Drilling and Drill Bits
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I'm always surprised to see people use ordinary twist drills for drilling graphite composites. They heat up excessively, they dull quickly, and they require significant backing to avoid tear-out. It doesn't matter if the bits are solid carbide: they will still only last for a handful of holes.
Several manufacturers produce bits specially designed for composites. They are more expensive than standard bits, but each bit will drill more holes before needing replacement or sharpening. Actual per-hole cost is lower, even if you don't consider improved machining efficiency and hole quality.
Below, I provide my recommendations for drill bits. There are other specialty bits available, but these are the ones I prefer based on lifetime cost and hole quality. At the end, I also have a brief discussion of the Nikon CoolPix 950 digital camera, which takes some of the best pictures of composites I've seen.
Tapered Drill Reamer
The tapered drill reamer is manufactured and sold by Starlite. I first read about this specialty bit in the Engineered Materials Handbook Volume 1: Composites. It was one of the first bits designed for composites, and in my opinion is still one of the best.
The drill reamer has three straight flutes. The first half-inch of the bit is gently tapered, with a sharper taper right at the tip.
This bit cuts very cleanly with little heat generated. I usually get on the order of 100 holes per bit without sharpening (typical laminate thickness of 0.25 in).
The image below shows the back side of a 0.25 in hole drilled with the reamer. (The test piece is a block of 15 pcf foam wrapped in 0.1 in thick graphite-epoxy braid.) The hole was cut without any backing. There's a little bit of fraying, but it doesn't extend beyond the edge of the hole (i.e., it was material that would be removed anyway).
I used an electric drill to cut this hole. Normally, I prefer air drills, but I wanted finer speed control. Because the drill reamers are solid carbide, they are brittle. The drill bushings we had for this part were slightly too large, and I was worried about the bit catching in the bushing and breaking. It seemed easier to avoid this problem by starting at very low speeds.
Dagger Bit
The guys in the shop are always looking for new bits to try. This time, they ordered some dagger bits in addition to the tapered reamers.
The dagger bits are also solid carbide. They are basically flat with two cutting edges.
These bits also cut nice holes. Resistance is a bit less than the drill reamers, and heat generation is comparable, perhaps a bit worse.
The back side of the hole shows the biggest difference. As shown in the image below, fraying is much worse, and may even extend beyond the edge of the hole. Backing is definitely required with the dagger bits.
Diamond Hole Saw
For holes much larger than 0.25 in, I like to use a diamond hole saw. As with the drill reamers, I usually order these from Starlite. They have good tolerances and an excellent lifetime.
The holes described above were actually pilot holes for 0.5 in hole saw. We removed the pilot bits (ordinary twist drills), and replaced them with upside-down dagger bits. The flat of the dagger gave a place for the set screw to grip, and the blank end could guide the saw without opening the pilot hole.
With a larger diameter hole, I would normally cut right through the laminate. In this case, though, the graphite and foam was getting stuck in the hole saw, forcing us to spend several minutes cleaning it out after every hole.
After a bit of trial-and-error, we finally came up with a simple solution. We started by cutting just through the first layer of graphite, stopping as soon as the saw went into the foam. We would then use a drill blank to pop the graphite "washer" off of the foam.
The trick here was to stop the drill as soon as we felt the loss of resistance, and to keep the drill vertical at all times. Sometimes the washer would come off in the hole saw, forcing us to stop and pry it out. It wasn't as hard as removing the entire thickness, but it still slowed us down.
We removed the graphite from both sides of the piece in this manner. Then we cut halfway through the foam, flipped the piece over, and cut through the remaining foam. This kept the foam from getting stuck in the hole saw: most of the time, it just fell through the cutout.
The diamond hole saws cut extremely clean holes. The image below shows the back side of a typical hole, cut with no backing.
Countersinking
One set of holes required a countersink. For that, we used a standard solid carbide countersink mounted on a mill.
In general, this cut pretty well. Most of the holes were very clean, with a few requiring some deburring (though there was no damage to the surrounding material).
The major problem we had was chattering of the part. I can't definitely attribute this to the countersink, though. The mill table was very small, so the part wasn't supported very firmly. If we had been able to clamp the part, we may have avoided the chattering.
Nikon CoolPix 950
I've been looking for years for a camera that will take good close-ups of composites. Many of the details are too fine to pick up, especially because there aren't many contrasting colors.
The first two pictures above--the close-ups of the drill reamer and the dagger bit--were taken with a Nikon CoolPix 950. The picture of the dagger bit, in fact, is the first picture I ever took with the camera.
To me, the detail in the pictures is nothing short of amazing. The weave pattern is obvious, and you can see the resin rich areas where one tow "dives" under another. Surface bubbles are also apparent. The white squiggly lines are actually scratches left over from sanding the surface.
These pictures were taken in macro mode, with the lens at maximum zoom and the built-in flash turned on. The camera was held about two inches from the part.
The Nikon CoolPix 950 is often compared to the Olympus C-2000Z. I've been shopping for a digital camera, and my choice came down to these two models. These pictures pushed me towards the Nikon.
My only complaint about the camera, though, is that it doesn't feel like a standard camera. The lens unit can rotate vertically, independent of the control panel and LCD. This makes it easier to take some pictures (it helped a lot in taking the first two on this page), but I'm used to holding an SLR. Olympus has just come out with the C-2500L, which can focus in closer than the C-2000, so I'm waiting to see how the new Olympus works before making my final decision.
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