Inventor Constraint Persistence: My Best Friend. My Worst Enemy.

Mastering Autodesk Inventor’s sketching environment is all about mastering Constraints. And mastering Constraints is all about mastering Constraint Inference and Constraint Persistence.

Don’t get me wrong – I’m all about keeping sketches as simple as possible. I even wrote this post showing how to build an entire Lipped, edged and veneered Panel using only one Sketched line (check it out, it’s pretty neat).

On the subject of keeping sketches simple, I also recommend that you check out this great post on Inventor Sketching by Curtis Waguespack ‘Simple Fully Constrained Sketches’.

However, there are occasions when Sketches get out of hand, and then it can be a really good idea to turn Constraint Persistence OFF.

Constraint Inference

For those not in the know – Autodesk Inventor’s Constraint Inference works pretty much like AutoCAD’s running OSNAPS.

As you sketch geometry Inventor will look for relationships with the surrounding geometry. Inventor will help you to create geometry that has coincident end or mid points and is horizontal or perpendicular or parallel and so on.

However, no geometrical constraints will be applied – unless you have Constraint Persistence turned on.

Constraint Persistence

With Constraint Persistence turned on Inventor will automatically apply constraints as it sees fit. Nothing wrong there – in a simple sketch, this is an extremely handy time saving feature.

Naughty Naughty

Inventor has a golden rule when it comes to Constraint Persistence

If it Can apply a Constraint, it Will.

This might might mean that the Geometric Constraints that are applied are not what you were expecting. If you draw a line between two fixed points, that line is not going anywhere. However, if that line is horizontal or perpendicular or parallel with another line Inventor will throw in the additional constraint for good measure.

Don’t be down on Inventor here, it’s only doing what it was designed to do.

You may not realize that this has happened until you try and ‘Flex’ your sketch and you get an error. Then you get to learn about the second most important skill in sketching – diagnosing sick constraints…

Hand Rolling Your Constraints

So here is my tip. If you need to sketch more than half a dozen lines or arcs in a sketch, Toggle Constraint persistence off and apply the remaining constraints yourself.

I guarantee that this will save you a great deal of frustration down the line when you know that the only constraints that have been applied are the constraints that you put there.

Toggling Constraint Persistence

You can toggle Constraint Persistence on and off from the ‘Constrain’ panel under the sketch tab.

Tip: You can temporarily turn off Constraint Persistence by holding down the CTRL key whilst you sketch.

In Inventor 2012, this has (inexplicably!) been hidden under the drop down at the bottom of the panel

Tip: If you are using this quite a bit you can move the controls back onto the main panel by right clicking on them and choosing ’Move to Main Panel’

My Best Friend and My Worst Enemy

So, there you go. When starting off a sketch, Constraint Persistence is my Best Friend. In a complex sketch, Constraint Persistence can be my Worst Enemy.

Consciously taking the decision to toggle constraint persistence off can be a great move. Just don’t forget to toggle it back on again when you start your next sketch!

…and yes – I am a big fan of The Oatmeal!