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Jan 23, 2007, 10:03 PM
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arx_n_sparx's Avatar
Brain fart on my part - the 2 diode rectifier also is referenced to 0. Either way, they are not seeing (in our case) 70 volts, only 35 or so. There is something screwy there. Got a link to what you're looking at?

Brad
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Jan 23, 2007, 11:12 PM
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drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
There is something screwy there. Got a link to what you're looking at?
Brad
http://www.electronics-tutorials.com...wer-supply.htm
Jan 24, 2007, 06:56 AM
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Purdie is wrong here. He's plugged in the wrong number - it's not peak to peak that we have to worry about. It *is* the right numberr for the ripple voltage.

Maybe he's just being overcautious? I dunno. The ratings are the ratings - you don't have to do fancy math to figure out what you need. As long as you don't draw more than 4A thru that bridge it will be fine - and I'll buy you a new one if you manage to smoke it (at less than 4A and using the 24V txfmr).

Brad
Jan 24, 2007, 10:17 AM
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Quote:
Originally Posted by L0keman
A lot of this stuff sounds dangerous if you're not careful. I would just drop a few bucks on a pre-built.

http://www.dickblick.com/zz608/01/

I know I saw a few other models that were bigger too a year or so ago when I was looking into getting one.
You can make your own machines, and better, for half that price. That $60 machine uses a tiny transformer and must use a very, very thin short wire. (I'm guessing 26 gauge nichrome.)

For the larger machines I've seen, you can build better for a quarter of the price.

This suite of tools, for example:
http://hotwirefoamfactory.com/produc...&cat=75&page=1

Nice, but for $300? Look at the power supply---it's a transformer on a dimmer---and not a very big transformer, I'm guessing, from the size and style of box. It should cost about $20, and all the other pieces should be cheaper.

If you're worried about dangers of wall-current AC wiring, just get a $12.50 router speed control from Harbor Freight and a $9 24V wall wart from Altex, and plug it in. There's no special danger there beyond the danger of having a very hot wire, and that's an irreducible risk of doing hot wire cutting. If you use thin wire, you can cut wings several feet long. (And you can turn the volts down to do a 5-inch scroll-saw-like thing, too.)

If you wire two wall warts in parallel to get more amps for thicker wire, you do have to do some wiring, but only on the 12-or 24-volt side, where there's no danger. If you screw up, you might feel an unpleasant tingle, but that's about it. (And if you're careful, you can avoid that completely.)

Don't let us going on and on about capacitors and rectifiers and PIVs and all that obscure the fact that you don't need the fancy stuff. It only gets you modest increase in wire length or wire thickness you can use.

Being able to vary the voltage is the one big win in functionality over just a transformer, and you can get that by plugging an off-the-shelf item into an off-the-shelf item. If you build something simple but fairly ampy, you'll be able to do a whole lot of stuff.

(Also, with respect to whether Ian Purdie's right that you need a higher-rated rectifier for a 24-volt system, and Brad betting me a rectifier that you don't, all we're talking about is the difference between a $2 rectifier and a $5 rectifier. Even if you do want all the marginally useful fancy stuff, but you're conservative, you can just overbuild a bit---spend $10-15 more on your rectifier and capacitors, and it's a total non-issue. That's actually what I did yesterday morning, because I didn't know any better. Brad just hates to see us waste even a few measly dollars, much less the hundreds it would take to buy equivalent tools off the shelf. :-) )
Jan 24, 2007, 07:05 PM
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Quote:
Originally Posted by drcrash
Brad just hates to see us waste even a few measly dollars, much less the hundreds it would take to buy equivalent tools off the shelf. :-) )
If you thought it was the local drunken bum who was dumpster diving for dinner, think again You'd be amazed at what you can find to eat in those big green bins.....

I have no money to waste - my wallet is more familiar with dust bunnies than dollars. The few shekels I can save every month do get spent on this hobby though.

Oh - I still say Purdie is wrong here
Jan 25, 2007, 01:03 AM
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By the way, the Radio Shack 276-1181 full wave bridge rectifier seems to be an 8 amp 200 volt part for a mere $2.59. (The catalog says 6 amps, but the blister pack I was looking at in the store said 8. I imagine they upgraded and haven't updated the catalog.)

Seems maybe worth the extra 60 cents, whether you need it or not. My understanding is that the easiest thing to fry in one of these setups is the rectifier.

(I got a way overkill $5 rectifier at Fry's, mainly because I liked the metal case & quick-disconnect contacts for experimenting.)
Jan 25, 2007, 02:10 AM
I found a 24V 1A AC Adapter from a local electronics surplus store for under $10. Works great with about 3ft of nichrome.
Jan 25, 2007, 07:54 AM
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Quote:
Originally Posted by ToJo
I found a 24V 1A AC Adapter from a local electronics surplus store for under $10. Works great with about 3ft of nichrome.
If you have a local Altex, you can get new 24 V 2A wall warts for about that. (And RS has non-warts for about that, but then you have to wire the AC side.)

