Re: engineering drawing question


"Don A. Gilmore" <eromlignod@xxxxxxxxx> wrote in message
news:B_Qcf.24272$1A1.14249@xxxxxxxxxxxxxxxxxxxxxxxx
> <neverhadanickname@xxxxxxxxx> wrote in message
> news:1131660639.363161.182560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >> To have it otherwise implies one can scale the drawing; scientists,
> >> engineers, designers, or shops do NOT scale drawings. Ever. No matter
how
> >> true. Art majors, however, may scale.
> >
> > I worked with a checker once who took "DO NOT SCALE DRAWING" to mean
> > nothing could be drawn to anything other than 1X scale. He sat there
> > with a scale, scaling the drawing, to make sure it wasn't scaled!
> >
> > It was a very young company (the company itself, not the employees; the
> > checker was ready to retire (or be retired)), and they had a lot of
> > like issues to work out. My stint there was thankfully brief.
>
>
> I've seen that marking on drawings before, but it always appeared to me
like
> a cover-your-ass disclaimer that isn't very practical in the real world.
If
> a drawing isn't to scale, why in hell AIN'T it to scale?
>
> If I'm looking at a drawing of a jig plate with 300+ holes of all
different
> sizes in it and I want to know the approximate distance from one
particular
> hole to some other cavity, I'll be goddamned if I'm going to waste time
> adding and subtracting a column of 35 figures when I can just slap a scale
> onto the drawing and measure it directly in two seconds.

Tolerances?

>I'll be goddamned if I'm going to waste time
> adding and subtracting a column of 35 figures

If you have to do that, the drawing is incorrectly drawn, period. AND it is
not communicating what needs to be communicated and HOW it is to be done.
AND both the mathematical and the measurement tolerances must be hell for a
drawing needing 35 calculations in order to locate two points.
E.g., proper format is to draw a bolt pattern referencing all holes in a
pattern to one reference hole, and referencing that reference hole to the
reference edge. In other words, any part has three reference edges. A
group of discontinuities having a specific purpose (e.g., a hole pattern) is
referenced in whole to an edge using one and only one of its
discontinuities - not have each one in the group referenced to the reference
edges.
And that is EXACTLY how I want it machined. If I have to go out on the
shop floor to show them how to do it per drawing, I will (and have)

Why? When I describe and approve a part, I approved the pattern shown to
be within the dimensions and tolerances _I_ listed for the purposes _I_ deem
appropriate, to be measured in the manner shown on the approved drawing,
with tolerances referenced within the group as shown - not have someone down
the line decide it is easier to make all measurements off an edge, or scale
a drawing because "it's easier".
They get paid to do it per the drawing which I decided was the easiest
way for all concerned, not do it "their private easy way".
( It goes to large production statistical fits as well as to one-time
tolerancing.)

The bolt pattern of the attached part does not care about the holes lining
up from an edge - it needs the holes it uses to line up with each other.
The location of the attached part on the part does not care if the bolt
goles line up - just if the pattern's reference hole is in the right place.

If the drawing you describe is properly drawn, the accurate distances from
pattern to pattern is quickly found by looking at two dimensions and one
subtraction for each axis.
If you want to find the distance between two non-reference holes, chances
are you are moving the intent of the design itself off-track.

An architectural drawing that dimensions the column lines to each other,
one that dimensions the overall size and lists column lines as typical, and
one that dimensions an overall size with all but one line dimensioned all
have different meanings. Yes, I have scaled them for purposes of my personal
design approach; I check their accuracy of scale first, I use them in lieu
of sketches, but I NEVER seal or approve a drawing using scaled information
from a drawing.


> I would think that
> there is much more possibility for error in a complex calculation than
there
> would be in reading a ruler.

I would hold the opposite, so perhaps it depends on the person's eyesight
and number comfort.
I do believe a benefit of discouraging scaling is that your error in
calculation is your error alone, and that your doing the calculations also
crosschecks the other persons' inputs to the drawing/design for possible
errors not yours - before steel is cut or concrete is poured.
But an error in scaling moves forward any one of several person's errors,
and has no crosscheck. The other lemmings follow the first lemming.


> That's why I, and most every engineer I know,
> keep a triangular scale in the top desk drawer. That's what it's for.

Yes, scale your own sketches when designing. But don't scale drawings made
by other people. Use the dimensions put on the drawing.

That's why they are there.


nuff....
I
> also keep a second, architectural scale for scaling plant layouts.
>
> Don
> Kansas City
>
>


.

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