topic Re: Sketch Lines resize and move illogically when adding constraints in Inventor Forum

Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 15:14:07 GMT

Why do lines resize and reorient erratically and illogically when adding constraints to sketches? See the example below. I want to simple constrain the floating line to have it's center point at the end of one of the other lines. When I add a coincident constraint, the line I want to stay fixed is moved and resized and the line I want to move is held fixed. It doesn't matter what order they are selected or any other choice I make. There does not seem to be a logic behind which line moves and which line is fixed.

If I add constraints to the line I want to stay fixed, the software continues to do weird things. The line I want to move changes size or is rotated. Why? This is the worst method of sketching I've ever encountered in 20 years of working with CAD.

There should be a basic logic to the process.

1. The line that is selected second should always stay fixed. Meaning I want to contrain point 1 to point 2, if I select them in that order, then I know which line will move and which will stay.

2. The sketches should not be resized when they are moved. I don't understand why at all this would happen. What sensible logic is there to change the dimensions of the a sketch when it is constrained to another sketch? It is random and stupid.

I can eventually add enough constraints to the first line to get the second line to move the way I want, but this is extremely inefficient and time consuming. It's just a dumb, lazy programming method and I don't understand why. It should be simple and quick to constrain two lines and have them stay the same dimensions and orientation, without first having to add a lot of dimensions and other constraints.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 16:12:22 GMT

Until you add the appropriate number/type of constraints, all sketch elements remain movable.
There is no such thing as a stationary sketched line. A line becomes stationary when it is described by constraints (both geometric and dimensional).
Before applying constraints, everything is movable and free.

There's a way to speed up this process.
You can use automatic constraints, which you can set preferences for (a small icon in the lower left corner of the constraints panel).
This way, some constraints will be created during the initial sketching phase.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 16:39:14 GMT

Thanks, but this doesn't answer the question I asked. What is the logic behind which lines Inventor decides to move and which parts it holds fixed? Why does it change the dimensions of lines in some instances, seemingly randomly? There should be a consistent basis of which line moves and which doesn't depending on the order of selection. This is done in other CAD software. Inventor is the only software I've used that simply moves and resizes sketches in an unpredictable manner. I shouldn't need fix all the lines in my sketch in order to constrain 2 lines and know that if I select "line 1" and then "line 2" that "line 1" will be moved to "line 2" and that "line 1" will maintain the same dimensions if it is free to do so. The fact that this is not the default setup is ridiculous.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 16:44:22 GMT

As I mentioned, before being defined (constrained), all elements are movable and free.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 16:57:39 GMT

That is a non-answer and not helpful.

If I have one line completely constrained and I select to constrain the second line to the first, why does the second line's dimension change?

It's not about things being free and moveable. Of course they are free and moveable. I am trying to move them so they have to be free and moveable. The point is about what logic is used to move the parts in the software. There is no logical reason for inventor to change the length of the line I moved. Why would it do this? It does it based on some code. Why would it be programmed this way?

Second, what is the logic behind which line Inventor decides to move when there are no constraints? Even in basic 20 year old AutoCAD, I knew that the first item I selected would move to the second item I selected. So I could control the sketch quickly and easily by the order of the operations. Why is Inventor so stupid in this regard? It has no logic to it. It takes me 3 times longer to complete sketches and other work just because there is so little thought put into this.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 17:33:36 GMT

@ryancrampton wrote:

If I have one line completely constrained and I select to constrain the second line to the first, why does the second line's dimension change?

The second line has no dimensions, so its length is free.
If you define its length by adding a dimensional constraint, its length will be stable.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 17:37:46 GMT

Yeah, I know. Why does the program change the length when it is moved? It is an arbitrary change with no basis for why it is done.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 18:16:58 GMT

This line has no length. It has a preliminary graphical representation with no defined properties.
Because you pull on one point, the other end remains loyal only to the verticality defined earlier.

Other 3D CAD software works similarly:
https://youtu.be/oZMtODonJKU?feature=shared&t=541
(9:01 - 9:46)

What software have you used before?

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 18:03:59 GMT

I'm not pulling on the end. I'm aligning the center point, but that's beyond the point. It's a line, defined in 2D space in a CAD program. Whether or not I've given it fixed values, it still has dimensions and X,Y positions. Some part of the parameters of the line should be held consistent when it is moved in order to be constrained to a second line. What reason is there, for Inventor to arbitrarily change the length of the line when it is moved?

I've used, AutoCAD, Inventor, Fusion, Solidworks, CATIA, Spaceclaim, Ansys Mechanical. Inventor is by far the worst of any of them for sketching.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 18:19:33 GMT

@ryancrampton wrote:

has dimensions and X,Y positions.