What gauge nichrome are you using, and for what kind of foam? (I'm thinking it must be thin nichrome like 26 ga.)
Jan 25, 2007, 09:20 AM
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frankyfly's Avatar

Bill Evans foam core cutter...


Look post #52...

https://www.rcgroups.com/forums/show...=502443&page=4
Jan 25, 2007, 09:30 AM
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Quote:
Originally Posted by frankyfly
That's a whole bunch of stuff, mostly not about foam cutters. Is there something special you want to point out about the foam cutter? (Say, compared to BobK's with the weighted lever for cutting tapers?)

Paul
Jan 25, 2007, 09:36 AM
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Dr I was impressed by the brilliant idea of the foam cutting table.
Jan 25, 2007, 09:44 AM
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drcrash's Avatar
Quote:
Originally Posted by frankyfly
Dr I was impressed by the brilliant idea of the foam cutting table.
I think it may be historically significant, and very cool, but my initial impression is that BobK's design is better. Having the plywood sheet that flops outward is unnecessary and makes the setup bulky, and a lever in a vertical plane is all you need to get different pull rates across the wing for taper.
Jan 25, 2007, 10:05 AM
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drcrash's Avatar
Quote:
Originally Posted by arx_n_sparx
I have no money to waste - my wallet is more familiar with dust bunnies than dollars. The few shekels I can save every month do get spent on this hobby though.

Oh - I still say Purdie is wrong here
I believe you, I think; I scrounged up a couple of things that seem to agree with you and not him. (I tried signing up for his Q & A forum to ask about it, but it doesn't seem to have worked.)

But back to scrimping.

I'm trying to figure out if I can interpret a $4 Harbor Freight meter reasonably precisely when it's measuring the rectified DC amps from the $2-3 rectifier, without the nastily expensive capacitors. (Locally or including shipping for a small mail order, a few thousand uF of capacitance seems to cost over $10 to $20, especially if you want >35 V capacitors, significantly increasing the system price.)

The nice web page you sent me a link to (in a PM) says that the meter will read the peak values of a rectified sinewave. I knew that it would read closer to to the peak values than average or RMS, but now I'm thinking it's probably standard behavior for meters to read the actual peak value, not somewhere in between.

If so, that's great. You can just multiply the amp reading by some appropriate conversion factor to get the RMS, and you're done. Not only can you use the bogusly high numbers for finding a "good" amp level for different length wires and sticking with it, you can convert and figure out the true number, to see if you're running your transformer too hot.

Does that sound right? If so, I'd think that for most people just doing foam cutting, the rectifier and meter are pretty handy---maybe worth the $7---but the capacitors are only worth it if you need the extra voltage for extra length or wire diameter. (And for most people, it wouldn't be worth getting that fancy. 24 volts is fine for pretty big wings and reasonable-sized wires.)
Jan 25, 2007, 10:28 AM
I got tired of trying to cut by my self and have a CNC in the works.
But still want to be able to make quick cuts. I have a 4 ft. bow and
thinking I would like to hang it with ropes & pullys and counter balance weight. I really like the idea of the bow under the table. I could put 2 dowels to hold on to on the ends to have a bit of control. Anything would be better than trying to cut a tapered wing with a dumb hand. Ideas anyone ? WOW ! Big snowflakes.

Ken
Jan 25, 2007, 11:11 AM
Impact Testing Specialist
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Quote:
Originally Posted by nubee 1
I have a 4 ft. bow and
thinking I would like to hang it with ropes & pullys and counter balance weight. I really like the idea of the bow under the table. I could put 2 dowels to hold on to on the ends to have a bit of control. Anything would be better than trying to cut a tapered wing with a dumb hand. Ideas anyone ? WOW ! Big snowflakes.
Ken
I'd think you could combine a lever-with-a-drop-weight like BobK's with a bow under the table (like in the article), if you have a table with no supports in the way in the middle.

You might want a C-bow rather than an H bow, so that it doesn't go down so far under the table. (I'm pretty sure that I'll never have a big table that doesn't have bunch of stuff stored on the floor under it.)

I'd think that arrangement would work with either a moving bow or moving foam, or with the bow moving in one axis and the foam moving in another plane. (Or vice versa.) Lots of possibilities there.

BTW, has anybody tried this sort of thing with a synchronous motor pull, to get the speed very even without relying on dragging the wire in the foam?

Then you could have the wire hot enough to vaporize the foam without touching it, and while your kerf would be bigger, it might be pretty uniform, so no problem if you compensate in your template outlines.

It wouldn't be CNC, but for relatively simple cores, it might work about as well.


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