New heatbox built

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yeoman
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New heatbox built

#1 Post by yeoman » Thu Oct 16, 2014 7:07 pm

Good evening everyone.

Recently I asked about the value of Techniglue, and after some investigation, I ended up getting some Smooth-On EA-40 from John McDonald in Sydney. Impossibly good service by the way. Ordered it over the phone Friday late morning, and it was on my doorstep Monday morning (Had been away for the weekend).

So anyway, while EA-40 cures at room temperature in 24 hours, I needed to hurry the process along for the sake of the bowmaking courses I run. The best way to do this is with a heat box.

I had deliberated over heaps of different options. One was to use a fan heater that you might put under your desk. Another was to use ceramic infrared lamps. In the end I settled on old-school halogen lamps for reptile enclosures. I got six ceramic light fittings and six 75 W lamps from the local reptiliary.

I had toyed with the idea of using a reptile enclosure thermostat. They cost about $50, but only went up to 50 degrees, generally. I found some thermal circuit breakers for under six dollars from an electronics supply place. It turns out these are tremendous, especially for the value. I got a breaker that shuts off at 60, and one at 70 degrees C.

I insulated the box with polystyrene insulation boards such as go under house slabs and in walls. Two sheets at $16 each were enough. Then over that went some aluminium faced bubble wrap which was about $45 for about twice the amount I needed. I might have gone overboard with insulation, but I was keen for the unit to be efficient.

Enough chatter! Onto the pictures!

Here's the whole:

Image

Here's the circuit breaker:

Image

It's tiny. The base is 16 mm diameter. Rated for 10,000 cycles, so should be good for a good long while. I bolted it to a short piece of right angle aluminium. I heard Al is a terrific conductor of heat. Holding it while cutting it, I found this to be true. The thermostat is set at the same height the bow staves will rest.

I was worried six lamps would not be sufficient. However the box got up to temperature in a few minutes, and once up to temp, only needs the lamp on for about 30 seconds at a time.

Image

I make backed wood bows mostly, as do the students on my course. This requires much less space than a glass bow in a large form. To the frame of the box, I screwed two pairs of right angle metal. Across these will go removable bars, and onto these go the glued up staves.

Here, a few bows and a board stave play proxy for a class full of laid up bow staves:

Image

I saw someone else did this on theirs, but theirs looked better. It's a tip from a broken bow attached as a handle:

Image

I set it going with the 60 C circuit breaker. I put a meat thermometer through a crack in the lid for a few cycles. Not in the same place as the circuit breaker though. Once up to temp, the box goes from about 56 to 60 approximately twice in 90 seconds. I'm yet to test the 70 C circuit breaker.

There is no fan in this box. But I left a board stave in for a while and felt it all over and couldn't find anywhere hotter or colder than anywhere else. The lamps are fairly close together and have a wide angle of radiation.

I've not yet tried it for real, with a glued up bow. But very soon I will. I suspect I will have some success cooking it for four hours, based on others' reported experiences. I'll try five just in case.

Overall, so far, I'm very happy with it. I need to put some handles on it to make lugging it around a bit easier though.
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greybeard
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Re: New heatbox built

#2 Post by greybeard » Thu Oct 16, 2014 8:28 pm

Hi Dave,

The hot box certainly looks like it will do the job.

What method do you use to hold the backing to the stave once glued, rubber tube, clamps or a two part form?

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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yeoman
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Re: New heatbox built

#3 Post by yeoman » Thu Oct 16, 2014 8:40 pm

Thanks Daryl.

I've used nothing but innertube strips for about eight years. I've never glued a backing on any other way, as it happens. Clamps are expensive, while tires are free.

There may be better methods mechanically, but none so expedient.
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Re: New heatbox built

#4 Post by greybeard » Thu Oct 16, 2014 9:34 pm

yeoman wrote:..... I suspect I will have some success cooking it for four hours, based on others' reported experiences. I'll try five just in case.
Dave, you may find that you may only need one and a half to two hours for the glue to cure.

