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cod
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Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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 Posted: Wed Nov 15, 2006 2:03 am Post subject: |
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all my searching on the web suggests 12AWG is rated at 15 amps. Maybe I'm just worrying too much and need to wire this up just like in the TJ book.
| Quote: |
12-AWG = 20 amps
10-AWG = 30 amps
08-AWG = 40 amps
06-AWG = 50 amps |
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drcrash
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Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Wed Nov 15, 2006 11:29 am Post subject: |
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I poked around looking for more info on dimmers as heat controllers. I haven't found anything definitive or authoritative, but I have found a few knowledgeable-sounding people saying that they will work, so long as they're rated for the watts.
So likely that one thing I read saying they wouldn't was by somebody who was misinformed/confused.
Dimmers are problematic for controlling electric motors, because they do weird things to the shape of the AC waveform. (They shut off the AC power for part of each AC cycle, reducing the average voltage.)
They also may cause radio-frequency interference. (Because a clipped AC waveform is eqivalent to a bunch of sine waveforms at different frequencies added together.) Dimmers generally have features to reduce the radio emissions, but it seems to me that they might not work work as well when you have a big long nichrome coil acting as an antenna, rather than a few tiny incandescent filaments.
BTW the timed on-off gadget I was talking about is apparently called a "percentage timer," and from just a little poking around so far, they don't seem to be particularly cheap. But then, high-wattage dimmers don't seem to be, either.
Also, I'm wondering about using infinite switches with bare nichrome coils. They may not switch as often as we'd like, and may rely on the fact that a calrod stove burner will stay pretty hot for a while when the current is turned off, and heat up slowly when it's turned on. (Smoothing out the heat pulses.) I suspect they'd be fine, though. Up to a point, the plastic itself will do the same thing. Beyond that, though, you might overheat the surface during the "on" part of the cycle. |
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cod
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Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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| Posted: Wed Nov 15, 2006 12:36 pm Post subject: |
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good info, thanks
dimmers are mentioned a handdful of times in this forum, sometimes I think as a typo error. Seems like at least one member was successful in incorporating it into designs. I see the point about frequency. My research suggested that there are indeed ideal frequencies of IR for various materials. I wonder if a dimmer would allow any sort of control over freq, or just phasing? It theoretically might aid in 'tuning' to the plastic. This is all quite fun to contemplate in theory, prolly doesn't matter too much in the end. It melts evenly or it doesn't , right?
I'm thinking I'll give it a go: 2 1k dimmers, 2 1.x k circuits.
I'm also wondering if surging amps on and off with switches will be more expensive on my electric bill ... not sure... have to do some electric co. research too. |
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drcrash
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Joined: 04 Sep 2006
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Location: Austin, Texas
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| Posted: Wed Nov 15, 2006 1:43 pm Post subject: |
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| Quote: | | I wonder if a dimmer would allow any sort of control over freq, or just phasing? It theoretically might aid in 'tuning' to the plastic. This is all quite fun to contemplate in theory, prolly doesn't matter too much in the end. It melts evenly or it doesn't , right? |
I don't the the frequencies in the power supplied to the heating elements matter to how fast the plastic heats. The little pulses or spikes or whatever will get averaged out, in heating up the nichrome, by the thermal mass of even a little bit of metal. (And what doesn't get averaged out there would get averaged out at the plastic.)
Using a dimmer will only affect the frequencies of IR radiated by the nichrome by affecting the temperature of the coils, averaged over a much longer period. Hotter things radiate at higher (average) frequencies than cooler ones. (And both radiate a broad range of frequencies, with long tails around a hump centered at the "color temperature.")
The Thurston James design is pretty "efficient" in terms of how many watts it needs to heat how much plastic. It gets away with significantly fewer watts per square foot than Hobby Vac, which Doug Walsh brags uses a lot fewer watts / sq. ft. than most pro systems. (Which then turn around and turn most of those watts off most of the time.)
The Thurston James design may not really be extremely efficient in term of total power consumption---kilowatt hours per square foot of plastic heated. It takes a while to warm up, and may be relying on re-radiating heat that is absorbed during the preheat.
