www.TK560.com

[knightshade]

I finally got around to wiring the oven with the NiChrome wire today.

I noticed on your site you had ohms readings:

[jegner]

The physics of the oven is not my bag of tricks, but as far as going by my readings and assuming that is gospel, I would not. Its a best guess based on a cheap meter, and I'm not sure I read it correctly. The book claims 4 parallel oven circuits, each will draw 5.2A @ 115V and have a power rating of 600W each, for a total oven wattage of 2400 W, drawing 20.8amps.

So if you are registering 11 ohms per circuit, then either you wire is too short, or something off by about 1/2. Double check your wire length, I bet yours is too short, hence the strectch wires and the too high amp reading.

I'm no electrician, so don't take my word for it. I get ohms and amps read backwards all the time!

[knightshade]

Thank you so much for posting the information out of the book.
That lets me do some of my own math.

[knightshade]

Rewired the oven earlier this week. Found some other issues and solutions - so I thought that I would share.

Each of the 4 segments had ~29.5 Ohms of resistance.
When connected to the wiring - I showed 7.4 Ohms across the entire oven circuit - so I should have been drawing somewhere under 17 Amps for the entire oven.

Initially I had it on a dedicated 20Amp circuit - and it would trip the breaker as soon as I turned it on.

As I had wired that circuit with 10 guage wire - I knew that I could safely draw 30 amps over it, so I changed the breaker out for a 30 Amp breaker. Unfortunately no change. As soon as I turn on the oven the breaker would trip.

My guess was that the change from 0 to 16 Amps all at once was what was causing the breakers to trip.

I went to the store to find a dimmer switch. To my dismay - I couldn't find one that would support 2400 Watts. (The biggest was 1000 Watts - and that was $40.) I instead decided to go with a cheaper (but safe) alternative.

Those of us who have looked at the plans for the oven recall that there are 5 electrical wiring points. Points B and D get connected to Neutral (white). Points A, C & E are where we connect Hot (Black).

If one assumes 5.2 Amps (600Watts) per oven segment - the wires for points A and E each draw 5.2 Amps, where the wire for point C draws 10.4 Amps (1200 Watts). I connected Point C to a simple on/off switch. Points A and E were each wired to their own 600Watt Dimmer Switch.

When I turn on the oven - I can turn on "Switch C" without tripping a breaker. From there I can turn on and dial up "Switch A" and "Switch E" and I do not trip the breaker (currently a 30 Amp.)

I strongly suspect that I could have used simple on/off switches for A and E - but I have not yet tested that. I also suspect that I could have continued to use a 20Amp breaker - and will be testing that tomorrow .

Another alternative that I had been considering was wiring it as a 20Amp 240volt (Red to point C, Black to points A & E and White to points B & D).

I know that Jim said that he has to run his oven off of a generator due to the draw. I assume that other's successfully wired the oven. Am I the only one that had issues with tripping breakers?

[jegner]

Great info. Anyone else have a breaker problem? I occassionally trip my 30Amp breaker on the generator, but I relate that to heat and poor ventelation in my generator room.

[crashmann]

Hi Everyone!

This is my first post to the board (and it's a whopper!) This topic held me up the most when building the table. I took an introduction to electronics class a very long time ago in college, and I used Ohm's law to calculate the amount of resistance in the heater coil (measured in ohms) to get as close as possible to 20 amps (maximum power draw resulting in maximum heat output) without tripping my breaker.

However, we're dealing with AC power, so the calculations don't quite line up...

20 amps = 120 volts AC / 6 Ohms

This would be an ideal setup.

My table is a little larger, designed for sheets 24" x 32"

The oven base is 28" by 36" I divided the heating circuit into 5 segments (one more than James Thurstons' design). However, the calculations should be very similar to a 4 segment coil.


Resistors in parallel, like what we have in the oven actually reduce the amount of resistance in a circuit - you're providing more channels to allow the energy to flow. That's why if you have 5 segments with 30 ohms of resistance each, the final resistance comes out to 6 ohms (30 ohms / 5 equal resistors = 6 ohms).

I consulted a couple of Electrical Engineering friends, and they agreed with my calculations. The worst that should happen is the breaker trips - there shouldn't be a massive explosion or fires :-p)

I stretched out the nichrome wire so when I connected the ohm-meter it read 30 ohms for one segment. This was roughly 9.5" of tightly coiled wire per segment. I continued around the oven ensuring I got 30 ohms for each segment ( A to B, B to C, C to D, D to E, and E to F). With all of the terminals connected (and the power cord UNPLUGGED) I measured the resistance from A to F. It was 6 ohms just like it was supposed to be!


