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IbrahimTayyab14-Alexandrite
14-Alexandrite
July 8, 2023
Solved

Help creating an assembly

  • July 8, 2023
  • 2 replies
  • 2867 views

I am trying to replicate a curved scissor mechanism from an old purse (image named open and closed below), I am having some trouble connecting the assembly particularly the translation for the curved parts (image named translation below) , it is working fine with the non-curved version as I selected the bottom and top surfaces respectively. With the curved versions even when I attempt to define unique planes for the translation it does not seem to work, I assume it is due to a problem with the geometry I have defined.

 

I would really appreciate it if the error that I have made can be pointed out, I am attaching the files below.

 

Best,
Ibrahim Tayyab

Best answer by IbrahimTayyab

Thank you very much, it definitely gave me a lot more think about and made me consider new approaches. 

 

For posterity, I figured out how to do the pin constraint for the curved features using a datum point, which works very well. 

pin.png

http://support.ptc.com/help/creo/creo_pma/usascii/#page/assembly%2Fasm%2Fasm_four_sub%2FAbout_Predefined_Constraint_Sets.html%23

 

You are correct is seems it is not possible to change the diameter by dragging.

completed.png

I am attempting to make a surface that curves in 2 axes instead of one and seeing how that affects the "dragging" and also because I realised my previous attempt was incorrect.

3d.png

 

So far the problems that I hoped to resolve by posting this have been resolved, again thank you for your help. I will update this with the complete assembly once I am able to complete it.

Wishing you a great day,
Ibrahim Tayyab

2 replies

I
IbrahimTayyab14-AlexandriteAuthor
14-Alexandrite
July 8, 2023

Fearing I might have not described the issue succinctly, I will summarise it here: I am unable to make a pin connection for the curved part on the end holes. Picture attached. I would appreciate any and all feedback.

I
IbrahimTayyab14-AlexandriteAuthor
14-Alexandrite
July 9, 2023

As an update, I was able to make the assembly using a mixture of other constraints however they are not reliable. Some parts seem to overlap when I reopen the file and this was not the case when I defined the assembly.

assembly.png 

problem.png

Also when I view the sub-assemblies they no longer seem to hold their shape when the file is re-opened in a new session. (they should look like a section of the assembly above). 

partial.png

The assembly does seem to work as I intended to however I think the method I applied is not reliable, I would appreciate methods to improve this. Files attached below.

As an addition, I defined each connection manually even though they were the same connections, I am not sure how I could have used creo to automate this, any and all suggestions for that are most welcome.

 

 

P
19-Tanzanite
pausob19-Tanzanite
19-Tanzanite
July 10, 2023

Not able to open your files as I am still using Creo 4.

 

But I had a quick go at this, and I defined the kinematic structure in this way:

pausob_0-1688970328634.png

Each link placed by a pin joint.

 

then the connecting links that complete the "x" are assembled, each with 2 additional spherical connections that link 2 of the previously laid out links above:

pausob_1-1688970530116.png

 

Note after this is all done, the whole assembly is not moveable.

But modifying the radial distance effectively closes or opens the "scissors":

pausob_2-1688970819413.png

So this construction requires regeneration to update, but you could build on this idea and devise a 1-input mechanism that makes it possible to "drag" the mechanism around.

 

However, I do suspect the actual device has slop in the "pin joints" which allows the extra degree of twist required (or the links themselves twist a little) - because the angle of the "pin" axis between the adjacent links changes as this device opens and closes - here it is illustrated by drawing the surfaces of the next link in the open position (green) vs closed position (red):

pausob_3-1688971403565.png

(this is done with straight links, so not sure whether using curved links would solve this)

 

Anyway, hope this gives you some clues, and thanks for showing this neat problem.

I
IbrahimTayyab14-AlexandriteAuthorAnswer
14-Alexandrite
July 11, 2023

Thank you very much, it definitely gave me a lot more think about and made me consider new approaches. 

 

For posterity, I figured out how to do the pin constraint for the curved features using a datum point, which works very well. 

pin.png

http://support.ptc.com/help/creo/creo_pma/usascii/#page/assembly%2Fasm%2Fasm_four_sub%2FAbout_Predefined_Constraint_Sets.html%23

 

You are correct is seems it is not possible to change the diameter by dragging.

completed.png

I am attempting to make a surface that curves in 2 axes instead of one and seeing how that affects the "dragging" and also because I realised my previous attempt was incorrect.

3d.png

 

So far the problems that I hoped to resolve by posting this have been resolved, again thank you for your help. I will update this with the complete assembly once I am able to complete it.

Wishing you a great day,
Ibrahim Tayyab

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