Serious BUG in Mechanism Slot Constraint in Creo 4

[Patriot_1776]

I was playing around with the chain issue, and ran into a major BUG with the slot constraint.  I used a Skeleton model for a datum curve "racetrack" to roughly simulate the path of a chain (wouldn't actually work with all the links assembled, but I was trying to test the constraints needed).  I made a sketch in the Skeleton simulating the path around 2 different sized sprockets, and made it a "spline".  I used a planar constraint and 2 slot constraints for the first link, and a planar constraint, a cylinder constraint to the first link, and a slot constraint to the racetrack.  Considering that for ALL other purposes, when I wanted to select this spline, it gave me the HALF spline (truncated at both ends in seemingly arbitrary locations), but also allowed me to pick the entire spline (i.e. to extrude a surface), but when I tried to pick it as the curve for the slot constrain, I ONLY got the truncated spline.  I could NOT pick the entire curve.  What FINALLY worked was that I copied the curve from the Skeleton model as a single curve spline, gave the feature a name, then picked the 3D Curves from the filter.  Then I was able to drag the 2 links around the racetrack as I wanted.  BUG.  As mentioned, this simple racetrack will work with a belt but NOT a chain because while the chain length if pulled straight will give a static number, since each link is a fixed length, when you bend it around a sprocket it gives issues.  The sprocket has the roller interface as a fixed pitch diameter based on an inscribed polygon with faces of the link length X number of teeth, the chain pins follow the inscribed circle, but the chain's length is determined by the cumulative length of it's number of links.  So, you can't just say that a chain is, say, 100 links at 1" distance between pins giving you a length of 100" for your racetrack.  It's a lot trickier than at first glance, but I digress, that's the topic of another thread...  Here, I just wanted to present the BUG.  Now, I'm on Creo 4, so this MAY be fixed in later versions....but I don't think so.  And if not, as I suspect, PTC, are you listening?

[Patriot_1776]

Played with it a little more, and it turns out I don't need the curve copy at the assembly level.  I was able to use the filter to select the 3D Curve from the Skeleton model, so I deleted the assembly copy to clean things up.  It's weird, that it's supposedly a single curve (like for use as a protrusion etc.) for anything else, because it was a spline (made up of 2 arcs and 2 lines) in the sketcher, but Creo somehow arbitrarily cuts it into 2 separate curves when you try and use it for a trajectory for a slot constraint.  Maybe there's a length limit?  Don't think so, as that would make ZERO sense (but then, neither does this...), and it still let me use the total length if I was able to pick both segments.  For some stupid reason, you cannot pick both segments without using the filter tool....go figure.  Yeah, I'd say this was a legit BUG...

[Nutron_Dynamics]

Can you post the STEP file?

[Tdaugherty]

You can hold Ctrl to multi-select the different curve segments when you are setting up the constraint. In this example, I created a similar sketch that has 4 segments (split at the tangent points of my trimmed sketch). If select them all, I end up with one curve and I can drag my component along the entire path.

[Patriot_1776]

Tried that, for some reason it absolutely refused to work for me.  What version are you on?  I'm stuck with Creo 4.

Also, now that I think about it, maybe the fact that I made it a spline FUBAR'd things.  It picked them as 2 non-equal segments, the ends arbitrary, not like picking defined arcs and defined lines.  No matter, using the filter worked.  And, if there was a bunch of segments, may end up being faster.  I ran into another problem that may be the show-stopper.

[Tdaugherty]

I'm using Creo 4.0 M150. The sketch was created with regular lines in sketcher. I've never tried to use splines for motion so I'm not sure what the advantages/disadvantages of using that method would be. 

[Patriot_1776]

For the chain to work, I need the path to move around slightly as I drag links around, it's not like a belt where a defined length can be dragged around a trajectory because it bends.  The fact that the chain has a specific length defined by the # of links, yet has the issue of the individual links rotating around the sprocket causes problems.  Yes, I could simply cheat and make a trajectory with a perimeter dimension of the length of the chain, and only constrain the last link to the penultimate link on one end and the slot on the other, but then as you dragged the chain around, that pin connection from the last link to the first link would change as you dragged the chain around.  Not by much, but it still wouldn't be correct, and I'm kinda retentive like that.  I want to do it correctly.  Cheating like that would work for 99% and likely be "close enough" for the vast majority of people's needs, but I don't play with horseshoes or hand grenades!🤣

EDIT:  Maybe not constrain the penultimate link to the slot on it's end closest to the last link, and then have the last link only connect to the pins of the penultimate and first link, so it could "float"?  Hmmm, then that link wouldn't 100% follow the trajectory, but the links would always be correctly connected to each other.....  Not exactly the way I'd want to do it, but better than not having the pins line up and actually, as a chain ALWAYS has a few links that are "sticky", and don't rotate as much as other links, it would actually be "real world" correct...

[Patriot_1776]

UPDATE:  Splines are almost completely useless, PTC should have made them POLYLINES like in the AutoCAD sketcher.  That way you can constrain other sketched items to them, yet maintain there base geometry (arc, line, conic, etc.) and simply then be able to make a perimeter out of them.  But Noooo.......