# CNC Foam cutter

Discussion in 'Composites' started by Foundationer, Apr 4, 2019.

1. Apr 4, 2019

### Foundationer

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I just made one - turns out it's really easy and pretty cheap! Who'd have known?

Basically you just follow the instructions the RC guys have worked out (link below) then make it a bit bigger. All in about £250 and with some mods it'll turn into a light duty milling machine - should be good for milling foam & ply & plastic.

https://www.rcgroups.com/forums/sho...Wire-CNC-(Arduino-Ramps1-4)-Complete-Solution

Should make mould making really, really quick, simple & accurate as well as cutting cores for control surfaces etc.

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2. Apr 4, 2019

### Vigilant1

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It looks very nice. Okay, at the risk of appearing to be a hopeless troglodite: For the one-off homebuilder of a full-size airplane, is there a significant advantage compared to Rutan's use of templates, a hot-wire bow, a helper, and a few practice runs to get the pace right? Maybe the pace/wire temp can be more precisely controlled so that ridges are less likely and the shape is more precise?
Again, I'm not dismissing it, but it looks like a lot of time and a not-negligible expense if the goal is to get a set of wing cores. For small scale kit production (? 20 sets per year? ) then the payoff would be there. An EAA chapter or region with a dozen composite builders would find it useful.

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3. Apr 4, 2019

### Foundationer

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AH - that's a really good question! The most honest answer is 'BUT LOOK! I MADE A CNC MACHINE!!!". So the rest doesn't really matter.

But as regards building an airplane... I really didn't like cutting by hand - the results were never great and it's fiddly and inaccurate and getting some templates CNCed isn't all that cheap either.

For what I'm planning to build I'll be cutting wing moulds, control surface cores, motor pylons, tailplane, fin and rear fuselage moulds. Cutting them this accurately means I can cut, face and go rather than having to fill and sand.

Is it worth it for wings? Maybe... It definitely will be worth it when it's got a new gantry and is milling wood and whatever.

4. Apr 4, 2019

### Vigilant1

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Thanks. And, once I have a new tool in the arsenal, I often think of new ways to use it. But this is truly cool in its own right.

5. Apr 4, 2019

### lr27

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It's possible to hot wire foam by radiant heat only, without the wire touching the foam. Gets rid of wire sag from dragging on the foam, plus gives a superior finish. Requires careful control of wire speed. Not sure this is possible by hand.

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6. Apr 5, 2019

### pwood66889

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7. Apr 5, 2019

### billyvray

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Nice
I want one also

8. Apr 5, 2019

### Foundationer

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Ok. Here's what it actually cost... And I'm not happy with you for making me actually add it up

£ 6.99 - m5 t Nuts
£ 5.59 - 8mm Cap Bolts
£ 14.99 - Belt & Pulleys
£ 38.99 - Arduino MEGA 2560
£ 16.99 - Power Supply
£ 37.68 - 2060 Extrusions
£ 27.38 - 2060 Extrusions
£ 14.28 - 2060 Extrusions
£ 25.01 - Gantry Plates
£ 46.22 - 4 Nema 17 Stepper Motors
£ 38.76 - Wheels & Gantry Plates
£ 294.88 [$385.97] And for amusements sake here's what it should have cost if I bought stuff off Banggood or AliExpress rather than the same stuff off Amazon Prime for three times the price... I got all the extrusions off a place in the UK that cuts them to size, taps them and delivers it all the following day! In the US somewhere will do the same - maybe the Openbuilds part store... £ 4.11 m5 t Nuts https://www.banggood.com/100pcs-M5-...-p-1048442.html?rmmds=search&cur_warehouse=CN £ 5.59 8mm Cap Bolts £ 13.92 Lead Screws https://www.banggood.com/300MM-T8-L...-p-1149817.html?rmmds=search&cur_warehouse=CN £ 5.78 Belt & Pulleys https://www.banggood.com/20T-GT2-Al...-p-1081330.html?rmmds=search&cur_warehouse=CN £ 16.43 Arduino MEGA 2560 https://www.banggood.com/3D-Printer...t-p-984594.html?rmmds=search&cur_warehouse=CN £ 8.39 Power Supply https://www.banggood.com/5V12V24V-P...-p-1120203.html?rmmds=search&cur_warehouse=CN £ 29.59 2060 Extrusions £ 27.38 2060 Extrusions £ 14.28 2060 Extrusions https://ooznest.co.uk/product-category/parts/mechanical-parts/v-slot-extrusions/ £ 25.01 Gantry Plates https://ooznest.co.uk/product/v-slot-gantry-plate/ £ 46.22 4off Nema 17 Stepper Motors https://ooznest.co.uk/product/nema17-stepper-motors/ £ 38.76 Wheels & Gantry Plates https://www.banggood.com/Machifit-V...-p-1410067.html?rmmds=search&cur_warehouse=CN £ 235.46 [$307.59] - TOTAL

