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Patriot_1776
Patriot_177622-Sapphire II
22-Sapphire II
November 4, 2021
Question

Serious BUG in Mechanism Slot Constraint in Creo 4

  • November 4, 2021
  • 2 replies
  • 3383 views

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?

CHAIN SLOT TRAJECTORY ISSUE-01.jpg

2 replies

Patriot_1776
Patriot_177622-Sapphire IIAuthor
22-Sapphire II
November 4, 2021

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...

A
Anonymous
November 4, 2021

Can you post the STEP file?

Patriot_1776
Patriot_177622-Sapphire IIAuthor
22-Sapphire II
November 8, 2021

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.......

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