I have a big problem with sketch control. Regardless of the software modeling used, sketch elements, profiles, etc., go ape-shit when you alter them. Regardless of the method, moving them about, copying them to a new sketch, or changing variables and parameters. Extensions and dimensions fly off into space.
Here’s an example of the problem, and what I learned to do to fix it.
Problem
I needed more sketch control; not constraints, but control. It is usually expected that a sketch will be properly constrained. Most graduates of the life lessons I have experienced are firm believers in this. They will also admit that when you change the situation the sketch is constrained under, bad things can happen. Some examples are:
When variables disappear while copying a sketch. The particular dimensional constraint is gone, and when the copied profiles are constrained into place, elements can go flying.
When a parameter or variable controlling a dimension goes beyond the expected limits. Perhaps something goes negative, which is often a serious no-no when linear dimensions are concerned. You might realize it, and correct the problem, but the dimension won’t readjust, as if it is stuck and can’t decide which side of the origin to go to. This is more prevalent in certain software that in others.
The possibilities are fairly broad.
One issue that I was recently experiencing dealt with curves and delta angles (also known as central angles and included angles). Curves under specific constraints can exist in two forms with the same chord. I changed the parameters substantially after a compressor redesign. The significant change caused all ‘un-controlled’ sketch curves to ‘flop’, or bulge, to the other possible definition. Notice in the image above, how the third and fifth curves have elongated, and some dimensions and control lines have shot off to the left side. This was one parameter change with a very well-constrained sketch. Other instances have been less forgiving.
The Solution – Sketch Container
I realized by accident that I can keep the many of the issues under control, including the ‘flip-flop’ behavior of multiple possibilities of geometries, as well as control lines that disappear or shoot off into space. A circle.
I placed all the profiles within the circle, and constrained the loose ends to it. The chord is the limit of the geometry. In my example, the circle origin is the middle of the chord line, and the circle radius is constrained to the end of the chord. Once the extraneous dimension lines are fixed to it, the stability is wonderful. Moreover, copying these was a dream, compared with the un-contained versions.
Conclusion
Does it work? look for yourself. Days after I began writing this up, I began the major overhaul on the compressor design, and the upper image represents what I received as a result of a simple variable (parameter) change. Every profile that was contained within a circle held up and required no assistance to reform itself properly. All remaining profiles ‘flip-flopped’, or bulged.
This won’t work in every instance. In one instance I tried to do this with a rectangle; not a useful option. Circular geometry have no limits along their length, whereas rectangles do. If the profiles are constrained to something with a limit, when the limit is reached, in odd circumstances like we are discussing, really bad things happen.
Give it a try and let us know how it goes.