I plugged into a 15 amp outlet, then used a chicken stick (12" wooden 2x4) and flipped the switch. You could smell the coil heating up for the first time. There were faint heat ripples in the air above the coils. I hovered my hand a few inches above (I don't have an infra-red thermometer) and it was warm, but not 300 degrees warm. It ran for about 5 minutes without tripping the breaker. Interesting - I would have figured my 20 amp calculation would have tripped the 15 amp breaker.

I was disappointed with the heat output, so I decided to reduce the resistance. I stretched the coils a little farther, this time at 25 ohms per segment (8 inches of tightly coiled nichrome). Total resistance = 5 ohms. Current = 120 volts AC / 5 ohms = 24 amps (calculated)

Again, using the chicken stick I energized the oven. It ran significantly hotter for 3 minutes, then tripped the 15 amp breaker. Excellent! I moved it to the 20 amp breaker, and it ran for 20 minutes without tripping. Plenty of heat output, and if you turn off the lights, you can barely see the coils glowing.


I tried reducing the resistance again, going down to 23 ohms per segment, for a total of 4.7 ohms. It ran for 6 minutes then tripped the 20 amp breaker. Just like that Toyota commercial, I heard a voice whispering "Too far..." Unfortunately, once you cut the wire, there's no splicing it back together

At this point, my electrical engineer buddy loaned me his amp probe, and it showed I was drawing 23 amps of current (I calculated 26.6 amps). I needed to fall back to 5 ohms.


I wanted to keep the nichrome as a continuous strand from A to F. I measured out roughly 40" (5 segments x 8" per segment) of tightly coiled wire, then stretched it out.

I thread the wire through the ceramic donuts, and measured 134 ohms of resistance from A to F. I mounted the wiring in the oven base and connected the terminals. Final check of the segments in parallel read 5.2 ohms. I energized the oven and the amp probe read 20.5 - a little beyond 20 amps, but not enough to trip the breaker (using the formula, it should be 23 amps, but AC power lowers the effective output). It ran for 20 minutes and did not trip the breaker - success!!!


To adapt my calculations to a 4 segment 20 amp oven, here's what you need:
Each segment of tightly coiled nichrome wire should be 6 3/4" and read 20.8 ohms
You'll need 4 segments, or 27" of continuous tightly coiled wire with a total resistance of 83.2 ohms
As you connect the wire to the oven terminals, make sure each segment is stretched evenly
After everything is connected, measure A to E - You should read 5.2 ohms of resistance

Plug in the oven and use a chicken stick to flip the switch.
Use a watch to see that it stays running for more than 15 minutes

If successful, build the rest of the oven, and you're done!
If the breaker pops, you'll need to try again, using a slightly longer coil of nichrome.

Knightshade, I'm not sure why your circuit breaker was tripping with your oven. At 7.4 ohms resistance, the current calculation would be 16.2 amps (and since AC power results in lower output, it should be a little less). It would have been pushing the limit of a 15amp breaker, but well within tolerance of a 20 amp. A 30 should have barely noticed the load! How far from the panel is your outlet? The length of wire will also add miniscule resistance to the circuit.

Another option I had considered was to split the coil across two separate circuit breakers. Then there wouldn't be as much current tripping the breaker. But it sounds like your dimmer switches are doing the trick. Plus, you can moderate the temperature with them. Very cool!

Ultimately, I just started conservative, then stretched the wire a little further and further until the breaker tripped. Then I backed off and it's been running great!

Disclaimer - The above details my experience wiring up and fine tuning my oven. I can not be held responsible for what happens with your own setup. If you have any doubt, please contact a professional electrician! There's a lot of current flowing, and it will kill you if you are not careful! NEVER touch the coils or terminals when the AC cord is plugged in!

I hope this helps you, and please send me a message if you have any questions.

Charlie
TI-386

[jegner]

WOW! Great info! That vac machne looks great! Thanks for sharing your pics, and wiring info. Have you pulled anything yet? Also, wondering why the larger 24x36 size?

Again, great job!

