The Clever Contraptions Corner

A Curiosity.diy Workshop Tool

The Clever Contraptions Corner

Manual — designing and cutting real mechanisms

Contents

What this tool is
Before you cut: the numbers that matter
Quick start
The movements
Module, and why it matters
Gear mesh clearance and the Gear Mesh Tester
Bearings, bolts and dowels
The Complex Workbench
Cut sheets and downloads
Building it
Troubleshooting
Glossary

1 · What this tool is

The Clever Contraptions Corner designs machines that move — gear trains, cams, cranks, linkages, drawing machines — and gives you laser-ready cutting files for them. You set the mechanism up on screen, watch it run, then download SVGs that are already nested onto your sheet and compensated for your laser's kerf.

There are two ways to work. Single movements builds one mechanism on its own — a gear pair, a cam, a rack. Combining parts chains several into one machine on a shared baseplate, using either the Composer Workbench (one layer) or the Complex Workbench (multiple layers, stacked gears, pens and arms).

Everything is cut from flat sheet. The tool assumes plywood or acrylic, bolts for shafts, and optional bearings and dowels.

2 · Before you cut: the numbers that matter

Four settings decide whether your machine runs sweetly or seizes. Wood thickness lives in 01 · Set up the scale; kerf and gear mesh clearance live in 04 · Prepare for cutting; bearings and other hardware live at the bottom of 03 · Build the machine. Get these right once for your laser and material, and everything afterwards just works.

Wood thickness: Your actual sheet, measured. Used for spacers, standoffs and stack heights.
Kerf: The width of material the laser beam burns away. Typically 0.10-0.25mm in ply. The tool grows outlines and shrinks holes by half of this, so finished parts come out on size.
Gear mesh clearance: Backlash between meshing teeth, set as a fraction of module. Default 0.08. This is the single most important setting for gears that turn freely.
Bearings: Whether gears get a bearing seat or a plain bolt hole, and the exact hole diameter to cut for a press fit.

Kerf is not the same as a hole fit

Kerf is the beam width — the same whether you cut a hole or a slot — and the tool compensates for it automatically. A press-fit hole size (for a bearing or a dowel) is a separate, tested number: an “8mm” bearing is rarely exactly 8mm, and wood compresses. Set kerf once for your machine; set hole sizes by testing.

Measuring your kerf

Cut a 50mm square with kerf set to 0.
Measure it with calipers. It will come out undersize, say 49.85mm.
The difference (0.15mm) is your kerf. Enter it in Prepare for cutting.
Re-cut and check: the square should now measure 50.0mm.

3 · Quick start

Pick a movement, or open Combining parts for a full machine.
Set the teeth counts, module and any lengths. Watch it run in 3D wood or 2D schematic.
Set your wood thickness in Set up the scale, your hardware (bolts, dowels, bearings) at the bottom of Build the machine, and kerf & gear mesh clearance in Prepare for cutting.
Open Cut sheets to laser and check the nesting and the piece count.
Download each numbered sheet. Cut at your usual settings for the material.
Build it using the Reference sheet and the materials list, which tell you every part, bolt, bearing and dowel you need.

New to gears? Cut a Gear Mesh Tester board first (section 6). Twenty minutes there saves a whole sheet of gears that bind.

4 · The movements

Movement	Converts	Notes
Spur gearing	rotary to rotary	Pairs and trains, 2-7 gears. Idlers set direction, not ratio.
Crank & connecting rod	rotary to reciprocating	The classic engine linkage; works both ways.
Cam & follower	rotary to reciprocating	Choose the motion law: heart, dwell, and others.
Rack & pinion	rotary to linear	Straight-line travel; rack length sets the stroke.
Ratchet & pawl	oscillating to intermittent	One-way motion, steps forward and holds.
Scotch yoke	rotary to sine reciprocation	Pure sine motion, fewer parts than a crank.
Internal ring gear	rotary to rotary	Same direction, compact centre distance.
Epicyclic / planetary	compound ratios	Big reductions in a small space; traces hypocycloid paths.
Four-bar linkage	rotary to complex curve	Coupler curves — the heart of drawing machines.
Composer Workbench	chained	Several movements on one baseplate, one crank.
Complex Workbench	chained, multi-layer	Stacked gears on shafts, plus pens and arms.

