The Machine · Recipes

Lapless Steel

Prototype stage

Updated 2026-10-19

One guitar string over aluminum extrusion. A steel rod rides under the string on a belt-driven plate, so every change of note is a glide, like a lap steel guitar with no one playing it.

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Printed

3DThe whole module. This is the version we started with. Only the rod holder is printed, and it's drawn in orange. The string is drawn thicker than life so it shows. Press Play to watch it glide and pluck.

Overview

Cost, roughly: About $210
Printing: About 1.5 hours
Building: A weekend
Difficulty: Moderate

Anatomy

Energy source: A servo plucks the string
Control: Where the rod sits under the string sets the pitch; a stepper moves it
Sounding body: One steel guitar string over a magnetic pickup
Reset: None needed

Fig. 1The slide. One printed piece on the V-wheel plate holds the steel rod across the string, just under it, and reaches forward to grip the belt. Drawn from the CAD model.

How it works

One guitar string runs 520 mm from a tuner to a single-string bridge, about 23 mm above a 600 mm aluminum extrusion.
The slide is under the string. A plate rides along the top of the extrusion on V-wheels. One printed piece on it holds a short steel rod across the string and grips the belt that pulls it. The rod sits 1 to 2 mm above the straight line from the tuner to the bridge, so the string’s own pull holds it down on the rod, with about 2 N. There’s no spring and no lever. Where the rod sits sets the note, like the slide on a lap steel guitar, but from below.
Every change of note is a glide. A stepper motor and a toothed belt move the plate, and the rod never leaves the string. The motor sits at the tuner end, as far as it can be from the pickup, and a 3D printer’s belt tensioner holds the belt at the other end.
The range runs from about two semitones above the open string to a little over two octaves above it.
A micro servo plucks the string. It sits in front of the string, near the bridge, and swings a pick up or down through it.
A magnetic guitar pickup under the string feeds its own jack, beside it, and from there the console. A strip of felt mutes the short length between the tuner and the rod, which would otherwise ring a note of its own.
An ESP32 drives the stepper and the servo. Running from the shared battery, it can join the wireless system.

Notes

This is a first design. The 3D model and the files show the version we started with. Bought parts vary, so check your tuner, bridge and pickup against the numbers in the CAD script before drilling.
Costs in the ingredients are rough estimates for budgeting, not checked prices.
Firmware comes with the broadcast control system. For a first test, any stepper and servo example sketch will do.

