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ptc-47663301-Visitor
1-Visitor
March 28, 2014
Question

Advanced ISDX questions

  • March 28, 2014
  • 19 replies
  • 9221 views

Hi I am trying to draw some curves with C2 curvature. I am using ISDX. The curve basically forms a corner (one end is on the vertical the other horizontal) so I want it to be symetrical on both end. I create the curve and make the ends curvature. In tangent >length I make sure both ends are the same number so I can be sure it's even. My curve isn't right though. I switch on "edit curve using control points" and 5 degrees. Now when I try and move the control points the two horizontal and 2 vertical points move together. This isn't what I want. If I change the end tangency to "tangent" rather than curvature, then I can move them independantly. Does this mean my curve is not C2, I am confused??


At this point I am also having difficulty making sure the control points are even on both sides. I seem to only be able to input coordinates which is not convenient at all. Is there a way to drag them together?


If I increase the curve to 7 degree, is there a way to align control points or snap them to another point?



As you can see I'm a little confused. Any help most appreciated...



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19 replies

DeanLong1-Visitor
1-Visitor
March 28, 2014

RR,


I am not sure I interpreted your posting correctly but you say your curves create a "corner". If you have a true corner they you will never be able to have any continuity at the corner. A corner is G0 or simply stated,hasa "positional" relationship.


G2 is a curvature continuous condition where theslope of both curves are indentical at their intersection. did you mean something else in your post?

bokm96061-Visitor
1-Visitor
March 29, 2014
I personally avoid ISDX and prefer history based construction of curves and
surfaces.
DeanLong1-Visitor
1-Visitor
March 31, 2014

Pete,


I am curious why you "avoid" ISDX (A.K.A. Style features). have you had some bad experiences using it?

In Reply to pete bokma:


I personally avoid ISDX and prefer history based construction of curves and
surfaces.
ptc-47663301-VisitorAuthor
1-Visitor
March 31, 2014
When I say corner I mean I have 2 90 degree lines which I am trying to put
a curve between in ISDX. Basically replicating a radius. I am not convinced
that you can get the same control with C2 curves outside of ISDX.


DeanLong1-Visitor
1-Visitor
March 31, 2014

Sure you can. ISDX has a little bit more freedom and numerical control for C2 but you can get it with and without ISDX.

In Reply to r r:


When I say corner I mean I have 2 90 degree lines which I am trying to put
a curve between in ISDX. Basically replicating a radius. I am not convinced
that you can get the same control with C2 curves outside of ISDX.


cdspk1-Visitor
1-Visitor
April 1, 2014

Can wee see your modell RR? or a similiar setup?

Greg_McMurray1-Visitor
1-Visitor
April 1, 2014

The reason you can't move the points independently is because, once you set the curvature constraint, the points are forever tied together according to the formula for the radius at the end of a bezier curve.


ISDX and a sketcher spline actually have the same functionality and control. The difference is ISDX buries the math behind the interface. In sketcher on the other hand, you can control the math by constraining the spline polygone in relations using the radius equation, trig functions, and trajpar.


It's defintely a lot eaiser just pushing and pulling points around and pushing buttons in ISDX rather than figuring out the equations in sketcher.


At the end of the day, the end result is the same. It turns out to be a matter of personal perference and whether or not you like to fiddle around under the hood.

bgruman1-Visitor
1-Visitor
April 1, 2014
Rachael,

I think I know where your trying to go, and tend to agree with you about
why your asking the question in the first place. I also agree with what
everyone else is saying and find myself somewhere I the middle. Sometimes
I find shortcomings using standard curves and don't want to dive into the
math. At the same time, I don't like the "click and drag" method of ISDX.
Its too "Artsy" for me. (Not to mention that when you need to make
changes later, changing a number by 3mm is much more accurate than
dragging a control handle on the screen - anyway, thats another issue)
With standard curve construction, You can define curves as Tangent with
Strech (or tweak), or define them as C2. (Stretch is not available when
you select C2 end conditions.) However, with ISDX curves you can define
Curvature end condition and also control a "numerical" stretch factor on
each end. (and with the help of the "porcupine" (curvature analysis) tool
you can get some really nice corners. But, its hard to numerically
control the endpoints of ISDX curves to keep them where you want them
without the "Click and Drag" artsy stuff.

