Thinline with Forearm AND Belly Contours

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Deed_Poll, Oct 11, 2018.

  1. Deed_Poll

    Deed_Poll Member

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    Hey guys! This is a first for me so I thought I'd document the process a bit and do a build-along :) I hope that others might find something useful in here, there will be a few challenges along the way and it's great to have others along for the ride. Please do not be shy! Sometimes I skip over important details by mistake, so any and all questions and comments are very welcome.

    This guitar body will be shaped like a Jazzmaster and routed for all standard USA Jazzmaster appointments. But the general principles could conceivably apply to all kinds of instruments, maybe even 335-style shapes with a fully carved top and back! So take from it what you will.

    The challenge is how to achieve the top and back contours with a hollow / Thinline construction. I've done one OR the other before as a Thinline, basically machining the body core in 3D inside and out, and capping it off with my standard 1/4" marine birch ply top (for belly cut) or back (for arm contour).

    This Thinline had a belly contour and shows the process pretty nicely:

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    Here's the inside of the body core (before cutting the belly contour on the outside). You can see I've extracted the shape of the belly cut and used it for the inside cavity cut, so that the cut leaves a consistent 1/4" back thickness.

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    Then I cut the corresponding contour in the back

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    Here's the top, with reference holes to glue to the core so the f-hole and control cavities are in the right place

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    And here's the whole thing clamped up and gluing the top to the back. After this stage, I simply line up the reference holes with the bed and cut the body from the front as usual.
     
    Jeff_G, Khromo and j.s.tonehound like this.
  2. Deed_Poll

    Deed_Poll Member

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    So I adapted the above process to try and tackle a Thinline with BOTH contours. What I would have to do is basically the same thing, but the top AND the back would have to be like the body core above. Instead of a thick core and thin top, I would have to split the difference and start with two boards approximately one-half the thickness of the finished body. So I got in touch with my supplier and ordered a stack of 1" thick, 44" long and 7"-8" wide heat treated maple.

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    Each of these boards would be sawn in half, and doubled back to make either a top or a back. It would take two whole boards to make a body, and each body would be 4-piece (two on top, two on the bottom) with a join down the centre and a 'pancake' join mid way around the body perimeter.

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    Here's a print-out to check that Nico is happy with the f-hole shape and location. I also used it to pick some suitable boards for the build and locate any knots and things to make sure they would not fall on the finished body.

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    These are the ones I chose, they're a good match for each other in terms of grain and colouration and are just the right size. I order 1" thick stock knowing that I will have to plane them off to 21mm-22.25mm (for a finished thickness between 1-5/8" and 1-3/4"). Obviously, for each guitar all the thicknesses will be the same! The range is just a handy reference.

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    You can see there will be a fair bit of planing and thicknessing involved to get these straight, square and planar!

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    And here's the first pair after planing and thicknessing ready for glue and clamping. This will be the top. I've left this to glue overnight, and the back will be clamped up tomorrow.

    I'm gluing the top first because it will be the first side to be machined - I will be cutting from the top, and will do the f-hole, pickup routes, and the control routes (cutting them just deep enough that the "hollowing out" pass from the other side will reveal the cavity).

    Then I will do the hollow pass (back of the top), then the hollowing pass of the back (top of the back), and then it will be time to clamp the top to the back.

    Stay tuned!
     
    Khromo and j.s.tonehound like this.
  3. Bluesful

    Bluesful Supporting Member

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  4. Deed_Poll

    Deed_Poll Member

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    Cheers Bluesful!

    Part 2! Great to have you all along for the ride on this one :)

    I thought it had all gone perfectly, but alas there was a problem! I'm quite sure it's fixable though. Just very, very annoying.

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    Things initially look good from the front! The first cut was doing the f-hole, pickup and control cavities from the front. Not to full thickness, because the Thinline cavities would intersect them from the back... Though this picture is taken after that.

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    At first glance, the underside looks decent as well. This is still attached via the reference pegs to the machine bed. I use 4.5mm wooden dowells for this since that way I can use the same tool I use for the neck bolt holes. The piece is also fixed down by vacuum (I'm a belt and braces kinda guy!)

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    I cut the treble side cavity with an end mill (square bottom). Where it overlaps with the control cavity, it is well within my standard 0.1mm allowance. The circle you can see by the cavity is to give the pickguard screw in that location something to bite into.

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    These bumps on the bass side perform the same function. The Jazzmaster was never really meant to be a Thinline, so I had to improvise a couple of things!

