I'm building an archtop octave mandola based on a scaled down (75%) jazz guitar shape. Carved spruce top, curly maple sides and back. Top and back are now carved.
I did a test build on a mandolin at 50% using much easier back and side wood (padauk). To do the tight radius bends on the mandolin I thinned the sides to c 1mm at the tight curve as I was finding it really hard to bend otherwise.
My question with the figured maple is whether I should do the same or not. I have been struggling with these sides and am changing from a home made bending iron set up and a "negative" mould, to a heat blanket and "positive" former. I hope this will enable me to go slower and gentler on the bends but wondered if thinning was also wise or whether the grain nature of the maple makes this unwise.
All help very gratefully received - any thoughts?
Sorry should also have said it's a cutaway design hence the tight radius on cutaway side
I have only made one cutaway guitar, and it was a resonator guitar instead of an archtop. I would not recommend thinning the wood, it will not have as much strength and the soundbox is already a compromise between being strong enough to withstand string tension and light enough to produce nice sound. I do all my bending on a piece of copper pipe with a bolt set in one side and held in a vise, and a propane torch turned down low stuck in one end and playing on the inside of the top. I use a 3" pipe for larger bends and 1-1/2" for tighter ones, like the cutaway or for fiddle sides. With a bit of practice it is not hard to bend 2mm thick sides, or even a bit more, with this method. I have not worked with padauk but I often use curly maple, cherry and walnut and they all bend well, with walnut being perhaps a bit easier.
I cannot speak to curly maple but I have bent Black Walnut, African and Honduran Mahoganies over a positive form, including a baritone uke with a venetian cutaway. Some Padauks are difficult, some are easy.
I laminate 2 batches of 3-4 layers of chipboard which I salvage from old Ikea type bookcases etc. These are screwed together and cut out on bandsaw to a half mold. The negatives are split for the mold and the positive at full thickness becomes the bending form. This works if your 2 sides are symmetrical. You can do the bass side 1st, then remove the cutaway and do the treble side.
Sides are thinned to about .070" to .080" thick. You may thin the tight cutaway horn area to .060" or 1.5mm and later reinforce it gluing in braces or fabric.
I spray the wood with a spray bottle of water (not heavily soaked) then wrap with 1 layer of aluminum foil.
Clamp one end of the side along with heat blanket to the mold. Set blanket to 175 - 190 Celsius (350F). When hot, push the side wearing heavy welding gloves into the shape of the mold, clamping as you go. Leave heat on for about 5 more minutes. Total bending time maybe 15-20 minutes and I haven't scorched one yet.
Let cool overnight before unclamping then clamp into the female mold and start the 2nd side.
Practice on scrap first.
Edited by - Fathand on 04/22/2021 12:54:53
If I understand this correctly, you are bending curly maple sides to the shape of a jazz guitar except 25% smaller, so you have some very tight bends.
There is a great chance that the curly maple will break or distort in the vertical axis.
I would laminate the sides from thinner material, arriving at a thickness the same as a solid side but much stiffer, which I do on banjo lutes, roughly the same size as octave mandolins, but without the tortured waist curve.
Here's a pretty good video—you would need to make a form.
You can use the same form to laminate the kerfing that allows the top and bottom to be glued on.
Edited by - Ken LeVan on 04/25/2021 17:51:41
Thanks all. Helpful thoughts. My heat blanket arrived from China this week along with some 0.2mm stainless steel sheet I need to cut in two that I plan to sandwich the wood with to support the bends.
Zach, thanks for the encouragement, it's good to know it's doable with practice and experience.
Rick, thanks for the detailed walkthrough, very helpful. I have built my former to include floating form clamps for the waist, top and bottom that I can progressively tighten to encourage the bend - does this sound sensible vs pushing by hand? I think I will have to start with the waist curve to do it this way which will make matching the grains on each side a little more tricky I guess.
Ken, interesting idea on laminating on the tight curve, on the mandolin (50% scale so super, super tight radius probably around 1") I was nervous about the c1mm thickness but it has held up for a couple of years now - maybe helped by the fact that the side is only around 40mm high meaning the kerfing extends nearly all the height of the side, but it would be reassuring to laminate a second layer to add additional strength on the deeper side of the mandola.
'c, d7, open' 28 min
'Leaving the Bunker' 2 hrs
'Old Washburn E 329' 2 hrs
'Ten Broeck & Mollie' 2 hrs