5 · Module, and why it matters

Module is tooth size: millimetres of pitch diameter per tooth. Two rules follow from it:

A gear's pitch diameter = module x number of teeth.
Two gears mesh at a centre distance of module x (teeth1 + teeth2) / 2.

Bigger module means bigger, chunkier teeth — stronger and easier to cut cleanly in ply. Smaller module means finer teeth and a more compact machine. Two gears can only mesh if they share the same module. The machine scale buttons in the Complex Workbench change module: at half scale, module halves and every gear shrinks with it.

In ply, module 2 is about the finest worth cutting. Module 3 is a good general choice; module 4-5 is very forgiving.

6 · Gear mesh clearance and the Gear Mesh Tester

A gear cut to perfect theoretical dimensions will not turn. Real teeth need backlash — a small gap between the driving and driven flanks — to allow for material roughness, char, and tiny errors. The Gear mesh clearance setting adds it by thinning each tooth very slightly, leaving centre distances exact so the motion stays true.

It is set as a fraction of module, which means the fit holds at any size. A machine that runs beautifully at half scale will run the same at full scale.

0: No clearance. Gears will almost certainly seize.
0.05: Tight. Precise, but stiff in plywood.
0.08: Good running fit for laser-cut ply. The default.
0.12 - 0.15: Free-running. A little lost motion, useful for rough material.
0.20+: Loose. Noticeable slack; fine for hand-turned toys, poor for drawing.

Using the tester

The Gear Mesh Tester (in the Combining menu, or the button at the top) cuts a small board of gear pairs at different clearances so you can feel the difference before committing a whole machine.

Press match my machine so the module matches what you are building.
Drag the clearance slider. The live close-up shows two teeth meshing, with the backlash gap shaded blue — watch it open as you go looser.
Set also cut to 5 to bracket your guess, or to 1 to test a single value.
Choose the shaft holes: match my machine, bearing seat, or bolt clearance.
Download and cut the board. Each pair is engraved with its clearance, centre distance and tooth count.
Mount each pair on its tile and turn them. Pick the one that turns freely without rattling.
Enter that number as your Gear mesh clearance in Prepare for cutting.

7 · Bearings, bolts and dowels

Rotating parts need something to turn on. The Bearings setting (bottom of Build the machine) offers three arrangements:

None: Every gear gets a bolt-clearance bore and spins directly on the bolt. Simplest, and fine for slow hand-turned machines.
In gears: Each gear gets a bearing seat; the baseplate keeps a smaller bolt hole. The bearing takes the load and the gear spins freely. This is the usual choice.
In gears + rod pivots: As above, plus bearings at linkage rod pivots. Smoothest, most hardware.

The bearing hole diameter is the size actually cut. Start at the bearing's nominal outside diameter and adjust by a tenth of a millimetre at a time until it presses in firmly without splitting the ply. Dowel holes work the same way for shafts and standoff posts.

Use the Kerf Hole Maker on curiosity.diy to test hole sizes quickly: it cuts a strip of holes in small increments so you can find the one that grips.

8 · The Complex Workbench

The Complex Workbench builds multi-layer machines: gears stacked on shared shafts, driven from one crank, optionally carrying a pen.

Gears

Driver is the input gear you turn. Mesh gears engage a chosen neighbour at an angle you set. Stack gears share a shaft with another gear, so they turn together.
Each row shows the live tip diameter, so you can see the machine's real size as you work.
Layers separate the gear trains vertically; spacers and standoffs are generated automatically.

