Finally wrote-up my "How to Design Parts" section...

Great stuff--well written.

I can help with the PDF if needed--I'll save as one & send to you if you'd
like. Just let me know.

If you'd prefer to save yourself, try PDF995 (www.pdf995.com, free) or
PdfFactory(www.fineprint.com, trial).

Thanks,

Brian

 

Yeah, they're using the default black background. Yellow and cyan look great
on a black screen. Too bad they're not printing on black paper.

Mike

 

Dale,

I've been down this road before. Here is the problem. Every machine
shop has their preferences that usually depend on the background of the
machinist(s) and the equipment being used. And of course the inspection
department is going to want to minimize their work. So it is possible
to make a print that one machine shop likes and another doesn't. This
brings me to the ANSI specification and the engineering side of things.
ANSI makes a big point that a print is not to specify manufacturing
methods. The engineering side is concerned with form, fit and function.
The part should be dimensioned primarily so that if it meets the
dimensions specified it works first time and every time. If you can use
Joe's methods and still assemble the parts after doing a tolerance
stackup then I agree that that method is going to be the least
expensive. Bottom line though is that engineering shouldn't be second
guessing manufacturing. And the machinist shouldn't be concerned with
the fit of parts, but rather making parts that meet print.

Now as far as tightening tolerances to be able to work off one edge I
have a little story. One place I worked would throw a .0005 tolerance
on in cases were working off one edge made the tolerance stack not
work. Then they would throw the part out for bid and take the cheapest
shop. What usually then happened is that the cheapest shop couldn't
hold .0005 if their life depended on it.

I think what you really want is to have working drawings for the
machine shop that may be dimensioned differently from the drawings the
part should be inspected to. This makes it easy for the machinist to
setup.

 

I have to agree here - unless you know your drawings will be going to a
dedicated place, and you tailor them to their process, they should dimension
what you want to hold.

When I started here, I was told that when you wanted to make sure the shop
paid attention to a dimension on a drawing, you were supposed to put HOLD
next to it. Aaaaaaaa, no. I will give you a properly dimensioned drawing
with the tolerances as needed to ensure form, fit & function, but I will not
dumb down the Engineering Dept because of laziness. Fortunately the
Engineering Manager agrees with me.

WT

 

I agree with this post. In fact, if you have a CAM capable machine
shop, it's might be best to only dimension critical features and let
the machine shops decide how to make the part for themselves by
directly referencing the solid model; with some overal general tol for
all unspecified features. The problem with drawings is that they are a
translation of intent. Back in the day, this was the best method
available, but nowadays, the best method is to directly reference the
solid model itself since that is practically as close as one can get to
the intended part (unless the CAD person doesn't know how to model
their desired results). Adding any noncritical dimensions on a drawing
just forces the machinist into a method that may not be necessary.
What I've been doing is using the drawing to specify critical dims and
provide notes, along with a note that refers the machinist to the solid
model for all unspecified features.

 

The original background color on AutoCAD was black, so white and yellow
colors where used frequently cuz they showed up best.

 

Dale,

I don't have control over who makes the part. Could be in house or job
shopped. Usually not my decision. One thing about 3D CAD is that the
machine shop can be given the model and make their own working drawings
is short order to minimize cost and still meet the customer's
requirements.

On Joe's example part I would probably consider the centerline between
the tapped holes as a datum from which the other features must be
located. This would fly in the face of what Joe was trying to do, but
would make the best fitting part with the loosest tolerances. Joe might
want to take that drawing, put the dimensions on a layer and hide it
and then redimension to make it easier for his guys to understand with
the caveat that when the part is done it is checked to the original
dimensioning scheme. Given the accuracy and repeatability of modern CNC
this should not worry Joe in the least as far as meeting requirements.

 

Yeah, I considered some sort of black (or dark) translucent material that I
could put on top of a light source (maybe one of those lighted tracing
boxes) and then place the prints on top of that.. :-\

Mike

 

Cliff wrote:

[...]

FYI; posting spam like you did in this thread and have repeatedly done
in the past is a good way to get your account at NIN nuked.

 

Enjoyed your booklet. I'm in a sheet metal job shop and we frequently
only send our own drawings to our shop for fabricating - not the
customer drawings.

Try http://www.primopdf.com/ for a free pdf creator. It works pretty
good.

Diego

 

What are you ranting about?

 

I think he's over in alt.machines.cnc ....