The order of Parametrics - Editing Parametrics?

[acwtzwegers]

It seems that the order in which parametric constraints are applied is important to how they work... this goes for geometrical constraints but also for the dimensional constraints.

- setup a complex parametric drawing (say 100 parameters)

- have a "primary" dimensional constraint (used in expressions of multiple others)... let's call it BOB

- you can at this point control the drawing by changing BOB's value

- now, remove BOB... for example by changing the object type of what it dimensions. (references to BOB  will change to "numerical value")

- set BOB again, same name, same value

- In the parameters manager replace the "numerican value" that used to be old BOB in other constraints with the new dimension's name

- while before you would be able to control the entire drawing by changing the value of BOB... now AutoCAD will not allow that and say "A dependant expression cannot be evaluated".

- I've had now multiple times experienced that earlier placed dimensional constraints are "stronger" then later placed ones.

So... is there a way to adjust the ordering of the parameters (like in SolidWorks Equations for example) so to repair the drawing... or would you be stuck to redo the entire parametrics?

Or: does editing a parametric drawing (changing a circle into an arc, exploding a polyline...) automatically mean that you have to redo the entire parametrics? (and what about if you get the order wrong the first time... because you didn't realize the importance of parameter X yet at the start?)

[acwtzwegers]

As added bonus: What AutoCAD thinks "tangent to" means 😛

I'm not bad at fully constraining objects... but in this "system" that doesn't help much. Not even "regen" helps 😞

[christina.davis]

Hi acwtzwegers, 

Your analysis is correct. The order and how you place and structure your parametric constraints and formulas is important, but we're also talking about a few different things here: 

1. There isn't a way to rearrange the order of importance of parametric constraints, other than redoing them. So if I place a perpendicular constraint, the first object I select has priority, and will move the second object. I would need to Undo and reverse my selection order to alter that. 
2. Because the constraints are tied to the object and its handle, if we change the object type, such as exploding or converting with PEDIT, etc.,we get a new object handle, and AutoCAD is not able to keep that link. 
3. Obviously if your entire plan is tied to that one "master" constraint, this will require some rework. But, by using 100s of constraints we're pushing AutoCAD to try to do more design than just simple constraints. Parametric constraints weren't intended to have that much control over the entire project. That kind of flexibility is closer to what you would find in Revit, Civil 3D, or Inventor. 

Regarding the tangent constraint, I can't quite tell what is happening just from the screenshot, but this could also be a result of the order or conflicting constraints. For example, if the radius is determined and locked by a formula, and the line we're trying to create tangency to has it's own set of constraints, AutoCAD might not be able to automate that for you.

I find most of the constraints work best when the geometry is in place, like a curve already being tangent to a line, and then you add the constraint to preserve that setup, rather than using the constraint to force the geometry in. 

Thanks 🙂

[acwtzwegers]

Thank you for your reply, even though it did not tell me anything that I did not know already (the example of entity change was just a specific way of editing).

Also: I am talking specifically about dimensional constraints, not geometric ones.

In regular calculations (maths, physics, even parametrics) it is common practice to be able to determine which value gets calculated before the other. Normally you would be able to edit that. (and truely: adding "display in order" and two buttons with "up" and "down" to the parametrics pallette is all that would be needed to solve this entire issue.)

Sadly, it's not even needed to go for 100s of dimensions... just 14 will do (and I need about 85 or 100).

The idea of a "master" constraint is not even the base of the question: it should not matter how many interconnected parametrics there are... the whole idea of parametry is that dimensions and interlinked... Else you just have a... hell if I know... what was the idea here?

The screenshot of the tangent geometric constraint was really a bonus... I was just getting annoyed with the fact that even a simple, simple drawing (added again), can't do what it's supposed to do... and that results are actually random. When I change the length of the primary, vertical, dimension in the middle of this drawing, undo, and do it again (to the same value!) - the results are different every time.

The shape was drawn (as polylines, to limit the "coincident") before the geometric constraints were placed, after which the dimensional constraints were placed. (as your advice, as would be mine). It is basically just 2 rounded-off rectangles at a distance from eachother... you're telling me i need Inventor for this???

People buy this software primarily for engineering... random results to strict calculations are just not acceptable.... the answer "buy our other products" neither: in that case I'd go shopping at a competitor that does not add functionality they don't actually support... before I find out I'm using another piece of functionality that is not actually working "in this piece of software" (buy another!? pfft).

PS, sorry if I seem angry at you personally here - that is certainly not the case. If you are correct, I am angry at Autodesk for not adding the note in the releases since 2010 "We've added parametrics, but don't use it for real... it's barely a toy" and the note to all AutoCAD releases "Don't take this software seriously, we don't either - please buy Inventor if you're serious."

And here goes: when I complained about the fact that Inventor (pre-release 2015) couldn't offset any sketch more complex then a brick, the answer was I should use AutoCAD for that... so what is it? Is Autodesk saying litterally that I shouldn't use either?

[acwtzwegers · Accepted solution]

Alright, sorry for blowing my top there - just the suggestion that I should use Inventor for this rubbed me the wrong way.

For anyone looking for actual solutions: apparently AutoCAD parametrics re-evaluate themselves (slowly.).. this means that there is a sort of work around.

After (really) editing a drawing and re-setting a "primary" dimensional parameter again it will not be "strong enough" any more to drive the drawing... in 1 go:

"A dependent expression cannot be evaluated.
The parameter is used in an expression which results in an invalid value for a dimensional constraint."

If you slowly change the value of the diameter in small increments however, it will (in many cases).

In the case that made me start this thread this meant that I could not go from a diameter of 228 mm to 260 mm in 1 go anymore (after edit and re-setting the parameter), but I could go to 230, wait, to 240, wait, to 250, wait and to 260 mm. (set with 1 unit = 1 mm)

The increments are not a given though. I've had to go per 0.5 mm on some other drawings/parameters. (Don't ask me what you should do if you're into large scale constructions, though I guess the unit size is more important then if they represent mm, inches or miles)

It's a work around, it's not a given and it should (in my opinion) be dealt with differently by Autodesk... but may you want to actually get into AutoCAD parametrics, it should be good to know.

[houdahouda2]

I have read your post, but I want to ask about  2 things :

1- The problem you have with dimensionel constrainst is when you use formulas. I have tried to copy an object with dimensional constraints many times. Now I have 130 dimensions with no problem but I have use just 8 formulas.

2- What I understand from your post is that there's no problem with geometric constraints, we can create the number we want.

Please can you tell if I'm right.

Thanks in advance.