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Hobby-Vac -- Construction Plans

 
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miranda_vey



Joined: 01 Oct 2006
Posts: 1
Location: Pittsburgh, PA

PostPosted: Thu Oct 05, 2006 1:03 pm    Post subject: Hobby-Vac -- Construction Plans Reply with quote

Hi boys,

I'm new here and have been reading and working on my own system. I am doing well with the vacuum table but still researching heating options. While reading heating options, I discovered this site and just had to share it with everyone.

http://www.build-stuff.com/1001plans.htm

Enjoy

Miranda
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jegner
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PostPosted: Thu Oct 05, 2006 1:31 pm    Post subject: Reply with quote

Glad to see you have made it here, Miranda. When you get some time I'm sure everyone here would love to see what you are working on.

Jim
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drcrash
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PostPosted: Thu Oct 05, 2006 3:31 pm    Post subject: can we dispense with ceramic insulators? Reply with quote

Miranda,

I don't think you could go far wrong with Doug Walsh's plans, if they fit what you're trying to do. My impression is that they're well thought out.

The Thurston James design that most people here use is similar to the Hobby-Vac oven. That design can be adapted to different shapes and sizes.

Both designs use a nichrome wire coil in a rectangular spiral, with closer spacing around the edges.

In the Thurston James design, the oven is wider at the bottom than at the top, which allows an extra row of nichrome coil past the edge of the plastic, to help heat the edges.

Walsh's Hobby Vac design seems to use a row of nichrome coil close to a reflective vertical wall. (Aluminum, I presume.) This will ensure that not only that row of coil but its "mirror image" through the reflective wall will heat the edge of the plastic. Putting one row close to the reflector approximates having two tightly-spaced rows, one just inside the actual wall, and one apparently just outside it. (It likewise mirrors the other rows of coil, further in from the wall, making virtual coils further out.)

Looking at the page you linked to, it sounds like the Hobby Vac has the nichrome coil mounted directly on the ceramic fiberboard, without having ceramic spacers/insulators in between. (This is workable because they coils don't actually draw a lot of current or get extremely hot; as heating elements go, they're not very hot.)

That's interesting, because the ceramic spacers are the biggest cost of the Thurston James-style oven; they cost considerably more than the actual heating element or the structural parts of the oven.

The nichrome coils in the Thurston James design are presumably not even as hot as the ones in the Hobby-Vac---they dissipate only about 40% more watts, over lots more area and inches of coil.

Maybe the Thurston James design could dispense with the ceramic insulators, too.

That's a tricky question, though, if you use inexpensive cement board for the oven floor, as Jim and others here do, rather than more expensive ceramic fiberboard.

(That makes me wonder whether really thin high-temperature board, like in a toaster, might be the way to go... just mount the nichrome directly to that, and then bolt it to the oven bottom on a few standoffs. The outer oven bottom could probably be made of wood, with cheap mineral wool insulation in between; the thin high-temperature board would act mostly electrical insulator and something strong enough to hang the nichrome on. It may be worth figuring out exactly what the thin stuff in toasters actually is, and whether sheets of it it can be cheaper than a bunch of individual ceramic insulators.)
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drcrash
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PostPosted: Thu Oct 05, 2006 5:28 pm    Post subject: what are the ceramic insulators for? Reply with quote

The stuff in toasters is mica. (They're still using it.)

I don't know if it's the right stuff, though.

But I have a more basic question. In Jim's oven, the coils are not held up off the hardibacker (cement board) by the insulators, right? They actually touch the hardibacker board, if I'm not mistaken.

So I guess the hardibacker can take being in direct contact with the hot coils, and the ceramic insulators are mostly just to keep from electrifying the bolts that hold them to the hardibacker board.

Is that right?

If that's true, then it seems to me that you could just use eyebolts or something to hold the coils in place on the board. You'd go ahead and electrify the bolts by having the nichrome touch them directly, but keep them from touching anything else and being a problem. All that would take would be a layer of insulator across the back of the board---maybe another layer of hardibacker, or just an air gap between that layer and some outer casing.

That could cut the cost of the oven substantially. (Or maybe I've missed something important.)
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crashmann
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PostPosted: Thu Oct 05, 2006 7:36 pm    Post subject: Reply with quote

You could just mount the nichrome wire directly to the Hardibacker board. The cement board is not electrically conductive. It is used around fireplaces to provide heat shielding, so the heat radiated from the nichrome wire would not affect the board either.

The only danger of using metal eye bolts to mount the wire is that you have live contacts poking through the bottom of the board. The James Thurston design also has live contacts, but only 5 of them. With eye bolts, you're looking at 50 potential shocking points! If you cover the bottom of the board with another layer of Hardibacker or wood, then you reduce the danger of being shocked if someone reaches under the oven.