Files

Parts

Ingredients

Kind	Part	Qty	Source	Each	File
Printed	Rod holder and belt grip, snaps the slide in under the string and clamps the belt's two ends	1	·	·	STL
Fabricated	Aluminum angle, 40 x 20 x 3 mm, 40 mm long, for the tunerDrill one 10 mm hole for the tuner and two 5.5 mm holes for the screws.	1	·	$3.00
Fabricated	Small aluminum angle for the jack, about 15 x 20 mmDrill a 9.5 mm hole for the jack and a 5.5 mm hole for the screw that holds it to the back face.	1	·	$1.00
Bought	V-slot aluminum extrusion, 20 x 40 mm, 600 mmAbout 400 mm and up all work; see Variations.	1	·	$15.00
Bought	V-slot gantry plate for 20 mm, about 65 x 65 mm, with four solid V-wheels and their spacersTwo of the spacers are eccentric, to take up play.	1	·	$20.00
Bought	NEMA 17 stepper motor, 40 mm body, and a steel L-bracket	1	·	$16.00
Bought	GT2 pulley, 20 teeth, 5 mm bore, for 6 mm belt	1	·	$2.00
Bought	3D printer belt tensioner for 20 mm extrusion, with its own toothed idler and a knobWe started with AEDIKO's X2020, made for the Ender 3's X axis; there are many like it. Its bar bolts flat on a bracket's shelf, so its idler lies flat in line with the motor's pulley, and the knob tightens the belt.	1	·	$8.00
Bought	A second NEMA 17 steel L-bracket, as the tensioner's shelfThe same as the motor's, so the tensioner's idler lands in line with the motor's pulley. Drill two holes in its shelf if the tensioner's screws don't line up with its slots.	1	·	$4.00
Bought	GT2 belt, 6 mm wide, 1.5 m, fiberglass-reinforcedMost GT2 belts are. Avoid one with a steel core, which the pickup could hear as it runs past.	1	·	$4.00
Bought	V-slot extrusion, 20 x 20 mm, 120 mm long, with two corner brackets, as a foot	2	·	$4.00
Bought	M5 screws and T-nuts for 20 mm V-slot	12	·	$0.30
Bought	M3 x 12 screws and nuts, for the belt grip	4	·	$0.20
Bought	Sealed guitar tuners for a 10 mm hole, pack of 6 (3 left, 3 right)We started with Jiozermi's black ones from Amazon. One is used; either hand works. The rest are spares, or for more strings.	1	·	$9.00
Bought	One-piece single-string guitar bridges, pack of 6We started with OVSAL's black zinc ones from Amazon: 49 x 10.5 mm, screws 36.5 mm apart, with saddle height and length screws. The string comes up through the bridge from underneath.	1	·	$22.00
Bought	M3 standoffs, 10 to 12 mm, with M3 screws and T-nuts, under the bridge	2	·	$0.50
Bought	A small washer with a hole of about 2 mm, under the string's ball end	1	·	$0.10
Bought	Electric guitar string, wound .036Your choice: see The stringTuned to about D3 on 520 mm, it pulls about 100 N, near its usual tension on a guitar.	1	·	$2.00
Bought	Steel dowel pin, 6 x 18 mmYour choice: see The slide	1	·	$1.00
Bought	Felt strip, or a guitar fret wrapMutes the short length of string between the tuner and the rod.	1	·	$3.00
Bought	Single-coil guitar pickupYour choice: see The pickupAny Strat-style single coil. It sits on double-sided foam pads on top of the extrusion, a pole right under the string.	1	·	$15.00
Bought	Mono jack, 6.35 mm, and shielded cableThe pickup's way out, on its own bracket beside the pickup. A plug-in wireless guitar transmitter in the jack keeps the element cable-free.	1	·	$3.00
Bought	MG90S micro servo, metal gears	1	·	$5.00
Bought	Aluminum bracket for micro servosBolted into the front top slot, near the bridge.	1	·	$4.00
Bought	ESP32 development board	1	·	$6.00
Bought	TMC2209 stepper driverQuiet, which matters with a pickup a few centimeters away.	1	·	$6.00
Bought	Micro switch with roller lever, and a mount for 20 mm V-slotThe home switch. The rod holder's arm presses it at the tuner end of the plate's travel.	1	·	$3.00
Bought	5 V buck converter, 3 AFor the ESP32 and the servo, which draws up to about 1 A when it plucks.	1	·	$3.00
Bought	Battery adapter for the shared power-tool platformThe stepper driver runs from the battery, through the fuse, the stop button and the relay.	1	·	$15.00
Bought	The listener parts every element carries, as on the Control pageA blade fuse holder and 2 A fuse, a latching stop button with a spare contact, the status light, the number switches and their resistors. The stop switch under Choices has a plain two-pole switch in place of the stop button.	1	Link	$12.00
Bought	Momentary push button, normally open, for the test buttonA short press plays each note once, and a long press relearns what the element can by itself. Any small panel button that closes while it's pressed will do.	1	·	$1.00
Bought	Relay module, 5 V, high-level trigger, contacts rated 10 AThe switch the motor's power goes through. It stands in for the MOSFET the Control page also allows, because a MOSFET module switches the ground side, which would leave the stepper driver grounded only through the ESP32.	1	·	$2.00
Bought	Resistor, 10 kΩHolds the relay off while the ESP32 starts.	1	·	$0.10
Bought	Electrolytic capacitor, 100 µF, 35 VAcross the driver's motor supply, to soak up the spikes the motor sends back.	1	·	$1.00
Bought	Project box, about 90 x 60 x 60 mm, for the electronics, with holes in its lid for the stop button and the test buttonYour choice: see The electronics	1	·	$6.00
Salvaged	A guitar pick, medium, and a mini binder clip to hold it on the servo hornYour choice: see The pluck	1	·	·
Total				$205.60

Tools

3D printer, PETG or PLA, for the rod holder
Drill, with 10 mm and 5.5 mm bits
Hex keys, 2.5, 3 and 4 mm
Small screwdriver
Soldering iron, for the pickup and the jack
Clip-on tuner
An amp, or a cable to the console, to listen

Choices

Decisions that are yours to make. Where we started is the version in the 3D model and the ingredients: a starting point, not the right answer.

