Dimensioning for the shop
Dimension from datums the fabricator can actually measure from, and only what matters.
By the end you can
- Dimension from a consistent datum rather than chaining measurements
- Decide which dimensions are critical and which are reference
- Adjust views and add sections so every dimension has somewhere sensible to live
On this page · 4 sections
There are many ways to dimension a part correctly and only a few that a person with a ruler can follow without accumulating error.
Datum, not chain
Chain dimensioning measures each feature from the previous one: edge to hole 1, hole 1 to hole 2, hole 2 to hole 3.
Datum dimensioning measures everything from one reference edge: edge to hole 1, edge to hole 2, edge to hole 3.
Chains accumulate error
Every measurement in a chain carries the error of the one before it. Three holes at ±0.5 mm each can put the last one 1.5 mm out — enough that a bolt pattern will not line up with the mating part.
Datum dimensioning gives each feature an independent error from a single reference. It is also how someone with a combination square actually works.
Pick the datum the fabricator will use
Usually a machined or sheared edge — something flat and real. Do not datum from a corner that is about to be rounded off, or from a centreline that exists only in CAD.
Dimension what matters, not everything
A fully dimensioned drawing of every edge is noise. Ask of each dimension: if this is off by a millimetre, does something fail to fit?
- Critical — bore diameters, bolt hole positions, anything that mates with another part. Dimension these clearly, from the datum.
- Reference — overall outline, cosmetic chamfers. Give them, but they do not need the same prominence.
Do not dimension the same thing twice
Giving both the three individual gaps and the overall length over-determines the part. When they disagree — and rounding means they eventually will — the fabricator has to guess which one you meant.
Making room
Dimensions need somewhere to go. Move views apart, add a section through a pocket whose depth is otherwise invisible, and blow up a detail rather than cramming four dimensions into a 5 mm square.
Check yourself
A plate has four bolt holes in a row that must line up with a mating part. How should they be dimensioned?
Next
The drawing describes one part. A bill of materials describes what to buy.
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