that the wear resistance of the material increases but can often have the side effect of being brittle and easily breakable in machining operations. Using a different reinforcement material, like copper or even diamond, can help prevent this.

Glass, or fibreglass, is a classic example of a material which is incredibly hard but very brittle and lacks toughness. Glass is made predominantly of rock, which has extremely high hardness levels, but other materials used in the creation of glass composites means it lacks toughness and is prone to easy shattering. However, glass is also very malleable and can be melted easily to form different shapes (e.g. on curved car windscreens).

At the other end of the scale, composites like Kevlar and carbon fibre have high toughness levels, in that they can withstand a large amount of wear and tear. Although they are ‘softer’ materials than the likes of glass, diamond and hardened steel, its properties gives it extremely high tensile strength. For instance, Kevlar is used in clothing for extreme activities like combat helmets, face masks, motorcycling and high intensity running. Kevlar’s creation is very complex, but its inter-chain bonds and cross-linked hydrogen bonds gives Kevlar its high strength properties. Its chemical composition includes hydrogen, carbon, oxygen and nitrogen. Not only is it strong, but Kevlar is also extremely heat resistant and malleable.

What industries and applications are Composites mainly used in?

Composite materials are often found in high-tech, forward-thinking industries, where the precision lamination of multiple substrates can create intricately unique materials for very specific applications. This could include NASA spacecraft development, new Boeing designs for commercial air travel, architectural features and renewable energy projects. Other key industries where composites are commonplace include automotive, marine, transport and defence (military).

In aerospace, for instance, Boeing have been increasingly using composites in the development of their commercial aircraft. Traditionally, fibreglass was used in a variety of key aircraft components, but manufacturers are increasingly using sandwich carbon composites, which feature more advanced chemical and physical characteristics for the aerospace industry. Aluminium composites are also commonly used. Similarly, the automotive industry is one of the biggest proponents of composites, with weight and fuel efficiency of paramount importance. Compared with steel, composites feature a low weight to mass ratio, corrosion and rust resistance, cost-efficiencies and increased strength and rigidity.

Meanwhile, defence organisations are tasked with making state-of-the-art military equipment. Polymers (more specifically fibre-reinforced polymer composites) are used extensively in a variety of equipment from guns to marine warships, navigation systems to UAVs. Military aircraft (both fighter jets and cargo planes) benefit from the use of carbon fibre’s corrosion and impact resistance and lightweight design. Kevlar provides similar properties to military clothing like ballistic helmets and full combat gear.

Composites are also playing a key role in new, emerging industries. Renewable energy sources are becoming more sought after across the developed world, and composites are help pioneering such technologies for the mass market. Wind turbines, for instance, draw a large portion of their power from spar caps within the turbine’s blades. Manufacturers have started creating these caps from carbon fibre, which have the key benefit of being lighter, allowing for blades to be longer and, in turn, boosts the total output and energy generation that can be achieved. Even the power cables and conductors are increasingly using aluminium composites. Aluminium features low thermal expansion, withstanding higher temperatures and therefore increasing the lifespan of this critical infrastructure.

What are the differences between Composites and other metals?

Composites vs Aluminium

Aluminium has traditionally dominated industries like aerospace and automotive. However, development of composites with significant weight reduction has made composites the new go-to for new high-spec transport development. Composites also offer higher flexibility, vibration absorption and better weight distribution.

Composites vs Cast Iron

Cast iron is an iron-carbon alloy, which some interpret as being a composite material. Cast iron has high hardness levels and high wear resistance, like some composites.

However, it has a low melting point so is less suitable than composites in high temperature applications.

Composites vs Exotics / HRSA's

Exotics, or HRSA’s are also alloys with nickel, iron and cobalt bases. Like composites, HRSA’s have corrosion resistance and high heat resistance.

They also have a high strength to weight ratio, but composites can feature higher ratios. Exotics are also conductive, whilst composites are generally non-conductive.

Composites vs Steel

Generally, composites offer several key advantages over steel, including being more lightweight, significantly stronger, resistant to corrosion and not conductive (in most cases).

Composites vs Stainless Steel

A common misconception is that stainless steel is a composite material; it isn’t. It’s a steel alloy. Many of the benefits vs steel apply to stainless steel, however stainless’ chromium content makes it similarly resistant to corrosion and rust.

What are the best milling tools for Composites?