Machine scale

The scale buttons resize the whole mechanism — module, arm lengths, pin radii — by an exact factor, relative to the original. Because clearance and hole fits are proportional or absolute as appropriate, a scaled machine still assembles correctly.

Pens and arms

A pen can ride on a gear pin, a linkage joint, or part-way along an arm.
Arms pivot on joints. A branch is glued rigidly to an arm — it is not a pivot — and carries the pen at its tip.
The arm that carries the pen has a widened pad at its tip with the pen hole through it — no separate collar to bolt on.
The pen hole can be round or a 3-8 sided polygon, measured across the flats — so a hex hole grips a hex pencil.

The X-Y pen carriage

Set How the pen is carried to X-Y carriage and two converters take over. Each one's output block grows a slotted arm lying across its own direction of travel, and a single pin dropped through both slots is the pen. Slot A fixes where the pen sits along A's axis, slot B along B's, and the crossing point is the pen.

Neither arm carries the other — both are driven straight off their own yoke, so there is nothing cantilevered and nothing stacked. The slots are cut long enough to cover the full stroke, and both arms appear on the cut sheets as pen slide X and pen slide Y. No linkage arms are needed at all in this mode.

You need exactly two: One converter cannot fix a point, and three over-constrain it. The panel says so if you have the wrong number.
They must cross: Two nearly parallel slides do not hold the pen. Set them at least 12° apart; 90° is the usual choice.
Stack order: The two arms sit on different levels so they pass each other freely, with the pen pin long enough to reach through both.

A shaft that carries a movement has a guide bridge standing just off the board above it. Stacking a gear above that shaft puts the gear where the bridge is — the gear list flags it and the buttons explain. Stacking below keeps them apart, or move the movement to a shaft with nothing over it.

How a shaft converter is held

Each converter's sliding block runs between a pair of guide rails, and those rails are a bridge standing on standoff posts bolted through the back board at each end. The block is captive between them for the whole stroke — it cannot wander off the end.

The order out from the board is: gear, then the slide deck, then the block running between its rails, then the caps, and only then the crank disc and its rod on top. That means a shaft carrying a movement runs on past the whole guide, so it is noticeably longer than a plain gear shaft — the bill of materials lists it separately. Where the guide crosses the shaft, the deck is cut with a clearance hole for it.

Three more pieces make that a real slide rather than a channel with no floor. A slide deck spans the same posts underneath, and the block runs on its face. Two slide cap strips sit over the rails and overhang the channel far enough to stop the block lifting off the deck, while leaving a gap down the middle for the rod or follower to pass through. Deck, rails and caps all bolt down onto the same four posts, so the whole guide is one sandwich: deck, then the block between the rails, then the caps.

Where that bridge sits depends on the type. A scotch yoke's block slides straight across its own gear's centre, so its guide straddles the shaft. A crank and rod pushes its block out to roughly the rod's length away, and a cam's follower out to the base circle, so those guides sit off to one side, centred on the run rather than on the gear. The schematic draws the rails where they really are, so if a block appears to be running outside its guide, that is worth reporting.

A long rod pushes the block a long way out, which means a long bridge and posts landing further across the board. If the panel warns that a post hits gear #N, shorten the rod or the throw, or turn the converter's dir so the bridge runs into open board instead.

Summing two throws on one axis

Give two converters the same dir and they share an axis. A summing bar spans them — pinned to the first block, slotted over the second — and the pen rides a slot along that bar. As the two blocks move by different amounts the bar tilts, so that axis carries a blend of both throws rather than just one.

The quickest way to get one is the + summing bar button in the pen panel: it adds a converter on a gear that is not already driving the pen, sets its direction to match the axis you chose, and switches it to move the pen — all three steps at once. Example 5 · Compound harmonograph is built this way.

It changes the character of the figure — one axis stops being a plain sinusoid and becomes a blend of two. Measured honestly it does not simply fill more of the page: on the machines tested so far, a well chosen gear ratio covers more paper than a summing bar does. Use it for the shape it gives, not for coverage. Keep it to two converters per axis — a third has nowhere to pin, and the panel will say so.