When you absorb IR, convert it to heat, and re-radiate it, the emitted IR is generally at lower frequencies than the IR that was absorbed. (The oven walls are much lower temperature than the nichrome---something like 400 degrees rather than 1500---so they radiate mostly at considerably lower frequencies.)
Doug Walsh claims that his designs radiate at frequencies that "typical plastics" are especially good at absorbing. (He designs the elements to run at about 1400 degrees, IIRC.)
I'm a little dubious about this. The only absorption spectrum I've seen for a particular plastic does have a high-absorption band somewhere around there, but also has a much broader absorption band a ways below that. More nichrome at lower heat may be better.
Most things actually absorb lower frequencies better than higher ones, on average.
Unfortunately, that includes aluminum. The lower the frequency of the IR, the more will be absorbed by the walls and re-radiated rather than being reflected. That makes somewhat it harder to reflect the IR where you want it to go with a reflector. But my impression is that this is a fairly small effect for our purposes, because aluminum's reflectivity only varies between very good and extremely good---a relatively shiny aluminum wall will still reflect the large majority of the IR.
So if you have reflective aluminum walls, most of the IR should get to the plastic, although it may bounce off, bounce off walls, and hit the plastic again until it's absorbed. As long as the aluminum is more reflective than the plastic, the plastic will absorb most of the IR after some number of bounces.
The important exception to this is for materials that are transparent to the IR frequencies you're using. In my experience, clear styrene and acrylic pass a fair fraction of the IR through. (Judging by how it heats up my bucks and gaskets in an over-and-under setup.)
This could be a problem, if your buck or platen can't take some heat.
It's also an opportunity. If you put an aluminum reflector under the plastic, you can redirect the transmitted IR back up to the plastic, and heat the plastic from both sides, to some extent. (I use a big cookie sheet for my little machine.) Cool.
Even better, it seems to me, is that you can heat the plastic in the middle, like a microwave oven. So "inefficient" frequencies of IR may actually be better. They may not be absorbed the first time they hit the plastic, but they will mostly be absorbed by the plastic eventually, and they give you a chance to heat from both sides and even in the middle.
Even the IR that is absorbed by the oven walls may not be a problem, if the oven is properly insulated. Once the oven is thoroughly preheated, and is in equilibrium, it will be re-radiating IR as fast as it absorbs it. (Without good insulation, it will be re-radiating and conducting a fair fraction of that heat to the outside, which really will degrade efficiency.)
That makes me think that either a reflective or an absorptive oven floor (or ceiling, in an over-and-under) will do pretty well. If it's absorptive, it will take a while to heat up, but once it does, it'll re-radiate about as much IR as a shiny one would reflect.
If you have straight walls, though, they should definitely be reflective. If they're reflective, they'll reflect the IR mostly to the plastic. (Since it's mostly coming from the other way.) If they're absorptive, they'll re-radiate equally in all directioins, which will mess up your heat distribution around the edges somewhat. |
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Vortilon
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Joined: 11 Nov 2006
Posts: 10
Location: Marana AZ
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| Posted: Fri Nov 17, 2006 9:06 am Post subject: |
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I used the keep it simple theory. Doug W who sold me the heater for my unit said he sells alot of the thermostatic controllers for the ovens because the customers want them however he said they are not necessary. He is right I have no controller on my machine and do not need one. I can control the rate the plastic heats by the distance it is from the cal rods. I drop the frame down an inch or two as it heats and watch the top of the plastic. If I suspect any Hygroscopic problems I let it preheat for a bit at about 2.5 ft from the oven. This might be wishful thinking on my part but so far I haven't had any problems. I have no microswitches, indicator lights, solenoid valves or anyother widgets to adjust or watch fail. I have formed a boat load of plastic in the last two months and have seen very little increase in my power bill. My cycle times are high enough so the oven gets turned off between pulls.