Due to the cost implications involved in composite machining, the only option engineers have when milling composites is to invest in specialist tooling. Standard HSS and carbide tooling will usually lack the geometries and feature set needed to tackle these unique materials. Milling tools for composites need to be tough, hard and have a finishing style geometry to prevent easy delamination or damage to the material’s fibres.

In most cases, we would always recommend a tool with a diamond coating, which will give you the ultimate hardness and wear resistance for machining most composite materials. Having a diamond coating on the cutting edge, as opposed to a diamond tool, will provide significant cost-efficiencies. Other features found on composite milling cutters include:

Compression geometry to prevent delamination on both upper and lower edges in fibre-reinforced plastics.

Up to 8 flutes for extreme finishing, which also prevents damage to the material fibres.

Utra-sharp laser-cut cutting edges help prevent delamination and ensuring the best surface finish.

Extreme PCD coating with ultra-high hardness levels.

Air cooling channels to dissipate heat effectively.

Different flute types depending on whether straight cut, drawing cut or pushing cut required.

Cutwel supply a comprehensive range of diamond coated (PCD & CVD coating) milling cutters for high performance machining of composites including carbon fibre, glass filled plastics, aramid, honeycomb, Kevlar and highly abrasive plastics. Special application cutters are also available with features like burr end cut, reduced neck, drill point end, v-style compression and drawing cut.

What are the best turning tools for Composites?

Like with milling cutters, it’s not really possible to compromise with cheap lathe tools when machining composites. Whether it’s a boring bar or turning tool holder, it’s important to get a performance holder which will be able to handle the cutting data of the inserts required to machine composites. This could be either a carbide shank boring bar or a premium, tough HSS substrate turning tool holder. We would recommend the Korloy high performance range of boring bars and turning tool holders.

The ideal choice for turning inserts is the PCD (Polycrystalline Diamond) range, which offers vastly improved cutting speeds, tool life and surface finish versus standard carbide inserts. They’re formed by sintering together diamond particles at high pressure and temperature. Because of this diamond substrate, it’s hard enough to effectively turn composites like carbon fibre, glass fibre and abrasive plastics. If budgets are really constrained, we do offer standard carbide inserts with a special AK chipbreaker and PD1000 DLC (diamond-like carbon) coating, which covers some copper alloys and abrasive plastics. Please check with our Technical Team to ensure these cover your material before use. Call 01924 869615 or click here to get in touch today.

What are the best drills for Composites?

Delamination is a real risk when drilling composite materials, especially when creating through holes. If a composite material is delaminated, it means the matrix or bonds in the material crack or break. This will essentially negate any benefits gained through creating the composite and can also cause the material to become more brittle and susceptible to breaking further. When picking a drill for machining composites, the main cost in the mind of the engineer shouldn’t be that of the tooling, but the cost of scrapping the composite material.

With this in mind, it’s essential to invest in drills which will effectively cut the composite without damaging the fibres which join the matrix and reinforcement materials together. Ideally it should feature a diamond coating, which will improve not only the tool life of the drill but also the accuracy of the produced hole. Combine this with a drill geometry designed specially for composites and the chance of delamination will drop considerably. Such special geometries may include point geometries designed for either multidirectional carbon fibres or unidirectional carbide fibres, as is the case with our Karnasch range of composite drills.

Cutwel supplies a selection of diamond coated carbide drills designed for tackling a wide range of composite materials including carbon fibre, glass fibre composites, graphite, Kevlar, high grade engineered and abrasive plastics. With both solid and through coolant options, as well as a crown point drill option, Karnasch offer a comprehensive market-leading range for composite drilling.

What are the best machine taps for Composites?

For threading composites like, for example, carbon fibre, we would not recommend machine taps in the first instance. Thread milling offers lower cutting forces and short chips, compared with machine tapping which performs the full threading operation in one hit. Due to the cost implications of having to scrap a composite job, thread milling also gives less chance of tool failure, as machine taps can sometimes get stuck or break in the tapped hole. As a thread mill will usually have a smaller diameter than the thread, this isn’t an issue. A combination of these factors, therefore, makes it easier and safer to thread mill composite materials than tap them.

Other benefits of thread milling which makes it the superior option for machining composites include:

Due to less force being required from thread milling compared with the single ‘hit’ in tapping, a lower amount of power is required from your machine.

As most thread mills are made from carbide, compared with HSS or HSS-PM for machine taps, the tool life and wear resistance of thread mills are significantly higher than taps.

