ACCEPTED SOLUTION
Accepted Solutions
Well, one will be usually inefficient when using an unknown tool. You'll get good support from this forum because Creo is not easy to "get to know" all by yourself.
I recall that in Solidworks, one easily specifies the constraints (mates?) between components and it "finds" the kinematic configuration of the assembly. Well, this is not how things work in Creo, and you have to be very explicit when creating kinematic connections. What's more frustrating is that they system does not give you good tools to fix the problem.
Actually, I still don't quite understand how it all works - I took the cylinder example provided elsewhere in this thread and tried to make it lock up by having the rod's constraints be "shared" between different models, but it still works. I include it to touch on the "regeneration value" aspect of a kinematic connection - maybe those are responsible for your cylinders "resetting" themselves.
BTW, note that you can hold CTRL+ALT and then left click on a component to drag it around subject to its constraints. Also, I assume you know how to use the "Drag Components and Mechanism snapshots" tool to capture different motion states so that you can then show them on your drawing (as different explode states - yeah, kind of a hack).
Motion can be a little tough to get the hang of in Creo but there are some tips that can help make the learning curve less steep. Here are a few:
- Separate things that move from things that don't move. I typically separate with subassemblies.
- Be careful with constraints. If you attach something that doesn't move to something that does, you'll lock down your motion. If a component needs to move, make sure all of its constraints are referencing your moving stuff. If a component doesn't move, make sure all of its constraints are referencing your stationary stuff. Don't span moving and non-moving with placement constraints (cylinders are the exception here).
- If something isn't moving that used to, you likely botched a constraint. Drag the insert bar up the model tree until it moves again. Then resume components until you find the thing locking down your motion and fix the constraint.
- Datum setup is everything. Keep things simple, organized and labeled. Use skeletons to your advantage.
- Opinion: avoid motion skeletons unless you have complex motions that require them. We teach our users to use motion skeletons as a last resort. We often make complex 3-axis motions without motion skeletons. If you've never heard of a motion skeleton, ignore this point.
- Use the out of the box mechanism constraints like pin and slider. They help keep things organized and are easier to understand than general constraints. I've seen some nasty general constraint messes.
- In Creo, it's more robust to fully constrain motion. Avoid using lack of constraint to show motion. I don't like to allow assumptions. I like to tell Creo what to do.
I've attached a very simple reference assembly for you to look at (Creo 4.0). Notice that I have moving and non-moving separated, self-contained, and referencing a common skeleton. The cylinder spans both (body can't move but rod needs to). In this case, I use two constraint sets. One to position the cylinder in the assembly and another to link the rod to the moving body. The cylinder itself needs to be mechanized to do this so take a look at how that is set up as well.
There are a bunch of different ways to do this but we teach two basic schools of thought:
- Mechanize the cylinder and tie the stuff that needs to move to the end of the rod.
- Mechanize the assemblies and use constraints to allow the cylinder to span moving and non-moving.
The second option is preferred in my opinion because the mechanism constraints are stored at the upper assembly level and will allow you to reuse the cylinder anywhere you want. Especially if you have different working strokes for each application of a cylinder. If you use the same datum setup or skeleton, I think this method is the easier of the two.
We have both Creo and SolidWorks running in Windchill. The rules that we follow for motion vary slightly due to software differences but are pretty much CAD-agnostic.
I hope this helps!
Ty
Ok so here is a good example of what I'm talking about. I have this gripper, using the slider (I believe) constraint system. The Coincident and angular offset are both planes of the gripper finger to the planes of the first component. I do this with both fingers. I use the gear mechanisms to synchronize movement. Works perfectly as just an assembly. I throw it into my main assembly and they loose synchronization.
Thanks for sharing - I was not familiar with the way you made the gear connection there. You see how in my example I had a dummy link.
Anyway, my notes:
when I placed your gripper into the motion_example in this thread, then the gripper did move, but in a very strange way. Basically, the 1:1 ratio was lost and one gripper would move very slowly compared to the other.
then I noticed that the gripper is in mm, while the motion_example assembly is in inches. I switched motion example to be in mm and the proper motion was restored.
Interesting that my dummy-linked version can be set to inches and still works...
So I'm thinking that there is some funny business with units going on. Definitely not an unheard-of-issue in Creo modeling.
How is the unit consistency in your actual models?
