# [another] Accuracy in Woodworking.

 Blog entry by antmjr posted 11-07-2010 12:29 AM 10739 reads 1 time favorited 7 comments

Hi all. Well, I was in the blocklist of GarageWoodworks, so the only way to explain my point of view was to start a twin discussion , hope not to bother anyone of you.

The terms are accuracy and tolerance (which btw are often mistaken for each other). I guess there is the same difference in English as there is in Italian: accuracy refers to the precision of execution, to our ability to build something according to plan, while tolerance refers to the max error one can tolerate, according to the realistic accuracy one can expect from himself. Each of us must set his/her own tolerance.

Here the etymo of accurate, from which accuracy comes (http://www.etymonline.com/):

accurate: 1610s, “done with care,” from L. accuratus “prepared with care, exact, elaborate,” pp. of accurare “take care of,” from ad- “to” + curare “take care of”. The notion of doing something carefully led to that of being exact (1650s)

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The term that intrigues me is tolerance (in its technical meaning). Accuracy apart, which is – realistically – the tolerance one must set in his/her project? is it always right to set the smallest tolerance, as the video suggests? Can one set a tolerance for a joint independently from the whole, I mean, without considering the rest of the building?

Here a somewhat extreme example that shows the problem of setting the right tolerance.

These fingers are pretty exact: the basic module is 6 mm (0,2362’), the finger is 5,8 mm thick (0,2283’), while the groove (I mean, the empty space between the fingers, I don’t know the exact term) is 6,2 mm wide (0,2441’).

Apparently there is a great tolerance: for each finger, the tolerance is 6,2-5,8= 0,4mm i.e. 0,0157’.
Unfortunately something went wrong though: in fact I had to hammer the two pieces to make the fingers enter the grooves (luckily the wood is black locust, hard enough to support my savage hammering).

Had I made larger grooves, everything would have worked without the need of forcing the fingers by means of a hammer. Now I know that I had to set the right tolerance for the basic module, and to decide the width of the groove accordingly. Here what happened:

Since the total error for the basic module was greater then the tolerance for each single finger, that total error could not be compensated by the groove between the fingers.

This example shows that it is silly to set a tolerance for a single finger joint without thinking at the whole first. In my example, if my smallest realistic tolerance for the basic module (with regard to my ability, tools, etc) is, say, +/-0,1 mm (0,004’), and I think it may be possible to do the same error for, say, 5 modules, the total error would be 0,5mm (0,02’), that is the tolerance a single finger joint must be able to handle in my case. A smaller tolerance for the single finger joint would simply mean that I have understood nothing about wood and woodworking.

So with regard to the advertising of that hightech-kerfmaker, “0,002’ is unacceptable”. Yes, 0,002’ is unacceptable, 0,02’ or better 0,025’ would be acceptable.

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(The other problem with joints so tight as that of the video is the glue: if one uses water based glue, the wood swells, and the joint may either break or not be assemblable, as most of us know)

-- Antonio