Module 19 · Lesson 19.1
From model to machine
What changes when the geometry is fabricated directly, and where the tolerance goes.
Why this matters
For most of the profession's history the model produced a drawing, and a person read the drawing and made the thing. That person absorbed a great deal: they noticed impossible dimensions, resolved ambiguity, and applied trade knowledge nobody had written down.
When the model drives the machine directly, that layer is gone. The geometry is fabricated as specified, including the parts that are wrong. It is a genuine gain in speed and precision and a genuine transfer of responsibility onto the model, and the second half is the one worth thinking about.
By the end of this lesson you should be able to
- Say what a fabrication model must contain that an analysis model need not
- Explain where tolerance goes in a digital workflow
- Explain rationalisation as a fabrication requirement
- Recognise the construction sequence as information the model usually lacks
What you should already know
- Physical, analytical and documentation models as different things (Module 1)
- Rationalisation of generated geometry (Module 3)
Three models, three purposes
This course has kept them apart from Module 1 onwards, and fabrication adds the third:
The analysis model represents behaviour. It may sit members on their centroids, ignore connections and omit anything below the mesh size. Module 12 has a case about the damage done by making it look like the structure.
The documentation model represents intent, dimensionally, so somebody can build from it.
The fabrication model represents the piece being made. Every hole, every weld preparation, every bolt, in the machine's coordinates.
The analysis model is the most abstract and the fabrication model the least. They are not versions of one another, and generating one from another is a translation with losses — which is what Module 5 is about.
Where the tolerance goes
A cutting machine holds a tolerance a person cannot. It is tempting to conclude that tolerance stops mattering. It does not; it moves.
To the interfaces. A perfectly cut member still meets a concrete slab poured on site to a construction tolerance of several millimetres. The variation has to be absorbed somewhere, and if the fabricated component has no adjustment in it, it will be absorbed by force or by grinding.
To the survey. A model-driven component fitting an existing structure is only as accurate as the survey of what is there. A laser scan is very precise about where the building was on the day it was scanned.
To thermal and erection effects. A steel frame is fabricated at one temperature and erected at another, under self-weight it did not have in the shop. Members fabricated to exact geometry fit an erected frame that has moved.
Digital fabrication makes the component more precise. It does not make the building more precise, and adjustment still has to exist somewhere. Deciding where is a design decision.
Rationalisation, again
Module 3 introduced it for parametric geometry and it returns here as a fabrication requirement.
A generated form has whatever geometry the rules produced: 289 distinct member lengths, no two nodes alike, every panel doubly curved. A machine will cut all of it. What it costs is not cutting time — it is 289 fabrication drawings, 289 stock items, 289 chances to erect the wrong one, and a sorting problem on site.
So rationalisation trades a little structural efficiency for a lot of buildability: standard lengths, planar panels, a small set of node types. It always costs something structurally and it usually saves more than it costs, and the trade should be quantified rather than assumed.
What the model does not contain
A fabrication model describes the finished state. Construction is a sequence, and the structure passes through states that are not the finished one:
- Members carrying loads they were not designed for, in temporary configurations.
- Stability that depends on a bracing member not yet installed.
- A concrete frame carrying construction loads at three days' strength.
None of that is in the model unless someone puts it there. Module 8's staged-analysis discussion belongs here too: the critical case for a member is sometimes a Tuesday in March, not the completed building.
Check yourself
A steel frame is fabricated directly from the model to a tolerance of ±1 mm, and connects to a concrete core built to ±15 mm. Where should the adjustment be?
Practice
A generated roof has 340 members of which 289 are distinct lengths. Rationalisation reduces this to 12 standard lengths at a cost of 4 % extra material. If the original material quantity was 210 tonnes, what is the rationalised quantity in tonnes?
Worked example
What rationalisation buys and what it costs
Given
- A roof has 289 distinct member lengths as optimised
- Rationalising to 12 standard lengths adds 4 % to the steel tonnage
- The original tonnage is 210 t
Find
Both sides of the trade, quantified
Worked example
Where the tolerance went
Given
- A steel frame is fabricated from the model to ±1 mm
- The concrete cores it connects to are built to ±25 mm
Find
What the precision of the steelwork achieves
Practice
Rationalising 289 distinct member lengths to 12 standards adds 4 % to a 210 t frame. How many tonnes is that?
Practice
A frame member is fabricated to ±1 mm and the core it connects to is built to ±25 mm. How much adjustment must the connection accommodate, in millimetres, taking the worst case in one direction?
Check yourself
What does a fabrication model contain that an analysis model does not?
Check yourself
Why is the critical case for a member sometimes during construction rather than in service?
Summary
- Analysis, documentation and fabrication models represent behaviour, intent and the piece — three different things
- Digital fabrication makes the component precise, not the building
- Tolerance moves to the interfaces, the survey and the erected geometry
- Rationalisation trades structural efficiency for buildability, and the trade should be quantified
- The critical case for a member is sometimes during construction, and the model does not know
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint