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
Do not cut this outline: this is the target exterior.
Adjust sizes by drawing lines parallel to template outline
(Trim template in future so this is not necessary?)
For a target interior body length of 1260mm, but remember we subtract ~20mm for (2x) 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

Side pieces therefore should start out as

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.

• Experiments have proved this necessary: rivetting closes the gaps, but the resulting angle is not 90-degrees.

• Don't bother on the vertical edges - angle-adjusted channel has a narrower channel.

Two options:

• Mill strips to <1mm, glue and hope for the best

• Mill strips to >1mm, glue, thickness with drum sander
  (which is just wide enough for wide end of top & bottom(

Proposed

Simplify cutout process by making hinge, latch and handle cutouts now instead of later
After confirming these measurements, make a set of templates

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

Drilling guide for double-angle joiner section

Long guide, so we can use it for hole spacing/offsetting as well as transverse position guide
Exterior piece is 3mm steel
Interior support piece is 6mm aluminium stock with cladding made from 10x20mm box section, cut lengthwise so one leg is max, the other 12mm

Drilling guide for double-angle joiner and tongue-and-groove sections

Shorter piece for drilling individual holes
Exterior piece is actually 9mm tongue-and-groove section, which allows 6mm section to nestle inside
Interiour support piece is 6mm stock with cladding from cut 10x20mm box section
The guide hole is positioned in the first narrow valley of the short leg of the 9mm section

Cut long lengths of double-angle joiner 40mm short of trapezoid pieces (to leave 20mm clear each end: ~1234mm)
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
for seven rivets each side, ending ~40mm in from each end of cut aluminium (~60++ in from trapezoid piece ends)

7 rivets = 193 spacing

Drilling technique:

• drilling will tend to enlarge template holes

• use template to drill starter holes only

• when a piece or set is done, drill all the way through using internal support piece only

Follow same procedure for trapezoid end edges

channel 40mm shorter than edge (~534 & 124mm)
rivets starting 35..40mm in from edge, each pair offset by 5mm
The following measurements from LHS edge give decent offsetting:
wide end: 40, 190, 340, 490
short end: 35, 85

Attach all perimeter channel lengths to trapezoid pieces (including rivetting)

Cut corner-edge lengths for verticals

Lid: 4x 61mm
Base: 4x 94mm

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

• Put some plywood scrap in the channels to assist in distributing from one side of channel to the other

• Note that for spreading, some additional padding will be needed to transfer jig geometry to piece

• Also, for smooshing, the jig will damage the aluminium finish, this can be legitimised by sanding and filing

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 ends with band sander, mallet them in place

Sequence

• Cut the four long lengths a few mm oversize

• Mallet them in place

• Position lid on top of box

• Slide/adjust so that lid is mating perfectly with base

• Measure the two end lengths, taking note that the bevels are internal

• Do the bevels on the band sander, hammer them in place

• Check that lid fits

• Mark external bevels on the (still oversize) but don't sand them yet

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

Base and lid, narrow end:

(nothing, corner brackets are enough)

Refit the tongue-and-groove pieces, clamp up, make sure everything is fully seated
Drill all holes through the wood
Drill a one or two pilot hole for four of the eight lip-corner braces (eg, all the lid pieces)
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 (nestle them against the rivetted ones in the lid)
Unclamp, rivet, clamp, drill remaining holes, unclamp rivet
Moment of truth: check that the lid fits!

Fit corner pieces:

Assuming the pieces are correctly angle-adjusted:

• Drill holes while holding each piece in place

• Minor "tilting" may be required, which should increase rivet tension

• Rivet in place

Waterproofing measures

• there's quite the gap around each corner, and the 3-way corner pieces aren't a watertight fit

• glue-gun some silicone sealer into corner cavity (black Sikaflex) to improve water resistance

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
  

• The two outer ones are as close as possible to the edge (leave 5mm gap from bracket)

• 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.

• Therefore, the middle one should sit 530..630mm measured from the wide-end outer corner
  (symmetry is overrated, lol)

• Current hardware:

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

• Start cutout so that entire 18mm in from ends of corner brackets

• Start smaller, enlarge by bevelling

Procedure:

• Drill corners

• Cut aluminium with angle grinder, finish with hacksaw

• Cut verticals with pull-saw

• Rough-in horizontals with 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

• Don't fit them yet, the holes will make fitting hingers a lot easier

And finally again for the handle:

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

• Vertical placement: align the top of the cutout with the bottom of the tongue-and-groove 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

• Might as well fit it at this stage

Fit anchors for cord-lid-stay (fit heavy-duty picture anchors with rivets)

Lid: approx 80mm from inner RH-front corner
Base: approx 60mm from inner RH-front corner

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 would have been where the middle hinge sat, had we centred it prefectly)
Cut 16x16 cutouts in the bottom corners, to avoid aluminium channel and rivets

• Cut bevel on tablesaw, with about 50mm rise over 300mm

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

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

• However, since we'll be screwing the piece in, we don't want it pulling the sides inwards either

• 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

• 190mm tall in the middle, height/shape to be finalised later

• Width to fit 200mm tailward of heel-barrier cushion piece (310++mm a good start?)

• At edges, trim so it clears tongue-and-groove channel

• again, don't fit it!

Starting on the cushions

When attaching velvet to foam:

• adhesive overspray is impossible to cleanly remove from foam

• mask off any foam surface that will be exposed

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

• mask off barrier base and sides to prevent glue residue (which will interfere when we attach it)

• 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

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

• put guitar in padded case bottom, check neck is horizontal

• put a padded shim under headstock headstock if required

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

• transfer this measurement to neck support piece

Designing neck-channel profile (see image below)

• mark actual position of neck-bottom on fitting, but oncomplete neck support piece

• centre this, then mark 23mm (25mm foam, slightly compressed) below that

• draw a 50mm radius / 100mm diameter circle that touches this point, above it

• mark a point 42mm above the lowest point of the circle

• using a 13" or 14" cymbal as a guide, trace from sides to this 42mm point

• the larger circle should extend up from the edge of the case bottom

• the smaller circle should cut into the larger (when padded with foam, it is the neck pocket)

• cut just outsite these lines roughly on the band saw, get closer on the band sander

Completing neck-channel support piece

• trace the outline on some pieces of plywood, so channel portion will be thicker

• decide how far these widening pieces go down: flat, aligned with tongue-and-groove, scalloped upwards, etc

• cut the extra pieces, glue to main piece

• finalise profile with band sander spindle etc

• paint the piece black: textured finish on exposed surfaces

• glue 25mm foam in channel

• glue velvet over exposed foam channel faces

• trim edges

• optional: 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

Lid cushions

Cut, measure, and resaw to correct thickness

• one 25mm piece, 900x90mm (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

Confirm that anchors for cord-lid-stay are installed

Fit neck-support and heel-barrier pieces

• Glue and screw to set

• double-check how the lid closes again

Fit hinges, permanently attaching lid

• Make sure cutout bevels allow the hinge to seat perfectly, and stand-offishness will prevent the lid from closing

• attach to lid first, then clamp lid to base and tight before rivetting

• Being able to poke your hands through the latch cutouts makes this a lot easier than if we'd already fitted latches

Fit latches

• At last...

Glue in all body cushions

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

Fit cord-lid-stay (the cord part)