This carbide substrate also gives thread mills the strength to tap hard materials like glass composites, again helping prevent the tool from breaking in the material.

If thread milling is simply not an option, due to the machining setup or other restrictions, some machine taps may work as suitable substitutes, but only as a last resort. For instance, our special aluminium taps feature interrupted flutes and deep flute pockets, designed to prevent clogging of swarf in the flutes. However, before performing any tapping applications on composites we would recommend first speaking with one of our Technical Engineers. Call 01924 869615 or click here to get in touch today.

How to improve your Composites machining process

Composite materials present many significant challenges for engineers of any industry and experience level; they react differently and, in some cases, unpredictably compared with traditional metals. A key reason for this is that most materials are homogenous, which means each material has its own designation and feature set (e.g. EN10 Steel or 304 Stainless Steel). With composites, the variation of matrix and reinforcement materials, all combinations contain their own properties like fibre type, weave pattern and fabrication methods, which means that non-homogeneity is a key problem.

Delamination has been touched on before, but is a common issue faced when machining composites, like sandwich or honeycomb materials, and can usually be addressed through slight adjustments in the machining set-up of the job. If the feed rates are too high, the thrust force by which the cutting tool pierces the material (usually during drilling) will cause the bonds between layers to crack. Selection of a feed rate which will prevent this is therefore essential, which can be found by consulting the cutting tool’s cutting data.

Similarly, if your cutting tool has been worn by repeated use, this could also cause feed rates to change and delamination. A sharp cutting edge is always the best way to go, which will not only reduce the stress on the material and machine caused by a blunt or worn edge, but also ensure the best possible surface finish. As stated before, a smooth surface finish is not only aesthetically pleasing, but minimises the damage to the composite’s fibres and chemical makeup. Cutwel supply a wide array of composite drills and milling tools specially designed for tackling these hard-to-machine materials with diamond coating, sharp cutting edges and finishing geometries.

Although using a dedicated composite milling cutter or drill will help increase tool life and prevent tool wear, careful tool management is required alongside appropriate tool selection. Because composites are so abrasive, engineers should also regularly check their tools for signs of wear, and adapt the tool path based on any patterns that are found.

High heat generation caused by composite machining causes a composite’s matrix to become weaker, another common cause of delamination. Tooling which dissipates heat efficiently, and is thermally conductive, will prove useful here. For instance, Germany-based premium tool manufacturer Karnasch supply a range of composite milling cutters with laser-ground cutting edges for incredible tool life, sharpness and surface finish. Coolant holes also allow for effective heat dissipation on some composites (although coolant is not always recommended with composite machining).

Finally, it’s not only the tool that you need to consider when machining composites, but also the tool holding. As composites are commonly extremely abrasive materials, there’ll likely be a significant amount of chattering between the tool and workpiece if using traditional tool holding like ER Collet Chucks, Drill Chucks or End Mill Holders. Using precision ER collets with runouts of 0.005mm or less can help, but ideally a hydraulic chuck with a 3 micron runout (or lower) will give you the best chance of success with composites. Hydraulic chucks use fluid which pressurises around the cutting tool, clamping it in place. This fluid also acts as a vibration dampener, meaning chatter is minimized, especially important in composite machining. Cutwel supply a wide range of hydraulic holders from YG-1, Schunk and premium German manufacturer WTE. With balancing up to 25,000RPM at h6 tolerance, 3 micron runouts and through coolant options, Cutwel provide the ULTIMATE tool holding solution for composite milling, drilling and threading.

Summary

Machining composites can be extremely difficult, and take a vast amount of experience or expertise to master. Thanks to the complex, non-standardised nature of the matrix-reinforcement mix, which creates materials with delicate fibres, low melting points and unpredictable machining conditions, you really can’t just ‘wing it’. Specialist composite tooling, and accurate cutting data and guidance, is of paramount importance.

Luckily, Cutwel’s Technical Team of expert time-served engineers are on hand to assist with any difficult composite jobs you may have. To get in touch, call 01924 869615 or fill in our form here and one of our engineers will get back to you.

CVD, PCD & diamond coated cutters for high performance machining of composites like Carbon Fibre, Glass Filled Plastics, Aramid, Honeycomb & Kevlar. 

High performance carbide drills with unique geometries for machining composites. Includes crown point, diamond coating and through coolant ranges.

Polycrystalline Diamond (PCD) turning insers are designed for turning high silicon aluminium, carbon fibre, glass fibre and abrasive plastics.