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PostPosted: Thu Nov 07, 2019 11:43 am 
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Joined: Fri Mar 03, 2006 7:09 pm
Posts: 5972
Location: Hegins, Pa
I have been bending for 20 years I found that many blankets are not the same,

so with this in mind a good point was made hand or machine so in this case lets say machine.

The blanket is the key
if you have a 2 1/2 watt per sq in blanket 2 may be what you need. I never understood this idea and I did try it and never say a benifit for it, so this is how I bend .
I use only 5 watts per sq in this provides plenty of heat for the thermal load. Once you start bending the heat transfers from the blanket to the slats thus into the wood. I don't measure the heat of the wood I rely on the heat on the blanket. I learned pretty early that moisture is a refrigerant and will give you a false reading so you may set the cell before bending and cause cracks.
So most woods will bend very similar in the temp range though a few woods tend to over bend and I will address that,

First I use .075 in thickness when I bend. This is the same that Martin uses. I used to bend at .090 and .095 and had more cupping issues.
I don't use alum foil but I do use untreated butchers wrapping paper and you can get that at uline. I also stopped using super soft2 as windex works just as well and doesn't stain the wood plus it is a good bit cheaper.
My method is this with exception listed below

I pre shape my side to the radius of the back thinned to .075
dampen the slap with a few spritzes and set the paper into the moisture . This helps to keep it from moving around. Then I spray the top of the paper. I will then spray the wood. We will assume you have them marked right and left and show side. So with the wood DAMPENED I lay that on the paper and spray the top of the wood and apply another sheet of paper also dampening that. Then slat then blanket. I set it into the machine and set on the hold downs. I like to place a weight on the lower bout. when the sides heat and are ready to bend thew weight will slide off.
I place a thermometer under the top blanket between the waist and the lower bout , and turn on the blanket to FULL and allow it to heat up. Usually in the 250F range things will be ready to go.
I do the lower bout first this is a walk down not a pull and fly so take about 15 to 20 seconds to set the hold down. Move the them to the upper bout and at about 275F you are ready . Also note that you want to check that the upper bout is still damp and has not dried out. So id you need to spray it do so. Again a walk down with the hold downs. You can also add a weight to the front I use about a 3 b chuck of Alum.
So now the upper is bent so run the waist.
At this point I let the heat go to 325F to 350F once it gets there I set the control to VAR and dial in a 275F. I give this a 15 min cycle and let it cool down. You can check to see how well your bend did before removing by sliding the lower bout hold down up and hold the lower slat down and see how the wood is sitting against the pattern. I have a spring back factor so it isn't going to be tight to the pattern but should be close.

Maple I will not use as much water so you use just a small amount. Some figured woods need a slower bend and this comes from experience. This is were having weights on the slats helps to let you know when it is ready to bend. If I need to heat cycle again I spray a little water in there and just run the heat up to 375F then cool down

The key is your patience and learning to know when the wood is ready to bend.

Common mistakes are bending too soon or taking too long.
From the time I turn on the machine until the sides are bent on the pattern is about 4 to 5 min. Then heat cycle takes about 5 more and the 275F setting and drying cycle 15 min. Cool down then is about 45 min
hope this helps.

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John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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PostPosted: Thu Nov 07, 2019 2:05 pm 
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Joined: Sun Mar 09, 2014 8:13 pm
Posts: 2034
John, I have a dumb question. Where do you place the weights on the slats? Toward the end, or in the middle? You do this on both bouts?

I have your bender, but I already owned the LMII temperature control and heating blanket, so I use them. I think I can extrapolate how you work your temp control to work with my system.


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PostPosted: Thu Nov 07, 2019 2:33 pm 
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Joined: Fri Mar 03, 2006 7:09 pm
Posts: 5972
Location: Hegins, Pa
just a tad behind the highest point to flex over the curve

_________________
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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PostPosted: Fri Nov 08, 2019 3:00 am 
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Joined: Sun Mar 09, 2014 8:13 pm
Posts: 2034
Gotcha. Thanks John.


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