Drilling carbon fibre

I wonder if I could tap the forum's collective wisdom on this question. I have made a successful clock using a carbon fibre tube for the pendulum, but I'm now making a start on the next which will have an unusual pendulum based on the "Robertson Clock" at Bristol University. Instead of the usual circular rod this has two invar strips that run "fore and aft" and are located in shallow slots cut in the bob. There are good reasons for this to do with ensuring that the impulse is exactly in the plane of the pendulum's swing, also because the "rod" is everywhere in contact with the air for maximum thermal transfer.

In my case I'm not building a replica but want to use a similar design, but using CF strip rather than the tube I have used before. To fix the strip to the bob and also at the suspension point I will need to use screw fasteners. This will require drilling the strip, which uses longitudinal pultruded fibres, with the risk of splitting, and also may weaken the strip once the bob weight is hung.

My thinking is to epoxy short strips of the CF material with the grain at right-angles to the main length at each point where a hole is needed before drilling, to help prevent splitting and reinforce the main length. My questions - are my concerns well founded? Is epoxy the right glue? What sort of drill should I use? What else should I beware of? I will be using 12 x 2 mm strip.

All suggestions gratefully received!

Possibly irrelevant, but I have an enormous roll of CF 'tape' should anyone have need

rgds

Bill

John

If the strip of carbon fibre laminate you are using is 2mm thick and 12mm wide I think your fears are unfounded, provided you follow some simple rules.

Keep the edges of any holes at least 2.0mm from the material edge, use carbide drills and clamp the laminate to a sacrificial backing to prevent shattering when the drill breaks through the laminate.

Epoxy resin should be suitable for bonding any additional fibres but test on some scrap first, best let the resin bonded parts cure in a warm place (30C) for 12 hours.

Emgee

I'd have thought that splitting would be very likely with any kind of drill, even if fed under extremely controlled conditions - not just rammed into the work. Consider using a diamond 'point' in a Dremel, etc?

Your idea of laminating cross fibres sounds good!

Edited By Kiwi Bloke on 14/04/2020 12:25:36

I used to drill larger holes (10 -12 mm) in CF plates with Diamond core drills.Quite sure I got them from ARC in England.

So for smaller holes I think I would also use the small diamond burrs, e. g. from Eternal Tools:

here

Regards,
Hans

I suppose it depends on the quality of the carbon fibre cloth used to make the rod. I've repurposed bits of broken (crashed) 2mm carbon fibre r.c heli frames and just drilled holes in those days with a 2 or 3mm jobbing drill in a dremel. Do be aware that breathing in carbon fibre dust ain't good.

pgk

John, I'm pleased you asked this question ! For my project (Wildings small tower clock) I have purchased some elliptical section CF rod with the aim of maximising the Q. Sadly this elliptical rod was only available in extruded form and I know from previous experiences that extrusions don't always drill well. In extruded form the fibres are all aligned so it has great strength but not much in the other. And there is very little resin separating the fibres. So it tends to get torn where the drill cutting edges move at right angles to the grain direction.

Of course, a lot depends on what quality of hole you are looking to achieve. A sharp carbide drill running at the highest speed with minimum force might give you a good enough result. But although I haven't tried it myself yet, a diamond burr, as suggested by Versaboss, would probably be even better as a diamond burr doesn't have one big ugly cutting edge.

Epoxies work well on CF extrusions which have almost no resin on the surface, (or in the material for that matter) so the glue can get right onto the fibres. I think I would be inclined to try put some epoxy in bore and on the outside surface you intend to drill to minimise tearing.

Personally I'm considering no drillings at all in my elliptical rod. I will bond metal fitting on to it and drill these. I'm hoping that metal parts will compensate for the small negative expansion coefficient of the extruded rod but I have to admit to not having done the sums yet

Gerry

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Sorry, pgk ... Unless something has changed drastically since I was introduced to them : Pultrusions don’t use cloth ... They use unidirectional fibres.

The clue to the manufacturing process is in the rather contrived name.

