Semi Hollow Junior build

[Jim_E]

I got lucky again and have an order for one of my Junior models except this one will be a typical semi hollow like a 335 instead of solid body. I’m quite happy for the order as I’ve been itching to build a semi version of my guitar and this will give me the chance to build the fixtures jigs and stuff to make more, either semi or full hollow.

This guitar is not going to be any bigger size-wise than my regular solid body model (which fits in a stock LP case), so I’ll be trying to keep the body weight up where there won’t be any neck dive – Here’s my solid body Junior.

This semi version will be a solid colour, probably red but maybe black and will be double bound, so I’ll be using a clear rotary cut Maple for the top back and sides like your typical 335. Aside from the Ebony fret board everything else will be Honduras Mahogany – the centre block, the linings and the one piece neck.

The sides will be bent laminated clear Eastern White Maple veneers – 6 layers of .021 thickness for a total of 0.126”. The arched top and back will also be from the same Maple veneers, but only 5 layers thick and both will be vacuum formed over solid molds.

I always like to start with the easy stuff, in this case that’s the sides.
As the general idea is to make both positive and negative molds with a space separating the two exactly equivalent to the finished thickness of the sides, everywhere, I’m thankful to have a cnc doing the cutting.

This jig took me a few hours at the computer and less than 10 minutes cutting time on the cnc, that doesn’t count the time I spent staring at the screen daydreaming about the actually bending the sides and whether or not this jig would work.

I wanted the jig to be the same thickness as the width of the sides (1 5/8”), so it’s built of two layers of ¾” Beech hardwood plywood, each of which has a layer of white laminated hot pressed on both sides to increase both the thickness/strength and the flatness/stability of the material. The two layers are glued and bolted together to make the correct total thickness of 1 5/8”.

Okay, here’s the finished product ready to go into action, the outline of the body is the exact size of my Junior model, developed from the same cad file with the corners are lopped off the jig for simple clamping.

The inside block are cut to leave the required .126” gap to the outline, with the clamping holes placed strategically (I think) to pull the bendy parts the right way and release once dry, just a few clamps and the entire glue-up can be done by one person in 2 minutes… is what I was aiming for at least.

With regard to bending the sides, my experience is that more layers of veneer is better than less in the same thickness. More thinner layers = more compliance and less to no spring back, so I’m using 6 layers of .021” for sides that will finish out at 1/8” total thickness and should be bullet-proof. The adhesive will be Unibond 800, a crystal urea resin glue which doesn’t creep and dries as hard as glass, it also gives you a lot of control over the open time.

After preparing the veneers by jointing them straight on one edge and ripping to an oversized width, it’s time to start praying, mix the glue and get clamping.

Believe it or not both sides went off without a hitch, I swear once everything is in place and the glue is mixed, I can get a side in the clamps in less than 2 minutes… the girls will sit under pressure for 48 hours and then we’ll see where things stand.

It killed me to wait and I could have done so earlier but I waited two days to open them up, here’s the end result.

Sides with absolutely no spring back, frozen in the exact right shape… good solid ones with no voids and a perfectly consistent thickness throughout, they ring like glass when tapped and are crazy hard/strong and stable… I declare victory over the straight wood!

The pieces go back in the jig which holds them perfectly so you can hand plane them down to the required width, nice and square on the edge also.

It’s time for a centre block so I head back to the computer and my original cad file. We’ve all seen how semi’s are built so the question of what to do for a centre block is pretty much answered by “form follows function”, no mystery it’s a plank.

Here’s my centre block in some old Honduras, finished it weighs 2.8 pounds which by my calculations should tip the scale in favor of the body and preclude the chance of neck dive. It’s a pretty simple piece that holds the sides together at the bottom, the bridge and buckers in the middle and the sides and neck at the top, that’s it, not very creative. Again cnc cut so it fit’s exactly in the jig, less the thickness of the sides.

I used my jig to mark where to cut the over-sized sides at the bottom centre joint, pulled out a square and a razor knife and tried to split a .5mm pencil line... time for a tester.

Well… it just falls together and stays there with little or no help, I love when you cut a bunch of parts and they just go together perfectly, it’s so much better than the other option.

Here we are with all the parts back in the original “donut” part of the jig, which I’ll use to glue in the centre block and all the linings, to hold the guts when I glue on the top and the back and also to hold the finished body when it comes time to cut the binding channels.

Well that’s it for the easy part, right now I’m working on a custom vacuum table/machine/contraption and molds to make arched tops and backs, I’ll post again once I make some more progress, thanks for checking in, Jim.

 

[pshupe]

Very cool. I am very interested in this thread. I'm sub'd. I had a vintage 335 for a month or so and it was a treat to play. I would eventually like to make a similar model. My shop is small and I may have to make room for some more specific hollow body tools and jigs.

Looking forward to updates.

Cheers Peter.

 

[KnightroExpress]

That jig (and your work) is fantastic! I love seeing the results of good planning.

 

[TheX]

I love seeing the results of good planning.

Me too!

 

[tonedragon]

Nice I like it

 

[frankencat]

Subscribed.

 

[mux164]

im looking forward to seeing this

 

[Tone_Chaser]

What a great jig. Can't wait to see this one come together.

 

[emoney]

Man, Jim, this is awesome! I love your jig building, and envy you guys with the CnC machines.
Kudos to you.

As an aside, can you share exactly how that lamination process works? Do you glue all
the pieces at the same time, in that meaning you apply the glue to each side and then
clamp together in one long piece or...?

Thanks for this thread, btw.

 

[Jim_E]

Thank you gentlemen for your kind words.

@emoney – I glued the side veneers all layers at once, by applying the glue as quickly as possible, stacking them and clamping before the water sets into the veneers and they start to go all wonky and become unwilling to cooperate.

