Drawing question - dimensioning oblique curves and such

I have a part that is the result of sweeping a rectangular section
about a helical path for about 1/8 of a turn. As such there are really
no good planar surfaces to align to the standard three view planes.
There are features located on the helical faces that need to be
located. What do folk do with this kind of geometry?

 

I work on mostly parts of this type. Usually I don't make 2D drawings,
but my last one required some. The part didn't have a flat surface, and
didn't even have any planes of symmetry about any axis, but it did
relate to the ground, so the ground determined the orientation of the
drawing views, just so there was some concrete context. I gave X, Y
dimensions, but also an aligned dimension for the overall length.

How does the guy with a CMM fixture this to check it? He could CNC a
fixture directly from the part file just to get true XYZ references and
properly orient the part. This part didn't mate with other parts, so
there were no location bosses, screw holes, etc.

A lot of curvilinear parts don't get drawings made of them at all, or if
they do it might be a few overalls, an approx weight range, C to C for
screw bosses, thickness, and a few simple tolerances. Since in many
ways dimensioning curvy parts is impossible, or at least unlikely to be
interpretable, it's often just not done. If it comes off the tool and
fits together, then it was ok. I guess most of the checking comes in
the design, making sure it looks good in the CAD system, making sure the
shrink rates used were apprpriate, and that the process was giving
expected results.

Machine designers keep reminding starry eyed CAD vendors that 2D paper
drawings probably aren't going away any time in the next 50 years, but
for plastics designers, the drawings already have disappeared.

There has been a lot of talk of 3D annotationless drawings where
tolerances are assigned by feature color. Size and Position are given
by the electronic CAD file. This seems to make a lot of sense, because
if a standard were acheived, it could be given from CAD to CAM and the
CAM software could automatically assign tools, feeds/speeds, clearances,
etc based on the tolerance colors.

It's already available to put a CMM type ball on a mill to check parts
as they're made. A lot of the pieces of the future of manufacturing are
here already.

Matt

 

I still have to specify where the plane is. And that depends on the
surface. So how do I document the surface in the drawing? Using a
tangent plane will get me a good representation of the feature, but not
it's location.

This geometry is easy to create in CAD, but hard to document because
all the construction stuff is gone once the part is built.

 

<http://www.esoxrepublic.com/models/>
CompoundAngleToGetHelixSection.zip

 

Nifty trick for a helix.

 

I guess we have gotten into an area that would make Gaspard Monge* roll
in his grave.

If you are molding a part you have a cavity set in a mold base so there
are at least a few data available from which to perform a coordinate
based specification of the part. What the final part ends up being is
still dependent on process parameters. So how would dimensions be
specified to check for example, warpage? If the part allows it, datum
pad could be added. But this is not always the case.

*http://www.factspider.com/ga/gaspard-monge.html

 

Well, yes, if by that you mean that the paper isn't held in such high
regard universally.

All that really gives you is a pull direction. If you're the guy making
the mold, you have to establish the rest.

Warpage would be measured like anything else; allowable deviation
(tolerance) from the theoretically perfect electronic model. On a
computer controlled CMM, select a spot on the CAD model, and it goes to
that point on the physical model to measure where reality is in relation
to theory.

When you're dealing with electronic instead of paper data and computer
control instead of optical comparitors or calipers / gages, you could
even start talking about non-planar datums.

Matt

 

Well, Monge founded descriptive geometry from which we get our
procedures for making and interpreting drawings. I hope his paper is
held in high regard.