Yes, but they are free.

Only by assigning constraints do their dependencies become defined.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 18:33:01 GMT

You are confusing or implying that because the constraint on the line is free, that I should expect it arbitrarily change when it is moved. And I'm arguing that this is poor programming and far from the standard across multiple CAD platforms. If you are one of the Inventor programmers who controls this type of logic, then I ask again, why do the dimensions arbitrarily change? Why can I not control which line moves by the order of the selection that is made? If you are not a programmer, then please don't tell me again that things are "free". I'm not an idiot. I understand what is constrained and what is not.

Re: Sketch Lines resize and move illogically when adding constraints

kacper.suchomski — Thu, 17 Jul 2025 18:45:07 GMT

I'm not a programmer, just a user.

You can control the behavior of objects in a sketch by applying constraints to them.
There are two types of constraints: dimensional and geometric.

Objects not defined by constraints remain free.
Both in terms of orientation, position and size.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Thu, 17 Jul 2025 19:02:25 GMT

So, another useless answer... thanks. Just because items are free in orientation, position and size, does not mean that they should change those parameters when applying a constraint. Even if that constraint caused one of those items to change. In addition, as I've mentioned several times, there is no obvious logic to which item is moved when two things are selected to be coincident.

Re: Sketch Lines resize and move illogically when adding constraints

CGBenner — Thu, 17 Jul 2025 19:22:51 GMT

@ryancrampton

May I ask what it is that you are sketching? It appears to be a 2D schematic, like a P&ID. I'm not going to say that this can't be done with the Inventor sketcher, but it's not really what the sketcher was designed for. Without locking down every aspect of these lines and shapes, you may continue to see this type of behavior. It behaves much more predictably with fully constrained closed profiles, but even then if it is not fully constrained the behavior can be very unpredictable.

I get your frustration, but @kacper.suchomski is genuinely trying to help, as am I.

Re: Sketch Lines resize and move illogically when adding constraints

johnsonshiue — Thu, 17 Jul 2025 19:59:14 GMT

Hi! It could be a bug. Without seeing the file, it is hard for me to tell. If possible, please share the ipt file here. I would like to understand the sketch solving behavior better.

Many thanks!

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Fri, 18 Jul 2025 11:56:38 GMT

Thanks. My point was exactly that this behavior is unpredictable. It shouldn't be. I mean this is a program, so it's either unpredictable because it was not considered when designing how the software works, or it's predictable in some backhanded way that only the programmer understands. I don't know where to submit technical problems other than the forum.

Yes, I am using this to do a P&ID and yes, I know that Inventor is far from the best program to do this in. It's not really by choice on my end.

But the fact is, that I see this unpredictable behavior in other aspects of how Inventor works. The same kind of thing happens when contraining parts within assemblies. There doesn't seem to be a logic to which part moves. I find it less troublesome as I will eventually constrain the whole assembly, but at the same time, it should be a standard practice that the order of the operations matters. When I select two parts to constrain them, it should be a matter of that I'm saying, constrain part 1 to part 2. So then part 1 should move to part 2. But instead, I find that Inventor moves the parts unpredictably.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Fri, 18 Jul 2025 11:58:33 GMT

Sure. I've attached the file. I would argue that it isn't a bug, it's clearly a programming decision or a lack of any decisions put into the programming for this aspect.

Re: Sketch Lines resize and move illogically when adding constraints

pball — Fri, 18 Jul 2025 12:16:08 GMT

I don't believe this was mentioned yet, but one thing sketches do is when you dimension a single line it will scale other lines relative to the line that was dimensioned. This can cause lines to move around and might be some of what you are seeing. An example is shown below.

Re: Sketch Lines resize and move illogically when adding constraints

ryancrampton — Fri, 18 Jul 2025 12:48:48 GMT

Thanks, but that is not related to the point I'm making.

Re: Sketch Lines resize and move illogically when adding constraints

johnsonshiue — Fri, 18 Jul 2025 16:22:20 GMT

Hi Ryan,

Many thanks for sharing the file! I took a quick look. I am still trying to reproduce the behavior. Please bear with me. But I do notice something undesirable. In the highlighted area, there are a few overlapped lines. This can lead to confusion, because you don't know which line you are editing and which line is being solved. Please delete the duplicate lines.

Another thing you may want to consider is to enable Relax Mode (click on the right-most button at the bottom of the window). In this mode, the sketch driving behavior will be less aggressive. You are allowed to override any sketch geometry including existing parametric dimensions.

Could you show me the exact sketch geometry to change in order to see the bad behavior? I would like to try it myself.

Many thanks!