Drilling of composite fiberglass material has been difficult and, for some, a mystery. The ability to accurately drill holes in composite fiberglass material has been the subject matter of numerous articles and how to demonstrations. Composites offer the benefits of part integration and minimization along with substantial savings in weight. Along with this is a reduction in the requirements for machining operations that need to be performed to complete an assembly.
However, drilling and cutting operations cannot be avoided completely in most cases. There are several types of machining operations that can be performed on composites, such as turning, drilling, routing, trimming, sanding, and milling. Most of these operations are similar to metal removal techniques but there are some differences that need to be addressed in order to make clean, high quality holes and cutouts in composites.
Delaminating of the outer surface and glass fibers directly below the surface are the main failure modes noticed when holes or cutouts are drilled or cut improperly. Often, excessive edge chipping around the perimeter of the cutout or hole is due to improper tools used and methods applied. Excessive fiber pulls or attached fibers not sheared off during the cutting or turning process can also cause delamination failure from the tearing action during material removal. Use of improper tools and/or methods is also a culprit with this failure mode.
All such problems can lead to downstream assembly problems, functionality problems, and can become aesthetically unappealing in the extreme.
The most common source of failure when making holes in an enclosure is a dull cutting tool. Dull tools tend to rip or tear the material rather than cutting or shearing the material and glass fibers. The main culprit is glass fibers embedded in the material. These glass fibers are very abrasive and can cause a tool to become dull very quickly. A little planning and understanding of the proper methods to machining composites up front can make all the difference in the final outcome of the operation.
1. Hole Saw: The easiest and least complex method to provide an opening in a composite enclosure is to use a fine-toothed hole saw. Lay out the size and location of the cutout; predrill a small hole in the center within the cutout area for the hole saw to start, and then carefully cut out the area to be removed. Time consuming, this is also the least accurate method, but can be accomplished in almost any environment. Keeping the saw perpendicular to the cutting surface, maintaining a consistent sawing action, and using a diamond impregnated hole saw or fine-toothed saw will provide the highest quality cutout with minimal edge chipping.
2. Drilling & Boring: Putting round holes in enclosure walls or through the enclosure door is the most common type of cutout used to provide entry into an enclosure and access to the components housed within. The most common tool used is the standard twist drill; a carbide-tipped or PCD diamond-tipped tool that will maintain a sharp cutting edge is recommended. HSS tools will also work but they will become dull very quickly, resulting in excessive edge chipping and a poor looking hole. We also recommend using high RPMs and low feed rates when using drills. This reduces the chipping around the cutout. The single most important factor, though, is keeping a very sharp tool.
Drill geometry and feed rates are the largest contributing factors when drilling composites. Using a drill with a positive rake angle and thin points or split points can help reduce cutting pressure and thus, delamination, at both entrance and exit. Feed rates must also be constant and may even be reduced upon exiting from a hole to reduce flexing of the part when the drill exits. Using a solid back surface to support the part when drilling can also aid in reducing delamination and chipping.
3. Routing: A carbide router bit and router produces very clean holes and cutouts but also requires the holes and cutouts to be manually laid out beforehand, and a steady hand to stay within the layout lines. The use of a jig or fixture to help guide the router is helpful to keep straight edges and clean cutouts. Diamond-impregnated router bits are preferred for longevity but carbide bits will work just as well. Jigsaws can also be used; again, diamond impregnated cutters are preferred but fine-toothed cutters can also be used.
4. Punching: A standard hole punch produces a good clean hole but can leave chipped edges if the punch is dull. Again, maintaining sharp tools is essential to producing clean cutouts. A pilot hole is required before using a standard hole punch. Manual or hydraulic punch actuators can both be used with composite materials. Punch press-type manufacturing equipment can be used if production volumes warrant. Proper support and sharp tools are necessary to produce chip free, clean holes.
5. Alternative methods: Additional methods sometimes used to machine composites are waterjet cutting and laser cutting. Both will produce very accurate, clean holes or cutouts but also require a high capital investment in both equipment and employee training. In waterjet cutting, a high velocity stream of water and abrasive material is forced through a small diameter (0.010 in.) jet. The stream of water quickly erodes the material with water pressure up to 60,000 psi and 2,600 ft/s. Laser cutting uses a concentrated raw light beam focused onto the part being cut. The cutting operation takes place by local melting as in thermoplastics, vaporization, and chemical degradation as in composites and thermosets.
