tabletop metallophone for gamelan practice \\ prototype 2 (build guide)

build guide

build time: 8 hours without power tools, not including paint or finish (yes, I’m slow).

tools

You will need:

  • drill bits: 3mm & 4mm
  • electric drill
  • hacksaw (or angle grinder)
  • tenon saw (or power saw)
  • straight files: smooth & bastard cut (or disc/belt sander)
  • side cutters
  • pliers
  • 2* F clamps
  • centerpunch
  • hammer
  • screwdriver
  • small adjustable spanner

You may also need:

  • heavy-duty bench vice (makes life easier)
  • jack plane (if you can’t find timber in the right dimensions)
  • A few drops of machine oil or WD-40 (for drilling node holes.)
  • Sandpaper, finishing equipment etc. – if you are so inclined.

materials

  • Steel flat bar – 3m length of 32mm(w)*3mm(t)
  • Timber – 1.4m length of 30mm(w)*16mm(t)
  • bullet head timber nails – 1mm(d)*30mm(l)
  • 12 units * M4 bolts – 65mm length
  • 12 units * M4 wing nuts
  • 24 units * M4 machine washers
  • 32 units * M4 hex nuts
  • nylon line, 1mm diameter * 1.5m length (or two lengths of nylon guitar 3rd string)
  • 20 plastic/wooden beads (or 10cm of standard 5mm flyscreen spline)

okay let’s do it

Just a few simple steps:

  1. cut and tune the keys
  2. drill node holes
  3. check geometry, then cut the frame pieces
  4. drill bolt holes in the frame sides
  5. assemble the frame
  6. string it up!

Let’s get cracking!

1. cut and tune the keys

This is the hardest part, but definitely the most fun. If you’re not using 32*3mm plain steel, you’ll have to take a totally random guess as to the length of the first key (the table below might help you find a ballpark), tune it to fit in your desired pitch set, and figure the rest out from there. It can get ugly, and you can expect to waste a few lengths of material along the way – share a table in the comments if you take this route!

Cut the keys a few mm oversized, and then file down the excess from the ends until they’re around 3-6 cents flat. Some things to bear in mind:

  • Metal heats up as it’s worked. This will expand and soften the material, lowering the pitch until the temperature drops again – so you should never tune a bar to sing exactly at or above your target pitch.
  • Steel from the hardware store varies quite a lot in density. Multiple bars cut to identical lengths can be as much as 50 cents out of tune with one another. I’ve personally cut a bar 1mm longer than the original, planning to file down the excess – and it sang sharp by a few cents!

Suwitra Jaya, the gamelan I play in, uses a combination of gong kebyar and semar pegulingan instrumentation tuned ‘diatonika‘ (approximating the white keys of the Western piano). The table below provides rough dimensions for ten plain steel bars in 32mm*3mm, with the fundamental (or first transverse partial) tone tuned within 5 cents of absolute Western pitch (A=440Hz), sounded at an ambient temperature of around 15°C (early winter in Sydney). Lengths will differ for all other materials and dimensions.

#pitchkey length (mm)
5B3254
6C4242
7D4233
1E4220
2F4214
3G4202
4A4191
5B4180
6C5174
7D5165
tuned bar lengths for 32mm*3mm plain steel. (From experience, similar ballpark range for 40mm*3mm aluminium.)
#pitchkey length (mm)
5254
6230
7D4216
1205
2
3
4A5174
5
6C6160
7
tuned bar lengths for 32mm*5mm plain steel.

There are in fact sources out there for those who wish to tune the second transverse partial tone to a harmonic interval – see related reading. I found that using plain steel in 35mm*3mm, the second transverse tone sounded very close to seventeen semitones (perfect 11th) above the fundamental – quite acceptable for my purposes. I also think 3mm material might be a little too thin to risk taking weight off the centre.

2. drill node holes

For ideal flat bars, nodes of the fundamental frequency are positioned at 22.4% of the length from either end (Hopkin 1996, pg 31). Note that on Western tuned percussion instruments, the undercutting of bars in the centre pushes the nodes outwards (La Favre 2007; Hopkin 1996)- so your xylophone bars probably aren’t going to make for a great reference!).

