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I am going to make a bending pattern (tenor ukulele) for a Fox style bending machine using a heating blanket. If I layer in this order: lower steel slat, wood, upper steel slat and then the heating blanket on top -----how much compensation (ie. - how much smaller) should I put into the bending pattern to get the best results?

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Formerly know as Mandodiddle.

Mandodiddle welcome to the OLF!

My stack looks like yours only with a third slat on top of the blanket. I don't use an bending form compensation and it works for me.

I know though that some folks do compensate their forms for the wood and/or stack. I am sure they will weigh-in here and help you out.

My bending molds are undersized by .100" all the way around.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

I make the inside mold first, then use a router template guide to make the outside mold 1/8" larger, which compensates for the thickness of the side and slat. .

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David White, Toronto

"All my favourite singers can't sing."

Thanks everybody. What a great place to exchange information. I think I'll try shrinking the pattern within the ranges you listed and see what happens. Again, thanks for your responses.
Bill

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Formerly know as Mandodiddle.

Another thing to think about....some folks over bend the wood on the form so that you can take it right off the form and put it in a mold. Check out the Taylor Factory Friday videos...you can see that the edges of the forms are not vertical and that they are overbending at the ends at a minimum...and probably the waist as well.

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http://www.PeakeGuitars.com

You answered your own question if think about it logically you said you would be using a slat/wood/slat/blanket. Every thing past the out side of the wood is inconsequential to the shape and size of the form

Form compensation = Thickness of one slat and thickness of the wood.
If this is the way you are going to lay out our sandwich.

Personally I would not place my blanket outside for 2 reasons. Loss of heating efficiency and wear on the blanket from the press cauls. I would go slat/wood/blanket/slat or slat/wood/slat/blanket/slat.

In any case if the wood is always the second component from the bottom of your sandwich the amount to compensate for would not change.