Composites materials are commonly consisting of two materials: the fibers and the bonding material. In the layup phase in production of the composites part, layers of composites cloth, are built one on top of the other, either manually or automated in a manufacturer specific fiber direction. These layers constructing the thickness of the final part, must be kept intact, unseparated. Damaged material will be weaker, with a potential probability of part failure. PCD drills are designed and manufactured to prevent this event in composite materials, where both the cutting edge and the PCD coating is radically improving tool life and wear resistance, Keeping the cutting edge sharper for longer period of time.
Machining of composites, with PCD tools or other tools, e.g. drilling, milling etc. might cause damage to the composite materials due to the nature of the cutting action and from various reasons, which we will address here.
Potential failure modes in machining of composites are:
Layer delamination.
Uncut fibers.
Uncut resin.
Spalling/splintering.
The most severe failure mode is layer delamination since it is more likely to occur, it has higher impact on material strength, and it is more difficult to fix.
Drilling operation in composites is widely use since composites parts are connected via riveting, therefore all holes are through holes and there are no threaded holes. Generally, the exit hole is more likely to suffer from delamination of layers when drilling in composite materials due to the tool movement direction and cutting forces applied on the lower layer of the material.
There are few factors which influence the probability of delamination to occur:
Drill geometry.
Drill substrate.
Cutting conditions and process.
Workpiece mounting fixture.
On this article – We will focus on the first two factors.
Drill geometry:
A drill with point angle for designed for metal (118 deg and higher) will likely to cause excessive forces on the drill exit, by tearing the last layers too aggressively and causing separation and delamination around the drilled hole.
A PCD drill with 135deg point angle (for Aluminum)
A sharper point angle drill (like 90deg) will have better results while reducing exit forces. However, angle transition, between the point and the cylindrical portion of the drill creates another built up pressure to the last layers.
Telcon Diamond drills, designed for composites, offer 8 facets point geometry, or double point angle which is longer point structure. The second angle, closer to the drill diameter is quite steep, reducing exit forces. Of course, one must consider the space beneath the part to verify there is enough space for the longer point angle drills.
Drills can be used in CNC machines or manually. While drills presented above are for CNC or ADU (auto drilling unit) usage, drills for composites which are used manually have different shape.
Still, though, the point angle must be sharp, for two reasons, one as discussed before, to reduce cutting forces on drill exit, and the second relates to the manual operation itself. Sharper drill, requires less pressure from the operator, therefore applying less force on the material itself, avoiding delamination issues.
One difference we in manual drills, to CNC drills is the flute helix (spiral) angle. While in CNC, machine has fixed location and rigidity in action, in manual operation spiral drills are very difficult to use to maintain a clear cut, hole concentricity and good surface. Therefore, most of manual drills for composites have straight, or very low spiral flutes.
Telcon’s manual carbide drill/reamer double point angles
Drill substrate:
Composites are being cut cleaner when tool cutting edges are sharp. The removed material flowing on the edge of the tool gradually rounds it and makes it dull. Rounded edge applies more axial forces on the material, causing it to be pushed instead of being cut and sheared.
Therefore, the harder tool substrate is, the higher its wear resistance.
Worn out drill cutting edge
Most common composites drill is made from Tungsten carbide. On a scale of 1 to 10, carbide hardness is 2-3, while PCD or CVD coated is on a scale of 7-8.
It is clear, therefore, that diamond tools can last much longer than carbide tools on composites.
If one wants to consider the options, since diamond tools are more expensive than carbide tools, the right way to compare both is by calculating cost per hole. The added value in tool life of diamond tools compared to carbide tools in composites, is much larger than the price difference.
Telcon’s drills for composites, used in CNC operations, as shown above, are all made from PCD segments or CVD coating, in order to maintain hole quality for longer period. A combination of an efficient geometry with the highest quality substrate, brings better, long lasting drilling results.
Telcon Diamond Ltd. is a world leading manufacturer of Diamond PCD and CVD cutting tools for the machining of composite materials: CFRP, GFRP, MMC, CMC and hard ceramics. TELCON produces a wide range of tools used by many of the world’s leaders in the field including Boeing, Airbus, Lockheed-Martin, Spirit, ATK, Embraer, Denel, ELBIT and IAI. Telcon’s tools include: PCD tipped Drills, CVD diamond drills, Carbide drills, PCD Countersinks, Carbide tipped Countersinks, PCD tipped End Mills and CVD diamond End Mills and Carbide End Mills.
TELCON possesses the most advanced Grinding, and EDM machines and state of the art Quality control equipment. TELCON’s vacuum brazing equipment is highly controlled to ensure repeatability and reproducibility of brazing operation. TELCON uses world class PCD and carbide grades and CVD coatings. In addition to tools manufacturing, Telcon operating reconditioning services for PCD and carbide tools.
