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Post by sean on Feb 7, 2010 10:41:08 GMT 10
Vanishing point method is good for saving the extra work of making an extra set of templates, but in my experience there are even more 'gotchas' that you have to work through to get it to cut accurate cores. I make more mistakes, and therefore waste more foam when vanishing point cutting. It's an excellent technique for cutting short highly tapred parts like stabs, fins and tip cores, but I'd hesitate to use it on a long cores with lowish taper.
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Post by thevon on Feb 7, 2010 15:50:52 GMT 10
Thanks Sean. I have given up in despondent defeat, and have redesigned the wing plan to 2 panels. One at 60cm (matching the width of the foam sheet) and a tapered 15cm tip, very Minij style. I've made 2 sets of templates and am shortening the bow to a 60cm cut.
What really bugs me, triggering the obsessive compulsion to switch from an incidental finding to the predominant psychopathology, is that sometimes the 750mm cuts work perfectly. That really bugs me. If it had stuffed up consistently I would have dropped the idea quickly. But some cuts were good, and some were perfect, and at least half were crap. I hate that sort of thing! It makes me want to isolate the problem! But after using up 1/3 of all my foam, I (sob) have to let go.
I'm using a standard RG15 foil and am planning to mount this wing on my old Minij fuselage.
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Post by bananaman on Feb 7, 2010 16:35:54 GMT 10
I guess I should assemble my CNC foam cutter, and let you cut some cores on it. (900x1200 bed) I have everything ready but the time to assemble it. (Its down on my priority list). Proper tracks and acme threads and everything.
I worked with a commercial operation (they gave me a lot of the parts) cutting high taper EPP, if you want frustrating, try cutting that stuff consistently, even on a CNC machine.
Isn't this hobby fun? So much to learn, so little time.
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Post by graememarion on Feb 7, 2010 17:15:17 GMT 10
For power I use a 32 Volt transformer with a light dimmer switch on the 240 Volt side. The plate on the side of the transformer says the secondary winding is... 32 Volts. 31.25 Amps. 1000 Watts. I know this doesn't compute, but that's what it says. It started life as a ships transformer. I bought it at a marine disposals for $15:00 thirty years ago and it is still going strong. My largest bow cuts foam 1540mm wide. (That is well over 1 1/2 Metres.) I use it mainly for cutting raw blocks to size. I described this bow in post #42. My other bows are 1435mm, 1200mm, 1165mm and 515mm. They use the clothes line and twist stick. (See below.) ______________________________________________
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Post by thevon on Feb 7, 2010 17:23:26 GMT 10
Graeme, have you cut the high-density RFM 160psi blue foam? That's the issue here. The high density foam has great benefits in its resistance to compression, but it's a challenge to cut.
Brian, I really like the idea of a CNC cutter. There's a guy in WA selling a good one for a good price 2nd hand, and I was very keen and asked Jeff, Sean and Shane about it but had to agree with them that it was really overkill. But I'd like to know more about yours ...
I just used the smaller bow setup, went to a lot of work cutting the blocks etc, and did the first cut and it was probably the worst cut I've seen yet for rippling. I don't know!! I suspect that the wire is actually stretching and losing its tension (not just with the heat, but permanently). Also the 3/8" legs probably need to be longer as you can only bend them a bit or the tension is too high for the wire. Then when the wire heats up and stretches, the tension drops.
I really, really seriously feel like giving up now. I left that cave in disgust to sulk in this one and write this whingey post!
Oh well. Andrew
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Post by sean on Feb 7, 2010 17:49:44 GMT 10
I have trouble cutting even 600mm cores out of the RFM foam. Less than 450mm seems to be easy. On the DS plane I'm building now, the centre cores are 500mm long and there was very slight evidence of waves in the middle of the cores. It was so slight that it wasn't a problem and could be lightly sanded out, but it showed that 500mm is the max length I can cut on my setup.
My shorter bows use 9/32" music wire and that gives plenty of tension. You don't need too much. I think the RFM causes the wire to drag more than other foam types, which is another reason why shorter cores are a good thing.
I read a post by Phil Barnes where he said he had trouble getting reliable cuts from Highload 60 (when cutting longer cores) so resorted to cutting shorter cores. If Phil is having trouble I think that says something! In contrast I find EPP a cinch - my first EPP core came out perfectly and the only time I cut a dud EPP core is due to stupid mistakes on my part.
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Post by bananaman on Feb 7, 2010 17:58:07 GMT 10
Rather than have duplication of something that won't be used a lot by one person, I'm happy to get mine going and have you guys cut your own. Due its size it'll have to stay here though. Like I said I have the rails, screws, mdf, steppers, electronics, program etc. Just have to move it higher in my "To do" list and put it together.
The EPP problem was the extreme taper and small tip they wanted, should have made that clearer. It does cut well, although I detest the smell. Could get good cuts with various densities of white foam but not EPP. Kept burning the TE and tip. One thing about CNC cuts, you can increase kerf allowance or profile thickness to compensate.
