Acoustic Bass Guitar Case

Materials

Current working approach: double angle joiner extrusion
6mm / 0.25" plywood to keep weight down

Angle channels mean sides stop before the corner ("symmetrical" corners rather than butt joints)
With this hardware each piece needs to be 3mm LESS than the target interior dimension
(1.5mm offset in joiner cross section)

Rather than building the carcase and then cutting the lid off,
We'll build box and lid separately.
The hardware for this build, square-interior hybrid tongue-and-groove channel adds 6mm to height

5mm at "outer overhang" side (lid)
1mm at "outer bevel" side (box)

NB: double-check all of these, part numbers / options etc.

Trapezoid = corners <> 90 degrees

Using the spreader and smoosher anvils, hammer and corner punches, bludgeon the hardware pieces into submission.
These jigs take a corner-piece, or length of corner-joining section, and adjust the angles.

• The spreader fits inside a 90-degree angle, and by way of hammering the outside, spreads it to 99 degrees

• The spreader allows a 90-degree angle to nestle inside it, and with a series of punches, compresses it to 81 degrees

Cutting plywood pieces

Trace two copies of trapezoid from (oversize) MDF template onto 6mm ply sheet
Adjust sizes by drawing lines parallel to template outline
For a target interior body length of 1260mm, but remember we subtract 3mm for joiner section overhead
This will give something like

Cut these two pieces out, clamp together and plane edges straight and true
If there is any asymmetry, clearly mark which side is "up" on both pieces

AV-Titan corner pieces require 92mm from (outer) top of lid to bottom of lid tongue-n-groove

calculations and experience: cut the lid end-and-side pieces to be 86mm high

Since we are aiming for 215 total interior height, we have 129 left to fill.

The box sides should therefore have a height of: 118mm high

In this build I could only find 6mm tongue-and-groove, and ~7mm double-angle joiner section,
so I glued strips of shim veneer to the pieces.

• Next build: experiment with not doing this

Paint the inside faces with black water-based enamel (don't need to go to the edges)
Finish the outside faces with Duratex: one brushed-on layer, one or two coarse-sponge-roller'd

To save on Duratex

• Consider building neck-support piece (actual width-bevel can be finessed later)
  Then we can finish the exposed parts now (see images below)

Commence construction

cut long lengths of double-angle joiner 40mm short of trapezoid pieces
create steel drilling guide
with double-angle joiner we cannot have rivets at the same position opposing each other

we must either offset them minimally (5mm) or in some sort of alternating arrangement
try minimal - seven rivets each side, ending 40mm in from each end OF CUT ALUMINIUM (60 in from trapezoid piece ends)

7 rivets = 193 spacing

follow same procedure for trapezoid end edges

channel 40mm shorter than edge
rivets starting 35..40mm in from edge, each pair offset by 5mm
four rivets for wide end, two for narrow

attach all perimeter channel lengths to trapezoid pieces

Cut corner-edge lengths for verticals

Lid: 4x 55 (+6 due to error = 61)
Base: 4x 100 (-6 due to error = 94)

Use the spreading and smooshing jigs to fit the +9/-9 degree trapezoid angles

Mallet in all side pieces but don't drill through the corner-channel holes yet
Hammer in the angle-adjusted vertical channels from the open end
Cut tongue-and-groove channel pieces, bevelling with band sander, mallet them in place
Position lid on top of box, and adjust lengths/bevels until lid mates perfectly with base
Mark all tongue-and-groove channel pieces so we know where they go, and drill rivet holes
Lid, front side:

Two, 530mm in from each end (corner brackets & latches will provide the rest)

Base, front side:

One, 530mm in from left/headstock end (ditto plus handle)

Base and lid, back side:

Two, 380mm in from each end (corner brackets and the three hinges will provide the rest)

Base and lid, wide end:

Two, 200mm in from each end

Refit the tongue-and-groove pieces, clamp up, make sure everything is fully seated
Drill all holes through the wood
Drill a single pilot hole for four of the eight lip-corner braces (eg, lid)
Unclamp, take off lid, rivet remaining double-angle and tongue-and-groove channel holes
Fit corner braces with single pilot rivet, reclamp, drill remaining corner brace holes, rivet
Clamp again, drill four pilot holes for remaining corner braces
Unclamp, rivet, clamp, drill remaining holes, unclamp rivet
Address any waterproofing measures

• glue-gun some silicone sealer along open gaps

• a little around the three-way corner perimeter so the corner pieces will fully seal

Fit corner pieces:

• Complex and annoying

• Multidimensional clamping

• Pilot rivets

Hinge and latch cutouts

Time to cut and drill for hinges, but do not attach yet

• We're using three, so the hinge-stay stresses are borne right next to the corner brackets
  Question: consider using four, as heel-barrier timber is about halfway

• The two outer ones are as close as possible to the edge, with the one in the middle

• The one in the middle is a potential problem, as it is right where we need the heel-barrier to be,
  which also functions as a through-brace giving support to the handle.
  Nevertheless, since the height of the hinge's bottom section is 66mm,
  and the internal depth of the box is 136mm, we should be OK.

• Current hardware:

NB: cutout depth for box is measured from the tongue-and-groove outer-ledge, not the highest point!

