fits-and-clearances

name	fits-and-clearances
description	Sizing mating features — ISO 286 hole-basis fits (H7 with g6/h6/k6/p6), printed-part clearances, bearing seats, dowel pins and heat-set insert holes.
triggers	["fit","clearance","tolerance","h7","g6","k6","p6","press","slip","sliding","bore","pin","dowel","bearing","insert","heat-set","interference","shaft","iso 286","reamed"]
version	1.0.0
license	Apache-2.0
author	AgentCAD core
requires	[]

Use this when two features have to mate — a pin in a bore, a bearing in a housing, a tongue in a groove, an insert in a boss — and you need the actual numbers: ISO 286 hole-basis fits for machined parts, the very different clearances printed parts need, and the common hardware seats. It does not cover fastener clearance holes (that is holes and the hole wizard's fine/medium/coarse, a different table), thread fits (threads-and-fasteners), snap-fit interference (snap-fits), or FDM process rules beyond fit (fdm-design-rules).

Pick the fit first

Function	Hole basis	Character	Typical use
Turns or slides freely, lubricated	H7/g6	clearance 0.005–0.029 mm at Ø10	shaft in a plain bearing, sliding pin, hinge
Assembles by hand, locates, comes apart	H7/h6	clearance 0–0.024 mm at Ø10	spigots, locating diameters, removable pins
Snug, no play, light force to fit	H7/k6	transition, −0.010…+0.014 mm at Ø10	dowels, clamped bearing seats
Stays put, carries load through the joint	H7/p6	interference 0–0.024 mm at Ø10	pressed bushings, gear/pulley hubs
Loose, dirty, painted, welded	H11/c11	clearance 0.080–0.260 mm at Ø10	brackets, sheet parts, agricultural

Hole basis (the hole is always H, EI = 0) because a hole comes out at the fixed size of a reamer, drill or broach while a shaft is easy to turn or grind to any size. Use shaft basis (h shaft, lettered hole) only when the shaft is bought stock — drawn bar, a bearing journal, a linear rail.

ISO 286 in one page

The basic size is the number both parts share. Each part gets a fundamental deviation (the letter — which side of basic its band sits on) and a standard tolerance grade (the IT number — how wide the band is).

• Hole H: EI = 0, ES = +IT. So H7 at Ø10 is 10.000 / 10.015.
• Shaft a…h: the letter gives es (upper), ei = es − IT. Shaft j…zc: it gives ei, es = ei + IT.
• Max clearance = ES − ei; min clearance = EI − es. A negative min clearance is interference.

Standard tolerance grades, µm (ISO 286-1:2010):

Nominal, mm	IT6	IT7	IT8	IT9	IT10	IT11
over 3 to 6	8	12	18	30	48	75
over 6 to 10	9	15	22	36	58	90
over 10 to 18	11	18	27	43	70	110
over 18 to 30	13	21	33	52	84	130
over 30 to 50	16	25	39	62	100	160

Fundamental deviations, µm (ISO 286-1:2010):

Nominal, mm	H (EI)	g (es)	h (es)	k (ei)	p (ei)
over 3 to 6	0	−4	0	+1	+12
over 6 to 10	0	−5	0	+1	+15
over 10 to 18	0	−6	0	+1	+18
over 18 to 30	0	−7	0	+2	+22
over 30 to 50	0	−9	0	+2	+26

k's deviation above holds for grades IT4–IT7; at IT3 and finer or IT8 and coarser, k has ei = 0. The machine-readable copy of both tables, plus the resulting clearance range of every H7 fit in every band, is tables/iso286.json.

The range trap: bands are over … up to and including. Ø10 is in over 6 to 10, Ø30 in over 18 to 30. Reading Ø10 one band up turns IT7 from 15 µm into 18 µm — 20 % of the whole fit.

