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David_M1-Visitor
1-Visitor
March 12, 2013
Question

An example of just how bad Pro/E sketcher can be

  • March 12, 2013
  • 3 replies
  • 5200 views

Forget curvature and conics, this goes back to basics:

To compare two monitor screen sizes, I thought I'd quickly overlay two rectangles in Pro/E sketcher.

I drew one landscape rectangle, then another with a common point at the lower left.

I used Relations to say that the first rectangle should have a ratio of 16:10 and the second should have a ratio of 16:9

Then I added diagonals and tried to add lengths of 24" for the first and 27" for the second.

And that was when I remembered just how bad Pro/E is at handling constraints between entities: very bad. After unlocking the height dimension, you get an "overdefined" error anyway, despite there being only one driving dimension for each rectangle. Joy.


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3 replies

Patriot_1776
Patriot_177622-Sapphire II
22-Sapphire II
March 12, 2013

Unlocking a dimension means nothing to an overconstrained condition. What overconstrained means is that you have too many strong dimensions/constraints total, and that the sketcher does not need one of them to make the geometry work.

I wish it had a mode, for more complicated sketches, that made it such that you can turn off all constraints hen importing geometry. AND, what I'd like is for it to be more like AutoCAD, where you could truly turn off all assumed constraints, and manually pick them all.

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David_M1-VisitorAuthor
1-Visitor
March 12, 2013

Nonetheless, it remains a problem that you cannot define something as simple as a rectangle by ratio and diagonal length. It doesn't even work with Reference Dimensions. Other packages can do this, even SolidWorks. And of course it can, because there's nothing logically wrong with this type of definition.

I could give far more complex examples of when Pro/E Sketcher can't manage something that should be fine, but I thought this one demonstrated things nicely. Perhaps I'll post another example in the future if I can separate the sketch into a .sec file.

I'd like to be able to have a manual mode for constraints too. And AutoCAD's select method. But that's another topic.

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dgschaefer21-Topaz II
21-Topaz II
March 12, 2013

This is a geometry restriction, not a pro/e restriction. As Frank indicated, a dimension that is not a reference dimension is driving the geometry, regardless if it's strong, weak or locked. You cannot control all three legs of a right triangle.

I'm not a big fan of sketcher relations as they are sort of buried constraints. You need to dig into not only the sketch but the relatsions dialog of the sketch to see what's going on.

In this case I'd use an angle to keep the proportions and the diagonal dimension to control the size. A 16:9 ratio would need an angle of 29.3574 degrees off the horizontal, for 16:10 it would be 32.0054 degrees off the horisontal.

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David_M1-VisitorAuthor
1-Visitor
March 12, 2013

No it isn't. If A = 2*B (where A is the horizontal and B is the vertical) then no size information has been added at all. A diagonal should be able to define the size. It is a Pro/E problem, not a logic paradox.

Patriot_1776
Patriot_177622-Sapphire II
22-Sapphire II
March 12, 2013

Sorry David, I have to disagree. If you have dimension A (width) and dimension B (height), then you have completely defined a rectangle and any other dimension is overconstrained, unless it's a reference dimension. In this scenario, while a relation may make A = 2*B, it doesn't factor into anything. You could comment it out and it wouldn't change anything. It is geometrically overconstrained, and you must either make the diagonal the reference dimension, or one of the other 2.

I'll have to disagree with Doug (just because I'm that retentive ) and say that he's right....except about the weak dimension. A strong or locked dimension will make it overconstrained, a weak dimension will disappear as soon as the sketch is fully constrained.

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TomD.inPDX17-Peridot
17-Peridot
March 13, 2013

This is a sketch that governs the size of an icosahedron from any face to the center. It is perfectly "stable" which means that changing the one controlling value, 20 in this case, maintains the entire sketch without issue. Even the model that is driven by this is wholly dependent on this one dimension.

thinkpad.JPG

I could not even tell you exactly what each relation or reference diagram defines without study. The idea is that each "diagram" in this one section is independently developed and tied together with both geometry and relations. You don't just do this. You have to "understand" your subject before you can represent it in a sketch.

Point is, this is part of the "art" behind what PTC can do just as well if not better than other programs. As with anything else, you just have to learn how to make it work for you. What use would it be if a program can make an icosahedron but only define the sphere that contains it? All the extra geometry in this sketch is based on this single requirement. Changing it for different requirements is as simple as adding one dimension and referencing the current driving dimension.

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