With heat strips the curing time for recurves is thirty minutes and for longbows [thicker limb] forty-five minutes.

Why you should expect a much shorter curing time is because you will be getting heat transfer on to the back and belly of the bow as well as on the edges.

When using a bulky two part form heat transfer is mainly through the edges of the bow. Although the bulky form will warm up heat transfer will be minimal.

Hope the above makes sense.

Daryl.

p.s. It may pay to do a heat test on the rubber strips when they are under tension.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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greybeard
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Re: New heatbox built

#5 Post by greybeard » Fri Oct 17, 2014 7:35 am

Dave,

One other point, have you given any thought to the dehydrating effect that elevated temperatures for an extended period of time will have on the staves.

If the timber becomes too brittle will re hydration neutralise / reverse this effect.

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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yeoman
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Re: New heatbox built

#6 Post by yeoman » Fri Oct 17, 2014 9:01 am

Thanks Daryl,

I had planned to do a test run or two in the very near future. I found an oldschool article, I think on http://www.vintageprojects.com where the bowyer, making a glass laminated bow, had a one-sided form and used tire rubber to bind the assemble down. But nevertheless I shall test it first.

With regards to the moisture...I had thought about that too. One thing I will be doing is wrapping the glued staves in cling wrap before binding with the rubber strips. Additionally, with a cook time of only a few short hours, I suspect not a lot of moisture will escape to make the stave dangerously brittle (though it certainly could happen if left to its own devices!). Lastly, on the course we cut the stave (from 19 mm flooring) to width and glue on the backing with the stave still at its full 19 mm thickness, and cut the thickness taper the next morning. This gives it a greater cross-sectional area.

So I think being wrapped in a non-porous film, short cook time (relatively speaking) and thick-as-possible-section should combine to preserve enough moisture in the stave for our purposes.

But I appreciate your forward-thinking on my behalf!

As an aside, how do you decant EA 40 from the tins? Pouring it I think would be messy. I was considering perhaps disposable spoons.

Dave
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Nezwin
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Re: New heatbox built

#7 Post by Nezwin » Fri Oct 17, 2014 9:30 am

Looking good, Dave! Seems like you've thought everything through, and you made a saving on the thermostat over the fancy digital one we discussed.

I'll have to take a look at some halogen lamps myself. Looks like they take up a fair bit of space in the box but it seems like you have plenty of usable space still there.

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Re: New heatbox built

#8 Post by greybeard » Fri Oct 17, 2014 11:22 am

Dave,

The sleeve of plastic shot measures was under $3.00 at the local Supa IGA store. It is cheaper to dispose of them than to clean them.

Old hack saw blades with the teeth filed off for decanting the resin and hardener into the shot measures as well as spreading the epoxy. The blades are also easy to clean.

The bottoms cut from 2 litre plastic milk containers make good mixing trays that are disposable.
Measures And Mixing.JPG
Measures And Mixing.JPG (181.33 KiB) Viewed 3515 times
Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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yeoman
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Re: New heatbox built

#9 Post by yeoman » Fri Oct 17, 2014 1:02 pm

Quick and easy setup there Daryl.

I asked about decanting before I'd opened the tins. I have done so now. I was surprised by how gloopy it was. I guess I was expecting something the consistency of 1 hour epoxy, but found it to be far thicker than cold honey.

Decanting is easier than I thought. I mixed some up using disposable plastic spoons into a disposable cup. Mixed and spread with a plastic disposable knife.

I guesstimate that half a plastic cup of mixed components (total) could be enough to stretch across three bamboo backed wood bows.

