How to Read a Technical Drawing to Prepare a Quote
In a technical drawing there are 5 points that determine 80% of the cost. Here's the reading checklist that avoids wrong quotes and bad surprises in production.
1. Dimensional tolerances: the hidden multiplier
The first scan of the drawing looks for the tightest tolerances. A generic dimension (±0.1 or general tolerance ISO 2768-m) is machined in a single pass; an H7 requires boring or fine interpolation, an IT5 may require external grinding. Rule of thumb: every tolerance class jump below IT8 can add 20-50% to the time for that feature — and sometimes an external operation with relative lead time.
2. Surface roughness: Ra and machining symbols
An Ra 3.2 is standard milling finish. An Ra 1.6 requires dedicated finishing passes; Ra 0.8 aggressive parameters and tools in excellent condition; Ra 0.4 or below almost always grinding or lapping. Also check WHERE the roughness is required: an Ra 0.8 only on a bearing seat costs little, the same value extended to the entire part changes the quote.
3. GD&T: the symbols that scare (with good reason)
Flatness, parallelism, concentricity, position: geometric tolerances impact setups and inspections. A concentricity of 0.02 between two diameters requires machining them in the same setup (or dedicated fixtures). A position ±0.05 on a hole pattern may require CMM inspection with report. Every GD&T symbol below 0.05 mm deserves a moment of reflection before writing the price.
4. Threads, grooves and special features
Count the threaded holes (every tapping is time + risk of tap breakage on tough materials), circlip grooves, relief grooves, unquoted chamfers required by notes. The notes at the bottom of the drawing are a mine of hidden costs: 'deburred edges', 'part free from burrs', 'laser marking of code', 'material certificate 3.1'.
5. Material and treatments: read the title block too
From the title block: exact material (C45 heat-treated is not C45!), supply condition, required heat treatment or surface treatment, any reference standard. A 42CrMo4 heat-treated to 850 N/mm² is machined with different parameters — and different tools — than the same steel annealed.
MachinePilot AI Vision automates this reading: you upload the drawing (PDF, STEP, DXF or even a photo) and the system extracts dimensions, ISO tolerances, Ra roughness, threads and critical features, highlighting them on the drawing with bounding boxes. The 4-level anti-hallucination guardrail blocks uncertain extractions instead of inventing numbers — and you verify in seconds instead of reading for half an hour.