Threads, fasteners and gears

Part scripts can import two build123d part libraries besides build123d itself. Both are installed with b123d-print and work inside the sandbox:

Library	For	Import
bd_warehouse 0.4.0	threads (ISO, trapezoidal, Acme, Whitworth, plastic bottle), screws, nuts, washers, heat-set inserts, clearance and tap holes, bearings, gears and worms, sprockets, pipes, flanges, O-rings, OpenBuilds V-slot parts	from bd_warehouse.thread import IsoThread, PlasticBottleThread from bd_warehouse.fastener import SocketHeadCapScrew, HexNut, HeatSetNut from bd_warehouse.open_builds import VSlotLinearRail, TNut
py_gearworks 0.0.24	involute spur, helical, herringbone, bevel, ring and cycloid gears and racks, with meshing (mesh_to), profile shift and backlash	import py_gearworks as gw

These libraries model the standard parts. Whether a part prints is a separate question, and the rest of this page covers it. The AI follows the same rules (the workflow rules, "Threads, fasteners and gears").

Why bd_vslot is not included

bd_vslot 0.0.2 (MIT) fails to build its rails with build123d 0.13, the version b123d-print pins: every rail opens an empty location list, which 0.13 refuses. bd_warehouse.open_builds has the same 2020 rail and T-nut, so use that.

Threads: model them, or not?

Model a thread when it is M6 or larger with a coarse pitch, or a bottle or jar thread. Two sources put the floor at M6 JLC Sovol, and Hubs at M5 (BP-T1). Below M5 the crests and roots are close to one extrusion line wide SOVOL-THR. Bigger and coarser is easier: M8 and up prints comfortably. Sovol recommends 2–4 mm pitches with rounded roots for custom threads, and a layer height of at most 1/6–1/8 of the pitch SOVOL-THR.

• Print the thread axis upright (Z) SOVOL-THR. Each turn is then traced in the layers; lying on its side, a thread turns into stair steps and sagging roofs.
• Expect an overhang warning on a 60° thread. Its flanks are 30° from horizontal, so the downward flank is 60° from vertical, past the 45° limit. Snapmaker calls these flanks "difficult to print accurately as overhangs" SNAP-THR. For an M10 × 1.5, check reports each turn's underside at 60°. Each flank is short, so a coarse thread still works, but its underside comes out rough, and this is why fine threads print poorly. A buttress profile has a 45° flank and a flat one. With the 45° flank facing down it has no overhang at all, as in the bottle thread below.
• Clearance: about 0.1–0.2 mm per side (0.2–0.4 mm on the diameter) to start SOVOL-THR. Use your measured sliding fit (load_settings(__file__).clearance, see Settings) and print a test pair. bd_warehouse threads take it as manufacturing_compensation, which is radial: it moves an internal thread outward and an external one inward by that much.
• Ends: fade or chamfer the start, so the first turn is not a blunt sliver that catches and breaks. Sovol suggests a 0.8–1.5 mm chamfer at the entry SOVOL-THR. bd_warehouse: IsoThread(..., end_finishes=("fade", "chamfer")). Bottle threads always fade both ends.
• Walls: 3–5 perimeters around a thread SOVOL-THR. Under constant tension PLA creeps and loosens.

from bd_warehouse.thread import IsoThread
from b123d_print import load_settings

CLEARANCE = load_settings(__file__).clearance  # per side
# an M10 x 1.5 external thread, 12 mm long, both ends faded, standing on Z
thread = IsoThread(major_diameter=10, pitch=1.5, length=12, external=True,
                   end_finishes=("fade", "fade"), manufacturing_compensation=CLEARANCE)

Bottle and jar threads are coarse (4–8 threads per inch) and short, which makes them easy to print. A finish such as 38-400 names a neck about 38 mm across and a thread style. The 400 finish has about one turn, 410 one and a half, 415 two REGENT. bd_warehouse covers ASTM D2911 ("M38SP400": M for the buttress profile of plastic closures, L for the trapezoidal profile of metal closures) and PCO-1881 (soda bottles, "28" with bottle_type="pco1881"). PlasticBottleThread.identify(...) finds the finish from measurements taken off a bottle. Measure the diameter over the thread crests (T) and the neck between the threads (E) BERLIN.

The jar_lid example: an internal 38-400 thread, top of the cap on the bed.

In print orientation the steep (45°) flank of the buttress faces down and the flat one faces up, so the thread prints without supports.

