Drawing standards

Why this matters

An engineering drawing is the common language between designer, printer operator and inspector. Read the views the wrong way round and the part comes out mirrored; leave out tolerances and nobody knows how much deviation is acceptable, so printed parts may not fit together. This course uses one hypothetical case throughout: the lab designs and prints a camera mount and a prototype drone arm in PLA. Numbers are hypothetical unless a source is given. The Python code can be downloaded from /downloads/uat-208/.

Projections and lines

The ISO 128 series (2020 editions) sets the principles of drawing, such as line types and sections. ISO 5456-2 sets out orthographic view layouts, with two systems you must tell apart:

• First angle, common in Europe and Asia: the top view goes below the front view, and the left-side view goes on the right.
• Third angle, common in North America: the top view goes above the front view, and the left-side view goes on the left.

A drawing must carry the projection symbol (a truncated cone and concentric circles) in the title block to show which system is used. Dimensioning follows ISO 129-1. Giesecke et al. (2023) explain both systems with examples.

General tolerances and stack-ups

Every size has a tolerance, the range by which the real size may differ from the stated size. If not every dimension is toleranced individually, general tolerances from ISO 2768-1 apply, noted in the title block as, for example, “ISO 2768-m”. The table gives values by size range and class f (fine), m (medium), c (coarse), v (very coarse). The standard also states that a part exceeding a general tolerance is not rejected automatically if its function is unaffected.

When several parts stack, deviations accumulate. The worst-case method adds tolerances directly: safe but overly cautious. The RSS method assumes each part’s deviation is independent and centred, so it adds them as the root sum of squares: narrower, but with a small chance of exceeding the result.

import math

ISO2768 = {                                   # class: [(upper bound of range mm, tolerance ± mm), ...]
    "f": [(3, 0.05), (6, 0.05), (30, 0.1), (120, 0.15), (400, 0.2), (1000, 0.3)],
    "m": [(3, 0.1), (6, 0.1), (30, 0.2), (120, 0.3), (400, 0.5), (1000, 0.8)],
    "c": [(3, 0.2), (6, 0.3), (30, 0.5), (120, 0.8), (400, 1.2), (1000, 2.0)],
}

def general_tol(size_mm, cls="m"):
    return next(t for upper, t in ISO2768[cls] if size_mm <= upper)

for size in (2.5, 5, 25, 60, 150):
    print(f"{size:>5} mm: f ±{general_tol(size, 'f')}, m ±{general_tol(size, 'm')}, c ±{general_tol(size, 'c')}")

plates = [3.0] * 4
slot = 12.5
t_plate = [general_tol(p) for p in plates]
t_slot = general_tol(slot)
gap = slot - sum(plates)
worst = t_slot + sum(t_plate)
rss = math.sqrt(t_slot ** 2 + sum(t * t for t in t_plate))
print(f"nominal gap {gap:.2f} mm, worst case ±{worst:.2f} -> min {gap - worst:+.2f} mm")
print(f"RSS ±{rss:.3f} -> min {gap - rss:+.3f} mm")

  2.5 mm: f ±0.05, m ±0.1, c ±0.2
    5 mm: f ±0.05, m ±0.1, c ±0.3
   25 mm: f ±0.1, m ±0.2, c ±0.5
   60 mm: f ±0.15, m ±0.3, c ±0.8
  150 mm: f ±0.2, m ±0.5, c ±1.2
nominal gap 0.50 mm, worst case ±0.60 -> min -0.10 mm
RSS ±0.283 -> min +0.217 mm

Fits and screw clearance holes

When a shaft goes into a hole, such as a bearing shaft or a hinge pin, the ISO 286 fit system is used. Hole and shaft codes set the position and width of the tolerance zones. Per ISO 286-2, for sizes over 6 up to 10 mm, an H7 hole is 0 to +15 µm and a g6 shaft is −5 to −14 µm, a small-clearance fit that can rotate.

For screw holes, ISO 273 gives three clearance hole series; for M3, holes of 3.2 mm (fine), 3.4 mm (medium) and 3.6 mm (coarse).

HOLE_H7 = (0.0, 0.015)                   # mm, 6–10 mm range per ISO 286-2
SHAFT_G6 = (-0.014, -0.005)
c_min = HOLE_H7[0] - SHAFT_G6[1]
c_max = HOLE_H7[1] - SHAFT_G6[0]
print(f"8 H7/g6: clearance {c_min * 1000:.0f} to {c_max * 1000:.0f} µm")

ISO273_M3 = {"fine": 3.2, "medium": 3.4, "coarse": 3.6}
for series, hole in ISO273_M3.items():
    print(f"M3 {series:<6}: hole {hole} mm, diametral clearance {hole - 3.0:.1f} mm")

8 H7/g6: clearance 5 to 29 µm
M3 fine  : hole 3.2 mm, diametral clearance 0.2 mm
M3 medium: hole 3.4 mm, diametral clearance 0.4 mm
M3 coarse: hole 3.6 mm, diametral clearance 0.6 mm

Module lab

Common mistakes

Summary

• First angle places the top view below the front view, third angle above; always include the projection symbol.
• ISO 2768-1 gives general tolerances by size range, for example ±0.2 mm for class m, 6–30 mm.
• Worst-case stack-ups add directly; RSS adds as the root sum of squares.
• An 8 H7/g6 fit has 5–29 µm clearance, and ISO 273 M3 holes are 3.2/3.4/3.6 mm.

Check your understanding

1. In third angle, where does the top view go?
2. What deviation does ISO 2768-m allow on a 45 mm dimension?
3. Five parts each ±0.1 mm: what are the worst-case and RSS stack-ups?
4. An 8 mm H7 hole with an h6 shaft (h6 = 0 to −9 µm): what is the minimum clearance?
5. What is the ISO 273 medium clearance hole for M3?