• Start cutout 15mm in from ends of corner brackets (slightly more for non-offset, ie 50:50 hinges)

• Start smaller, enlarge by bevelling

Procedure:

• Drill corners

• Cut aluminium with angle grinder, finish with hacksaw

• Cut verticals with pull-saw

• Rough-in horizontals wity jig-saw

• Smooth aluminium with rasp

• Smooth wood and enlarge/bevel to fit with Shinto saw-rasp

• Again, do not attach yet

Repeat similar procedure for latches:

• Current hardware same dimensions as latches

• However, we only fit two, cutouts starting 250mm from each end

• This time we can drill and fit them:
  NB: before drilling, front edges should be clamped firmly so lid will be tight when latched

And finally again for the handle:

• Centre hole approx 763mm / 537mm in from the neck/tail ends respectively

• Vertical placement: bottom of piece should be more or less same as top of bottom corner channel

• Current hardware, hole should be 125 x 70mm

• That means hole should extend from 20mm..90mm above top of bottom channel

• Cut hole a little small, enlarge with file/shinto to get perfect position

Check lid fit:

• tong-and-groove ends at butt-join corners and latch-cutouts may foul with opposing corner-brackets / latch halves.

• Filing a bevel at these points should allow the lid to close easily and fully

Making the heel-barrier

Cut heel-barrier timber, ~370 x 100, with bevelled sides so it fits upright

Inward-facing edge about 625 from inside of wide end.
This is precisely where the middle hinge sits, meaning the height, at least on the hinge-side must be 70mm (for offset hinges)
Question: should we use four hinges to avoid this?
Cut 16x16 cutouts in the bottom corners, to avoid aluminium channel and rivets

• Cut bevel on tablesaw, with about 50mm rise over 370mm

• finalise fit, making sure that piece is perpendicular to centreline

• check that we're not making sides bow outward (interfering with lid closing)

• mark outlines on bottom and sides

• drill screw holes from inside, countersink from outside
  holes are best positioned if drilled from the inside,
  however this can cause a lot of unwanted blowout on the other (already painted) side.
  Either use a sacrificial backing piece, touch-up the blowout with paint,
  or attempt to transfer measurements to the outside and drill inwards.

• don't screw or glue it in yet

Starting the neck-support piece

Cut neck-support piece using same plan, but don't finalise height yet

• 200mm tailward of heel-barrier cushion piece (180mm if four-hinge design)

• again, don't fit it!

OK to drill the "easy" hinge holes - those that are just through wood, not aluminium section

Starting on the cushions

Cut main base body cushions

• 25mm foam, use trapezoidal template, extending 540mm from tail toward heel
  will stop short, to be filled in with thicker piece after fitting heel-barrier

• cut a piece of velvet slightly overside, glue together, trim fabric flush

Cut two base tail cushions, and two base lower bout cushions

• 25mm foam, ~282mm long, 105-110 high

• 2* the harder stuff (tail) and 2* normal (lower bout)

• cut maching pieces of velvet

• all pieces have wrap over the top

• for tail cushions, also wrap over the inner join, so strap button won't catch on trimmed edges

Cut heel-barrier cushion

• harder foam, about 280x380, to wrap around the heel-barrier timber, edges flush with base

• attach form, trim to fit, then wrap with velvet & trim again

Nestle all pieces (so far) in place, measure and cut the tree thicker cushions

• 1x heel-base cushion, approx 400x70mm (bevelled to fit)

• 2x upper bout cushions, will be approx 300x105, with a cutout to clear the heel base cushion

• cover with velvet

Completing the neck-support piece

Designing neck-channel profile (see image below)

• draw a 100mm circle in one 9mm plywood, approx 350x170mm

• cut it in half lengthwise, leaving each half with a semicircle

• cut out those semicircles with a bandsaw & band sander

• cut off an extra 9mm, so semicircles are slightly less than half

• use 360mm circle as guide to profile the piece (14" hihat is a good cheat...)

• trim pieces to those lines

• glue the pieces to the neck support piece, at appropriate height (see next para)

• cut along profile lines, sand smooth

Nestle all pieces in place again, plan height of neck-support

• shim headstock, measure from base to underside of neck (expect approx 170mm)

• allow ~23mm of foam (25 compressed by neck)

• trace the neck profile, extended out by ~23mm, onto neck-support piece

• glue an extra piece of timber, so channel portion will be thicker

• cut channel on bandsaw, shape with belt sander spindle etc

• paint it black: textured finish on exposed surfaces

• glue 25mm foam in channel

• glue velvet over exposed foam channel faces

• trim edges

• (next time: consider fabric across entire top surface)

Nestle completed neck-support in place, check strings are level lenghth-wise

• hopefully, strings section will have correct clearance for 25mm foam

• for two sides of soundboard, decide whether to use 25mm, 50mm or something in between, foam

Lid cushions

Cut, measure, and resaw to correct thickness

• one 25mm piece, 900x9mm (bridge to nut)

• cut three 50mm pieces, 150x300mm, two positioned either side of strings, one between bridge and tail
  They will almost certainly be too thick: rest lid in place, measure how far off closing the lid is, then resaw to correct thickness with bandsaw!

• Cover with velvet, glue into the lid using low-tack masking tape to protect borders from overspray

Do a test-fit of both heel-barrier and neck-support pieces

• screw them in, then test that the box-sides are neither pushed out or pulled in

• this can affect whether the lid closes cleanly

• shim or shave accordingly

• If they don't interfere with hinges, leave them screwed in, otherwise remove

Final steps

Fit hinges, permanently attaching lid

• this is tricky, as it is almost impossible to rivet while the lid is closed

• doing it while open risks having the lid skewed to one side, causing problems closing

• if using inner brackets, consider attaching with glue or double-sided tape

• if using washers, consider attaching one-per-hinge with double-sided tape

• the idea is to get the positioning right, so we can then fit the remaining rivets with confidance

Fit neck-support and heel-barrier pieces

• Glue and screw to set

• double-check how the lid closes again

Glue in all body cushions

• do any final black paint touch ups if required, inside and out