Level 2Unit E · Session 25
25

Unit E · Designing a part that fits

Measuring, and designing to fit

What we are making: the measurements and the skill. The part itself comes next session.

Why this unit sits here

In Session 10 you built a distance meter, and you found two things out: aimed squarely at a wall it is accurate to a centimetre, and aimed at an angle it is nonsense.

Since then it has been propped against a book, or held in somebody's hand. Both of those move. Every reading you have taken has depended on how well somebody was holding it.

This unit fixes that properly, and it makes a point worth making: a measurement is only as good as the thing holding the sensor.

the printer bed, seen from above and to the sideXleft and rightYback and forthZup, the layersEverything you design lives in three directions.X and Y are the flat ones. Z is height, and it is the one that mattersmost — the printer builds in Z, one layer at a time.Millimetres. Always millimetres. Never centimetres, never inches.
Millimetres in three directions. Everything here is mm — never cm.
New words
CADComputer-Aided Design. Drawing a part on screen before making it.
ToleranceThe gap you deliberately leave so two parts actually fit together.
ClearanceHow much bigger a hole is than the thing going into it.
Layer heightHow thick each printed layer is. Usually 0.2 mm.
OverhangPart of a model with nothing underneath it. Printers hate these.

Do it — measure the sensor properly

You need six numbers, and you need them right. A ruler will do; callipers are better if your teacher has them.

  1. The sensor board: length, width, thickness.
  2. The two barrels: diameter, and the distance between their centres.
  3. How far the pins stick out behind the board — the stand must not crush them.
  4. Write all six in your notebook, with a labelled sketch.
  5. Now measure them again, independently, and compare. If the two sets differ, measure a third time.
⚠ Careful
Measuring twice is not being careful, it is being correct. A 1 mm error on a hole spacing means the part does not fit at all, and you will not know why.

Why the hole has to be bigger

A 10 mm rod, and three holes drawn for ithole 10 mmwill not fitthe printer over-fillshole 10.4 mmfits, snugly0.4 mm of clearancehole 11.5 mmfits, and rattlestoo much clearanceMelted plastic spreads slightly as it cools, so a printed hole always comes out a little smaller than you drew it.Draw holes about 0.4 mm bigger than the thing going into them. Then measure what you got, and adjust.
Plastic spreads as it cools. Draw holes about 0.4 mm oversize and measure the result.
📷
images/l2-e1-tinkercad-hole.png
Screenshot: a cylinder switched to Hole, dropped into a box, with the diameter box showing 16.4

The four rules of a printable part

  • Nothing floats. Every part of the model needs something below it, or the printer prints into air.
  • Walls at least 2 mm. Thinner than that and it snaps in your hand.
  • Overhangs under 45 degrees. Steeper than that needs support material, which is slow and leaves a rough surface.
  • Holes 0.4 mm oversize. Then measure the print and adjust.

Try it

  1. In Tinkercad, make a plate the size of your sensor board, with two holes matching your barrel diameter and spacing.
  2. Print nothing yet. Just get the numbers into a shape.
  3. Swap screens with another team and check each other's numbers against their notebook.
📷
images/l2-e1-tinkercad-tour.png
Screenshot: Tinkercad with the workplane, the shape panel on the right and the ruler tool each named
Challenge optional — only if you finish early
  • Measure the same part with a ruler and with callipers. How far apart are the two numbers?
24. Where it goes wrong