Manufacturing tolerances on site
Two sets of tolerances meet
Section titled “Two sets of tolerances meet”Production tolerances
Section titled “Production tolerances”The length of the produced member, the position of the operations, the geometry of the cross-section. These are machine tolerances, in millimetres or less.
Site tolerances
Section titled “Site tolerances”The plane and dimensions of the foundation, the position of existing structures, the accuracy of setting out. These are traditional construction tolerances — an order of magnitude larger.
Practical problems almost always arise where the two meet.
Why an inaccurate base hurts
Section titled “Why an inaccurate base hurts”An accurately produced frame assumes the surface it sits on is plane and the dimensions are right. If the slab is not plane, or the dimensions drift:
- the panel does not bear evenly,
- openings do not land where the windows were made for,
- restraint builds up during erection, loading the screwed connections,
- the accumulated deviation shows up at the end of erection, where there is nothing left to absorb it.
So the accuracy of setting out and of the foundation plane is not the groundworks contractor’s private business but a condition of the whole erection. Setting out and foundation tolerances.
How to handle it
Section titled “How to handle it”A levelling layer or shims
Section titled “A levelling layer or shims”Levelling designed under the bottom track — mortar or shims — absorbs small deviations. Design this in advance; do not invent it on erection day.
Check before erection
Section titled “Check before erection”Survey the foundation before the panels arrive. Half a day of measurement can prevent a week of trouble.
Fix the tolerance regime in the contract
Section titled “Fix the tolerance regime in the contract”What deviation in plane and dimension the structure will accept. Settle this between the contractors before anyone starts.
Avoid accumulation
Section titled “Avoid accumulation”Check dimension chains as a whole, not member by member: twenty small deviations add up to a big one.
What is checked on the production side
Section titled “What is checked on the production side”The length of the produced profile, the position of the operations, the characteristics of the cross-section. If these are right, production has done its part. In-process quality control.
The point
Section titled “The point”An accurate technology needs an accurate environment. Production accuracy alone is not enough — the accuracy of the base is part of the structure too.