MYP Design Measuring and Marking Out Notes

Marking Out Is the Cheapest Place to Fix a Mistake

1. Marking out means drawing the cutting lines, hole centres and joint positions onto the material before any tool touches it.
2. A wrong line costs you a rubber and thirty seconds, while a wrong cut costs the whole sheet of 18 mm birch ply.
3. Cutting, drilling, bending and joining all work to the lines you put down here, so an error at this stage travels through the whole build.
4. Photograph the marked out material before you cut anything, because for criterion C that photo is evidence you followed your plan instead of guessing.
5. Marking out also forces a decision: the moment you draw the tenon shoulders you have settled how long the rail really is.

The Marking Out Kit: One Tool Per Job

1. A steel rule both measures and acts as a straight edge, and you lay it with the graduations touching the surface so your eye cannot read the mark at an angle.
2. A try square has a thick wooden or plastic stock and a steel blade set at 90 degrees, and you use it to carry a line across timber square to an edge.
3. An engineer's square is all steel with a thin stock, so it sits flat against machined metal and tells you whether a filed face is truly square.
4. A scriber is a hardened steel point that scratches a fine permanent line into metal, far thinner than the half millimetre smudge a pencil leaves.
5. A marking gauge carries a pin on a stem with a sliding stock, and once the stock is locked it draws a line a fixed distance from the edge for the full length of the piece.
6. Dividers scribe circles and arcs from a fixed radius, and a centre punch dimples each hole position so the drill bit has somewhere to bite.

Datum Faces and Edges: Everything Starts From One Corner

1. A datum face is the one surface you make flat and true first, and the datum edge is the edge you then make square to it.
2. Every measurement afterwards comes off that face and that edge, never off a line you drew five minutes ago.
3. Cabinetmakers mark the datum face with a looping pencil squiggle and the datum edge with a caret pointing at it, so the reference surfaces are obvious hours later.
4. On a length of 44 by 20 mm pine, gauging the 20 mm thickness from the datum face on both sides makes the two lines meet even if the timber is slightly bowed.
   1. Gauge from whichever face is nearest and the lines miss each other, and the piece ends up wedge shaped.
5. Metal gets its datum the same way: file one edge flat, then check it with an engineer's square before you scribe anything from it.

Accumulated Error: Each Mark Inherits the Last One's Mistake

1. Say you need six holes at 40 mm spacing and you measure each one from the hole before it.
2. Being 0.5 mm out on each of the five gaps leaves the last hole 2.5 mm from where the drawing puts it, and the errors never cancel because they all lean the same way.
3. Measure from the datum instead and you mark 40, 80, 120, 160 and 200 mm, so each hole carries only its own small error.
4. The same trap catches you cutting five 100 mm lengths from one strip, because every saw cut swallows roughly 2 mm of kerf that nobody measured.
5. Holding a new piece against the one you just cut copies that error forward too, so go back to the drawing for every part.

Callipers and Micrometers: Reading Below a Millimetre

1. A steel rule is honest to about 0.5 mm, which is plenty for a shelf and useless for a shaft that has to turn inside a bearing.
2. A vernier calliper reads to 0.02 mm on the vernier scale, or 0.01 mm on a digital one, and it measures three different things.
   1. The outside jaws grip a dowel, the inside jaws open inside a drilled hole, and the depth rod slides out of the tail to measure how deep a housing has been cut.
3. A micrometer reads to 0.01 mm and is built for thickness, and you turn the ratchet at the end until it clicks so every reading uses the same gentle pressure.
4. One full turn of the thimble moves the spindle 0.5 mm and the thimble is split into 50 divisions, so each division is 0.01 mm.
5. Close the jaws and check the tool reads zero before you trust it, because a calliper that has hit a concrete floor rarely does.

Wood, Metal and Polymer Each Take a Different Mark

1. On wood, use a sharp HB pencil for general lines and a marking knife where the cut must be exact, since a knife line severs the surface fibres and gives the chisel a wall to sit against.
2. Marking gauge lines run with the grain and will wander into a soft growth ring if you lean on the pin, so take two light passes instead of one heavy one.
3. On metal, brush the surface with layout blue or colour it with a permanent marker, then scribe through the coating so a bright silver line shows against the dark ground.
4. Centre punch every hole position in metal with one light tap of a ball pein hammer, or the drill will skate across the polished surface and start the hole in the wrong place.
5. On acrylic and other polymers, leave the protective film in place and draw on the film with a fine marker, because a scriber leaves a scratch that is still visible after polishing.
6. Where one shape repeats, cut a template from 3 mm MDF once and draw round it, which takes the measuring out of every piece after the first.