Worked example — Ø10 H7/g6

1. Band: over 6 to 10. IT7 = 15 µm, IT6 = 9 µm, g es = −5 µm.
2. Hole H7: 10.000 / 10.015.
3. Shaft g6: es = −5 → 9.995; ei = −5 − 9 = −14 → 9.986. So 9.986 / 9.995.
4. Min clearance = 10.000 − 9.995 = 0.005 mm; max = 10.015 − 9.986 = 0.029 mm.

CAD carries one number per feature: model the basic size (or the mid-band, as the snippet does) and put the limits in the callout or a SPECS entry — geometry is not where a tolerance lives.

Printed parts: the process is the tolerance

Diametral values in mm — the gap you must model between the two nominal diameters. Diametral, not radial: half of it appears on each side, and halving it twice is the single most common printed-fit bug.

Process	Sliding	Locational	Light press	As-printed hole
FDM, 0.4 mm nozzle	0.20–0.40 (start 0.30)	0.20	0.10–0.15 interference	0.1–0.3 mm undersize
SLA / DLP	0.10–0.20	0.10	0.05 interference	~0.05 undersize
SLS, PA12	0.30	0.20	0.10 interference	~0.10 undersize, plus trapped powder
Machined	ISO 286 above	—	—	reamed to size

• Compensate the hole. An FDM Ø5.0 bore measures ~4.7–4.9 (extrusion width, corner overshoot, shrink). Model it 0.1–0.3 mm oversize — or 0.5 mm undersize and drill/ream it after printing when it must truly be round.
• Orientation decides roundness. A bore printed axis-vertical is round; printed on its side it sags at the top and has an elephant-foot lip at the bed. Real bearing and shaft seats print axis-vertical.
• An H7 callout on an FDM part is fiction. IT7 at Ø20 is 21 µm; FDM spread is ±0.2 mm — ten times wider. Tolerance the function, not the drawing: decide what the feature must do, print one fit coupon (bores stepping 0.05 mm), set the parameter from what fits.
• Printed press fits creep. Plastic relaxes over days under interference; if the joint must hold torque, add a screw, key or flat.

Bearings, pins, inserts

608 bearing (8 × 22 × 7 mm; ISO 15 boundary dimensions, ISO 492 Normal class rings are minus-toleranced, bore and OD both 0 / −0.008):

Seat	Machined	Printed
Housing Ø22	H7 = 22.000 / 22.021, stationary outer ring, light load, non-split housing	model Ø22.0, measure, aim 0–0.10 mm interference; lead-in chamfer + clamp slot
Shaft Ø8	h6 = 7.991 / 8.000 stationary inner ring; j6/k6 when it rotates under load	use a steel shaft or an M8 bolt — printed shafts flex and wear
Shoulder	contacts the ring face only, never the shield or seal	same

Dowel pins. A hardened parallel pin (ISO 2338, m6) in a reamed H7 hole is a transition fit: it locates to a few µm and still presses out; use H7/p6 for a permanent one. Two pins locate a joint, but the second hole must be a slot (or a diamond pin) or hole-spacing tolerance over-constrains the pair.

Heat-set inserts for thermoplastics — typical brass knurled inserts; the vendor datasheet always wins:

Thread	Insert OD × length	Hole Ø	Min boss OD
M2	3.2 × 4.0	3.2	6.5
M2.5	3.5 × 5.7	3.5	7.0
M3	4.0 × 5.7	4.0	8.0
M4	5.6 × 8.1	5.6	11.0
M5	6.4 × 9.5	6.4	13.0

The hole is straight (no taper), ~1 mm deeper than the insert so displaced plastic has somewhere to go; boss OD ≥ 2 × insert OD with ≥ 1.5 mm of wall. Do not add the FDM shrink compensation on top — the vendor figure is already an as-printed diameter, and that undersize is the interference the melting insert consumes.