The blend is weighted by where the pen happens to be along the bar, so it is a coupled sum rather than a plain one. That is a good thing for the drawings — it puts harmonics in that a simple sum would not give you.

Getting a richer figure

Choose an untidy ratio: By far the biggest change. Two gears sharing a common factor close in a handful of turns and look sparse; two that share none take dozens of turns and weave a dense lattice. Example 4 runs 47T against 42T — a ratio of 1.119, sharing nothing — which needs 42 turns to close and fills about two thirds of its square. Change the driver to 49T and it closes in 6 turns and looks bare by comparison.
Turn up the cycles: A detuned ratio needs turns to show itself. Example 4 runs 40; auto works out the natural period for you.
Put the paper on a turntable: Mount the paper on one of the gears and the whole figure is drawn into a rotating frame, sweeping it into a broad annular rosette.
Sum an axis: Changes the shape rather than the coverage — see above. Worth trying for its own sake, but the ratio is the bigger lever.

Racks, and gears that oscillate

A rack rolling on a gear's rim scrolls one way for ever — on a 40T wheel that is nearly half a metre in a single turn — so it leaves its pinion almost at once and nothing brings it back. Set pushed by to one of the shaft converters instead and the rack is bolted to that converter's sliding block: it travels the converter's stroke and returns every turn, and the block's own guide rails hold it, so the rack needs no guide of its own.

Use ＋ rack & gear (placed clear) to add both at once. A pushed rack meshes nothing but its own pinion, so it does not need to sit on any wheel's rim — the pair stand on their own beside the machine, where there is room to build them. The rack runs parallel to its converter's travel, since that is the direction it is pushed in, and it is cut long enough to stay meshed. A shaft converter is wired up as the driver if one exists; without one the rack has nothing pushing it and will not move at all, and the panel says so.

Once a rack moves like that it can drive a gear. Press ＋ gear driven by this rack and you get a pinion standing against the rack with no other connection to the train. Because the rack reverses, so does the pinion — it oscillates, sweeping out and back rather than turning. A smaller pinion sweeps further for the same stroke. Slide it along the rack to place it.

A rack has teeth on one face, so a second gear normally shares that face and has to stand far enough along the bar to clear the wheel the rack rolls on — which can want a rack longer than the sheet it is cut from. Press ⇄ other face and the rack is cut double-sided instead: teeth on both edges, a thicker web between them carrying the mounting holes, and the pinion sitting straight opposite its host with the bar trapped between the two. That is usually a third of the length, and it is what the tool picks for you when the single-sided version will not fit the sheet.

If nothing is pushing a rack it will not move, and nor will its gear. Push it with ＋ a yoke / a crank / a cam mounts that movement on the roomiest wheel available, aims its travel along the rack and wires it up — one press from a dead rack to a moving one. A pushed rack can only run along its block's travel, since it is bolted to it, so turning the converter's dir swings the whole rack round.

There are two ways to bolt it on. Pushed at its side is a bracket holding the bar alongside the block: across and along set how far that bracket reaches, in any direction, so the rack and its gear together can be put anywhere on the board while still being driven. Pushed at its end makes the bar an extension of the pushrod itself, in line with the travel and starting just clear of the block — nothing stands beside the machine at all, and the bar reaches out as far as it is long. ⇄ push from the other end sends it out the opposite way.

A pushed bar rides the drive plane above its wheel, exactly where its block is, so it passes clear over the gears on that layer — and the pinion meshing it stands on a taller shaft, a layer up. Only something stacked above that plane can foul it, and the panel says so if anything does.

A rack rolling on a rim is held to that rim, so to put one anywhere you like press ⤢ stand it free. It lifts off the wheel and keeps the place it was in; stands at then moves it across and up, angle turns it, and its gear comes with it. A free rack has nothing rolling it, so it needs a converter to push it — otherwise it will sit there, and the panel will say so.