There are also some scrap bits of aluminium for legs & 3mm sheet for motor mounts as well as a bunch of 3d printed parts to mount motors / pillow bearings / etc.

9. Apr 5, 2019

### Foundationer

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I'm running a guitar high e string and the wire doesn't touch the foam - you can look right down the length of the cut and it's straight as a die. That's running at 75mm/minute and something like 13v / 3a though I'll check that.

Cut quality looks like this:

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10. Apr 5, 2019

### Tiger Tim

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I have zero composite airplane experience, but after reading up a bit and seeing some how to videos online I wonder if CNC hotwired cores would save some measurable amount of weight. With a more precise surface to begin with there should be less filler required later on, no? Given the cost of making an airplane from nothing the $400 machine might be worth it for the performance gains when finished, especially if you turn around and sell the rig for$350 when you're done with it... if you're ever done with it once you figure out what all it can do.

11. Apr 6, 2019

### pwood66889

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From my reading, T. Tim, the filler/sanding later on is due to the resin sag over the cloth bringing out the weave there of. Vacuum bagging helps smooth out the surface.

12. Apr 6, 2019

### Vigilant1

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Very nice, thanks. The wire length (junction to junction) is about 1.2 meters? And is that 13v/3a metric or Imperial volts/amps?

13. Apr 7, 2019

### Tiger Tim

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Fair enough. I thought that wavy cores were to blame in a lot of cases too.

14. Apr 7, 2019

### Vigilant1

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I think waves and dips in cores do cause a lot of extra work and weight. Also, from my understanding, failure to cut or sand propper "joggles" into the foam cores so that overlapping layers of fiberglass/CF end up smooth on the top. Often, folks think it won't matter, won't be seen etc and they are in a hurry to make progress, so the can gets kicked down the road. Then, after the finish is smooth (and especially when it is shiny), the dips and bumps are apparent and they start adding fairing material. And sanding. More material. More sanding. etc.
From the folks I've listened to who have done it, in the long run a lot of time and weight is saved by getting the cores as close to perfect as possible.

15. Apr 7, 2019

### BJC

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Right you are.

There is a law in process control that, paraphrased, states that the earlier in a process that a deviation is corrected, the less effort is required. Applies to building airplanes too.

BJC

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16. Apr 7, 2019

### rickofudall

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As a buddy of mine told me, "perfect cores make perfect parts". Not quite as simple as that but I"m sure you get the idea. His solution was to cut all parts oversize by an 1/8" or so and than sand the cores to finish shape. Much more labor intensive than just letting the cutting wire do it all, but his LEZ sure was pretty.

Rick

17. Apr 10, 2019

### Foundationer

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One problem that's appeared is the DOW FLOORMATE 300 foam I've been using for moulds seems to have some stresses built into it. If you cut a 100mm block down the middle it bows outward. Seems that if you slice both the surfaces off you can mitigate a lot of that but still a bit annoying. Anyone got any cunning ideas for how best to keep the bottom surface nice and flat (no problem for a part cut from the middle - seems the pre-stressing is symmetric about the centre of the billet).

18. Apr 10, 2019

### Vigilant1

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Yes, others who have cut shapes from billet foam have seen the same thing. IIRC, some brands and types have a better reputation than others.
While a vacuum table would be a nice industrial solution for holding things down flat, I'd be inclined to try simple double-sided tape first. It is often sold in office supply stores and places that sell carpets. The carpet stuff has some tenacious adhesive, you'd want to reduce it's grip somewhat by blotting a few times against some cloth (so you don't risk tearing divots from your foam).

19. Apr 10, 2019

### Foundationer

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You, Sir, are a genius. I spent a good amount of time thinking about a vacuum table but at no point considered the pile of double sided tape in the corner.

Another question: Anyone got a favourite contact adhesive for [polystyrene] foam? Something that doesn't come out too thick & gooey or melt the stuff.

20. Apr 10, 2019

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