I’ve done quite a but of bent laminating over the years so I had a pretty good idea of what to expect, if you’re trying this the first time you might want to read up and do some testing like anything else.

There are veneer softeners and other methods you can use to make the work more compliant and much depends on the actual veneer you’re using, but at the end of the day you just have to try and keep trying until your experience leads you down the right path.

It’s nice to have a cnc cutting the parts but I’ve made dozens and dozens of jigs without a cnc, this jig could have been made by hand but the time saved and the accuracy of the finished product means you can’t ignore the advantages you can get from a cnc.

Regarding making the same jig by hand - the only tough part is getting the space created for the veneer side to be exactly the right width and doing so consistently but it’s not impossible by hand, it just takes more time.

 

[LtDave32]

Nice work, Jim. And nice bit of jig-making!

 

[valvetoneman]

Do you glue together flat and put them in the mould cold or is it wet, very interesting and different.
I'm used to building acoustics but we bend either with a fox bender or on a bending iron before putting it in a body jig, very similar construction but love the perfect inside body jig, I just use braces.
Really liking this, great work.

 

[Jim_E]

Do you glue together flat and put them in the mould cold or is it wet, very interesting and different.
I'm used to building acoustics but we bend either with a fox bender or on a bending iron before putting it in a body jig, very similar construction but love the perfect inside body jig, I just use braces.
Really liking this, great work.

Thank you, the sides are 6 layers of veneer, I stuck 5 of them down to a board and rolled a coat of glue onto all of them in one shot - stacked them as quickly as possible and added the 6th layer, the good face.

Then they go into the jig wet and full length, stacked as a complete package as fast as possible.

That's why it helps to have everything ready to go and be able to close up your jig or press quickly (read; with not too many clamps), because as soon as the glue hits the veneer it starts to absorb the moisture and curl, it progressively becomes harder to wrestle into place, so if you can do the whole operation in say 2 minutes then you're golden.

 

[fatdaddypreacher]

impressive at many levels.

 

[valvetoneman]

Thank you, the sides are 6 layers of veneer, I stuck 5 of them down to a board and rolled a coat of glue onto all of them in one shot - stacked them as quickly as possible and added the 6th layer, the good face.

Then they go into the jig wet and full length, stacked as a complete package as fast as possible.

That's why it helps to have everything ready to go and be able to close up your jig or press quickly (read; with not too many clamps), because as soon as the glue hits the veneer it starts to absorb the moisture and curl, it progressively becomes harder to wrestle into place, so if you can do the whole operation in say 2 minutes then you're golden.

Alot quicker than bending ribs by hand, I used the bending iron first, that was fun, it's the lazy foxbender way now thank god.

Can't wait to see this done.

 

[Ole'Lefty]

CNC aside, your work is impressively well-pre-planned and executed. Now subscribed; I have to see this outcome.

 

[Jim_E]

Okay I’ve made some progress on my vacuum set-up to press the arched top and back.

My plan is to build a vacuum table that will be self-contained unit with integral pump / reservoir and electrics etc. The overall size of the table will be about 26” x 34”, built as a freestanding unit it’ll be 36” high and will accept interchangeable molds.

It’ll have a hinged lid built like a stretcher (similar to a silk screen machine), that will suspend a clear 30 mil polyurethane film.

Each mold insert is essentially a vacuum manifold, 2 3/8” thick x 22” x 30” with a quick release connection to a vacuum line so they can be interchanged within the table, built from 3 layers of the same plywood I used for my body jig.

The shape for the mold comes from a cnc cut body set into the centre. The body has a carved top but no cut-outs and it’s set into the mold so that only the carved top portion sits above the level of the top surface.

I did it this way because after the top carve is done it needs to be finished by hand and it just seemed easier this way, but if you think about it, doing it this way might open up a host of possibilities for the source of your mold…

Building the first mold.

Here’s the 3 layer stack that will make the first mold, the one for the top of the guitar.

The top layer will go back on the cnc and have the body outline cut out, and then I’ll bolt all 3 layers together with a bead of silicone between them around the perimeter, just outside the bolt line.

I’ll seal the raw plywood outside edges with a few coats of rolled on white glue and it should be airtight.

Vacuum comes up from the bottom into those dados you see on the top.

Here’s what the innards look like;

The bottom layer with holes around the perimeter to bolt the 3 layers together.

The pocket in the middle sets the height of the body within the mold, the small circle at the bottom will have a metal insert and the quick release vacuum line will come up from the bottom into that insert, the vacuum travels around that ¼” x ¼” dado coming out of each side of the small circle.

The middle layer is there essentially just to make up thickness/strength and transfer the vacuum up, it has the same holes around the perimeter to bolt the layers together and a giant hole in the middle for the inserted body to pass by.

The 5.0mm holes around the middle of the ring line-up with the vacuum dado on the bottom layer and transfer the vacuum through to the top dado’s.

To press a top or back the idea is;

*Place the mold insert on the table and hook up the vacuum line,

*Place the glued up veneer pack (in this case 5 layers of .021” Maple) over the mold in the correct location,

*Place a breather mesh over the veneer pack extending at least partially onto the vacuum dados,

*Lower the lid stretching the suspended poly film over the work and sealing it to the face of the mold.

*Engage the vacuum pump, the suction comes up from the bottom and into those dado’s which pulls the polyurethane film down to the face of the mold sandwiching the veneer pack, then it’s just a matter of time before you have your arched part.

The table with hinged stretcher lid is next.

Here’s a side shot of the body sitting in the mold to give you the general idea.

 

[KnightroExpress]

This is so cool. I can't wait to see it in use!

 

[rockinlespaul]

Wow, very nice work!

 

[b_rogers]

Mind blowing good thread...Props.