Rick
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drcrash
Guru
Joined: 04 Sep 2006
Posts: 705
Location: Austin, Texas
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| Posted: Fri Nov 17, 2006 11:53 am Post subject: temperature controls |
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| Quote: | | I used the keep it simple theory. Doug W who sold me the heater for my unit said he sells alot of the thermostatic controllers for the ovens because the customers want them however he said they are not necessary. He is right I have no controller on my machine and do not need one. I can control the rate the plastic heats by the distance it is from the cal rods. I drop the frame down an inch or two as it heats and watch the top of the plastic. |
I suspect that works fine for a lot of things, but wouldn't for some things I do. Walsh's strategy is to mainly regulate heat by varying the exposure time. The oven is a bit "too hot" for something, but as long as the plastic is thin enough, you can pull it out before it burns. The hot spots will be significantly cooled from the other side while the cooler spots more or less catch up.
That's pretty much how the straight Thurston James design works, too.
I'd expect this to be a problem if the oven is way too hot for a given plastic (such as vinyls), and the plastic is not very thin. For example, I'm playing around with vacuum forming 1/4" Sintra, which is a slightly foamed rigid vinyl. It's a pretty good insulator with a low thermoforming temperature range, so the top can easily burn before the heat makes its way through to the bottom. I also vacuum-form 1/4" EVA foam, and even when I try to heat it at the recommended temperature, the outside starts to smoke a bit before the bottom is warm enough to form. (Even when I heat it from both sides.) And the smoke is toxic...
If you regulate the heat by moving the plastic away from the elements, and you don't have shiny walls that extend down that far, I'd expect there to be evenness problems. (And if you do have shiny walls downn that far, it doesn't work---almost the same amount of IR still goes to the plastic.)
Do you actually use the whole footprint of your oven, and get good detail at the edges and corners? I didn't realize how bad the heat distribution for my little over-and-under was until I started really using the corners.
Another reason to use temperature controls is just for experimentation with a new oven design. Once you find the right settings, you may be able to just set it and forget it for most purposes, like Walsh.
(BTW, earlier this week I saw 1000-watt, 115-volt single-burner tabletop stoves on sale at a Walgreens drug store, for $10 with a $3 mail in rebate; $7 isn't bad just for the infinite switch. Three of those would work for a three-zone temp-controlled version of the TJ oven. To me, it's worth $21 for the ease of tuning the oven, and the flexibility for weird situations I'm prone to getting into.  ) |
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Vortilon
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Joined: 11 Nov 2006
Posts: 10
Location: Marana AZ
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| Posted: Fri Nov 17, 2006 9:59 pm Post subject: Re: temperature controls |
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| Quote: | | I'd expect this to be a problem if the oven is way too hot for a given plastic (such as vinyls), and the plastic is not very thin. For example, I'm playing around with vacuum forming 1/4" Sintra, which is a slightly foamed rigid vinyl. It's a pretty good insulator with a low thermoforming temperature range, so the top can easily burn before the heat makes its way through to the bottom. I also vacuum-form 1/4" EVA foam, and even when I try to heat it at the recommended temperature, the outside starts to smoke a bit before the bottom is warm enough to form. (Even when I heat it from both sides.) And the smoke is toxic. |
I would suspect you are right on the money with some of these exotic foam backed vinyls and EVA foam. And no I have not used the full foot print of my machines table in fact it is not even drilled to the edge. This is a mod I plan to perform at a later date. The plastic does heat very evenly to the edges. My machine works well for the parts I am currently making.
I would love to hear more about the Vinyl that you have formed. I have dreams of making door panels for my Alpine with an arm rest. I think one sided heating is maybe not possible for some of these materials. I read an article about Ford's vacuum form plant making dash boards for the Town Car and they develope a terrible scrap rate. The solution came with a complex heat management system that was installed on their existing machines. Unfortunatly we don't have millions of dollars to bring in outside gurus to solve our problems.
Was the vinyl of the type you see in automotive interiors? Kinda spongy after forming? Can you post some pics of the stuff?
I am way over my head here but that's what makes it fun.
Rick
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drcrash
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Joined: 04 Sep 2006
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Location: Austin, Texas
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| Posted: Sat Nov 18, 2006 10:31 am Post subject: |
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| Quote: | I would love to hear more about the Vinyl that you have formed. I have dreams of making door panels for my Alpine with an arm rest. [...]