The slide

What does the string rest on?

OptionHow it soundsTrade-offs

Steel rodWhere we started

SoundBright, with a metallic edge, and a scrape on every glide over a wound string

Cheapest; a 6 x 18 mm dowel pin is ready-made

Brass rod

SoundWarmer, with a softer attack

Soft, so the string slowly wears a groove in it

Glass rod

SoundSmooth and warm, the bottleneck slide sound

Costs more and can chip; cut it to length with care

Spinning roller

SoundThe scrape is gone; the string rolls over it instead of sliding

One more small part, a rod turning in two small bearings

Any 6 mm rod fits the holder. For another size, change the rod size near the top of the CAD script and print the holder again.

The string

Which string, and which note?

OptionHow it soundsTrade-offs

Wound .036, tuned near D3Where we started

SoundFull and growly, the lowest of the three

Cheap, and sold singly at any guitar shop. Glides scrape.

Flatwound .036

SoundDarker and smoother, with nearly silent glides

Costs more, and is mostly sold in sets

Plain steel, .017 to .024, tuned higher

SoundBright and singing, with silent glides

Quieter through the pickup, and the whole range moves up

Whatever the string, aim for 80 to 100 N of pull. A guitar string tension chart gives the pull for each gauge and note.

The pickup

How does it hear the string?

OptionHow it soundsTrade-offs

Magnetic single coilWhere we started

SoundThe lap steel tone, clear, with a long ring

About $15. It ignores the motor and the servo, but hums near the stepper's wires.

Piezo disc under the saddle

SoundBrighter and more acoustic, a little thin

About $1, but the string has to press hard on the saddle, and it hears the motor and the servo through the frame

The pluck

What does the servo swing through the string?

OptionHow it soundsTrade-offs

Medium guitar pickWhere we started

SoundA bright, clear attack

Cheap, and wears slowly

Thin pick

SoundLighter, with more snap and less volume

Easy on the servo

Felt or leather pick

SoundA soft, round attack, like a thumb

Quieter, and needs a deeper catch on the string

The servo and its pick are the Motor Pick's servo flick, run from this element's own ESP32. The Motor Pick's wheel of picks, with its own ESP32 on its own number, can take their place near the bridge, and adds tremolo.

The stop switch

What cuts the motor's power at the element?

OptionTrade-offs

Latching emergency-stop button, with a spare contactWhere we started

The red mushroom head anyone nearby knows to hit. It stays down until it's twisted to release, so the element can't start again by accident. Its main contact cuts the power and its spare contact tells the ESP32. About $8.

Two-pole power switch, a rocker or a toggle

Smaller, plainer and cheaper, about $2. One pole cuts the power and the other tells the ESP32, so it does the same job. It doesn't read as a stop to someone who doesn't know the Machine, and it switches back on as easily as off. Rate its contacts above what the element draws, as the button's are.

Keep the second pole either way. The ESP32 knows where the slide is only by counting the steps it sends, so the second pole is how it learns the power was cut, and finds its home switch again before it moves. Without it, a slide pushed by hand while the power was off could be driven into the end of its travel. Either way the relay stays, since it's how a stop from the Bridge reaches the element when no one is at it. A two-pole switch wires as the button does: one pole in place of the main contact, from 11 to 12, and the other in place of the spare contact, from 13 to 14.

Print

Material: PETG
Layers: 0.2 mm
Walls: 4
Infill: 40%

There's one printed part, and it prints without supports. PETG holds its shape in a warm room and under the string's steady push; PLA is fine for a first try.

