brackets-and-mounts

name	brackets-and-mounts
description	L/U/Z brackets, gussets, bolt patterns, adjustment slots and NEMA motor mounts — sizing, clearances, load path, and the build123d order that survives a rebuild.
triggers	["bracket","mount","gusset","nema","motor","flange","l-bracket","stand","foot","bolt-pattern","slot","standoff","angle"]
version	1.0.0
license	Apache-2.0
author	AgentCAD core
requires	[]

A bracket is a load path with holes in it, and almost every bracket that disappoints is either too thin at the corner or has its bolts too close to an edge. This skill covers choosing between the L, U, Z and gusseted-plate forms, sizing gussets, laying out bolt patterns and adjustment slots, and mounting NEMA stepper motors (bolt square, pilot register, shaft clearance). Use it when the part's job is to hold something else in place. It is not the process manual: fdm-design-rules for printed parts, sheet-metal for bends and flat patterns, holes for the wizard's full API, fits-and-clearances for a shaft-in-bore fit, fem-workflow when a rule of thumb is not enough.

Choose the form

Form	Use when	Watch
L (90° corner)	one member meets a perpendicular one	weakest in the open direction — gusset it
U (base + two uprights)	a shaft, bearing or motor supported at both ends	the two uprights must be built from one profile or they will not stay parallel
Z (offset)	joining two parallel faces at different heights	both legs bend; keep the offset leg as short as the clash allows
Flat plate + gusset	a face mount whose load is mostly in-plane	stiffest per gram; needs a mating flat surface
Cut angle stock	one-off, metal, cheap	thickness and root radius are fixed by the stock

Load path: deflection goes as L³/t³

For a leg of width b, thickness t and free length L under a tip load F, δ = 4FL³/(E·b·t³) (Roark Table 8.1, cantilever with I = b·t³/12).

• Doubling t → 1/8 the deflection. Cutting L by 20 % → 0.51×. Doubling b → only 1/2. Thickness and length dominate; width barely helps.
• The L's open side is the compliant direction. A gusset takes the bending term out of the path and replaces it with an axial one — in practice an order of magnitude, not a percent.
• Failure and fatigue start at the inner corner. Keep r/t ≥ 0.5, which puts the stress concentration near Kt ≈ 1.5; r/t = 0.1 is near 2.5 (Peterson's).
• Bolts do not add stiffness if the joint slips. Two properly torqued M5 beat four hand-tight M3.

Gussets

• Triangle legs = 0.6–0.8 × the free length of the leg they brace. Below ~0.5 the gusset barely acts; above ~0.85 it only adds mass and blocks access to the fasteners.
• Thickness = the wall/leg thickness. Thicker buys no stiffness and costs a sink mark in moulding, a thermal mass in FDM, and a heavy cut in machining.
• Two gussets on the outer faces, or one on the centreline. The outer pair also resists twist, so prefer it whenever the load can be off-axis; the single central web is for narrow brackets whose bolts sit near the edges.
• A bolted gusset carries load over its Whitmore section: spread 30° each side from the outermost fastener of the first row to the last row, and check yield on that width, not on the whole plate (AISC Design Guide 29).
• End the gusset toe 1–2 mm short of the leg's free end, and keep 1.5 × head diameter clear of every bolt head and washer face.

Bolt patterns

• Edge distance ≥ 1.5 d (hole centre to edge) in metal — AISC Table J3.4 runs ≈1.25–1.5 d and EN 1993-1-8 sets e₁ ≥ 1.2 d₀ as the floor, so 1.5 d clears both. Use ≥ 2 d in plastic and FDM, with a washer under the head.
• Pitch ≥ 3 d centre-to-centre (AISC minimum 2⅔ d; EN p₁ ≥ 2.2 d₀), and ≤ 14 t so the plies stay closed between fasteners.
• Symmetric patterns only: two or four bolts on a rectangle, or a bolt circle. A three-bolt triangle is fine on a round flange and a mistake on a plate — it defines a rotation you did not mean to define.
• Put the bolts where the moment is: on an L, the base pair furthest from the corner takes the prying load, so push it as far out as edge distance allows.
• Generate positions with patterns.grid / patterns.bolt_circle — pure arithmetic, rounded to 9 dp — not GridLocations/PolarLocations, which also rotate each copy (see patterns).

