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I am about to build a bending form for an archtop guitar and have two questions. First should I build in some overbend, and if so how much.. yeh I know that was two questions... Second, and I hope I don't start an arguement, but some people say heat blanket on top of the wood, some say under the wood. What are the advantages of each, what do people prefer.. Thanks,, Mike

Mike, there was a recent discussion about spring back and building it in to your forms.
For me, I dont worry about it.
As for blanket on top or bottom, it depends, If you want your sides to follow your form as closly as possible then put the blanket on top. If you put it under the wood you will add the thickness of the blanket to the size of the form.

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Lance@LuthiersForum.com

Yes you should build in a little spring back but I can't tell you how much as it's probably differnt for all of us based on moisture content of the wood, bending temperature, probably a few other things as well such as wood type, thickness, etc..
I probably do this different than every one elese, but I'll put it out there anyway, this works very well never broken a set doing this. I place my blanket next to my form, preheat my slats by cranking them down against the form/blanket to 250F, open up, slide my wood wraped in paper between the slats bend the waist mostly complete then teh upper bout, then the lower bout and then finish off the waist. I also compensate my form for this method.

I think you'll get a fair amount of variation on this question which just goes to show there's more than one way that works.Jim_W38721.9211805556

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Jim Watts
http://jameswattsguitars.com

Compensation is needed but the most important thing is the mold. That is the first thing you need as you then compensate to that.
   I can't give away all my secrets but you need to shrink at least the amount of the wood , and slat ,
john hall

If when the side has been bent and gone through a couple of cycles, you heat it back up and put it into the mould at full temperature (don't forget to wear gloves) crank up the cauls in the mould it will cool down and set in the shape of the mould. No spring back to worry about.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Remeber: it's the shape it's in when it cools off that counts. Never remove a side from the mold warm.

From my meagre experience, it seems that each different species of wood behaves differently... Mahogany seems to spring back quite a bit & if left unclamped, will slowly straighten out for days. EIR seems to hold its shape quite well.
When building a new bending form, I draw my pattern on the plywood (or MDF) & make a mark about 3/4" above the apex of the lower bout, around 1/2" above the upper bout & 3/8" or so below the lowest point of the waist. Then I use the half pattern kinda like a french curve, to help draw the exagerated shape. This worked very well on the last two forms I built. My Zebrawood jumbo sides ended up a little overbent, but after a day clamped in the workboard, the sides fit perfectly.
I am careful to continue the bend of the form at both ends, (the bending form is at least an inch deeper than the half pattern)so I don't end up with flat sections at the heel & butt joints.
Overbending works well for me. Its easy to straighten the sides out as necessary in your building form or workboard.
I put my blanket under the top slat. (FWIW)

ok, so pretty much as long as I make sure it is cool either while on the bending form on in the body form I 'll be fine, now what about the thoughts on heat blanket over the wood vs under the wood. So far I think I have one vote for each. And I do realise that if I put it under the side I will have to compensated the form for the thickness of the blanket.. I bet either works fine, just need to make up my mind..      Mike

Hi Mike

I use the blanket on top, so from the top down

Blanket
Steel Slat
Wood
Steel Slat

Depending what I am bending I use either foil or Soaked Brown Paper between the slats and wood.

Just to give you another option, some people use two blankets one under one over

Hey Russel; I've heard of the "blanket on top" method from several guys on this forum. What do you feel is the advantage? My only reluctance in trying it is the thought of applying pressure with the waist caul directly on my precious blanket.

      Mr Al Carruth is 100% correct. You want to have the wood in the shape you want it to hold until it cools. I will pull a set out when it is warm but it is set into the mold right away and blocked in to let cool.   Here are a range of temps I find that works well
   Most woods   275 to 325 F ( 5 minutes )
   You seldom need 2 cycles if you are using a heating blanket.

   Mahogany    325-350&nb sp;   ( 5-7 minutes )
    paduk     & nbsp;  350       ( 5- 7 minutes )
   Babingga     &nbs p;350        ( 5-7 minutes )

   High figured woods
    very wet hold to 250-275 for 3-5 minutes then bend and heat to dry off

    I find this works well for most of the bending I do

Hi Daniel

One advantage is you don't have to compensate your forms for the blanket thickness, altough I guess this is no real hardship.

The blanket seems to stand up to being clamped from above no problem the machine I have was made by John Hall at Blues Creek, and the cowels are nicely curved, the front and back cowels are round dowels and the Waist and Cutaway are steel backed by a round dowel.

the other thing I like about having the blanket on top is you get a very positive contact with the form.


I know people who swear by all three methods, I am sure they all work well, just a case of finding the one you are most comfortable with.

Hope this helps a little

One thing that has not been mentioned here that I feel is important, is the slat material Spring steel has near no shape memory so it returns to flat. Stainles will want to hold a bit of shape but is less likly to cause galvaistic staining. I use mostly spring steel and wrap my wood in either foil or craft paper. I have heard of some that dont wrap the wood and use stainless steel slats. but if you use spring steel you darn sure better wrap your wood in something to protect it from staining. the wrap also helps keep the moisture in.