Hi again juergenwrt:
Fair points all, and I have no dispute with you that a drawing that is unambiguous is a far better drawing than one that is incomplete or internally contradictory.
I also do not dispute that the ISO system or the ASME system or the ANSI system or any other system will help to fulfill that requirement that the drawing be complete with a callout for every feature the designer deems relevant.
However, there is a great tendency among professionals to adopt and cleave to their own mystical jargon to help elevate themselves...I was a dentist in a former life and I can assure you it's true in that profession as I believe it to be true in engineering too.
I may seem to be drifting into irrelevancies here, but that professional arrogance which too often comes with the mastery of the secret code is not helpful.
There is no rational reason to specify what you want in a way that the recipient of the drawing needs to look it up...this is decidedly not like an electrician wannabee who does not understand the meaning of OHM and AMPERE and VOLT and licks the wire to see if it's hot.
It is more like saying..."You want to know how big it should be... fuck you...I'm not going to tell you the easy way...look it up yourself you lazy little shit".
"You get to go to the trouble because I'm too important (or perhaps too wonderful) to do it for you, even though I pondered and set the specification for a reason I deemed to be good and I really do want it this way".
Now I know it's not intended to be interpreted that way and of course, not many engineers are guilty of that arrogance, but some are, and the plea from us machinists is that you do not do that or seem to do that even if you didn't mean it that way.
Whether it's because the designer has overt contempt for the machinist or maybe just because it never occurred to the designer what a right royal pain in the ass it is to be leafing through Machinery's Handbook all the time while the boss is crying about how come the job's not done yet, it's still a nuisance and decidedly not necessary to help the designer communicate his/her intent.
So my policy, whenever I make a drawing, is to call out a target dimension and an explicit tolerance for everything the machinist needs to have in order to make the part the way I wanted them to.
I'll put notes on it wherever I want to if I think it will help the machinist.
I never want him/her to have to go consult something else somewhere...everything should be on the drawing, not just unambiguously but as plainly obvious as I can make it.
It's an approach that is technically "wrong" but it's served me very well for decades and every shop I've worked with has appreciated it
You can still call it out using the proper engineering shorthand...you can even have a naive expectation that every competent machinist knows the shorthand, but in my experience you certainly can't count on it so I prefer to augment it so it can be built with the minimum of hassles by anyone who can work the necessary gear properly.
Just look at some of the shit drawings produced by qualified engineers all over the world...if they cannot consistently do it, there may be something not quite so wonderful about the shorthand.
If it's like a stick in the spokes of an already stressed and harassed machine shop, can you wonder that it's unpopular?
Next time you get a chance, have a look at how drawings defined with GD&T are quoted compared to drawings that are dimensioned conventionally.
I've seen RFQ's for simple work outright rejected and I've seen them quoted at 5 times the cost of a conventionally dimensioned print for the exact same parts.
Not all those who refused to bid were incompetent idiots either!
Some were great at what they do, capable of magnificent workmanship on challenging parts but the incomprehensible scribble on the picture put them off and it wasn't worth it to them to figure it all out.
Yes, it's technically a better drawing if it conforms, and if you invest fully in understanding it as a machinist you will technically be a better machinist, but you'll spend a lot more effort deciphering and everything will cost a lot more...even when it doesn't need to.
Cheers
Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