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Post by thevon on Feb 7, 2010 18:34:11 GMT 10
Ah, Sean, didn't realize you still had problems at 600mm. Yes, I agree if Phil Barnes finds it hard that says something! I persevered with mixed results. Here's the setup. It's actually a great feathercut setup. The drop bar assembly is from some 25mm square aluminium that I had lying around. It works well. You can see 's some black lead sheet wrapped around the bar and you can slide it along to change the weight. This is the shocker I got when I switched to the 60cm bow! I got much better cuts with the 75cm one!!!! So next time I was careful to cut the heat back to go as slow as possible, and the results were pretty good. So I changed templates and didn't change anything else, and got a shocker again!!! Aaaarrrggh!!
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Post by bananaman on Feb 7, 2010 18:41:50 GMT 10
Wow! get those corrugations going the other way and you could make a scale Junkers.
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Post by bananaman on Feb 7, 2010 18:57:34 GMT 10
That wire sure was wondering around in there and dragging.
A bit thicker wire, a bit more tension and a bit more heat, or try less weight. Is the wire humming? i.e. is the power supply good or does it chop and hum. I see you are using a dimmer switch. That cut is almost resonant looking.
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Post by thevon on Feb 7, 2010 19:47:03 GMT 10
Haha I can make corrugated blue foam for the scale guys!
With the thin wire yes, the transformer is humming.
I just tried again with heavier wire (the .025 surgical wire) and heaps of tension. Plus, I cut the power well back. I don't understand the electrics but the thicker wire cuts on a much lower setting (no humming either) on the dimmer switch. It gave a really good cut, so I'll try a core again.
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Post by graememarion on Feb 7, 2010 19:54:54 GMT 10
I just had a look at my stock of blue foam. This what it has printed on it..... "Dow styrofoam SMTG"......."Styrofoam 03030.9 N".
It cuts easily so it is probably not the same as your high density stuff.
I fail to see how you can adjust the tension of your wire once you have switched on the power. I like to tune my hot wire like a guitar string after I have adjusted its temperature. That is why I like the clothes line and stick design.
How do you support the wooden spacer on your bow when you are cutting? Does it just drag across the bench on a piece of plastic? Perhaps if it was supported by a skyhook the cut might be smoother?
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Post by jirvin4505 on Feb 7, 2010 21:16:52 GMT 10
Avb. I feel for your frustration!! Reminds me of trying to learn how to make molds - 4 goes at getting the gel coat on Or trying to learn spraypainting into waxed molds - still getting fisheyes!! Or then there eas learning vac bagging - still have the panels in the shed, use them as ballast! Or was learning to do wet seam joins - practiced everynightvat work for a month to learn that one ;( Without seeing what is happening. He goes. The only time I have had ripples was cutting 25psi foam (same as graemes smtg) many years ago when I incorrectly used soft stainless fishing trace. The wire kept loosing tension when it got warm I cannot see any problems in your setup- it looks identical to mine. Yours is a little neater the initial wire drag on the rippled core suggests to much weight or not enough temp! Are you getting angel hair on the foam cuts? I see angel hair as good sign that everything is ok I am off for the next 3 days. Tomorrow I am going to cg composites on way home after nights do you want me to drop in on way past for a consult? I know you are a busy man cheers jeff
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Post by thevon on Feb 7, 2010 22:19:07 GMT 10
Success at last I feel. I have managed to cut a full set of cores with 60cm panels and 15cm tapered tips, for the "SC1". One slightly ripply side but not bad.
The .025 wire with really solid tension on the 3/8" legs seems much more consistent. It will cut at around 8 volts whereas the thin wire needs over 12V. That's why I burnt notches into the templates the first time I tried it ... I left it on the same voltage as the thin wire, and that was way too hot. Once I'd worked that out I got good cuts with the .025.
Also been concentrating on keeping the cut as slow as possible.
Graeme, there's a guide wheel under the wooden brace. You have to angle it to aim the bow so it doesn't run into the panel. Jeff/ Sean etc, you might notice I have a piece of thin ply whiteboard for the wheel to run on. It's on a slope so it gives the bow a bit of extra help. The tricky bit was that I worked out that while it's cutting slowly you can twist/warp the ply to guide the wheel one way or the other. Works well.
Jeff, I'd love you to drop around. But don't go to any trouble as I'm getting there. Probably won't be finishing field work till 5:30 most days this week (then back to office work aarrgh).
Sometimes I get the hairy fuzz and yes it does seem to be the good cuts. The ripply ones are never fuzzy.
I made a small light bow for the highly tapered tips and did them by hand. Seems to work OK. But I suspect you guys wouldn't be happy with my standard of OK!
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Post by graememarion on Feb 8, 2010 1:17:36 GMT 10
This is how I support the fixed end of the bow for cutting deltas etc..
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