By using the ceramic "hats" (as I like to call them) the mounting bolts are separated from the wire, and do not conduct electricity, except at the terminal points.

I found the ceramic donuts were a better option because you don't need to use bolts. Just wrap a 6" length of baling wire around the donut, drill holes through the oven base, and mount up! With the donuts, you can adjust the nichrome coil after installation (stretching it further to reduce resistance, and increase the heat).

Or, if you want to be super economical, just mount the nichrome wire with the baling wire directly to the board. Then mount a board underneath to avoid "shocking the monkey" Razz

Charlie
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Mattax
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PostPosted: Fri Oct 06, 2006 12:43 am    Post subject: Reply with quote

I highly suggest on your exit point for the electrical hook up to mount an electrical box. 4x4 inch box with a cover plate will be fine. You can purchase insulated spacers to get the wire through. Also, you can purchase heat shielded/insulated wire for the exit points - also highly recommended.
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drcrash
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PostPosted: Fri Oct 06, 2006 6:04 am    Post subject: mounting nichrome directly to cement board Reply with quote

Charlie says:
Quote:
Or, if you want to be super economical, just mount the nichrome wire with the baling wire directly to the board. Then mount a board underneath to avoid "shocking the monkey"


Yes, I think that'd be the way to go, if there's no heat problem at the attachment points. There's no point in using 60 individual insulators when one big insulator is cheaper and maybe easier.

After all, the other side of the oven is a bigger danger. It's open, and if somebody happened to stumble and put a hand in it, they could touch the electrified coils directly. It should have a grill of hardware cloth or something over the coils. (Also helpful for reducing the mess of a meltdown, of course---it's much easier to replace a piece of hardware cloth than all that carefully-routed coil.)

If you did use the baling wire attachment system, what would be the best way to do it? The obvious thing would be to drill two holes per attachment point, run the wire up through one, around the nichrome, and down through the other, and twisting it together on the back side. But you wouldn't want to double the number of holes you have to drill; 60 is plenty.

Using one hole, you could either twist the wire onto something cheap that won't slip through the hole, like a little washer, or some kind of (insulating) rod that you could twist several attachments around. I suppose that if the wire is pretty stiff, you could just splay it out and that would be secure enough, if nobody tugs on the nichrome or baling wire from the other side. But if the wire is that stiff, it's going to be more of a pain to twist it together.

BTW, I actually have the donuts and pass-through insulators sitting here in some junk heating elements from an electric central heating system. But I'd rather save them for something where they're really needed, like mounting coils on an aluminum-lined oven, where you need the electrical insulation. And I figure that a lot of people would like to save $50 on their ovens.

I was impressed that Doug Walsh can sell his heating element for the Hobby Vac for only $65, presumably including some profit. Maybe what he's calling "ceramic fiberboard" is cheap cement board, too... and if not, where does he get "real" high-heat-rated ceramic fiberboard cheap enough?
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jegner
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PostPosted: Fri Oct 06, 2006 6:19 am    Post subject: Reply with quote

Doug Walsh's design is very nice, the Proto-form is a version I looked at a long time, but spending $150 for plans, was not in my budget.
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drcrash
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PostPosted: Fri Oct 06, 2006 8:32 am    Post subject: DIY design: conventional (IR) oven, low-volume platen, etc Reply with quote

Jim says:

Quote:
Doug Walsh's design is very nice, the Proto-form is a version I looked at a long time, but spending $150 for plans, was not in my budget.


Mine either.

I don't think that Proto-form is really aimed at hobbyists; it's a bit too expensive a system, and is more appropriate for people who are making industrial prototypes (hence the name), or people doing modest-volume production runs like Harley Harley guy. (For high-volume production it's cheaper to farm it out to a specialized industrial shop that can make a zillion parts in no time for serious money but a small cost per part.)

My impression is that there's nothing special in the Proto-form design that you can't get from other sources. It's just some good solid engineering using principles that are well-known among People Who Know These Things, put together in a nice package for people who don't. (Which is a very fine thing, and Walsh seems to have done a fine job. I'm just too cheap, and too inclined to do my own research.)

For example, the oven appears to be a pretty standard configuration---evenly placed calrod bars about 4" apart, about 7" away from the plastic, plus a tweak or two to put heat around the edges, in a rectilinear metal box. Simple and effective---and easy to design for yourself if you understand a few simple principles.

You don't need a fancy reflector, if you don't mind the box being 2x or so as deep as the spacing between the elements. A simple rectilinear reflective box works about as well as anything possibly could.