Anything meshed to an oscillating gear oscillates as well, scaled by the tooth ratio and reversed; anything stacked on its shaft shares its angle exactly. So a whole sub-train can be made to sweep back and forth from one rack.

A free-standing rack has stands at controls to move it across and up, and its gear comes with it — the gear is bolted to the rack, not to the board. An oscillating gear carries things like any other: mount a cam, crank or scotch yoke on its shaft and that converter reverses too, or put a pen pin on it and the linkage follows its sweep.

Length is measured end to end, but everything that has to fit on a rack is measured from its middle: the pinion sits so far off centre, and the bar slides half its stroke each way. So it needs to be twice the pinion's offset, plus the stroke, plus about three teeth of engagement at each end. The panel works that out and offers to lengthen it; it also says when the bar is laid across a wheel, and offers to move it somewhere clear.

When a rack on its own is worth having

Rolling on a wheel that turns, a rack is no use: it scrolls away at a fixed rate and never comes back, so it leaves its pinion within a few teeth and nothing returns it. There is one case where a bare rack does work, and it is worth knowing — roll it on a wheel that rocks. A pinion driven by another rack oscillates rather than turns, and a rack meshing its rim runs out and back with it, travelling its pitch radius times the swing, cycle after cycle.

That makes a rack the way OUT of an oscillation as well as the way in: rack → pinion → rack. Stack a bigger wheel on the pinion's shaft and the second rack travels further than the first; a smaller one and it travels less. So a pair of racks either side of a stack is a straight-line throw multiplier, and the far end of it is a bed or carriage that shuttles.

＋ rack now goes to a rocking wheel if the machine has one. If it has none, it comes with a yoke to push it, because the alternative is a part that cannot work wherever it is put.

The rack's track, and joining it to a movement

A rack has to run dead straight or it climbs out of its mesh, so every rack is now cut with a track: guide rails either side of the bar with cap strips over them, standing on dowel posts off the back board — the same channel a converter's block runs in. The rail on a toothed face comes in segments, because the pinion is bolted to the board and sits where that rail would run, so it stops short either side of it. The panel says how many rails and posts the track needs, and the back board grows to reach them.

A movement joins the rack in one of two ways. A bolted bracket makes the rack part of the block: rigid, simple, and it must run parallel to the block's travel. A rod with pins makes it a slider-crank instead — pinned at the block and again at a hole in the middle of the bar — and then the rack does not have to be parallel to anything. Set its angle and where it stands, and the rod follows.

What reaches the rack through a rod is the block's travel foreshortened by the angle between them: square on it is the full stroke, at 45° about three quarters, and across the travel almost nothing at all — there the rod comes up square to the bar and the pair sit on a dead point. Standing the bar out along its own line keeps the rod shallow and the motion honest, which is where re-fit the rod puts it.

Against a wheel of fewer than 17 teeth the rack's own teeth are cut shorter than full height. A full-height rack tooth digs into the flank of a small wheel — the classic undercut interference — and shortening the rack is the standard cure. The panel says when it has done it.

Pivots partway along an arm

A joint does not have to hang off the end of another arm. Set an arm's anchor to along arm and give it a percentage, and it pivots from a point partway down that arm instead. That is the trick behind the classic multi-arm drawing machines: each extra pivot multiplies the harmonics, so a machine with only three wheels can fill most of its page.

Example 6 · Arm chain drawing machine is built this way. Three wheels at unrelated ratios carry two pins; a first pair of arms meets at a joint, a second pair hangs off a pivot 45% along one of those arms, and the pen rides further out again on a short welded arm. Move that percentage and the figure changes completely — it is the most expressive single control on the machine.