So, with all that said, several years ago I came up with a technique for
solving this problem for me. This would probably be best served in some
sort of training video, but I don't have time for that now, so I'll just
list out the basic steps below, then try to attach a .prt file so that you
can see an example. You can then apply these basic steps into projected
curves, COS, 3D curves, etc... whatever your situation calls for.

- feature 1 - sketch the shape of the box with sharp corners.
- feature 2 - create another sketch, use edge from the first sketch, then
trim back the ends and dimension from the corner that you want to create
the C2 radius. (these dims will control your tangency (or c2) connection
points.
- feature 3 - Create your ISDX corner curve.
- Snap the endpoints of the ISDX curve to the curve entities from feature
2 (again... hold the control key and snap to the curve, not the endpoints.)
- drag the endpoint of the ISDX curve to the trimmed endpoint of the
feature2 sketched curves (again, let off your control key and drag it
there... don't snap it there)
- highlight the ISDX endpoint control circles, drop down the dashboard,
and set it to "curvature", and enter a "length" value to control the
stretch.
- now highlight the other end of the ISDX curve and control it the same
way

Now, the trim distances from feature 2 and the "length" values from
feature 3 can be manipulated to give you pretty much any corner shape your
looking for... all while maintaining the C2 connections at the endpoints

There you go. Three simple features that can be applied in many different
ways. Its all in the technique of creating the features that makes it work.

The attached part is Creo2. but I've been using this same technique ever
since I was first introduced to ISDX in early WF2 (or maybe earlier... I
don't exactly remember) anyway, it works in a lot of previous versions.

Hopefully this points you in the direction you want to go. Now that I'm
thinking about it, maybe I should put some details and variations together
and make a presentation for next years PTC Live... Hmmm....

Good Luck
Bernie

Bernie Gruman
Owner / Designer / Builder
www.GrumanCreations.com




ptc-47663301-VisitorAuthor
1-Visitor
April 2, 2014

Hi Bernie, that is the exact method I am using. My issues are these though.


You can choose between curvature or tangent of the end point. If you switch control points on (when you are editing the ISDX curve next to where you choose number of degrees) you get even more control. I attached a screen shot to help explain. I can only move points B and C independantly if the ends are tangent, if the ends are curvature B and C move together. When I look at the curvature graph (shown in a screen grab) and I keep the points at default but flip between curvature and tangent I see no difference. If I move point B closer to point C when the end is tangent I get a nicer curvature analysis (it gradually decreses to zero) on the ends. What I find strange is that this is a characteristic of a C2 curve but I can only achieve this with tangent ends not curvature ones?


Another issue I am having is that when I extrude this curve my curvature analysis gets messed up. It looks stepped and the ends no longer go to zero (see screenshot). This happens if I copy the curve too. Basically whatever I do to that curve loses it's nice curvature.


Any ideas on this?!

bgruman1-Visitor
1-Visitor
April 2, 2014
Racheal,

For this, I would refer you back to Greg's comments. I think what your
seeing is intended functionality. I can't speak for right or wrong, but
rather, it is what it is and (as far as I can remember) has always been. I
think it just boils down to the math that controls Tangency conditions vs
Curvature conditions.
Then, I will also reference Greg's comments on "Personal Preference".
Personally, I don't mess with the ISDX control points. To me, they just
"muddy the water". I try to keep it as simple as I can and really only mess
with Tangent vs Curvature end control, Trim lengths, and stretch values as
stated in my earlier post. Overall, I have been very happy (and very
successful) with that method.

As for your other issue, (someone please correct me if I'm wrong...) I
don't know that Extrude features have ever been able to maintain Curvature.
IMHO... If I'm working a project that is styled enough that I need to
create Curvature construction features, then its also worth me taking the
time to build proper Boundary surfaces so that I can control all of the
edge conditions.
If I'm working something that is not so styled then I stick to extrudes and
radii. --- or maybe Conic Radii with a Stretch 🙂

Good Luck
Bernie

Bernie Gruman
Owner / Designer / Builder
www.GrumanCreations.com




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