I have been a machine designer for (gasp) 44 years. One of the basic rules I learned by, was to anticipate what the person actually building from the detail drawing was going to be measuring with!! I did a lot of structural steel work---and it was all dimensioned in fractions of an inch.--Why?---Because the man laying out the beams and columns in the steel shop was going to be doing it with a tape measure.--Same thing for weldments, anything flame cut, and anything vaguely architectural. If you were designing something that would be built in the machine shop, on a lathe, mill, shaper, etc, then it was designed in decimals of an inch. For non critical work, it was dimensioned to two decimal places. For "critical" dimensions, it was taken to 3 decimal places. For really critical work, as in matching jig bored dowel holes and bearing fits, the dimensions were taken out to 4 decimal places. Then, in 1975 or thereabouts, the whole damned world turned upside down, and everthing had to be designed in hard metric!!! This was a total joke--as soon as the blueprints were issued to the shop floor, you would see the poor machinists setting around with pocket calculators converting all this metric $hit back to British Imperial measurements so they could build it on their machines, which were all set up for British Imperial meaurements only. Of course, this was in the "pre-computer" era, when all design work was done on a drafting board. I went back to school when I was 50, to learn how to design in "computer world". I was absolutely terrified at first, but I learned to design first in 2D for about 3 years, then on to 3D about 10 years ago. Now computers are great, but there is a flaw---When you were working on a drafting board, you KNEW which dimensions were critical, consequently you KNEW which dimensions had to be 2, 3, or 4 decimal places, and you acted accordingly. In "computerized" drafting, its one setting, and all the dimensions on the drawing will have however many decimal places you set the "default" to. Then you have to go back over all the dimensions, decide which ones must have more or fewer decimal places than the default setting, and edit them accordingly---and most times, that doesn't get done. Which makes everybody crazy!!! Either you get a whole whack of not critical stuff dimensioned to 3 decimal places, or you get really important, critical dimensions left at the default setting of 2 decimal places. Now the onus is on the machinist, rather than on the engineer/designer to decide which dimensions are critical, and which are not.---Now, some of my customers want their stuff designed in metric. Some want British Imperial ---Some want a combination of both, because even though canada has been converted to metric for 35 years or so, it is very difficult to buy structural shapes that are "hard" metric. Old guys in the shops (yeah, old like me) are still very uncomfortable with the Metric system, so they prefer to see drawings in British Imperial--as in inches, feet, and pounds. Young fellows who are newly graduated wouldn't recognize an inch if it come up and bit them on the a$$.--They have been teaching ONLY metric in our schools for the last 35 years. So---Is there a clearly defined answer to "What measuring system, what level of accuracy, do you use fractions or decimals?--No, not really. It depend on who you are doing the work for, where the work will be built, is it general use or military spec.--what colour the sky was when you got up this morning, and whether or not you put on clean undies when you got up today.---Brian
