How to cut a perfect hole in glass/ carbon/ kevlar laminates.
Hello everyone.
Here is a new article on how to cut a perfect hole in glass carbon and kevlar laminates.
The basics is very simple: use the right tool at the right speed.
The tool is by far the most important part of the equation. We custom designed diamond coated carbide brad point jobbers specifically shaped for this job. They are high quality aerospace drill bits made in the U.S.A.The jobbers are carbide hard diamond coated and brad point shaped to penetrate the toughest carbon fiber and kevlar fiber laminates. The flute shape enables an easy extraction of the drilling material up and out of the hole, reducing burring. This makes very clean holes even in softer glass fiber and kevlar laminates. They stay centered without having to make a hole point.
The second point of the equation is to use the proper drilling speed: I recommend 500 RPM maximum with these bits. Any faster and the laminate is going to heat soak. This is very detrimental to the drilling process as the material will soften and get sticky ( resin reaching its plastic point ). Then the precision of the hole will be ruined as soft fibers get stuck between the flute and the hole walls.
Similarly, it is important to allow the material to stay cold on laminates thicker than 3 mm. I typically recommend to stop the drilling process every 3 mm depth and check for temperature. Usually heat soaking starts at about this depth.
It is important to vacuum the part for any drilling deeper than 3 mm to ensure that the de-burring zone stays unobstructed. This improves the deburring process and also avoids heat soaking.
You will want to use a slightly softer material behind the part to even out the drilling pressure and ensure a soft exit transition of the jobber at the other side of the part. This is very important to ensure that both surfaces of the part are free of any chipping after the drilling process.
Also DO NOT use any lubricant. Dry drilling is important on laminates as lubricant would certainly chemically interact with the matrice of the material and soften the fibers of the laminates. This would also reduce the de-burring process.
Here is a video of a hole drilling in a very hard 3 mm prepreg aero grade carbon part:
Hole cutting in 3 mm carbon laminate with a column drill. from Oli Ni on Vimeo.
Note the hard wood block below the part and the slow rotation speed ( 500 RPM ). The part being 3 mm thick, not vacuuming is necessary.
And here is a video of the same hole drilling in the same 3 mm prepreg aero grade carbon part but this time using a hand drill:
Hole cutting in 3 mm carbon laminate with a hand drill. from Oli Ni on Vimeo.
As you can see, the process is as easy, clean and fast as with the column drill.
Using a cutting mat below the part to drill is important to avoid surface chipping on the other side of the part. There is no need to vacuum the part neither in that case. The hole cut is super clean as you can see.
I will add a video of the same process with a kevlar laminate when I have one in hand. With kevlar, I tend to drill a bit slower and avoid applying too much pressure to the part. Low pressure is important to ensure a clean cut of the fibers and thus a clean hole.
Note that I do not recommend the use of carbide rotary grinders with kevlar laminate. These only just mess up a hole by freeing up fiber strands without cutting them. The only way to cut clean holes with kevlar is to use these drill bits.
If you need to cut a conical recess for countersink screws, we have some top-of-the-line 6 flute carbide cutters:
They come in two two sizes.
The small size has a 1/4" flute is particularly adapted to super scale embedding of micro countersunk screws.
I also use it to machine offset hinges securing screw recess, and for scale panel access fasteners.
The other size is 1/2" flute and is very handy for bigger countersunk screws like nylon screws, retract fasteners that need to be flush, or any other heavier duty job.
Both tools are USA designed and made titanium oxide coated carbide cutters.
http://www.ultimate-jets.net/collections/machining-hardware/products/6-flute-carbide-countersink
Here is a video showing the use of the 1/4" flute countersink on the Scorpion Mk2 fuselage:
Cutting a countersink recess in composite laminate. from Oli Ni on Vimeo.
With 6 flute bits, you need to be careful about the rotation speed. Too fast and the flute will vibrate and the hole will have ridges. A slow speed is recommended on these cutters.