MichaelG.

Thanks Gerry. You may be interested that for the rod in my first clock I started by epoxying short "slugs" of aluminium into the bore at each end - the tube is 10 mm OD and 8 mm OD and I have some 8 mm ali rod that was a nice smooth fit. When this was thoroughly set I carefully cross-drilled at the top end for (if I recall aright) an M3 bolt that holds the rod in its socket in the upper chops. The lower slug I had drilled and tapped M4, initially for a rating screw but actually the bob is fixed and the clock regulated through its electronics. But I guess your rod is solid? How will you make the hole in the fittings for the rod? Depending on how precisely the rod is actually elliptical then CNC could do it.

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oops, when I referred to 'extrusions' I did of course mean 'pultrusions' ! I was involved in some discussion on rubber parts just before I wrote my reply and the wrong term was clearly still stuck in my mind.

John, yes my rod is solid 8mm by 5mm, and I have been pondering the connections. I constantly wrestle with functionality versus appearance. A cnc-made elliptical hole would be the most elegant solution, with ugliest being a press-fit into an epoxy-filled circular hole. I did wonder if I could carefully drill the rod along its axis and glue something in this hole but these pultrusions split like a banana with very little encouragement when given any significant internal pressure. I might need to do a trial or two on this idea first.

I'm planning an elliptical pendulum bob too, filled scraps of tungsten, not to match the rod but to minimise drag of course. I might bond the rod directly into this then I only have one connection to worry about at the top.

I'm in no rush to decide these details just yet as there are just so many things to think about. By the time it's finished this clock will bear about as much resemblance to JW's small tower clock as I do to Donald Trump.

Gerry

My brain skipped the word 'pultruded' and assumed this was spun on a mandrel...apologies..

pgk

Thanks Barrie, sounds like a carbide burr is a good way forward. I have got a CNC mill and some 1mm carbide cutters but no way could I get a metre length of material on the table! So should I be using an end-cutting cylindrical burr for this please?

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Thanks Barrie. I just want to drill a plain hole without risk of splitting the fibres apart. So I was thinking that a flat-ended cutter that can cut on its end could make the hole while minimising the tendency to wedge the fibres apart.

You could use an edge cutting tool and use a boring action on the CNC where it works it's way down in a spiral which is what I think Barrie meant by "generating the hole with a small cutter" that way you get the speed at the edge of the tool.

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As the strips are 1 metre long I won't be able to fit then on the CNC mill table!

Are you wanting to drill the end then?

If not why can't the strip overhang the end/side of the table?

Some more detail of orientation and hole size/number may help thos eof use who don't know the clock design.

Edited By JasonB on 16/04/2020 12:35:22

I use Diamond-Cut End mills or router bits as sometimes known, for composite work, especially items made from Carbon Tow. Below are photos of one half of a Landing Gear set for a small UAV - the hole made with these type of bits, routed out, not drilled in full size. The hole is 6mm, done with a 3mm cutter.

The tip is available in Drill tip or Fish-Tail, I use the latter as it cuts a circle out on exit, rather that the point of a drill pushing the fibers apart.

I also use the diamond cut - the flutes are 'pull-up' but the diamond grind provides a measure of 'push-down' to the material. There is no tearing or lifting of fibres at all, provided the cutter is sharp.

The bits are also available in reverse flute, ie, 'push-down', but are more costly.

I have never had success with so-called carbide burrs - great care is needed, slow feed, and SLOW rpm, maybe around 1000RPM, as the resin just gums up and results in a mess.

These Diamong Cute End mills cutters are commonly used to route out PCB peripheries in PCB manufacture.

They can be used free-hand, in high speed hand held tools, like a Dremel, must be run at high speed - above 8000rpm for a 2mm cutter, but beware of freehand work on the larger diameters - I would not do more than 3mm - a 6mm cutter easily grabs and wrecks the job.

Carbon Tow is the stuff most pultrusions are made from - makes for incredibly strong parts in tension, very resistant to bending, buts can split in the fibre direction.

Joe