The printed parts

Rod holder and belt grip, snaps the slide in under the string and clamps the belt's two ends
Which way up
On its side, so its outline lies flat on the bed
Supports
None
Time
About 1.5 hours
The slide clicks into two cradles whose mouths are a little narrower than it. If it won't go in, file the mouths a little; if it's loose, print again with the mouth a little narrower (SNAP in the CAD script). Each belt end slides 10 mm into a slot in the jaw and is pinched by two M3 screws.
STL ↓

PrintThe rod holder (STL)

Assembly

1
Fit the four V-wheels to the gantry plate, two of them on eccentric spacers, and set the plate on top of the extrusion, with the wheels in the upper slots of the two 40 mm faces.
TrickyToo tight and the motor stalls; too loose and the rod chatters against the string. Set it with the plate in the middle, then try both ends.
CheckTurn the eccentric spacers until the plate doesn't rock, but still rolls freely from end to end.
2
Bolt the two short pieces of 20 x 20 extrusion across the underside as feet, one about 50 mm in from each end and clear of the motor, with corner brackets.
CheckIt stands without rocking, and doesn't tip forward under the motor.
3
Bolt the motor's L-bracket to the front face at the tuner end, with the motor hanging under its shelf and the shaft pointing up. Fit the pulley on the shaft.
4
Bolt the second L-bracket to the front face at the bridge end, and the belt tensioner flat on its shelf, with its idler just past the end of the extrusion.
5
Drill the aluminum angle, fit the tuner with its post pointing to the front, and bolt the angle into the top slot at the tuner end.
TrickyThese tuners are made for a guitar headstock about 13 to 15 mm thick, and the angle is 3 mm. If the bushing bottoms out before it grips, stack washers between the angle and the tuner's body. The small screw on the tuner's ear keeps it from turning; drill a 2 mm hole for it, or rely on the bushing's grip.
CheckThe top of the tuner post is about 23 mm above the extrusion, and the tuner doesn't turn in its hole when you tune.
6
Bolt the bridge on its two standoffs into the top slot at the other end, with its saddle about 50 mm short of the extrusion's end and its string hole toward the end. On 600 mm, that's about 520 mm from the tuner post.
7
Print the rod holder and belt grip. Bolt it to the middle of the plate with two M5 screws, its jaw forward over the plate's front edge, and click your slide into its two cradles.
TrickyIf your plate has no holes 18 mm apart along its middle, drill two.
8
Slide one end of the belt into the jaw from one side and pinch it with that side's two screws. Run the belt round the tensioner's idler and the motor's pulley and back into the jaw's other side, pull it snug, and pinch that end too. Turn the tensioner's knob to tighten it.
TrickyThe jaw, the tensioner's idler and the motor's pulley all have to sit at the same height, or the belt climbs off the pulley. Slide the pulley along the motor's shaft to match the idler, and put washers under the tensioner's bar if the idler sits too low for the jaw.
CheckPressed in the middle, the belt gives a few millimeters. Push the plate by hand, and it never jumps a tooth.
9
String it: thread the small washer onto the string, feed it up through the bridge from underneath so the washer and the ball end sit under the bridge, then over the saddle and the slide and onto the tuner post. Wind it so the string leaves from the top of the post. Tune it to its note: about D3 for a wound .036.
TrickyThe slide has to push the string up 1 to 2 mm from the straight line between the post and the saddle. Raise or lower the bridge saddle until it does.
CheckPush the plate from end to end by hand while plucking. There's no buzz anywhere.
10
Tuck the felt under the string between the tuner and the lowest point the slide reaches.
CheckPlucked between the felt and the slide, the string thuds instead of ringing.
11
Fit your pickup. A magnetic one sits on foam pads on top of the extrusion, under the string, between the end of the slide's travel and the bridge, a pole right under the string and 2 to 4 mm below it. Solder it to the jack.
CheckPlucked, it comes through the amp or the console.
12
Bolt the servo's bracket to the front face near the bridge, so the servo sits in front of the string with its shaft pointing toward the tuner. Clip your pick to the end of the horn's arm, flat side up, so it sweeps up and down through the string.
TrickySet the pick to catch the string by 1 to 2 mm. Deeper is louder, but the servo strains and the pick can snag.
13
Fit the home switch in its mount on the back face at the tuner end, with its roller where the plate's back edge presses it before the slide reaches its lowest note.
CheckPush the plate toward the tuner by hand. The switch clicks before anything else touches.
14
Mount the electronics box on the back face at the tuner end, with the stop button through its lid, and the battery adapter at the bridge end with the fuse holder right beside it. Fit the jack on its bracket beside the pickup, and wire it all as the Wiring tab shows.
TrickyKeep the pickup's cable away from the stepper's wires, or it hums.
15
Before any code, push the plate by hand, pluck, and listen.

