View Full Version : Tooling, Pass Type and Production method for Composite Hydrofoil

11-03-2017, 06:28 AM
Hi guys,

I put this post in here, as I have 3 different questions to help me create a CNC'd Hydrofoil for kitesurfing.

Background: I made 2 masts and 4 hydrofoils using different techniques including 3D PLA printing, vacuum bagging, pine and plastic cores and composite filament winding for some pieces. I have had limited success in that they all work and have been ridden, but due to flex it's time to aim at the higher performance end of the spectrum now.

The plan is to use a sheet of G10 ( prototype #2), or Marine ply ( prototype #1), and CNC cut out a long, thin 3D symmetrical foil shape. 1 meter long / 1 cm thick / 10cm Chord ( width ) and it must be 3D profiled to be finished to the correct shape, down to less than 50 microns.

Question 1, Because the part is so long and thin, I have ruled out using a 4th axis to rotate and machine the part due to flex + vibration. I was planning on cutting 1/2 way through the part to the centerline, flipping over , placing in a female mould half with only the ends of the part ( off the end of what I need ) bolted down, and then cutting the ends off after. Alternatively I could make a channel in the female mould, connect my kracher vac and try to vacuum hold the male piece in the female mould for final cut. ( with some guide lugs off the end to position accurately ). Any more tips or other methods that may work to get the basic shape done.

Question 2, The part thickness is only 10-11mm maximum and G10 stock 12mm thick, reducing down to probably 1mm or 0.5mm part thicknessat the trailing edge. If I am using plywood or G10, what sort of minimum thickness could I machine without part bits breaking off, and does anyone have any guides regarding stepdown to get a thin plywood trailing edge ( I am guessing G10 won't matter as it will not chip away ). Take into account, that after machining the part, I will be cabon fiber compositing over the part with the carbon wrapping the front, and trailing off the rear edge to achieve a fine trailing edge sandable finish.

Question 3, I don't have a lot of stock to remove, and I have some 2 flute Bull / round nose that I was going to use on the plywood, but can anyone recommend a tool for the G10 - taking into account there is not much stock removal to achieve - and I'm pretty much at the 3D profiling level with a max single cut height of 5mm to get to the centerline of the part, and minimum of 1mm?

Quesition 4, I have a 750Watt draper woodchip extractor, with the clear bag under. I replaced the 20 micron stock filter bag, with a 1 micron bag. Should this be sufficient to filter the air of the G10 particles?


Robin Hewitt
11-03-2017, 11:14 AM
After a quick Google for G10 I believe this to be what I would call FR4 glass fibre as used by PCB manufacturers? I can see why you are worried about the dust. As it is eminently glue-able, why not make this in two pieces so that you have downward facing flats to fix to the bed and holding it down accurately becomes easy peasy.

Alternatively, if you are CNC'ing it, leave pillars on one side to support it when you turn it over. You are going to be sanding it down anyway so pillars are not a problem, you can even drill and thread.

Have you considered aluminium alloy, it would be a lot cheaper and in T4 temper, probably stronger.

I should warn you, everyone hereabouts waits for me to put my foot in it before they chime in. They don't like to go first. Better advice will probably follow.

11-03-2017, 12:07 PM
Thanks Robin,

The G10/FR4 is just basically fiberglass, and I have the filter installed because I don't want everything to get covered in glass dust more than anything else. The marine ply is much cheaper than the fiberglass so if I can get that to work, I will probably go down that route instead.

We tested the glue 1/2 method with a plastic core. My main concern here was that the both the strength and shape of the mast is critical, and if I vacuum glue two long halves together, it's possible to get them out of alignment, but it's also possible to glue them back together with a lengthwise bend in there, and if there are any voids in the glue, side/side delamination could create issues also. Although it's probably more work, that's how I arrived at the solid core outcome. 2 halves may be an option if I can address the above 3 points though

Regarding the pillars and sanding, I am not planning on sanding the piece down after machining it, as I thought pillar removal would be inaccurate and time consuming for me ( both to clamp all the pillars down, and cut them off ) - which is why I thought the female mould would be a better option, and the female mould with vacuum clamping probably the best option.

We thought about using Ali but it is more expensive due to shipping to get out here, and it's completely incompatible next to Carbon fiber in a salt water environment.

Thanks for the suggestions

11-03-2017, 04:45 PM
Do we think this will work?

I did a parallel finish job without any roughing, contact point with stepover 0.5mm, full depth contact cut, and cut the entire side, then flipped the piece, and did the same again, so once cut the foil has no support ( behind the tool path ), but immediately on the cutting face ( in front of the tool path ), it is supported full length for the entire length of cut? Then once it gets to the far end, I chop both ends off and the piece is complete? No vac table, no supports and no mould required? The only gotchya is that I have to finished the part with a single pass in each direction.


Boyan Silyavski
12-03-2017, 05:25 PM
Before all that : http://www.surfersteve.com/smwavedesign.htm,

next question is - are you building an 1 off or a mold???

Anyway if i was doing it:

1. be 100% sure about design and final shape then make it from hard foam

2. When happy with the above , make a wood mold, 2 parts to be clamped

3. Then experiment clamping different stuff and playing with materials inside that shape untill i balance rigidity, weight and price

Thats it

12-03-2017, 07:14 PM
Cheers, we would use foam for ease of machining, but can't get any here, and it's $$ to import.
Making the bit under the water, not the board :)

Boyan Silyavski
12-03-2017, 07:54 PM
Its a building insulation foam available anywhere?? for 120x60x4cm for example here in Spain for 4 euro at any construction shop. Up to 100mm thick. No super duper foam. this for the test shape pieces. can be glued together to form thicker pieces. Check local lettering companies. big letters for shops and restaurants are made from better variant of this foam.

16-03-2017, 03:46 PM
I live in a desert, they have no foam whatsoever anywhere and the Marine ply - or importing G10 is about my only options.

Anyone on here who has cut plywood care to comment?

16-03-2017, 04:54 PM
I live in a desert, they have no foam whatsoever anywhere and the Marine ply - or importing G10 is about my only options.

Anyone on here who has cut plywood care to comment?

How about making your own foam using the expanding polyurethane stuff used to keep door frames in place while building?

16-03-2017, 04:59 PM
I have used that stuff when I made my first board, it's not rigid enough. The core needs to be rigid like solid wood plastic metal or composite