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    Here's the disaster area. So my method is to lower the tool pass around the f-hole, so that the edge of the f-hole looks like it is 1/8" thick rather than the 1/4" thick the bulk of the top actually is (Fender did this as well with their thinlines).

    But you can see here, I was unlucky this time and got a splinter right along the edge of the f-hole... that will be visible from the top as you will see, so I'll need to fill that in to the correct thickness and reshape it flush with the edge before I can join top to bottom. Really annoying! :fp:

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    Here it is from the other side, clearly visible and horrible! I'll try and build it up with superglue and sawdust unless anyone has any bright ideas...

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    Inside is like a Zen garden! I'll machine the visible side (inside of the back) a lot finer than this, but I perversely like the idea of leaving some invisible evidence of how the thing is made.

    The back is now patiently gluing up, more on that in part 3!

    Cheers
     
    Khromo, j.s.tonehound and Bluesful like this.
  5. ffoont

    ffoont Member

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    I love build threads by talented builders. Thanks for showing this one off.
     
  6. B. Howard

    B. Howard Silver Supporting Member

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    [​IMG] I've done a thinline with an arm bevel. While tough to see the bevel if you look through the F hole you can see the solid brace I installed to help keep the top in shape after it's glued.

    I made a shallow V cut with a router on the inside of the top where I wanted the bevel to start and then used my hot pipe to bend the wood. That is Clouded Chechen on that drop top.

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  7. Deed_Poll

    Deed_Poll Member

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    Cheers, ffoont! Good to have you along for the build :)

    B. Howard, that looks really nice! I don't have that same level of experience with bending and setting wood, so I would lack the confidence to try something like that! But it looks like you've done a fantastic job on that Tele. Bravo :)
     
  8. Deed_Poll

    Deed_Poll Member

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    So this is the fix on the splintered f-hole. I'm quite happy with this now, it shows up a little grey but that's exaggerated in the photos. The shape is the important bit and that's come out very nicely.

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    I just used the old sawdust and superglue trick, and brought it back to shape with a file and some 180 grit and a tiny block to keep the edges true :)
     
  9. Deed_Poll

    Deed_Poll Member

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    Part 3!

    So today I cut the inside of the back, and was able to line up the top and back with my reference peg holes for gluing :)

    Let's kick it off with an artistic photo of the interior space with both contours!

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    I like how the f-hole projects onto the bottom, this pic has avatar potential I think haha!

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    So here it is as it came off the machine. You can see I used a much finer cut for this one, it's only about a 1mm step over on the finish pass.

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    The grain it revealed is also rather nice! This will be what you see when you look through the f-hole, so it's just as well!
     
  10. Deed_Poll

    Deed_Poll Member

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    [​IMG]

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    Here are some pictures with the back and front mated up using the 2 reference peg holes I cut in each interior. These will keep the faces lined up perfectly while they're clamped and glued.

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    I was pleased to discover that you can actually see quite a lot of the hollow interior through the rhythm circuit cavity

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    And here's testing whether I got the centre lines to join exactly. Up top (neck pocket end) they're out by about 0.3mm

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    But at the butt of the guitar they line up exactly. This isn't a reflection on how accurate the machine is in any way, just on how accurate I was in lining up the centrelines of the blanks with the centreline of my machine.

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    Finally, here they are "book matched" together. You can see the pegs and reference holes that will keep it straight.

    Incidentally, I was thinking how cool it would be on a future project to basically spray both these faces in CAR or gold, but leave the exterior natural or a sunburst finish! So the inside of the f-hole, control cavities and maybe pickup cavities are a bright colour - and maybe use an acrylic pickguard so you can see through to the coloured cavities. Does that sound insane??

    Cheers!
     
    SgMaster and De - Fre like this.
  11. Deed_Poll

    Deed_Poll Member

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    So, plenty of rough edges still on this so bear that in mind! There will be hairy edges around the routes, marks from the planer and so on that will still need to be sorted out.

    This is how my bodies look when they come off the machine. You can see there's still an awful lot of work to do, lots of sanding and of course the roundovers as well.

    I always incorporate the edge roundover into my CNC cuts for the contours, that way you get a consistent radius all the way around the body and a nice transition on the back where it meets the neck pocket. It's not exactly vintage-correct, but I like it.

    I got sick of trying to get the colours accurate to real life with my phone camera, so these are just arty black and white so sorry about that. I'll do some better colour pictures once the roundovers and sanding are done.

    I'm very pleased with how this has all turned out at this stage since I'm trying a lot of new things with this build and although it's been a lot of work, I'm learning a great deal that I can take forward.

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    P.S. I rather like that the release cut intersected one of the pegs I used to align the top and back ;)

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