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Drilling of carbon fibre reinforced polymer (CFRP) composites

Drilling into composites is a real challenge, easliy resulting in delamination at the hole outlet, or splintering leads to a defect workpiece. This article give some tips to help solve these issues.
Key words: carbon fiber composite material, CFRP, composite material processing, delamination, splintering, fraying, drilling tool
Article source: Modern Machine Shop
By Peter Zelinski
Published: August 12, 2008
Drilling into composites is a real challenge. Drilling a metal workpiece simply removes the material and emptying the drilled holes, while drilling the laminate workpiece, the woven layers are pushed forward, resulting in delamination at the hole outlet, or splintering leads to a defect workpiece, as shown in Figure 1.

Delamination and splintering around hole exit

Delamination and splintering around hole exit

Figure 1.

Earl Wilkerson is a CNC program and tool specialist for General Machining. The company has 240 employees, a significant number of whom are composite processing specialists. Earl likens drilling into the composite to drilling into a piece of plywood with no support at the bottom. It’s not like drilling a hole into a piece of metal. Some specialist companies, such as Onsrud Cutter, have developed a variety of compound drills specifically for the characteristics of various composites.
The following are several CFRP composite drill designs:
■ As shown in Figure 2, the central drilling point plays a similar role to the central drilling, and the two drilling points along the edge are similar to the wing knife, cutting the edge of the hole to avoid the hole edge fragmentation;

Figure 2, Kevlar Dirll of Toolind

■ Drill reamer with special geometry to complete drilling and reaming in one operation

■ 8 facet double angle drill to elimilate delamination and splintering when making holes in CFRP

Figure 4, TOOLIND aircraft drill

■ Step drill with tight tolerance

Follow these tips to make the cutting tool work more efficiently:
■ Positive tool geometry Angle can reduce cutting force and avoid delamination;
■ The larger the angles, the higher the quality of the hole drilled: spiral Angle, clearance Angle, rake Angle;
■ The smaller the drilling angle, the better the hole outlet quality. However, the edge will weaken if the drill Angle is too small. Karthik Sampath of Kenner thinks 90 degrees is the best Angle for CFRP materials. For metal drilling, the most typical drilling Angle is 135 degrees.
The diamond coating can extend tool life by more than ten times. The coating thickness is between 5u and 16u. In practice, the 12u diamond coating provides the best cost performance.
When machining composite holes, the main problem is usually at the exit of the hole, however, this does not mean that the entrance of the hole is no problem. Incorrect tools or machining methods often roll up and layer the material’s surface at the hole’s entrance or chip and scratch. This is mainly because the feed rate is too low. Increasing the feed speed can effectively solve the problem, but it may also reduce the machining quality of the hole outlet, so it is necessary to find an optimal feed rate, taking into account the inlet and outlet quality of the hole.
Mr. Wilkerson of General Machining recommended a compound drill that his company bought from International Tungsten Cutting Tools. The drill has a “hook” with a positive edge Angle that pulls the fiber toward the tip, creating a smooth, clean hole. Even so, GE does not rely on the drill to achieve the final size because there is still a risk of disqualification. Usually, the drilling diameter is left with a margin of 0.5-1.0mm, and then the corn milling cutter (see Figure 6) is used as circular interpolation to machine the final size accurately.
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