The composite cutting craft

In this Q&A, Dr Peter Müller-Hummel, senior manager business development aerospace and composites at Mapal, discusses with Ed Hill Hihow the company has developed new tools for drilling holes in advanced materials used in aerospace.

What particular problems for cutting tools do composite materials create when drilling?

The main problem that can occur is delamination as the cutting tool enters the material, delamination inside the hole and delamination as it exits. The main issue is delamination on the exit because this is the most likely and the most visible. Delamination on the entrance is less common. Delamination inside the hole usually occurs when cutting tools become worn. A tool at the start of its life works well, but as it becomes less sharp delamination in the middle of composite materials being drilled can occur.

CFRP is known as a very abrasive material, so how do you lengthen cutting tool life in these circumstances?

Carbon fibre is hard like diamond along the length of the fibre strands. This means we need to use a harder cutting material which can overcome this. Steel drills are too soft, solid carbide tools can work to a degree, but the best material option is a diamond coating, and of course the thicker the coating the longer the tool life.

This coating has to be very tough with continuous wear characteristics on the cutting edge. It cannot chip away or flake off from the substrate. We try to apply the thickest coatings that we can but using thick coatings can also cause residual stresses inside the coating, so if the coating is thicker than 15µm you start to get problems with it flaking off the drill substrate. There has to be a balance between the thickness of the coating and its ability to stay on the substrate of the tool.

What types of composite materials are drilled most by your cutting tools? CFRP, Kevlar, Aramid, glass, or combinations of metal and composite stacks?

Where and when to use composites is being considered more carefully by manufacturers and OEMs today. They really only bring benefits where they are the most cost-efficient. CFRP is by far the most common composite material our tools cut because of the work we do in the aerospace sector. Particularly in civil aircraft production there is less use of exotic composite materials such as Kevlar, and other aramids.

We also process composite materials used in automotive and wind energy, but not to the same degree. Composite tools are only a small part of our tool range compared to those used for metals. The main sector we supply is automotive and the materials used there are around 90% aluminium and steels.

What is your approach when it comes to carrying out trial composite cutting for your customers?

We try to avoid unnecessary trials if we can. The good thing about drilling composite materials is that using the tool under the approved conditions, there is no oxidation, chemical reaction or build up taking place at the cutting edge as with materials such as aluminium or titanium [Fig 1], there is only abrasion.

The helpful factor about the rate of abrasion is that there is a linear correlation between tool wear and the path and time the tool is in contact with the material it is cutting. So, if each tool has a certain amount of contact length with the composite there is a predictable rate of wear until the end of its life.

Naturally, if a material is more abrasive this contact length is shorter. Additionally, if a tool is larger in diameter that means we have more contact length each revolution and that also means tool life gets shorter. It’s a bit like using a snow shovel. The snow shovel wears because of the length in contact to the abrasive ground independent to the load of snow.

We have designed our drilling tools for the most difficult carbon structures/lay-ups to machine which is IMS unidirectional carbon fibre materials on the hole exit. Unidirectional materials (UD) in effect have become our benchmark when cutting CFRP. If you can cut IMS UD materials successfully then you can cut most other Commercial Aerospace composite materials, structures and lay-ups successfully too.

Most of the time the customer comes to us and asks for a solution. That is why we have a big R&D department at our headquarters, so we can address their issues and our team can come up with some inspired ideas.

What about milling as a process for making holes in composite materials?

When you carry out circular milling/drilling of holes it is easier to ensure there is no delamination. However, it’s not as cost-efficient as straightforward drilling. Drilling is much faster, but it creates more pressure on the material, so we have to design the drill that creates less pressure on the material as it exits.

How does the increasing use of automated drilling operations affect your tool’s performance?

We are working with the aerospace OEMs on robot applications, particularly automated drilling of the fuselage. The OEMs like our tools because they have a high complex process capability (CPK), which means their reliability is very high. The design of our tools also means that we can offer dry drilling without the need for lubricant. That means we can drill thin stacks of CFRP and aluminium materials under dry conditions greatly reducing the problems that using lubricants can have on the shopfloor and further reducing the price per hole.

How do you avoid having to use more than one type of drill on a stacked material?

The main problem you have when cutting stacked composite/metal materials is that when you cut carbon fibre it remains colder than the adjoining metal. This is because you are breaking the material as opposed to melting it at the cutting edge. The heat generated in the metal material means the hole has a tendency to be fractionally larger than the one going through the composite. To counteract this, we have developed an additional micro-reamer on the tool that reams the composite layers of the hole. On these drills there is a tiny step on the outside diameter that reams the composite to the same size as the metal. This means you can get the H8 (ISO hole tolerance specification) quality in one shot.

What advances do you expect to see in cutting tool development for composite materials?

For increased tool life it would be good if we could develop harder materials for coatings, but there isn’t really anything available harder than diamond. The only thing we can do is increase the thickness of the coating and improve the adhesion of the coating to the tool. These are the most important aspects for further development.

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