from agentcad.toolkit import holes, patterns

def build(p):
    ...
    pts = patterns.grid(2, 2, p.pitch, p.gauge)
    part, recs, warn = holes.clearance(part, pts, "M5", fit="medium",
                                       plane="top")
    return part

Clearance holes and slots

• Never type a clearance diameter. holes.clearance(...) reads ISO 273 (fine/medium/coarse) or, with std="ansi", ASME B18.2.8 (close/normal/loose): M3 → 3.2 / 3.4 / 3.6, M5 → 5.3 / 5.5 / 5.8, M6 → 6.4 / 6.6 / 7.0 mm. medium (normal) is the default; fine when the pattern must locate the part; coarse for weldments, printed parts and stacked tolerances.
• Slot width = the clearance hole diameter; overall length = width + required travel. Keep the length within 2–3 d, with 2.5 d the code ceiling for a long-slotted hole (AISC Table J3.3). Beyond that, use two positions or a real adjuster.
• One round hole + one slot per pair gives a datum plus tolerance take-up; two slots give genuine adjustment and no datum. Decide which you meant.
• Every slot needs a washer spanning it (AISC J3.8) — and on a printed part the washer is also what stops the head sinking into the slot.
• The hole wizard has no slot family, so a slot carries no hole record and will not appear in drawing callouts. Cut it as sketch geometry:

with BuildSketch(Plane.XY):
    with Locations((x_slot, gauge / 2), (x_slot, -gauge / 2)):
        SlotOverall(slot_w + travel, slot_w)
extrude(amount=t + 1.0, both=True, mode=Mode.SUBTRACT)

NEMA motor mounts

Full table with inch originals, flange sizes, boss heights and tap depths: tables/nema.json. The frame number is the flange width in tenths of an inch.

Frame	Flange	Bolt square	Bolt circle	Pilot ⌀	Screw	Shaft ⌀
NEMA 8	20.3	16.0	22.63	15.0	M2	4.0
NEMA 11	28.2	23.0	32.53	22.0	M2.5	5.0
NEMA 14	35.2	26.0	36.77	22.0	M3	5.0
NEMA 17	42.3	31.0	43.84	22.0	M3	5.0
NEMA 23	56.4	47.14	66.67	38.1	M5 (#10-24)	6.35

All mm, from NEMA ICS 16 Part 4 (converted from inches).

• The pattern is a square, not a circle: bolt_circle = square × √2. Use the square with patterns.grid(2, 2, s, s) unless you are deliberately patterning polar.
• Pilot register: bore = pilot ⌀ + 0.2–0.5 mm diametral (0.2 machined or laser-cut, 0.3–0.5 FDM). The pilot locates the motor; the four screws only clamp. The boss is only ~1.5–2 mm tall, so a plate thicker than the boss still registers on the first 2 mm — that is enough, and a plain through bore is fine.
• Shaft and coupler clearance: the pilot bore already clears the shaft (⌀5 for NEMA 8–17, ⌀6.35 for 23) and a plain jaw coupler. When a pulley or a larger hub must pass through, size the bore to hub ⌀ + 1 mm — but keep at least 1.5 × wall thickness of material between the bore edge and the nearest bolt hole edge, or the four-bolt pattern turns into a ring of ligaments.
• Screw length = plate thickness + 4–5 mm. Motor face tappings are shallow (≈4.5 mm on a NEMA 17); a screw that bottoms out feels tight and clamps nothing.
• Keep the motor face metal-to-metal against the bracket — that joint is the motor's heat path. Do not slip a printed spacer in between.

