I guess Eton is asking why it is difficult to cleanly drill holes in composite parts made from unidirectional tape.
Composites are similar to wood in that it has fibers that are oriented in one direction. These fibers are stronger than the glue that holds the fibers together. So, while cutting or drilling, it important that the cutting tool does not snag fibers, which results in their being peeled out for some distance away from the cut. Carbon and glass fibers are far stronger and more and more collimnated than wood fibers so the so peel-out and splintering are a more significant problem.
In woven composites the fiber bundles cross over and under every few mm, which stops any further peeling. This makes the issue less noticeable. In unidirectional composites it is easy to peel-out slivers of fiber from the composite surface that are over a foot long. Thus it critical to make very clean cuts to avoid getting any fiber peeling started.
Delamination on the backside is also a major problem when drilling composite materials, and a backup plate is almost always required. There are special geometries used on tool cutting edges for composites (very sharp) to allow the fibers to be cut with a force that is low enough to avoid debonding the fiber from its matrix.
Thank you for Compositepro’s reply; We tried several different PCD drill tip configurations, and the drilling results were not ideal; the original low modulus carbon fiber can achieve perfect drilling, but it is not suitable for high modulus; I doubt it is The bore shrunk during machining, or the tooling didn't fit it well. In addition, the life of its perfect drilling is not ideal, and the life is also poor.
Sorry, my question description is very general; my specific question is that when drilling carbon fiber unidirectional tape, the sheet is extremely prone to delamination
Which [rotary] drilling process are You really concerned about: hand or automated or intermixed?
OBVIOUSLY automated conventional rotary drilling processes can be developed to attain/maintain high levels of quality/consistency among all structural mixes, 100% composite/multi-composites and any level of composites mixed with metals.
Hand-drilling composites can get sketchy without loads of training and proper tools/tooling.
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
Waterjet may be useful for edge and large holes [lightening/pass-thru]… but not fastener holes. However laminate thicknesses for clean cutting is likely to be limited. Controlling edge-fraying is critical.
Water-jet works well with most thickness of 'flat' aluminum honeycomb panels with aluminum or composite core.
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
IFRs, I worked at an composite experimental GA company for a couple of years. The airplane was mostly carbon fiber but had some fiberglass. We tried using the water jet machine to cut fiberglass but quickly eliminated it as a possibility because it created delaminations emanating radially from the jet stream. For example, if we tried to cut a Ø.5 hole in a piece of fiberglass that was 10 plies thick, we would get a circular-shaped area of delamination with a diameter of 1.0-1.5 inches. As the water/garnet stream is moving along it tries to find an escape path and ends up forcing its way between plies creating delams. As I recall, they/we didn't try it out on carbon fiber because we were concerned the same thing would happen. It was easy to visually see the delams in the fiberglass because the delam area was a different shade of tan than the intact fiberglass. We were worried with the carbon fiber we may not be able to detect the damage so didn't even try it and ruled it out as a cutting method.
I didn't expect a lot of replies, thank you for your active participation.
The drill is used on the cnc, and if you switch to the back of the material immediately, delamination will occur.
As for Stephan Nelle's reply, you can try the pcd drill bit, we have tested it with a large diameter, and the effect is ok (this should not be considered an advertisement, emmm)
Perhaps drill on both sides slowly so you don´t get breakout on the farside. If you are getting delmamination in general there is too much heat and speed in drilling.
I would expect poor results with a drill - as the cutting edge comes perpendicular to the fibers it will no longer be cutting them but prying them up in a way that they are very strong compared to the matrix/resin ability to hold them. Once loose they are pushed clear leaving shards around the hole.
In wood and thin sheetmetal the same prying action can take place and is far worse on the far side as the pressure of the advancing drill tend to delaminate it even before the cutting edge can get there making a huge mess.
Of course this is what Composite Pro said - just repeated for emphasis.
So, with that in mind, I would look at diamond dust coated grinders to at least start on the the initial layers - like a countersink or a dent. This way the upper layers of the composite, now out of the way with some small clearance and not seeing peeling forces, can act as a local backer for the lower layers.
Same relief is done on hand planing end grain in wood to keep a chunk from splintering - put a chamfer where the cutting edge leaves the material so that there is support right there.
But it would also work to have a chamfer where the backer is that is deeper than the cut so the edge of the board never sees a cutting load - the required depth of the chamfer will depend on how much support is needed. I tried to find a better video or picture but everyone is concerned about tearout from the chamfer on the endgrain - which, if you start with the diamond dust coated grinder you won't have.