Each arm says how far along itself it is pinned. At 0 the pin is at the very end, as on an ordinary arm. Raise it and the wood runs on past the pin at both ends, so the pin is through the middle of a solid member — 50% puts it dead centre. That is how a Balke machine's arms are made, and it is what lets several wheels drive one linkage without fighting each other. The panel shows the length of the stick you would actually cut, which is longer than the pin-to-joint distance once the pin sits inside.

An arm can also carry pivot holes — marked out before anything hangs on them, exactly as you would at the bench. Add one, slide it to where you want it along the arm, and it appears in every attachment list further down as pivot 1 on arm 1A. That means a machine can be built forwards: lay out the arms you want, drill the holes, then hang the next pair on them.

A machine does not have to carry a pen at all. Choose no pen, just the linkage and it still moves and still cuts — useful when the arms are driving something else, or when you are still working out the shape.

That one setting decides a member's length. Anything else fastened to that member — another arm, a pen branch — simply picks a spot on the wood already there, given as a percentage of its whole length: 0% is one end, 100% the other. Push it below 0 and the anchor moves out beyond the arm's own pin, on the far side — so that pin ends up partway along a straight member rather than at its end. That is the arrangement in Robert Balke's drawing machines, and it is what lets several wheels drive one linkage at once: a wheel bearing on the interior of a member shares the work rather than fighting the others. Push it above 1 and the anchor runs out past the joint instead.

Example 7 · Balke-style interior drive works this way. Three wheels at 49, 36 and 29 teeth all push at once; the member from joint 1 runs back through wheel #0's pin and half an arm's length beyond it, and wheel #2 drives a second pair off that far end. The three ratios share no common factor, so the figure never quite repeats — it is drawn for a set number of turns rather than until it closes, which is how those machines behave.

A joint's arms can also start at other joints rather than at pins. That is what builds the full Balke arrangement: two elbows, each found from a pair of wheel pins, then a third pair of arms rising from those two elbows with the pen at their apex. Six pivots — three pins on the wheels and three elbows — and the pen carried by members that are themselves carried by members. It looks like a closed loop in a photograph, but it resolves as a tree three levels deep, which is why the tool can solve it at all. Example 8 · Six-pivot Balke machine is built this way.

Chains like this can run out of reach: if the arms cannot close at some angle the pen simply vanishes for part of the turn. Lengthen both arms of the offending joint, or move the pivot fraction, until the trace is unbroken all the way round.

Lissajous figures

To draw a Lissajous — the figure of eight and its relatives — the pen needs a pure sine along one axis and another along the other. Only the scotch yoke gives that: its output is a perfect sinusoid, where a crank's is spoilt by the connecting rod and comes out about a third as clean. Example 4 is set up this way.

Ratio: The two gears' tooth counts set the frequency ratio. 40T and 20T give 1:2, the classic figure of eight; 45T and 15T give 1:3; 45T and 30T give 2:3.
Phase: Rotate the second gear's mesh angle around the first. This opens the figure from a degenerate arc, through an open loop, to the crossed figure of eight, and back.
Size: Each yoke's crank radius sets the swing along its own axis.
Fixed home: Both arm pins sit at radius 0, pivoting on the gears' own shafts, so the linkage holds the pen at a fixed point and the yokes do all the drawing. Any larger radius mixes the arms' own motion back in.

A scotch yoke in the workbench has no crank disc: its pin is pressed straight into the gear it sits on, since that gear is already turning on the shaft. The pin reaches out through a clearance opening in the slide deck into the yoke's slot. Only a crank or a cam carries a separate disc, because their rod or follower has to pick up motion outboard of the guide.

The slotted arm bolts to its block through a standoff — a short dowel that lifts the arm clear of the caps and the crank disc on its way past. It cannot sit on the block's centreline, because the disc is spinning in the plane it passes through, so it is offset across the travel where its distance from the shaft stays constant. On a scotch yoke there is nothing outboard of the plate for it to clear, so it simply sits beside the slot.

