Dimensioning Standards

[Brian Rupnow]

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

 

[Tin Falcon]

Do you feel better now Brian ?
I work in the scale business and we mostly use imperial pound weights (avoirdupois) some work requires metric. Like balances that weigh by .00001 grams. But sometime things will drive you crazy like a customer Using only kilos on there scales but we only have pound weights to test and calibrate with . And some scales will only let you calibrate them with a factory set quantity of pounds other need a factory set number of kilos. A few scales will actually let you calibrate with whatever quantity you tell it you are setting on it.
Tin

 

[zeeprogrammer]

One of the most important points you made Brian was in the first sentence. Knowing who the audience is.

My big issue is relying on computers too much. They are a tool...not a solution.

Actually, my bigger issue is the lack of answer's to 'why'. I run into this all the time at work. We do certain things or do things a certain way...but no one remembers why. Does it still apply? Or worse, if you change it...are you making a serious mistake?

That's one of the things that gripes me about instructions manual (guess which), or teachers in high school and college. Unless you can include 'why'....you've provided less than half the solution.

Er...not you you...you know what I mean. You just touched a thought.

 

[mu38&Bg#]

I worked at a US fab shop that was German owned. All of the equipment in the shop was metric, even the Bridgeport and Clausing lathe which never got used. The guys had worked there long enough that they knew what they were doing. It always struck me as funny to put W6x9 x 1200mm, etc., on a bill of materials or drawing. It worked for them.

Solidworks (Brian, if I remember you run SW) allows you to change each and every dimension's precision as you place it in the model, even define the dual dimension. I realize this is a pain, but if you have a drawing that has 2, 3, and 4 place dimensions you'll be doing it anyway. Then you just pull it up in the drawing and it will be correct depending on if you've chosen a metric, imperial, or dual dimensioned drawing format. I love the ISO hole/shaft tolerance feature.

I have a different problem. I like working in metric. So I design my parts in metric. My lathes are inch. Occasionally you see a metric SB, or Clausing on thEbay, but they are far more than I want to spend. I have 3 large face metric long travel indicators to put on my lathe. I could do the same with cheapy caliper DRO's. My little CNC mill is native metric. I still have to deal with 1.190625mm offset for a 3/32 endmill. I might go to metric tooling also. If I ever get around to making a CNC lathe it will also be metric. The biggest problem I have is threading. My old Atlas will do some equivalent (within 1%) metric threads. The Clausing does not, at least not with the metric gear set I don't have. That drives me to do the CNC lathe, which won't care even if the ballscrews are imperial.

 

[Artie]

Aus has been metric (if memorys serves me) for the same period as has Canada.... Im 47 so I therefore 'grew up' (yeah right) in an almost completely metric world......

Why therefore when I ordered some timber the other day did I ask for 4 metre lengths of 4" by 2" (or as we call it here 4 be 2)???

Might take a few generations to completely clean this one up.... when working on car engines I measure clearances in thousands of an inch..... seems to make more 'sense' to me.... (4 thou of ring gap per every inch of bore et al)

Weird... 8)

 

[BAH101]

Try working on a French built helicopter with a USA built engine installed, you have to carry both standard and metric tools. Adds weight to a tool box, especially when you have to carry it.
I remember being in a grocery store here, a lady from the States was asking one of the young employees how many pounds in a kilo. The kid didn't have a clue what she was asking. Yes, I helped
Bryan