10.3 Slots, Tabs, and Tolerance Allocation

10.3 Slots, Tabs, and Tolerance Allocation

Quick Answer: For a slot or tab, the position zone is two parallel planes for the width, combined (when needed) with two parallel planes for the length — a bidirectional zone, not a cylinder. Allocate a fixed assembly tolerance across a stack-up so the sum of feature tolerances does not exceed the available clearance. Choose position to control a slot's center; choose profile to control its boundaries (size + form + location together).

Position zone for a slot or tab

A cylindrical position zone (Ø) applies to a feature's axis and assumes equal tolerance in every radial direction. A slot does not have a single axis; it has a center plane for its width and, separately, a center plane for its length. The Y14.5-2009 position zone for a slot is therefore a pair of parallel planes spaced by the tolerance value, centered on the true position of the slot's width. If the length must also be located, a second pair of parallel planes is added for the length — this is the bidirectional zone (width × height).

• Width zone: two parallel planes spaced = position tolerance, centered on the true center plane of the slot width. The slot's width center plane must lie within this zone.
• Length zone: two parallel planes spaced = position tolerance for length, centered on the true center plane of the slot length. The slot's length center plane (or its end surfaces) must lie within this zone.

When the slot is symmetric (e.g., a rectangular keyseat), the width and length zones together form a rectangular tolerance zone at the slot's center — equivalent to a rectangular true position with separate width and length tolerances.

Specifying the bidirectional zone on the drawing

The drawing typically shows two position callouts for the same slot, one for each direction:

  Width: position 0.1 to A|B|C  (two parallel planes, 0.1 apart, normal to width)
  Length: position 0.4 to A|B|C (two parallel planes, 0.4 apart, normal to length)

Or, where the slot is located by a single bidirectional control, a single callout with a note 'TWO PARALLEL PLANES' may be used. The width tolerance is typically tighter than the length tolerance because a keyseat must fit a key of fixed width precisely, but the key can be shorter or longer than the slot and still function.

Worked example — keyseat slot

A shaft has a 6 mm wide × 25 mm long keyseat. A parallel key 6 × 6 × 22 must fit the keyseat. The slot is located to A (shaft axis), B (a datum shoulder), and C (a clocking flat). The drawing specifies:

• Width: position 0.05 to A|B|C (two parallel planes 0.05 apart, normal to the 6 mm width).
• Length: position 0.4 to A|B|C (two parallel planes 0.4 apart, normal to the 25 mm length).

Analysis:

• The 6 mm key must fit the 6 mm slot, so the slot's width is the functional dimension; the width tolerance is held tight (0.05) to ensure the key slides without play.
• The key is 22 mm long in a 25 mm slot, so the slot has 3 mm of length clearance; the length tolerance can be loose (0.4) because the key engages regardless of small length variation.
• A cylindrical Ø0.05 zone would be wrong: it would imply the slot's center can move 0.025 in any radial direction, including along the length — overly restrictive on length and functionally unnecessary.
• A cylindrical Ø0.4 zone would be wrong the other way: it would allow 0.2 of width shift, which is far too loose for a 6 mm key fit.

Tolerance allocation across a stack-up

When an assembly tolerance is fixed (e.g., a cover must fit a housing within a total of 0.3 mm), the designer allocates that tolerance across the contributing features. The sum of allocated tolerances cannot exceed the assembly allowance. Allocation is a design choice that can be weighted toward harder-to-machine features.

Allocation example

A cover bolted to a housing must seat fully with 0.3 mm total clearance available for mislocation. Three features contribute to the mislocation stack-up:

1. The cover's bolt-hole pattern (machined).
2. The housing's tapped-hole pattern (cast then tapped).
3. The cover's locating dowel hole (machined).

The sum of allocated tolerances (0.08 + 0.18 + 0.04 = 0.30) equals the available assembly clearance of 0.3 mm, so assembly is guaranteed at worst case. The designer weighted the allocation toward the cast housing (the hardest to hold) and away from the machined dowel (the easiest). If the housing could not be held to 0.18, the assembly clearance would need to increase, or a different process (machining the housing boss) would be required.

Allocation vs. statistical stack-up

The example above is a worst-case (arithmetic) stack — all tolerances at their extreme simultaneously. For larger stacks, a statistical (RSS) analysis may be used: RSS = √(T1² + T2² + … + Tn²), which yields a smaller combined tolerance and lets individual feature tolerances be looser, at the cost of accepting a small probability of out-of-tolerance assemblies. The Senior exam tests both; know which method the question assumes.

Position vs. profile for slots

Position controls the slot's center (center plane or center point). Profile of a surface controls the slot's boundaries — the actual surfaces of the slot — and therefore controls size, form, and location together. The choice depends on what must be controlled:

Senior-level traps

1. Using a cylindrical Ø zone for a slot. A cylinder assumes equal tolerance in all directions; a slot usually needs different tolerances on width and length. The bidirectional parallel-plane zone is the correct control.
2. Allocating more tolerance than the assembly allows. The sum of allocated tolerances must not exceed the available assembly clearance. A stack that sums to more than the clearance will not assemble at worst case.
3. Confusing position and profile for a slot. Position controls the center; profile controls the boundaries. A Senior question may show a slot whose size is drifting and ask which control is missing — if the width dimension's ± is not held, position alone does not control it, and profile would be the correct single control.
4. Ignoring the length zone. A slot located only by a width-zone position callout has no length location control; the slot can shift along its length. If length location matters, a second (bidirectional) callout is required.
5. Mixing worst-case and RSS in one stack. A stack-up must be consistently worst-case or consistently statistical; mixing methods in one analysis produces an incorrect combined tolerance.