Express the fit in PARAMS

Never hard-code a diameter pair. Take the basic size plus a fit enum and compute both members in build(p):

_BANDS = ((3.0, 6.0), (6.0, 10.0), (10.0, 18.0), (18.0, 30.0), (30.0, 50.0))
_IT6 = (8, 9, 11, 13, 16)
_IT7 = (12, 15, 18, 21, 25)
_DEV = {"sliding":    ("es", (-4, -5, -6, -7, -9)),     # g6
        "locational": ("es", (0, 0, 0, 0, 0)),          # h6
        "transition": ("ei", (1, 1, 1, 2, 2)),          # k6
        "press":      ("ei", (12, 15, 18, 22, 26))}     # p6

PARAMS = {
    "bore": {"default": 10.0, "min": 3.0, "max": , : ,
             : },
    : {: , : ,
            : [, , , ],
            : },
}

 ():
     i, (lo, hi)  (_BANDS):
         lo < nominal <= hi:              
             i
       nominal <= _BANDS[][]  (_BANDS) - 

 ():
    i = _band(nominal)
    it6, it7 = _IT6[i] / , _IT7[i] / 
    which, values = _DEV[fit]
    dev = values[i] / 
    ei = dev  which ==   dev - it6
     nominal + it7 / , nominal + ei + it6 /    

 ():
    bore_d, shaft_d = _iso_sizes(p.bore, p.fit)
    ...

The printed branch is the same shape with a flat table instead of ISO 286:

_FDM = {"sliding": 0.30, "locational": 0.20, "transition": 0.05, "press": -0.12}

def _printed_sizes(nominal, fit, hole_shrink=0.20):
    return nominal + hole_shrink, nominal - _FDM[fit]      # bore, shaft

snippets/pin_and_bore.py is the complete part: block, bore, mating pin, fit and process enums, SOLID_LABELS = ["block", "pin"].

Checklist and failure modes

• Named the function (turns / locates / stays), then picked the designation — not the reverse.
• Nominal read from the right band (over … up to and including).
• Clearance stated diametral, applied once.
• Lead-in chamfer 0.5–1 mm at 30–45° on the entering member — an assembly aid, never part of the fit.
• Printed: hole compensated, bore axis vertical, one fit coupon measured.
• Two locating features → one hole plus one slot.

Recovering in build123d:

• A fit modelled as zero clearance is a degenerate boolean — coincident cylindrical faces are exactly what OCCT fuses unpredictably or refuses. Model each member at its own size and record the press in a spec; selectors-and-occt-failures has the playbook and safe_bool.
• Use holes.drill(part, points, diameter) for a fit bore — millimetres, no fastener table behind it (right: a Ø22 bearing seat is not an ISO 273 row), and its record reaches the drawing callout. holes.clearance(..., "M5", fit="medium") is fastener clearance, a different standard — not a way to size a bearing seat.
• A mouth chamfer that fails is usually eating past the wall: reduce the length (min(0.6, d / 12)) and wrap the call, or fillet with safe_fillet, which searches down to a radius that works.
• A bore that vanished was cut before the material existed — drill after the last fuse, then re-check metrics.n_solids.

Sources

• ISO 286-1:2010, ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits — IT grades and fundamental deviations.
• ISO 286-2:2010, Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts — the limit tables above and in tables/iso286.json.
• ISO 2338:1997, Parallel pins, of unhardened steel and austenitic stainless steel — m6 / h8 pin tolerances.
• ISO 15:2017 (rolling-bearing boundary dimensions) and ISO 492:2014 (radial-bearing GPS and tolerance values).
• SKF Rolling Bearings catalogue, "Bearing seats — tolerances and fits" — shaft/housing class by load condition and which ring rotates.
• Machinery's Handbook, 31st ed., Industrial Press — "Preferred Metric Fits".
• Heat-set core-hole diameters: ruthex brass insert datasheets (M2–M6) and Böllhoff QUICKSERT technical data for thermoplastics.
• Printed clearances are measured practice on 0.4 mm-nozzle FDM, SLA/DLP and SLS PA12 — starting values, confirmed with a fit coupon.