Wiring

The fuse comes first, right at the battery adapter, at the bridge end; everything after it lives in the electronics box at the tuner end. The ESP32 and the servo run from the 5 V converter. The motor's power goes through the stop button and then the relay, so either one cuts it. The pickup is wired on its own, straight to its jack. The status light, the number switches and the battery reading are wired the same way on every element, so they're on the Control page, not here.

PowerSignalMotorGround, all joinedPins drawn faint are left empty.

Wire list

FromToWire
Battery adapter, +
Fuse, 2 A, in
PowerRed, 16 AWG
The fuse holder sits right at the battery adapter, so the wire is as short as it can be and nothing unfused runs along the extrusion.
Fuse, 2 A, out
5 V converter, IN+
PowerRed, 22 AWG
Along the back slot to the electronics box at the tuner end, about 60 cm.
Fuse, 2 A, out
Stop button, 11
PowerRed, 18 AWG
Along the back slot with the wire above, to the stop button in the box's lid.
Stop button, 12
Relay module, COM
PowerRed, 18 AWG
The stop button's main contact opens when pressed, cutting the motor's power itself, so a program that hangs can't ignore it.
Relay module, NO
TMC2209 driver, VM
PowerRed, 18 AWG
Capacitor, 100 µF, +
TMC2209 driver, VM
PowerThe capacitor's own leg
Solder its legs straight onto the driver's VM and GND pins. The stripe on its side marks the − leg.
5 V converter, OUT+
ESP32 board, 5V
PowerRed, 22 AWG
Only once the converter is set to 5 V.
5 V converter, OUT+
Relay module, VCC
PowerRed, 22 AWG
5 V converter, OUT+
MG90S servo, +
PowerThe servo's red lead, extended in red 22 AWG
Straight from the converter, not from the ESP32's 5V pin. The servo's pull when it plucks would reset the board.
ESP32 board, 3V3
TMC2209 driver, VDD
PowerRed, 22 AWG
The driver's logic runs at the ESP32's 3.3 V, so its signals match.
Battery adapter, −
5 V converter, IN−
GroundBlack, 22 AWG
Battery adapter, −
TMC2209 driver, GND
GroundBlack, 18 AWG
The motor's return. Use the driver's GND pin next to VM.
Capacitor, 100 µF, −
TMC2209 driver, GND
GroundThe capacitor's own leg
5 V converter, OUT−
ESP32 board, GND
GroundBlack, 22 AWG
5 V converter, OUT−
MG90S servo, −
GroundThe servo's brown lead, extended in black 22 AWG
ESP32 board, GND
TMC2209 driver, GND
GroundBlack, 22 AWG
The driver's other GND pin, next to VDD.
Relay module, GND
ESP32 board, GND
GroundBlack, 22 AWG
Resistor, 10 kΩ, 2
ESP32 board, GND
GroundThe resistor's own leg
Home switch, COM
ESP32 board, GND
GroundBlack, 22 AWG
Stop button, 14
ESP32 board, GND
GroundBlack, 22 AWG
Test button, 2
ESP32 board, GND
GroundBlack, 22 AWG
ESP32 board, GPIO 25
TMC2209 driver, STEP
SignalAny color, 22 AWG
ESP32 board, GPIO 26
TMC2209 driver, DIR
SignalAny color, 22 AWG
ESP32 board, GPIO 27
TMC2209 driver, EN
SignalAny color, 22 AWG
Low turns the motor on. The ESP32 holds it high while the element waits, so the motor doesn't hum into the pickup.
ESP32 board, GPIO 33
Relay module, IN
SignalAny color, 22 AWG
Resistor, 10 kΩ, 1
Relay module, IN
SignalThe resistor's own leg
Holds the relay off while the ESP32 starts, when its pins float.
ESP32 board, GPIO 18
MG90S servo, SIG
SignalThe servo's orange lead, extended in any color, 22 AWG
The ESP32 stops sending pulses between plucks, so the servo goes limp instead of humming.
Home switch, NC
ESP32 board, GPIO 32
SignalAny color, 22 AWG
The switch's normally closed contact, which opens when pressed. A broken wire then reads as pressed, so the plate stops rather than driving into the end.
Stop button, 13
ESP32 board, GPIO 13
SignalAny color, 22 AWG
The stop button's spare contact, so the ESP32 knows it's been pressed.
Test button, 1
ESP32 board, GPIO 15
SignalAny color, 22 AWG
The test button. The ESP32's own pull-up holds the pin high until it's pressed. The board reads GPIO 15 as it starts, but only to choose whether to print, so holding the button then does no harm.
TMC2209 driver, 1A
Stepper motor, A1
MotorThe motor's own cable
1A and 1B go to one coil, 2A and 2B to the other.
TMC2209 driver, 1B
Stepper motor, A2
MotorThe motor's own cable
TMC2209 driver, 2A
Stepper motor, B1
MotorThe motor's own cable
TMC2209 driver, 2B
Stepper motor, B2
MotorThe motor's own cable
Pickup, hot
Output jack, tip
SignalShielded cable, its center core
On most single coils the white or yellow lead is hot and the black one is ground. Check yours.
Pickup, ground
Output jack, sleeve
SignalShielded cable, its braid
The pickup's ground stays with the jack, apart from the electronics' ground.
String and bridge, bridge
Output jack, sleeve
SignalAny color, 22 AWG, from a bridge screw
Grounds the string through the bridge, which cuts hum when a hand or the console's cable is near.