In the case of Vortilon's oven, the turned-up ends of the calrod bars produce extra heat there along the long edges of the plastic, and the first and last bars (along the short edges) are placed closer to the plastic.

This general kind of thing is very common. There's a general rule that for evenly-spaced heating elements in a plane, the plastic should be at least 1.5x or 2x as far from the heating elements as the elements are from each other. (Ideally 2x or more.) This gives the IR from the heating elements a chance to spread out before it hits the plastic, and for the bands of IR to overlap where they start to weaken.

(So if you have nichrome coils on a two-inch spacing, the plastic should be at least 3 or 4 inches from the coils to avoid "striping" or "banding" problems. Same idea.)

Conversely, if you want to heat unevenly, say to put extra heat around the edges, you can either space the heating elements closer to each other there, or mount them closer to the plastic. Apparently Walsh does one thing on the sides of his Proto Form oven, and the other on the ends. (Assuming Vortilon's using them as they're meant to be used.)

For folks like us, nichrome is probably a better option than calrod. To save a few hundred dollars on heating elements, we'd rather drill a few dozen more holes, stretch some coil, put a cheap electrical insulator across the back whole thing, etc.

Nichrome should do about as good a job of heating plastic as calrod. They're both just IR emitters, and calrod bars are actually just nichrome coils, in a nicer package---embedded in a ceramic electrical insulator, and encased in a protective metal jacket.

The advantages of calrod are mainly that it's more durable and that there are fewer electrical issues installing it, because the outer jacket is not electrified.

If you can deal with the hassle of making sure fingers and things don't get into your nichrome coils, and they don't sag and touch metal, nichrome does just as good a job heating plastic.

Nichrome will wear out sooner---eventually, after decades, or after thousands of heating/cooling cycles. I don't think any of us here have to worry about that. Nichrome coil is plenty good, and should give equally good results for the kinds of low-volume things we do.

Likewise, my understanding is that Walsh's designs use some kind of low-volume ducting inside the platen, to minimize the air that must be sucked out of the platen itself. This too is common in industrial systems, and it's not hard to figure out. (I've seen discussions of it somewhere on the web, but don't have a link handy.)

Here's how:

0. Don't use a spacer in your sandwich like the Thurston James design.

1. Rout a grid of 1/8" x 1/8" grooves, an inch apart, on the bottom side of the top layer of MDF. Drill your holes into those grooves. This ensures that air from any hole can flow to anywhere on the board through a network of tiny ducts. (Actually, the grooves only need to be on 1-inch centers in one direction, to ensure that the holes you drill hit the grooves. Crossing the short dimension of the board, they can be further apart.)

2. Rout a few 1/4" x 1/4" grooves on the top of the bottom layer of MDF, radiating from wherever you put the hole to attach the vacuum, halfway to the corners and sides of the board. (The hole doesn't need to be in the middle of the board, and it's usually convenient if it's off-center. That allows you more freedom in placing things under the platen.) This ensures that there won't be a bottleneck where the flows from the small grooves combine around the vacuum attachment.

3. Glue it together, making sure it's sealed around the edges.

I've done roughly this for my 18" x 24" platen, and it's easy if you have a router. If you don't, any way of making grooves is fine---they don't have to be pretty, or square. (My little grooves are actually 1/4" x 1/16", because I don't have a 1/8" router bit.) You can do it with a dremel, or the edge of an angle grinder, or a table saw, or whatever.

I think all the information you need to design a great vacuum former is available free on the web. (Mostly on this site.) You just need a basic grasp of the physics, to know how to adapt the various design elements to your needs.

The one thing I that think would be a big improvement to the Thurston James design is heating the plastic from both sides. Proto-Form doesn't do that, either.
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jegner
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PostPosted: Fri Oct 06, 2006 8:53 am    Post subject: Reply with quote

Speaking of Doug's design a production machine a few years ago, called a Boba-matic.

Here is a pic:



or at least someone took that proto-form idea, and made a this machine. Some times these show up on ebay.

I agree, why sell the knowledge, when most of the info is available for free online. I don't blame anyone for it, in fact, I have Doug's book on Vac-forming and it is full of good ideas, including a 2 stage shop vac assisted high-vac rig. A great resource for making small machines, reviews of plastics, theory on vacuums etc.

Jim
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tallpaul
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PostPosted: Thu Nov 02, 2006 6:27 pm    Post subject: Reply with quote

If anyone is intrested in seeing a Hobby-Vac look here:

http://www.tk560.com/phpBB2/viewtopic.php?t=420
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