That's pretty good economy.
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bigbob
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Re: New heatbox built

#10 Post by bigbob » Fri Oct 17, 2014 1:10 pm

Dave that is one great hotbox. Wish mine was as sophisticated! Once the bow currently in the over is 'done' I will have to do a bit of a repair job on mine. Mine has the bulbs in the upper section with the wiring under alfoil. I used batten mount light sockets and after several years of work the heat is causing the socket material to become brittle and breaking out. Where did you source the aluminium bubble wrap? I think I will have to use an insulation board as well to better reflect the heat away from these sockets.I just use the bayonet mount 75w halogen bulbs [10] of and they are very effective.
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yeoman
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Re: New heatbox built

#11 Post by yeoman » Fri Oct 17, 2014 1:31 pm

I'd heard about the 'household' variety light sockets perishing over time. That's why went with the ceramic light fittings, which happen to have Edison screws. I bought the fittings and the bulbs from the same place, just to make sure they fit.

I just the polystyrene board from these folks:

http://www.justrite.com.au/content.php/category/id/42

The 30 mm board was just over $16 a sheet.

I got the aluminium faced bubble wrap from Ebay:

http://www.ebay.com.au/itm/Foil-Air-Bub ... 2a45d9d482

In future when this link no longer goes anywhere, it is "Foil Air Bubble Cell Radiant Reflective Insulation".

I have quite a bit left over. Seeing as it's waterproof, and insulating, I am considering using it to line a steam box for a better steamed recurve setup.

I've only got six 75 W lamps, and it turns out that's plenty. There might be a differece between lamps made for heating and lamps made for lighting though.
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Re: New heatbox built

#12 Post by greybeard » Fri Oct 17, 2014 8:03 pm

yeoman wrote:....With regards to the moisture...I had thought about that too. One thing I will be doing is wrapping the glued staves in cling wrap before binding with the rubber strips...... So I think being wrapped in a non-porous film, short cook time (relatively speaking) and thick-as-possible-section should combine to preserve enough moisture in the stave for our purposes....
Dave, without over thinking the situation "could wrapped in a non-porous film" result in condensation forming under the film and creating problems. Something to think about.
yeoman wrote:...I guesstimate that half a plastic cup of mixed components (total) could be enough to stretch across three bamboo backed wood bows.
In the overall scheme of things the epoxy is a low cost item so do not scrimp on the amount used, better to squeeze excess out than have dry joints.

Daryl.
"And you must not stick for a groat or twelvepence more than another man would give, if it be a good bow.
For a good bow twice paid for, is better than an ill bow once broken.
[Ascham]

“If a cluttered desk is a sign of a cluttered mind, of what, then, is an empty desk a sign?” [Einstein]

I am old enough to make my own decisions....Just not young enough to remember what I decided!....

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rodlonq
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Re: New heatbox built

#13 Post by rodlonq » Fri Oct 24, 2014 10:37 pm

Nice work Dave.
I had toyed with the idea of using a reptile enclosure thermostat. They cost about $50, but only went up to 50 degrees, generally. I found some thermal circuit breakers for under six dollars from an electronics supply place. It turns out these are tremendous, especially for the value. I got a breaker that shuts off at 60, and one at 70 degrees C.
I have set up a system with a surface mount hot water system thermostat attached to a sheet aluminium enclosure, they are generally adjustable from 60 - 75C. They usually have about 8C dead band but the thermal mass of the box and contents seem to flatten out the air temperature quite a bit. The other advantage is you can find them in good working order on most busted hot water systems (if you have a couple of brother-in-laws in the plumbing game :smile: ).

Daryl said;
In the overall scheme of things the epoxy is a low cost item so do not scrimp on the amount used, better to squeeze excess out than have dry joints.
I could not agree with this strongly enough. If you are making enough bows to avoid your smooth-on going off, it is the most valuable part of any bow you want to make. When the kit gets down to the last bit and there is not enough for the next glue up I save it for putting on points and adaptors on arrows, never had one fall off yet.

Keep up the good work mate, your posts are always inspirational for those of us still getting there.

Cheers... Rod

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