Get these right

Set the converter first: Most 5 V converters come set higher. Turn its screw with the battery in and a multimeter on its output until it reads 5.0 V, and only then connect the ESP32 and the servo. More than about 6 V destroys the board.
The servo isn't behind the stop button: Only the motor's power goes through the stop button and the relay. When the button is pressed, the ESP32 stops sending the servo pulses and it goes limp; a hung program could still twitch it, but the pick is light and slow.
Finding the motor's coils: Motor wire colors vary by maker. Touch two wires together and turn the shaft by hand; if it suddenly turns stiffly, those two are one coil. If the plate runs the wrong way, swap the two wires of one coil.
Never unplug the motor with power on: Pulling the motor's plug while the driver has power can destroy the driver. Switch off, or press the stop button, first.
Keep the pickup's cable apart: Run the pickup's shielded cable on its own, away from the motor's cable and the driver. The stepper's current pulses are the loudest hum a magnetic pickup can catch, which is also why the motor sits at the far end from it.
Set the driver's current: The TMC2209 has a small adjusting screw that sets the motor's current. Set it before the first run, following the driver's own instructions, to about half to two thirds of the motor's rated current. That's plenty for a plate this light. Too high and the motor and the driver run hot; too low and the plate skips steps.
Plug the pickup into an instrument input: A guitar pickup sounds thin and quiet through an ordinary line or microphone input. Use an instrument input (sometimes marked Hi-Z), a DI box, or a wireless guitar transmitter.

Mounting

Everything fixes to the V-slot with T-nuts and screws, so any part can slide to a new place or come off with a hex key. Only the plate and the rod holder move, and nothing wired rides on them, so no cable has to follow the slide.

The plate: On top of the extrusion, with its wheels in the upper slots of the two 40 mm faces. The rod holder, the only part that rides on it, bolts to its middle with two M5 screws.
The motor and the tensioner: Two matching steel L-brackets on the front face, each with M5 screws into T-nuts in the slots. The motor hangs under the one at the tuner end, as far from the pickup as it can be. The belt tensioner bolts flat on the shelf of the one at the bridge end, with its idler just past the end of the extrusion.
The feet: Two short pieces of 20 x 20 extrusion across the underside, one near each end. Two corner brackets on each hold it to the bottom slot.
The tuner: Its aluminum angle sits on top at the tuner end, with two M5 screws into T-nuts in the top slot.
The home switch: In its V-slot mount on the back face's lower slot at the tuner end, its roller just where the plate's back edge arrives.
The bridge and the pickup: The bridge stands on two M3 standoffs on T-nuts in the top slot. The pickup sits on foam pads on top of the extrusion, so it can slide along to find the sound you like.
The servo: Its bracket bolts to the front face's upper slot near the bridge, past the end of the plate's travel, holding the servo in front of the string.
The electronics and the stop button: One box on the back face's lower slot at the tuner end, past the plate's travel, beside the motor and as far as it can be from the pickup. The stop button and the test button sit in its lid, where a hand finds them without reaching past the moving plate.
The battery: The adapter sits on the back face at the bridge end, past the plate's travel, on two T-nuts, with the fuse holder beside it. The pack slides on and off without moving anything else, and it balances the motor's weight at the other end.
The output jack: On its small bracket on the back face's lower slot near the bridge, a few centimeters from the pickup.
The wires: Clip-in slot covers hold the wires in the back face's lower slot. The motor's cable goes straight into the box beside it. The battery's fused wires and the servo's cable run along the back from the bridge end. The pickup's short cable to its jack stays at the bridge end, away from all of them.