A yoke driving a carriage is cut as an H rather than the usual T. The T's crossbar bears on its guide rails over its own narrow width — on a big yoke that can be 23mm of bearing spanning 125mm between the rails, where a slider wants nearer one and a half times its span. With a rod pushing on it that is fine; with nothing steadying it, it racks in its grooves and can walk out of them. The H puts a full-depth cheek along each rail, so the guide becomes two long widely spaced bearings instead of one short one, and the rod tail is dropped since a carriage yoke has no rod to push. Cheeks are cut 60–80mm long.

The arm carries a small H-shaped foot at its block end, echoing the yoke's own H: a pad about 27mm long at each of the two fin stations, wide enough to take a fin either side of the slot, joined by the arm itself. Only the pads are widened — the arm stays slim everywhere it does not need the width.

The arm is joined to its block by four carriage fins — thin slices of ply standing on edge, two at each end of the yoke's slot and one either side of it, each tabbed into a mortise cut in the block below and another in the arm above. A fin is only one sheet thick, so it fits in the narrow strip beside the slot where a round post never would, and standing on edge it is far stiffer than a dowel of the same footprint. The tabs mean the joint is located by the wood rather than by the glue; glue or pin them once everything lines up.

Since a scotch yoke has no crank disc, a carriage driven entirely by yokes brings its pen slides in close and the whole machine stands about 42mm off the board. Put a crank or a cam on either axis and the slides move back out to clear its disc, adding roughly 18mm. On a carriage the paper is drawn see-through, so the mechanism behind it stays visible.

Example 4 ships with the X-Y carriage switched on, so the slides that carry the pen are cut along with everything else. A converter set to moves the paper is still modelled without hardware — the driven paper bed has no parts yet.

Driving a pin from a converter

A pin normally orbits a gear, so it feeds the linkage a circle. It can instead ride the output block of a converter — the slider of a crank, the yoke of a scotch, the follower of a cam — which feeds the linkage a straight reciprocation. Choose the converter in the pin's own dropdown, next to the gears.

The second field then reads along the slide rather than distance from centre: 0 sits on the block's centre, and moving it shifts the pin up or down the slide axis to reach either end of the block. The hole is cut in the block for you.

A circle crossed with a straight reciprocation draws a very different family of figures from two circles — closer to a Lissajous than a rosette. If the pen disappears, the linkage cannot reach: the two arm lengths need to span the distance between the pins across the whole stroke, not just at the position you happen to be looking at.

Reading the drawing

A closed figure repeats after a set number of turns. An open figure never quite closes, so it weaves denser the longer it runs — set run for N turns to control how much it fills in.

The shape of the back board

The back board does not have to be a rectangle. Three shapes are offered:

Rectangle: The full plate, big enough to cover every gear with a margin. Strongest, simplest, most wood.
Curved: A disc around every gear, blended together into one smooth organic outline. Struts bridge the mesh links automatically as soon as the discs stop touching, so it stays in one piece.
Minimal: Only strips running shaft to shaft — least wood, and you can see the mechanism through it.

The offset (−60 to +40mm) is measured out from each gear's tip circle. At 0 the outline traces the teeth. Negative pulls the discs inside the rims, giving a rounded skeleton; positive pads them out until they merge into one broad blob. On the minimal shape the same number sets the strip half-width.

Whatever the outline does, every shaft keeps a full pad of wood around it — wide enough for its spacer rings to seat right the way round — and every standoff post keeps a pad plus a strut back to the nearest shaft, so nothing is left floating.

Wind the offset down far enough and the board will fall into separate pieces. The panel warns you when that happens and gives the count. Sometimes that is what you want; usually it means winding the offset back up until they knit together.

Splitting the baseplate

If the baseplate is bigger than your sheet, use Split the baseplate to tile it into sheet-sized pieces with alignment features.

A curved or minimal board is clipped to the tiles, so the join runs along the cut and the gear-tooth seam option lines its teeth up with the cut edge. Two cautions: where a seam crosses a narrow strip the teeth can be wider than the wood there, so reduce the tooth depth or shift the tile size; and right at the very edge of the board, where a seam runs out into the curved outline, the two halves can be out by up to a millimetre or so. The teeth themselves interlock exactly — it is only the last few millimetres at the perimeter.