Was the vinyl of the type you see in automotive interiors? Kinda spongy after forming? Can you post some pics of the stuff? |
OK, I posted some discussion, with pictures, starting a new thread:
http://www.tk560.com/phpBB2/viewtopic.php?t=441
I'm not vacuum forming laminated vinyl-with-foam; I'm just vacuum forming vinyl sheet foams at the this point. And I want to try laminating them by vacuum forming one layer over another, with a little spray adhesive applied in between. That seems easier, but I could be entirely wrong about that.
I'm thinking about trying to vacuum form soft foam over dense stiff foam, and/or putting a tough non-foam over a rubbery foam. That seems like it would be good for things like auto interiors, paintball helmets and body armor, etc.
I'm curious why Ford had so much trouble. This doesn't seem that difficult.
One potential problem is warping, if a new layer shrinks after being wrapped around an old layer. For some things, you could use a rigid back panel or a few struts inside to resist warping, and a fairly strong, dense foam.
(I'd think you'd want that for things like dashboards and armrests, anyway---you'd want a strong dense rigid foam that crushes under major impact, and a thinnish rubbery foam over that for minor bangs, and something thin and tough over that for protection and appearance.)
Another way to do it would be to vacuum form a shell and fill it with two-part (urethane?) foam, or maybe roughly-fitting hunks of dense foam filled in with low-expansion spray foam from the hardware store.
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Mattax
Expert
Joined: 09 Mar 2006
Posts: 160
Location: Virginia
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| Posted: Mon Jan 15, 2007 10:05 pm Post subject: Re: oven wiring |
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| cod wrote: |
Also, wah are people's power bills like, working with these ovens ? I'm in the most expensive area in the country, and in an industrial space. I want my partner to know waht he's getting into with allowing me to use the shop for this kind of draw. |
A lot of power companies meter reading works from reading the highest leg draw of amps. Standard homes are 220volts - 2 legs - 100volts per leg.
Therefore, you could potentially run up a high power bill by running all of your high amp circuits off one leg of your panel. That is why running 220volt oven is cheaper than 110volts - it runs half the amps.
Also, you could balance your panel in your home by checking the amp draw on all of your breakers at the highest energy draw time of the day. Write down which 110 volt breaker is on which leg and then do a check on each one (no need to check the 220volt as it operates off both legs). Then figure out how much draw you have by adding up each leg, then switch breakers to the opposite leg to balance out the draw. You will see a noticable difference in your electric bill.
Safety reminder/disclaimer: If you are not familiar with electric panels and electricity in general, I recommend hiring an electrician to do this for you. I do not want to hear of anyone getting electrocuted. So be safe!
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Mattax
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cod
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Location: San Francisco
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| Posted: Wed Jan 17, 2007 1:39 am Post subject: power bill, thermostats et al |
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I made two 10 amp circuits. I can plug into two legs. So plugging into two legs off the main panel will make my power bill smaller ? Especially at midnight?
so would 220v still be cheaper? I guess YRMV with local power infrastructure. Im in a commercial space. I'm subletting from a friend, so I want to make sure I work efficiently (and cheaply!)
I decided to skip all the complex circuitry for now, and wire directly to 2 edison plugs(standard 13 amp extension cords cut in half). I can pull the plug
(local quad box with GFCI oultets) on the inner or outer ring separately -hopefully simple and safe. I've got a tighter outer circuit, with 1 inch between coils- the inner circuit has 2" between coils.
so it goes
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Mattax
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| Posted: Thu Jan 18, 2007 1:35 am Post subject: Re: power bill, thermostats et al |
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| cod wrote: | | I made two 10 amp circuits. I can plug into two legs. So plugging into two legs off the main panel will make my power bill smaller ? Especially at midnight? |
I did say a lot of power companies meters reads this way - not all. 
But for plugging into two legs, make sure that each breaker is on a seperate leg and then, yes, the draw would be the same as far as the meter is concerned.
But again 220v draws less amps. I=E/R I know someone posted about the laws. Link please?
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TK487
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Stomper
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Joined: 01 Oct 2005
Posts: 473
Location: Woodburn, Oregon USA
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| Posted: Thu Jan 18, 2007 8:52 am Post subject: |
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I posted a thread with an ohm's law wheel a while ago, to help make electrical calculations.
here it is:
http://www.tk560.com/phpBB2/viewtopic.php?t=112
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