The electronics

Hidden in a box, or on show?

OptionTrade-offs

Closed boxWhere we started

A project box, bought or printed, at the tuner end with the stop button in its lid. Tidy, and it shields the driver's noise a little more, but it hides the electronics.

Open plate

The boards on standoffs on a flat plate on the back face, with the wires dressed along the slot. The electronics become part of the look. It wants neat wiring, and nothing guards it from knocks.

DIN rail

A short length of 35 mm DIN rail on T-nuts, with each board in a snap-on holder. It looks like a control panel, and any board comes off with a flick. The holders add a few dollars.

Whichever you pick, keep the stop button on the outside, where anyone can reach it.

Tuning

The test button: A short press glides up through its notes, plucking each once, low to high. Hold it for two seconds and it homes the slide against its switch instead. It works only while the element is resting, with no Bridge in range or with the Bridge holding it silent. While it plays or is stopped, the button does nothing. A stop ends a test at once, and pressing again ends one early. The pitch table is still measured with a clip-on tuner, at the console.
A buzz as it glides: The slide isn't pushing the string up far enough. Lower the bridge saddle a little, or put a washer under each end of the rod holder. Keep the lift between 1 and 2 mm.
Every note a little sharp: The slide is pushing too hard and stretching the string. Back the lift off toward 1 mm, then measure the pitch table again.
Notes off from the table: The firmware turns a note into a plate position. Measure a few notes with a clip-on tuner and let it fill in between. Notes crowd toward the bridge, about 25 mm apart at the low end and about 6 mm at the top.
A second note behind the slide: The short length between the tuner and the slide is ringing. Press the felt a little more firmly against the string, or add a second strip.
Thin, or a short wobbly ring: Thin means the pickup is too far away; raise it. A short, wobbly ring means it's so close its magnet pulls on the string; lower it. Somewhere between 2 and 4 mm.
A hum or a buzz between notes: Move the pickup's cable away from the stepper's wires. Stop sending the servo pulses between plucks, so it goes limp instead of humming through the pickup.

Variations

A longer extrusion: Nothing here depends on 600 mm. The string is about 80 mm shorter than the extrusion, and the belt about twice its length plus 10 cm. A longer string pulls harder for the same note, so tune lower or use a lighter string to stay around 80 to 100 N. On a 1 m extrusion, the same .036 sits near E2 and the slide travels about 750 mm.
A linear rail: An MGN12 rail bolted along the top, instead of the wheels, runs smoother and needs no adjusting. It costs a little more, and needs a carriage plate for the rod holder to bolt to.
A belt-driven kit, used as sold: Belt-driven V-slot kits come with the plate, wheels, belt and motor mounts together. Their belt runs along the top, where the string goes, so turn the kit on its side, with the plate on the front face and an arm on the rod holder reaching over. It needs taller feet, and the string sits about 10 mm higher.
A ready-made linear module: Enclosed belt-driven modules come assembled, sold by travel length. Bolt the rod holder to the carriage; there's nothing to adjust, but fewer lengths to choose from.
A lift: A second servo that drops the rod holder away from the string lets the slide jump to a note without gliding.
Homing without a switch: The TMC2209 can find the end of travel by feeling the motor stall, so the home switch can go. It needs the driver's settings line wired to the ESP32, and some tuning so it stops gently.
Several strings: Identical modules side by side, each with its own slide, make a multi-string instrument. The tuners and bridges come in packs of six, enough for six strings.
The E-Bow: The E-Bow, an element of its own, clamps behind the extrusion and holds the note ringing with two coils instead of plucking it, so every glide sustains. It only fits near the bridge, either side of the pickup, so the pickup hears some of its drive as a whine and the low notes swell slowly. Its page says more.

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The Servo MalletServos

Shared by every elementEvery element listens to one broadcast signal. The system →