Jim

[knightshade]

[crashmann]

Hi Jim,

I designed the table to be a little larger to accomodate the longer pieces - chest plate, thighs, shins, etc... With the extra area of the plastic, I figured it would droop lower, so I made the oven sides 6 inches tall instead of 4".


Yes, there's a little more wasted plastic when you don't have a platen full of bucks, but you don't have to worry about the plastic stretching too thin. Also, with the dimensions 24"x32" your plastic supplier can cut 6 pieces from a single 4'x8' sheet (with no waste).

Check out the wiring plan I came up with. I couldn't get the wire nuts to the point where I felt comfortable, so I used 2" stainless steel screws for the terminals, then wrapped the 10 gague wiring around the post. I put a star lockwasher in between each wire to ensure it didn't work it's way loose from heating and cooling:


Then I just ran the wiring direct from A to C and E, and also from D to B and F. Be sure to have one extra to go to the power switch:


We've been working on Clone Armor, and my friend Mike, TK-1536 has been doing an awesome job sculpting parts out of 4 pound insulation foam (similar to the hard green foam at the bottom of a fake flower vase). Then we pull .060 HIPS to make the master molds. Remove the foam, pour in Hydrostone plaster and let it cure. Then we extract the bucks and pull .125 HIPS to make the final parts. They've been turning out great!


You can see the ridge from the MDF riser under the forearm parts. That's been a major help for extracting the bucks!

Pile o' parts!


Check out Mike's website for finished pieces.

And a video from last week's armor party! (requires the latest version of QuickTime)
(Direct link to the video file)

Knightshade, I think my wiring run is about 50' from the panel to the outlet. Then I've got a 20' 12 gauge extension cord wired to the power switch of the oven. I would think 60' to 75' should not be a problem. You'll just need to tune it until you're below the breaker threshold

I'm going to try pulling some .060 clear Lexan for a special project, and the plastic supplier claimed the Lexan should be heated at 200 degrees. Your dimmer switches would be perfect for this type of forming. Then you can control the oven temperature. I'll probably just leave a gap between the oven and the clamping frame and see how it goes...

Good luck!

Charlie
TI-386

[knightshade]

Made my first attempts at pulling today.

A few parts are useable - but it was more of a learning exercise.

I think that my oven is a little on the cool side. It takes over 20 minutes to heat each sheet of plastic - and at that I needed to give it a helping hand with a paint stripping gun.

I need to find a better way to hold the plastic in place in the holding frame. I had it slide out of one edge of the frame for each of the attempts. One time managed to maintain enough of a vacuum - but the other two were complete losses

But learning was the important thing - and now I know a few things that I need to change.

[jegner]

Amazing stuff! Great idea on the high denseity foam. I never thougth of that. maybe I need to use it for my helmet molds. Great job.

As for the low heat on the oven, you should only have to heat the plastic for 4 minutes. So I's say shorten the nichrome wire.

Good stuff!

Jim

[crashmann]

Yup, I'd agree with Jim that you should try to shorten the nichrome wire. Also, be sure to put the Hardibacker lid on top of the plastic clamping frame when heating.

The tough thing with shortening the wire, is you only want to take off one or two inches at a time. Then you have to do it for each segment - tedious! Make sure each segment has the same resistance with your voltmeter.

We've been getting the foam from http://www.americanfoamgroup.com Go to the sculpting foam, 4 pound density. The stuff is expensive! If you're buying one 12x12 block, you should really consider buying two - do the math on the price break...

Good luck!

Charlie
TI-386

[knightshade]

Work (i.e. the type that pays) has been keeping me busy - but I'm planning on spending tomorrow night re-wiring the oven. Like you said - not difficult, but it will take some time.

[jegner]

I know what you mean regarding 'real work'. I have 2 jobs my self.

[knightshade]

Just one job here... but it has been keeping me busy 55-70 hours a week for the past 2 months. Thankfully that project is almost over.

I rewired the oven today. 24 Ohms per segment. 6.1 Ohms across the entire oven when fully wired. Ran it for a bit without tripping any breakers - and it definatly feels hotter.

crashmann - I just noticed what you used for standoffs for most of your posts.
That looks like it would have been a lot easier to use with how I needed to restrech my wire

I also started to work on improving my (wooden) holding frame. Mine wasn't holding the sheets of plastic tight enough. I've gotten some bracketing to re-enforce it better - and am adding some clamps to the sides. Hopefully version 2.0 works a bit better. I plan to get a few test pulls in tomorrow night.