The Slot That Lost 4 mm of Travel
A 120 × 60 × 8 mm aluminium plate needs a central through slot so a 12 mm pin can travel 40 mm. The sketch outline is anchored to the origin with horizontal/vertical relations and dimensioned 120 × 60. The slot was drawn as two parallel lines capped by two tangent arcs, made symmetric about the plate's vertical centerline, then dimensioned with a 40 mm overall length (tangent extremity to tangent extremity) and a 12 mm width. The sketch is fully defined and the part rebuilds clean. Six weeks later the pin is upsized to 16 mm, so the width dimension is edited from 12 to 16. The 40 mm overall dimension is untouched, but the assembly's limit mate now allows only 24 mm of travel and the pin bottoms out short of its stop. The drafter insists the travel cannot have changed because the drawing still shows 40.
1. The usable travel fell from 28 mm to 24 mm when the width changed from 12 to 16, even though the overall dimension still reads 40. Show the arithmetic and state the dimensioning scheme that would have prevented it.
Read the worked answer
The overall (tangent-to-tangent) length of a slot is not the travel. Each end cap adds half the width beyond its arc centre:
overall = centre_distance + width- Original:
40 = CD + 12→CD = 28 mm. That 28 mm was the real travel, not 40. - After the edit:
40 = CD + 16→CD = 24 mm. Travel dropped by exactly half the width increase (2 mm at each end).
The 40 mm dimension pins the outside of the slot, so the functional quantity (centre distance) is a derived, drifting value. Correct scheme: dimension the centre distance to 40 mm directly - sketch a construction line between the two arc centres (or use the arc centre points) and dimension 40 between them - then let the overall size be driven, or add it as a reference dimension. With a 12 mm pin the overall becomes 52 mm; with 16 mm it becomes 56 mm, and travel stays 40 in both cases. The tell to look for on any slot: if the dimension can be picked on a curved end face, ask whether the design intent is centre-to-centre or over-the-ends.
2. The drafter argues the symmetric relation about the plate's vertical centerline should have kept the slot correct. Explain why that is wrong, and what happens if you keep the 40 mm overall dimension and also add a 40 mm centre-distance dimension.
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Symmetry constrains position, not size. The symmetric relation only forces the two straight walls to sit at equal offsets from the centreline; it says nothing about where the arc centres are relative to each other, and it cannot preserve the centre distance when the width is edited. Symmetry is doing the job of keeping the slot centred - a different job from defining travel.
Adding both dimensions over-defines the sketch. With tangent end arcs and a width dimension, the three quantities are locked by CD = overall − width, so a 40 overall plus a 40 CD is contradictory/ redundant. SolidWorks will either flag the sketch as over-defined or silently solve to whichever it prefers and drive the other, which is worse because the error is invisible.
Rule to carry into the exam: in a chain of tangent geometry, dimension to the functional entities (arc centres, pin axes, mating planes) and leave derived extremes free or marked as reference. If you must control the envelope, dimension the envelope and the width, and accept that the centre distance is now the derived value - then check that nothing downstream depends on it.
3. After re-dimensioning, the plate rebuilds and the assembly's limit-distance mate (0 to 40) still reports the pin bottoming out early. What would you check first, and why?
Read the worked answer
The mate, not the sketch, is now the suspect. Check which entities the limit mate is actually attached to.
- If it references the slot's end wall (the arc face) or a tangent edge, that surface moves whenever the width changes, even with the centre distance correct. Highlight the mate's references and re-pick them.
- If travel is defined by the pin's axis, mate the pin's axis to a plane or sketch point located at the extreme arc centre, not to a curved face. Curved faces give the solver a tangent contact that shifts with every width edit.
- Confirm the mate's direction/flip - a flipped limit gives an inverted range that reads as "bottoming out early".
- Finally, check the pin-to-slot clearance. A 16 mm pin in a 16 mm slot with radiused ends contacts only at the tangent point; full travel requires slot width greater than pin diameter. If the slot is 16.0 and the pin 16.0, the pin will bind before the limit value regardless of how the sketch is dimensioned.