Small threads: inserts, nuts, self-tapping

Below about M6, don't model the thread BP-T1. Use one of these:

• Heat-set inserts for anything opened more than a few times. A screw driven straight into plastic holds about as well in pull-out, but strips at about 1 Nm, against 3 Nm for an insert CNCK-HELI. Use the insert maker's hole size plus HOLE_COMP (CNC Kitchen: M3 × 5.7 → 4.0 mm, M4 × 8.1 → 5.6 mm, M5 × 9.5 → 6.4 mm) CNCK-TABLE. Make the hole straight and blind, about 1 mm deeper than the insert, with no chamfer needed CNCK-TIPS. Keep at least the maker's minimum wall around it (M3 1.6, M4 2.1, M5 2.6 mm) CNCK-TABLE, and install at print temperature + 10–20 °C CNCK-TIPS.
• Captive nuts: a hex pocket across-flats + 0.2 mm in total (BP-T7). A pocket that opens downward can print over a one-layer sacrificial bridge.
• Self-tapping screws into a pilot hole: M3 ≈ 2.6–2.9 mm, depending on material and orientation. Print a test pair (BP-T6).

bd_warehouse holes and hole compensation

bd_warehouse has the fastener tables, so take sizes from them instead of typing them in:

from bd_warehouse.fastener import HeatSetNut, SocketHeadCapScrew

screw = SocketHeadCapScrew("M4-0.7", 16)
screw.clearance_hole_diameters   # {'Close': 4.3, 'Normal': 4.5, 'Loose': 4.8}
screw.tap_hole_diameters         # {'Soft': 3.3, 'Hard': 3.5}  (for a tap, not a self-tapping screw)
screw.head_diameter, screw.head_height
insert = HeatSetNut("M4-0.7-8.1", "CNCKitchen")   # also "ruthex", "McMaster-Carr", ...
insert.nut_data["m"], insert.nut_data["material_thickness"]   # 8.1 long, 2.1 minimum wall

Its hole objects (ClearanceHole, TapHole, ThreadedHole, InsertHole) cut a hole in place. Mind how they meet hole compensation, so a hole is compensated once:

Hole	Compensation	Do this
ClearanceHole(screw, fit=...), TapHole(screw, ...)	none; cut at the table's nominal size	read the size from screw.clearance_hole_diameters[fit] and cut it yourself with + HOLE_COMP, like every other hole (the insert_bracket example does this)
InsertHole(insert, manufacturing_compensation=...)	radial, added to the radius	pass HOLE_COMP / 2 (HOLE_COMP is on the diameter). It cuts the table's drill size, which can differ from the maker's hole: 5.79 mm for the CNC Kitchen M4 against the maker's 5.6. Where that matters, cut the maker's size yourself
PlasticBottleThread, IsoThread, ... manufacturing_compensation	radial	your clearance per side (above). It is a fit, not hole compensation: don't add HOLE_COMP as well

Don't also turn on the slicer's hole compensation.

The insert_bracket example: two CNC Kitchen M4 inserts in blind 5.6 mm holes on top, and two counterbored M4 clearance holes sized from bd_warehouse.

Gears

• Module ≥ 1, preferably 1.5–2 for a 0.4 mm nozzle. Module 1 suits light duty only, and below 1 the teeth are a few lines wide SOVOL-GEAR. Bigger modules are stronger HTM.
• Face width about 10 × module GS-FW, or 3–5 circular pitches SOVOL-GEAR (about 9–16 × module).
• Backlash: printed gears need more than metal ones KHK-PLASTIC. Thin each gear's flanks by 0.1–0.2 mm, for 0.2–0.4 mm in total SOVOL-GEAR. py_gearworks: backlash= on each gear is a coefficient of the module along the line of action (0.1 at module 1.5 is 0.15 mm), and mesh_to sets the center distance to suit. Add a Gap check between the meshed gears in assembly(d) to see what is left.
• Few teeth: at a 20° pressure angle a gear with fewer than 17 teeth is undercut at the root. A positive profile shift avoids that KHK-INV: profile_shift=(17 - z) / 17 in py_gearworks, which moves the center distance (mesh_to accounts for it).
• Helical and herringbone gears run smoother and quieter and carry more load. A herringbone's two halves cancel each other's axial thrust TEC-HB. They are harder to line up SOVOL-GEAR. A herringbone must be meshed before it goes onto its shafts HTM. py_gearworks: HelicalGear(..., helix_angle=..., herringbone=True).
• Print flat on the bed for the most accurate tooth profile. Standing on edge, the flanks come out rough and the bore needs support SOVOL-GEAR. 4–6 walls make the teeth solid perimeters. Chamfer only the bore's top edge: a chamfer at the bed widens the hole downward, which is an overhang.
• Material: PLA is stiff and strong but softens at 55–60 °C. PETG gives (75–80 °C) instead of snapping. ASA is 95–100 °C SOVOL-GEAR. Plastic gears change size with temperature and moisture and lose strength as they warm KHK-PLASTIC. Industrial plastic gears are nylon or acetal (POM).

