How to check convexity / concavity of edges in SW ?

Hi,

Does anyone know how to check convexity or an edge in SW.

In case, this terminology is not clear, let me explain a bit. Convexity
of an edge is determined by the angle between the face-normals. If the
edge is convex, it comes out of the part and if it is concave, then it
doesn't. While filletting a convex edge, material gets removed while
filletting a concave edge, material gets added. Like edge, vertex also
has convexity / concavity depending on whether it protrudes out of the
part or not.

I would like to know if there is a way to determine the convexity of an
edge ... any hint / trick would be of great help.

Thanks,
aditya

 

In case, this terminology is not clear, let me explain a bit. Convexity

According to who's definition? What you are talking about is dihedral angle or
simply angle of intersection, no?

http://en.wikipedia.org/wiki/Dihedral_angle if you expand to include the angle
between CV's of a section curve.

 

Hi Hequler,

Thanks for the message.
No, i am not talking about the dihedral angle, although it is also
defined wrt the angle between the adjacent face-normals.

Frankly, i could not get a reference on the net which I can give to
clarify what I intend by convexity/concavity ...
but the meaning that I want to convey is essentially as follows which I
wrote in my initial post as well.
"While filletting [or even chamfering] a convex edge, material gets
removed while filletting [or chamfering] a concave edge, material gets
added"
Here,
filletting is the modelling operation [in SW] which removes edges
[sometime vertices] by introducing faces that are tangential to the
adjacent surfaces.

If you could give some links which talk more about the definition of
convexity that I have in mind, it will be of great help as well.

thanks,
aditya

 

Do you want to do this manually or with a macro? Do you also need to
know when the edge is both convex and concave?

If you do a construction why not create a plane normal to the edge, do
an intersection curve and measure the angle between the two curves?

You could also do a spline on a surface and bring it up to the edge.
Then do a 3D line tangent to the spline. If it goes into space it is
convex and if it cuts into the adjacent suface it would be concave.

 

Thanx for reply. I am preferably looking for a manual [but quick] way
rather than a macro.
I work related to translations of models. So, i wanted a way to give a
quick-check whether convexity of edge/vertex is being preserved when
the model gets translated to other types.

 

The easiest way is to convert an entity into a 2D or 3D sketch, and show
the curvature comb on it. The comb always sits on the outside of the
curve. You can also show inflection points, which show where convexity
changes from one side to another.

 

Hi Matt,
'curvature comb' is specific to 2D, that too for non-linear curves
only. It definitely helps deducing convexity in 2D scenario [which is
the standard one and used more frequently], but doesn't really help me
as I am looking for convexity of edge on a 3D solid.
Please note as per the definition of edge-convexity, same type of edge
can have both types of convexities.
Just to illustrate, all 12 edges of cube are convex.
If we consider extrude of an L shape, it will have 18 [=6*2 for L
sketches + 6 due to extrusion ] edges. The edge that gets created at
the lap-joint of L due to extrusion is the only concave edge for the
part. Rest 17 edges are convex.
regards,
aditya

 

What happens is you simply try to apply a fillet / round feature? Does SW give
you a preview or can you create it as a surface feature?

FWIW, what you are talking about is, in Pro/E parlance anyway, dihedral angle
(for which there is an analysis feature).

Re checking translations; I've never seen problems such as you seem to want to
check for as long as the system isn't `healing' on import. Rare are the systems
today that can't write and read a list of control vertices and knot vectors
correctly? If I wanted to check I might create some section curves, export them
along with the surface set and compare to post translation section curves.

 

another maybe quick check - offset the surfaces and test for gaps and
intersections.

 

Hi Cronik,

Thanks for the suggestion, indeed that's a good idea, but how to go
about offsetting faces in SW ?
I am aware of shelling which offsets the entire feature, but it might
not be feasible to do shelling in all scenarios to find convexity of
edge.

Regarding translations and going bit deeper, we have some more types of
convexities, in addition to just plain convex / plain concave. ...
viz. convex-smooth, concave-smooth, plain - smooth. When a fillet
operation takes place to remove convex edge, the newly generated
smoothened face has edges with convexity convex-smooth. When an edge is
created on a planar face, it has convexity 'plain-smooth'. I understand
that this jargon [plain-smooth, convex-smooth] is not standard but I
hope you get what I am saying. There are cases in translation when a
convex edge becomes convex-smooth etc.

regards,
aditya

 

Curvature comb works in 2D or 3D sketch. Try it.

As for it being for non-linear entities only, yes, I assume that is a
given since linear entities have no curvature and thus no
concavity/convexity.

From your example of the L shape, I wouldn't call what you are looking
for convexity. If you are looking for inside vs outside corners, for
flat-faced parts you could measure the angle between the face normals of
adjacent faces, <90 deg or >90 deg would determin inside/outside.

Maybe an easier way would be if a fillet on a given edge adds mass to
the model, the edge is an inside corner. If a fillet removes mass, it
is an outside corner. Cases where an edge could transition from being
an inside to an outside would be rare, but do exist.

 

In case, this terminology is not clear, let me explain a bit. Convexity

Isn't Tools -> Deviation Analysis what you are looking for?

-h-

 

Hi Matt,

I agree, i am not looking for what is generally referred to as
convexity / concavity ... but rather inside / outside edges.
But just checking the angle betn the face-normals doesn't help as this
value is =90 for every edge on the L extrude. So, something more
[sense] is needed to determine that.

Filletting is not an option for me as that's what I am using the
convexity information for.
As said by you, transition from inside to outside [or vice versa] can
occur along the edge ... and that's why, strictly speaking, the
property [of insidedness / outsidedness] should be evaluated at a given
point along the edge.

regards,
aditya

 

Another case, if an edge is a "tangent edge" (faces on either side are
tangent to one another), the fillet technique won't work, and should
give the error "smooth edges cannot be filleted".

 

Face normals have a sense, so for outside corners it would be 90 or -90,
and for inside it would be 270 or -270. But for complex shapes, the
edge condition doesn't necessarily have anything to do with the face
normal, normals only work for flat faces.

Deviation analysis is a cool thing, but it measures all right angles as
90 deg, without regard for it being 90 or 270. I don't think it will
measure anything over 180. I don;t see a way to make it distinguish
between an inside and an outside corner.