HANDS:
hands
hands title:
§7. Rule 6: Measuring I. Practicalities
hands date:
1 July 2022
hands text:
One accurate measurement is worth
a thousand expert opinions.
a thousand expert opinions.
01
Measuring is a mechanical skill that requires a degree of hand–eye coordination and a bit of technique to ensure consistency. There are just a few simple things to keep in mind.
02
Your measurements determine your cuts and the cuts determine the end product. You cannot produce a decent product if your measurements are poor.
03
The most important aspect of measuring is consistency. It is therefore irrelevant if the tolerances for the project that you may be doing right now are low. It is necessary to maintain the correct routine until it becomes second nature. Until such a habit is formed, deliberate effort is required.
04
Not all measuring instruments are equal. In many shops, you can buy, say, a spirit level very cheaply, with an accuracy of, for example, 1.5 mm per metre. Such a level is useless.
05
The accuracy of many measuring instruments can be evaluated manually. For example, to check a spirit level, you should level it against a flat surface and make marks. Then flip it 180 degrees and align it with the marks you made. The position of the bubble should remain the same. Obviously, the greater the distance between the marks, the more accurate the check will be. The accuracy of a square is checked using the Pythagorean theorem: a triangle made using this square must have sides that are multiples of 3, 4 and 5. Again, the accuracy of the check will be determined by the size of the triangle and the accuracy of your tape measure.
06
You, however, should know your tolerances and how your tools contribute to the overall result. It is therefore important to have at least one tool with a known level of accuracy. I suggest this to be a tape, the accuracy of which is determined by established standards. In Europe, this is Directive 2014/32/EU.
07
A tape’s compliance with European standards is indicated by Roman numerals on the tape itself. If your tape measure corresponds to Class I, then the maximum permissible error (MPE) at a distance of 5 metres will be no more than 0.6 mm and at 50 m no more than 5.1 mm. For Class II, it is no more than 1.3 mm and 10.1 mm respectively, and for Class III no more than 2.6 mm and 20.6 mm.
08
You should always establish a reference point. Switching reference points mid-process compounds error. Once a reference face is chosen, all layouts, marking and checking should proceed from it.
09
I am a great fan of story sticks, templates and all sorts of physical gauges. They eliminate reading errors, parallax and cumulative inaccuracies. When parts relate to one another, transferring size directly from one part to the next is often more accurate than relying on numbers.
10
Math is more reliable than tools: I like checking for squareness using the Pythagorean Theorem and/or by measuring diagonals.