Process differences

• FDM — the corner is printed, not bent, so orientation is the design. Lay the L on its side face: the profile becomes the first layer, there are no overhangs at all, and the corner's stress runs along the extrusions instead of across the layer bond. Make the wall and gusset an integer number of perimeters, and add ≈ +0.2 mm to hole diameters. See fdm-design-rules.
• Sheet metal — the corner is a bend: inside radius ≥ t, bend relief at both ends, and every hole ≥ 1.5 t + r from the bend line or it pulls oval. A gusset has to be a separate welded piece or a formed stiffening bead. See sheet-metal.
• Machined / plate — the inner corner carries the cutter radius (≥ 3 mm is typical), gussets are free geometry but expensive material removal, and the cheapest stiff bracket is often two flat plates bolted together rather than one solid.

Build order that survives a rebuild

Sketch the L profile on Plane.XZ and extrude(amount=w/2, both=True) so the part is symmetric about Y. Then, in this order:

1. fillet the inner corner — before the gussets, because after them the corner edge is split into segments and the radius no longer fits;
2. fuse the gussets;
3. cut the slots;
4. drill last, so hole circles never contaminate a fillet's edge selection.

Failure modes and their recovery:

• A gusset placed on a plane whose normal you guessed wrong fuses in mid-air and OCCT reports success: is_valid True, volume up by exactly the right amount. Assert n_solids == 1, or build it with features.rib(..., to=<mm>) and read the guard's warning (ribs-bosses-draft).
• features.rib(to="part") intersects the part's bounding solid, not the part: on a convex bracket it adds 0 mm³ and always warns.
• Pass holes a real Plane or a "top"/"left" predicate, never a face index. The normal points out of the material, the drill runs along -z_dir, and points are (u, v) in that plane.
• Select the corner edge by coordinate, not by ordinal:

inner = [e for e in part.edges().filter_by(Axis.Y)
         if abs(e.center().X - t) < 1e-4 and abs(e.center().Z - t) < 1e-4]

• Use safe_fillet for any radius near the leg thickness — it binary-searches down and returns the part unchanged instead of failing the build. More in selectors-and-occt-failures and robust-parametrics.

Checklist

• Edge distance ≥ 1.5 d (2 d printed), pitch ≥ 3 d, slot length ≤ 3 d, washer over every slot.
• Gusset legs 0.6–0.8 × the free leg, thickness = wall, clear of every head.
• Inner corner r/t ≥ 0.5.
• Clearance diameters from holes, never typed in.
• Motor: pilot + 0.2–0.5 mm, coupler clears the bore, screw = t + 4–5 mm, face contact metal-to-metal.
• Rebuilt at both parameter extremes: n_solids == 1, is_valid, no dropped hole instances in the warnings.

Snippet

snippets/nema17_bracket.py — an L-bracket carrying the NEMA 17 pattern (31.0 mm square, ⌀22.3 pilot) drilled from the motor face, two adjustment slots in the base, and a gusset on each outer face, all driven by one thk. Defaults build one valid solid of 33 696 mm³ with no warnings; bolt_sq, pilot_d and screw retarget it to any row of tables/nema.json.

Sources

• NEMA ICS 16, Motion/Position Control Motors, Controls, and Feedback Devices, Part 4 — motor frame and face-mounting dimensions.
• Oriental Motor, PKP Series Stepping Motors catalogue — dimensional drawings for the 20/28/35/42/56 mm frames (vendor corroboration of the table).
• ISO 273:1979, Fasteners — Clearance holes for bolts and screws.
• ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs.
• AISC 360, Specification for Structural Steel Buildings — §J3.3 (spacing), §J3.4 and Table J3.4 (edge distance), Table J3.3 (hole and slot dimensions), §J3.8 (washers over slotted holes).
• EN 1993-1-8, Design of joints, Table 3.3 — e₁, e₂, p₁, p₂ limits.
• AISC Design Guide 29, Vertical Bracing Connections — gusset plates and the Whitmore section.
• W. D. Pilkey & D. F. Pilkey, Peterson's Stress Concentration Factors, 3rd ed. — fillet radius versus Kt at a stepped section in bending.
• W. C. Young & R. G. Budynas, Roark's Formulas for Stress and Strain, 8th ed., Table 8.1 — cantilever deflection.
• R. G. Budynas & J. K. Nisbett, Shigley's Mechanical Engineering Design — bolted joints, preload and thread engagement.