9 · Cut sheets and downloads

There are two different files, and they do different jobs.

Reference sheet (annotated): One long drawing with every part named, dimensioned and annotated — which hole is which, what glues where. Print it or keep it on screen while you build. Not intended for cutting.
Cut sheets to laser: The numbered sheets you actually cut. Tightly nested to your sheet size, kerf compensated, one SVG per sheet.

Options

Sheet outline: Draws the sheet border. Turn it off if your laser software places jobs itself.
0,0 marker: A crosshair at the origin for lining the job up on the bed. Handy with the outline off.
Engrave part labels: Engraves each part's number and tooth count. Off by default, since engraving adds machine time.
Zoom / fit width: Inspect the nesting before committing.

Colours

Red lines are cuts. Blue lines are engraving or scoring. Map these to your laser's layers as cut and raster/score.

If a piece is larger than your sheet, a red warning appears above the downloads. Choose a bigger sheet size or split the baseplate — otherwise that piece will be cut short.

10 · Building it

Weed and de-soot the parts. Wiping cut edges with methylated spirits removes char, which is a surprisingly large source of friction.
Press bearings into the gears. They should need firm thumb pressure, not a hammer.
Fit shafts and standoffs to the baseplate. Dowels glue in; bolts pass through.
Stack the layers in order, using the generated spacers to set the gaps.
Check each gear turns freely on its own before adding the next.
Fit arms and linkages. Branches are glued rigidly; joints must stay free.
Drop the pen through the widened pad at the tip of the pen arm.
Turn the crank slowly by hand for a full cycle before running it properly.

A candle or paste wax rubbed on tooth flanks makes a real difference to how freely wooden gears run.

11 · Troubleshooting

Symptom	Likely cause	Fix
Gears bind or lock up	Not enough mesh clearance	Raise Gear mesh clearance to 0.10-0.12; de-soot and wax the teeth
Gears rattle, drawing looks fuzzy	Too much clearance	Lower to 0.05-0.06 and re-cut
Gear wobbles on its bolt	No bearing, or hole too large	Switch Bearings to 'In gears', or reduce the bearing hole
Bearing falls out	Hole cut too big	Reduce the bearing hole diameter by 0.1mm and re-cut
Bearing won't go in	Hole too small	Increase by 0.1mm; do not change kerf to fix this
Parts come out undersize	Kerf set too low	Measure a test square and correct the kerf
A piece is cut short at the edge	Piece exceeds the sheet	Bigger sheet size, or split the baseplate
Pen slips in the collar	Pen hole too large	Reduce the pen hole size, or match the shape to the barrel
Drawing never repeats	Open figure	Expected — set run for N turns to control the density
Screen looks unchanged after an edit	Cached page	Hard reload, adding ?v=2 to the address

12 · Glossary

Module: Tooth size: mm of pitch diameter per tooth. Meshing gears must share it.
Pitch circle: The imaginary circle where two gears effectively roll on each other.
Centre distance: Distance between two meshing gear centres: module x (T1 + T2) / 2.
Backlash: The small gap between meshing tooth flanks that lets gears turn freely.
Kerf: The width of material removed by the laser beam.
Kerf compensation: Growing outlines and shrinking holes by half a kerf so finished parts are on size.
Involute: The tooth curve that gives a constant speed ratio through the mesh. All gears here are involute.
Across flats (A/F): The width of a polygon measured flat-to-flat, not corner-to-corner. How hex pencils and bolts are sized.
Press fit: A hole slightly smaller than the part, so it grips without glue.
Coupler curve: The path traced by a point on the connecting link of a four-bar linkage.
Hypocycloid: The path traced by a point on a circle rolling inside another circle, as in planetary gearing.

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