Dimensioning is taught as a drafting skill; tolerancing is a manufacturing decision. Most designers who came up through design school rather than a machine shop inherited a title block with a default tolerance in it, and that default quietly governs every dimension on every drawing they issue. Here is the discipline that replaces it — and it fits in one rule.
The rule: two kinds of tolerance information, no more
A fabrication drawing needs exactly two things said about tolerance. First, a general note referencing the Acrylic Fabrication Tolerances — the published standard that defines typical ranges per process: laser-cut components at approximately ±1/32 to ±1/64 inch, CNC-routed at approximately ±1/32 inch, saw-cut and fabricated assemblies at approximately ±1/16 to ±1/8 inch, thermoformed components at approximately ±3% to ±5% of the formed dimension. Second, explicit callouts on the few features where fit actually matters. Everything beyond those two is over-specification: precision requested on dimensions that have no fit partner, purchased at fabrication-cost and calendar prices nobody chose deliberately.
Which features earn an explicit callout
The explicit list is shorter than most designers expect — typically a handful of dimensions per part, not dozens:
- Mating parts. Slots and tabs, nesting components, anything that assembles into something else made on the same drawings. Fit between parts you control is the classic case for a stated clearance.
- Hardware interfaces. Hole patterns that must align with real hardware — hinges, standoffs, latches, fasteners — where the hardware’s own dimensions set the requirement.
- Third-party interfaces. Mounting to site conditions, engaging another fabricator’s structure, dropping into millwork built from a different drawing set. Wherever your part meets a dimension you don’t control, the callout is how the two drawings agree.
Everything else on the part — outer profiles, decorative features, dimensions with no fit partner — runs happily inside the general note’s process band.
What blanket tight tolerancing actually does
Here is the mechanism, so the habit dies for a reason rather than on authority. A tolerance is an instruction about process selection and inspection. Dimension a part at a blanket tight tolerance and features that could have been saw-cut now route to slower machining; parts that needed a glance now need measurement against numbers that were never functional. The shop can build it — that is not the issue. The issue is that the drawing ordered precision the part cannot use, and precision is a pure cost and calendar factor. Nobody decided to spend it; a title-block default did.
Using the process-tolerance connection forward
The same table that sets expectations also hands designers a design tool. Knowing that machined features hold tighter bands than formed ones means precision gets placed where the process delivers it: the tight fit goes into the CNC-routed insert, not the thermoformed body; the formed part gets contour and the machined part gets registration. Designers who internalize the per-process ranges stop fighting their processes and start assigning jobs to them — which is most of what design-for-fabrication means in practice.
The one-line fix
Open your drawing template. Replace the inherited default in the title block with a general note referencing the published fabrication tolerance standard, and reserve explicit callouts for the mating, hardware, and site interfaces that earn them. That single edit upgrades every drawing you issue from now on — parts arrive fit for their function, quotes stop pricing precision you didn’t want, and the drawing finally says what you meant. The same treatment applies to the template’s finish and edge callouts, where the published edge finishing methods are the vocabulary to reference. Send the next drawing through and see what a two-callout print quotes like.
