A hole marked 25 on a drawing is not an instruction. No machine makes anything exactly 25, and the person reading the sheet has to know how wrong it is allowed to be before the part stops working. A dimension without a tolerance is not an instruction, it is a wish with a number attached, and the workshop will either guess or ring you, and one of those is worse.
Emre is drawing a mounting bracket for a dough divider at a bakery in Bursa: a steel plate 160 mm long by 60 mm, four holes along it for M8 bolts, and a bore in the middle for the divider's shaft. Every decision on that sheet is a pair of numbers rather than one, and the second number of each pair is where the money is.
The second number, and which way it goes
The shaft is 25 and the bore is 25, and a 25 mm shaft does not go into a 25 mm hole. The way out is not to fiddle with both: the hole runs from nominal upwards and the shaft from nominal downwards. Emre's bore is 25 H7, which is 25.000 at its smallest and 25.021 at its largest, and the shaft it receives is undersized by a matched amount. That is the hole basis system, and its whole point is that a hole is made by a tool of a fixed size while a shaft can be turned down to anything, so the cheaper thing to vary is the one you vary.
The bolt holes are the same idea without the precision. They are 8.5 mm, not 8, because an M8 bolt does not go through a hole its own size, and the clearance is the difference between a bracket that bolts on and a bracket that gets filed on site by somebody who will mention it.
Then the one that catches everybody once. Dimension from one datum, or the tolerances stack. Four holes at 40 mm centres can be dimensioned as 40, 40, 40 from each other, and each of those is plus or minus 0.1, so by the fourth hole the error can be plus or minus 0.4 and the bracket does not meet the machine it bolts to. Dimensioned instead from the left edge, the same four holes read 20, 60, 100 and 140, each plus or minus 0.1 from the same face, and the stack never happens. Nothing about the part changed. The drawing changed, and the drawing is what gets made.
The band stops being an abstraction the moment parts exist. Emre gauges three bores out of the run: the first off the machine is 25.004 mm, the twentieth is 25.018 mm and still inside, and the fortieth is 25.03 mm, which is outside by 0.009 mm and is scrap. Nothing looks different about the third one. The shaft turns in it, and the divider shakes itself loose in a fortnight. That is what the second number buys: a part rejected in the workshop by a gauge reading from 24.98 to 25.04, rather than by a bakery in six weeks.
The sheet is a language, and the workshop reads it fast
A drawing is not a picture of the part, it is a set of agreed marks, and somebody in a workshop reads dozens of them in a shift. Dashed is hidden, chain is a centre line: a dashed line is an edge that exists and is behind something, a long-short chain line is the axis of a hole or the symmetry of a shape, and a thick continuous line is an edge you could touch. Draw a centre line as a thin continuous line and the reader is told there is a slot there.
Read the number, never measure off the paper. A drawing at 1:5 has been printed, scaled by a driver, photocopied, and opened on a phone in a workshop, and the only dimension that survives all of that is the one written down. This is why a scale is stated rather than implied, and why a drawing with a dimension missing is returned rather than measured.
The one that ruins whole parts is quieter than either. The projection symbol says which side the views were laid out from. Europe lays out its views in first angle and North America in third, which puts the right-hand view on opposite sides of the front view, and a part read in the wrong convention comes back mirrored: every hole in the correct place and the whole bracket a mirror image, fitting nothing. The symbol lives in the title block, which also carries the scale, the units and who checked it, and those are not administration. They are the four questions the shop asks before cutting anything.
What this topic is made of
Everything above is the waist of this subject. A building, a bracket, a shopfront, a stair detail and a kitchen all arrive from different worlds and they all pass through the same conventions, and then they come out again as different things: a toolpath, a cutting list, a planning submission, a joiner with a printed sheet. That is why the conventions a drawing has to follow to be read is the first cluster rather than a reference section at the back, and why it is worth learning even if the eventual work is entirely in a model.
Below the waist the topic divides by what is being made. Parts, assemblies and things that have to fit is where tolerance is the daily subject. Buildings, modelled rather than drawn is where the model generates the drawings instead of accompanying them, which changes what a mistake costs and where it is found. Geometry driven by rules instead of by hand is the one people arrive at last and it changes the others, because a shape defined by a rule can be asked for in eleven sizes.
Three bores, one gauge
A tolerance is a band, and the third part is scrap by 0.009
The band is the instruction. Drawn as a single line at 25 it would say there is a cliff there, and a reader would treat the cliff as a pass mark rather than as a region.
Millimetres, on the 25 H7 bore of Emre's bracket. The bakery is invented; the H7 band is the standard one for a 25 mm hole.
- First off the machine25.004 mmInside, near the bottom of the band
- Twentieth25.018 mmInside, and drifting as the tool wears
- Fortieth25.03 mmOutside. The shaft turns in it, and the divider vibrates itself apart
It says nothing about how the bore was measured, which is the argument that actually happens. A bore is round in one direction and not in another, and two people with two gauges will hand you two of these numbers.
The stall is modelling without drafting
This is where people in this topic stop making progress without noticing. Modelling is satisfying and it is visible: a bracket appears on the screen in an evening, it rotates, it looks like the thing. Drafting is none of that. So somebody arrives at a workshop with a beautiful model and an exported file, and the shop asks for a drawing with dimensions and tolerances on it, and there is not one, and the honest answer to why not is that nobody ever had to read the model.
The mirror image of it inside the software is the unconstrained sketch. Fully constrain the sketch before extruding: every line either dimensioned or locked to something, until the sketch turns the colour the program uses for solved. A shape that merely looks right is held up by nothing, and the first change to it moves geometry that nobody asked to move, usually a fortnight later and in a part that had been signed off.
What to open in 2026
AutoCAD has been the drafting standard since 1982, when it replaced the drawing board, and what keeps it there is not the program but the file: everyone you send a drawing to can open a DWG, including the shop that has not upgraded since 2014. It is a subscription, free for students. Revit is the other Autodesk seat, and it replaced drawing plans, sections and elevations as separate drawings that could disagree with one another, because the model produces all three and they cannot.
Fusion 360 is where a modeller and a toolpath live in the same file, which replaced buying a separate CAM package alongside the modelling one. It has a free personal licence that has been narrowed more than once, so check what it covers on the day rather than trusting a video. FreeCAD is the open source route and it became a serious recommendation rather than a hopeful one with version 1.0 in late 2024, which finally dealt with the topological naming problem that had been the first objection anybody raised to it for a decade.
What none of these decides is whether the drawing is readable, which is a separate skill from whether it is correct. Line weights on a sheet are hierarchy with a standard attached, and the reasons a thick line reads before a thin one are the same everywhere, in the principles that hold whatever you are designing. The image a client is shown rather than the sheet a shop is sent is materials, light and an image a client understands, and it answers a different question from any drawing here. And the question underneath all of it, which program a given output actually requires, is the software, and the parts of it actually used, with the difference that in this topic the wrong file format is not an inconvenience, it is a part that cannot be made. Everything a shop cannot read gets guessed at, which is the same failure that marks, layouts and printed things meets at the printer, arriving a week later and costing rather more.