It’s a fairly simple process, and since we’re suspending the bars from cord, the hole position is quite forgiving – you can do the whole thing without making any measurements.

1) Suspend the bar upright by pinching at the node point.

2) Repeatedly strike the bar in the centre of its length while shuffling your position around until you find the suspension point which yields maximum presence and sustain from the fundamental tone (lowest pitch the bar produces).

3) Mark this point with a centerpunch and hammer at the centre of the bar’s width, then repeat for the node on the other end of the bar.

5) Check your accuracy by suspending the bar at the marked point, striking and shuffling.

Repeat for all bars.

Tuned key clamped to a block and marked for drilling. I used a whiteboard marker to help me find the punched indents.

Once you are satisfied that all node holes have been accurately marked, it’s time to drill out the holes! Use a drill bit which is a little over double the diameter of your nylon line. I used a 3mm bit for 1mm nylon line, which worked just fine. (Note: I’m no metalworker, but it might be a good idea to use lubricant for drilling, especially if you’re working with bars thicker than 3mm.)

3. check geometry, then cut the frame pieces

Great, we’ve finished the keys and we know where all of the node holes will be!

For my most recent prototype, here’s a scale drawing of how everything roughly fits together for ten keys (tuned as above) at a uniform width of 32mm:

Updated plan coming soon!

(That internal angle should actually be 86.57491189663236° according to trigonometry, but if you’re using a softwood, 86.5° is probably close enough.)

Note that the sides of the frame should lie roughly under the positions of the node holes we drilled in step 2.

If any of your materials or tunings have differed so far, you’ll want to make your own measurements and calculate the internal angles. You can pretty well eyeball it without worrying too much – just make sure you allow for at least 5mm between the edges of the each key, and allow some space for wing nuts and mounting hardware.

Frame side pieces: 510mm long from end to end. 30mm excess on either side; then mark for six holes spaced at 90mm intervals.

Time to put that tenon saw to work!

4. drill the frame sides for mounting hardware

4mm bit (to match the threading on the bolts). For 32mm bars, I allowed for 90mm between hole centres, which gives us plenty of wriggle room when it comes to spacing the bars. Remember to drill straight down!

5. assemble the frame

Start with one side: tap nails until they poke through on one side, impression guide holes on one brace end (careful with the orientation here!), tap nails all the way in. Repeat for second brace, then move on to side B. For side b, tap only one nail in for each brace position first, impression guide hole for one brace, check alignment and fix all the way in…. then add the two remaining nails.

Now for the bolts, which we are using as mounting hardware. Thread on a washer, poke the bolt through the underside of the frame, thread on another washer and hex nut, secure the nut tightly, thread on a wing nut until the wings are about level with the end of the bolt. It’s not a bad idea to hot-glue the wing nut in place, or secure it tightly against another nut… but that might only be necessary if you plan to travel a lot.

6. string it up!

This is definitely the least fun part, but at least it’s a simple process.

1) poke the nylon cord through the top of the lowest (leftmost) key.

2) thread a bead* onto the leading end of the cord, then poke that end back through the same hole. Be sure to leave about 20cm of excess hanging to the left of the key – we’ll use this to tie the keys to the frame when we’re done!

3) Mirror the above with a separate cord through the other hole.

4) Repeat that whole lot again for all of the other keys, adding them one at a time until everything is loosely attached.

Now for the properly frustrating part: slap that cold, clanging mess onto your metallophone frame, and manually adjust the spacing of each key. Remember to apply a little tension and check regularly as you go, because the spacing will change once you tie the ends down.

When you’re satisfied with the key spacing, it’s time to tie it all together! I just improvised a knot which involved folding the cord over the wing nut, under the frame, and back up around the bolt before finishing with a simple half-hitch. Let me know if you figure out a better system!

La Favre, Jeff (2007). Tuning the Marimba Bar and Resonator.

Hopkin, Bart (1996). Musical Instrument Design: Practical Information for Instrument Making. Chapter 4: Idiophones.

von Helmholtz, Hermann (1863). On the Sensations of Tone.