The gear_pair example: a 12-tooth pinion with profile shift and a 30-tooth gear, module 1.5, with 0.15 mm between the teeth.

bd_warehouse also has SpurGear, HelicalGear, RackGear and a worm drive (Worm, WormWheel). py_gearworks has no worms.

2020 V-slot extrusion

• The slot: 6 mm across the opening, an M5 tapped bore in the middle (4.2 mm drilled) SYSTEAL. On V-slot the slot opens into a 45° V groove at the face. Suppliers differ in detail (some bores are 5 or 6 mm), so measure yours.
• Fasteners: M5 screws into T-nuts. A drop-in or sliding T-nut has a narrow neck that sits in the lips' opening and a flange under the lips. Screw length = plate + a few mm into the nut, short of the slot's floor. M5 × 8 is the common pairing for a 3–4 mm plate (retailer and forum advice, not fetched).
• Locating: a key that drops into the slot keeps a bracket square to the rail. Shape it as a V with 45° sides to match the groove: it centers itself, and it prints without support with its sides facing up. The T-nut's neck fills the slot about 1 mm below the face, so the key can only be shallow. Use your sliding fit (clearance) normal to the flanks; there is no published value for this.
• Stand-ins: put the rail and the T-nuts in assembly(d) from bd_warehouse.open_builds (VSlotLinearRail("20x20", length), TNut() turned across the slot), and the overlap and Gap checks prove the key fits.

The vslot_mount example: a V key 0.2 mm off the groove's flanks and 0.3 mm above the T-nut's neck.

Sources

Fetched in October 2026 unless marked. Values from a single vendor blog (Sovol) are starting points: print a test.

Claim	Source
Smallest modeled thread M6	JLC3DP, 3D printing design guideline
M6+ for a 0.4 mm nozzle; pitch ≥ 1 mm, custom 2–4 mm; layer ≤ pitch/6–1/8; 0.10–0.20 mm radial clearance; vertical axis; 0.8–1.5 mm entry chamfer; 3–5 walls; PLA creep	Sovol, 3D printing threads and screws (vendor blog)
Threads standing up; chamfer as a guide; 60° flanks hard as overhangs	Snapmaker, 3D printing threads
38-400: neck diameter and finish; 400 one turn, 410 one and a half, 415 two	Regent Plast, neck finishes; Pretium, Neck know-how
T, E, H, S finish dimensions	Berlin Packaging, Neck finish guide
Direct screw ~1 Nm, insert 3 Nm (PETG, M3)	CNC Kitchen, Helicoils, inserts and nuts in 3D prints
Insert holes and minimum walls (M3 4.0/1.6, M4 5.6/2.1, M5 6.4/2.6)	CNC Kitchen datasheet, as copied by 3DJake (the CNC Kitchen store returned HTTP 429)
Blind hole ~1 mm deeper; straight; no chamfer needed; iron at print temperature + 10–20 °C	CNC Kitchen, Tips and tricks for heat-set inserts
Insert at print temperature + 10 °C	Prusa, Heat-set inserts M3
Module 1.5–2 for 0.4 mm, 1 the lower limit; backlash 0.1–0.2 mm per gear; face 3–5 circular pitches; print flat; materials	Sovol, 3D printing gears that actually work (vendor blog)
Bigger module stronger; helical smoother; herringbone meshed before mounting	How To Mechatronics, How to 3D print gears
Face width 10 × module	Gear Solutions, Face width in gear design
Undercut below 17 teeth at 20°; positive profile shift	KHK, Involute gear profile; Gear Solutions (KHK USA), Addendum modification and undercut
Plastic gears: more backlash; thermal and moisture change; nylon and acetal	KHK, Design of plastic gears
Herringbone halves cancel axial thrust; more load, less noise	tec-science, Herringbone gears
V-slot 2020: 6 mm slot, 4.2 mm bore for M5	Systeal, V-slot 20×20; Electrokit, V-slot 2020
M5 × 8 for a 3 mm plate on T-nuts	Makerstore, Screw guide (not fetched: HTTP 403)