48610 Chap.7 Marking Out, Measurement and Manufacturing
Marking Out, Measurement and Manufacturing
This is the step where a correct drawing becomes a wrong part, and it is where the prototyping project actually lives: workshop and machine inductions sit in Week 5 of the lab sequence, and from Week 8 onward groups are cutting, drilling and assembling.
Marking out, also called scribing, means scratching lines onto a workpiece, and the manufacturing guide gives it two purposes: to outline the shape and mark feature positions as a guide for removing material by hacksawing or filing, and to help set the workpiece up on a machine by aligning it to scribed lines, which matters most when establishing a datum on a casting or forging that has no accurate face to start from.
It is two operations, not one: measuring carries the sizes off the drawing onto the metal, and marking scribes the witness lines.
Everything is measured from a datum. The basis of all dimensional measurement is the datum plane, whose most important physical embodiment is the surface plate, a hard flat surface, levelled, against which precision inspection, marking out and tool setting are carried out.
The instrument ladder runs from a steel rule at about a millimetre, or half of one by eye, to vernier instruments with a least count down to 0.01 mm, to micrometers where one thimble turn advances the spindle 0.5 mm and the fifty thimble divisions are therefore worth 0.01 mm each.
The instrument is chosen against the tolerance, because inspection compares a measurement with the tolerance on the drawing to decide whether the part is acceptable.
What this chapter covers
- 01
Marking out and its two published purposes
- 02
Measuring and marking as separate operations
- 03
The datum plane and the surface plate
- 04
Engineer's square, scribers, dividers and the hermaphrodite calliper
- 05
The vernier height gauge and working from the plate
- 06
Steel rules and plain callipers: transfer against direct reading
- 07
How a vernier scale gains a decimal place
- 08
Micrometer mechanics: 0.5 mm per turn, fifty divisions of 0.01 mm
- 09
Using the ratchet so two people get the same reading
- 10
Choosing an instrument against the tolerance band
- 11
General tolerances in the notes, and why inspection needs them
Two micrometer readings, one of them a trap
- +1Read the sleeve correctly. Each millimetre on the sleeve is subdivided in half, because one full turn of the thimble moves the spindle 0.5 mm. The first student has counted the uncovered half millimetre graduation; the second has ignored it and read only the last whole number.
- +1Read the thimble. Fifty divisions span one turn and one turn is 0.5 mm, so each division is 0.5 divided by 50, which is 0.01 mm. Division 32 therefore contributes 0.32 mm for both students.
- +1Add. The first student reports 7.5 plus 0.32, which is 7.82 mm. The second reports 7.0 plus 0.32, which is 7.32 mm.
- +1Compare each against the tolerance. The drawing allows 7.75 to 7.85 mm, so 7.82 mm is accepted while 7.32 mm is 0.43 mm undersize and would condemn a good part. Note the direction: missing the half millimetre graduation always reads low, by exactly 0.5 mm.
Key terms
- Marking out
- Scribing lines onto a workpiece to outline its shape, mark feature positions, and provide references for setting the job up on a machine. Also called scribing.
- Datum plane
- The reference surface from which all dimensional measurements on a workpiece are taken. The surface plate is its most important physical example and must itself be made to high accuracy.
- Surface plate
- A hard flat surface, levelled, against which precision inspection, marking out and tool setting are carried out, durable enough to withstand constant contact with metal parts.
- Least count
- The smallest increment an instrument can resolve. A vernier calliper reaches 0.01 mm or better, while a steel rule manages about 1 mm, or 0.5 mm by estimating between graduations.
- Vernier scale
- A second scale whose divisions are slightly shorter than those of the main scale, so that exactly one pair of lines coincides at any setting and identifies the fractional part of the reading.
- Micrometer
- A screw based measuring instrument in which one turn of the thimble advances the spindle 0.5 mm, with fifty thimble divisions each worth 0.01 mm. Also a unit of length equal to one thousandth of a millimetre.
- Ratchet
- The part of a micrometer that limits the force applied when closing on the work, so that the reading is repeatable between operators rather than depending on grip.
Marking Out, Measurement and Manufacturing FAQ
Why not just measure from the previous feature each time?
Because the errors accumulate. Marking six holes at 50 mm intervals, each step from the one before, with each step up to about 0.3 mm out in the same direction, puts the last hole roughly 1.5 mm from where the drawing says even though no individual measurement was bad. Mark out from a single datum edge instead, scribing at 50, 100, 150 and so on from that one face, and each feature then carries only its own error.
How do I choose which instrument to use?
By comparing its resolution with the tolerance band, not with the nominal size. A dimension toleranced at plus or minus 0.05 mm has a band only 0.1 mm wide, which a steel rule resolving 0.5 mm at best cannot see at all. That is a different and more serious problem than measuring imprecisely: no reading from that rule can distinguish a conforming part from one well outside the band.
What is the difference between a plain calliper and a vernier calliper?
A plain calliper has no scale and transfers a size from the work to a rule, so the accuracy of the result is the accuracy of the rule plus whatever was lost in the transfer. A vernier calliper reads the size directly on its own two scales, with a clamping screw to lock the jaw once positioned, so there is no transfer step. For any number you intend to quote in the project report, read it directly.
Why does the drawing need tolerances for the workshop to do its job?
Because inspection is a comparison. Manufactured parts are measured with tools chosen for the accuracy required, and the readings are then held against the tolerances written on the drawing to decide whether the part passes. With no tolerance anywhere on the sheet there is nothing to compare against, which is why the manufacturing guide states plainly that a drawing containing dimensions but no tolerances is incomplete.
General tolerances live in the notes and cover every dimension that has no explicit one.
Assessment move
Get your inductions done in the week the lab sequence schedules them, because nobody in your group can manufacture anything before theirs is complete and it is invisible as a constraint until it stops you at the door. Practise reading a vernier and a micrometer until you can do it without thinking, using the virtual instrument the guide links to if you cannot get to the workshop.
Before marking out anything, choose and label the datum face on the workpiece itself so the next person to pick it up uses the same one. And measure your finished parts against your own drawing rather than against your memory of it: the comparison is the point of having written tolerances down.
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