Loft twist

rachel_hollis

I'm using a book called Onshape tutorials. Chapter 8, exercise 1.

The base is a circle whilst the top has 4   3point arcs with 2 tangent lines too.  How do you loft this complex shape. I get arbitrary twists a curves going up.

R. H

John_P_Desilets

@rachel_hollis

Like @bruce_williams mentioned, lofts are complex shapes that require additional framework to ensure a robust solid feature. 

Loft profiles should have the same number of vertices in order to prevent twist. If the profiles do not have the same number of vertices, guides and paths may be used to help drive the loft. A circular profile however has no vertices so how do you match to the receiving profile?   

The answer is quite simple. Split the circular profile to have the same number of vertices as the second profile. 


This profile was a circle split into 4 regions using the split command in the sketch toolbar



The second profile is a rectangle with 4 vertices 


The vertices match up correctly after creating the loft. 




This is a very basic example of splitting profiles.  Loft guides are needed for more complex geometry.   Hope this helps!

rachel_hollis

Thought I ought to pop on here and thank you both for the help, I've only had onshape a week and exercises like this are test. 
https://cad.onshape.com/documents/5e52be110a7a6769ba152ace/w/99dba5b325a59dd1674e5970/e/5bde45beb53f725d0a391c4e

bruce_williams

@rachel_hollis

Welcome to the forum! 

You have a common issue, although I am not familiar with that specific lesson.  You will need to add geometry to the circle to use for 'Match vertices' in Loft command.  Anything which creates a vertex coincident with the circle including split the circle, add a point or line with coincident end point, etc.    

A circle naturally has no vertex and the Loft command can find it ambiguous as to what to match.

Lofts are tricky as they have many controls which may be added.  Refer to Loft 'Match Vertices' help here

John_P_Desilets

@rachel_hollis

Like @bruce_williams mentioned, lofts are complex shapes that require additional framework to ensure a robust solid feature. 

Loft profiles should have the same number of vertices in order to prevent twist. If the profiles do not have the same number of vertices, guides and paths may be used to help drive the loft. A circular profile however has no vertices so how do you match to the receiving profile?   

The answer is quite simple. Split the circular profile to have the same number of vertices as the second profile. 


This profile was a circle split into 4 regions using the split command in the sketch toolbar



The second profile is a rectangle with 4 vertices 


The vertices match up correctly after creating the loft. 




This is a very basic example of splitting profiles.  Loft guides are needed for more complex geometry.   Hope this helps!

rachel_hollis

Thought I ought to pop on here and thank you both for the help, I've only had onshape a week and exercises like this are test. 
https://cad.onshape.com/documents/5e52be110a7a6769ba152ace/w/99dba5b325a59dd1674e5970/e/5bde45beb53f725d0a391c4e

rachel_hollis

Thank you.

bruce_williams

@rachel_hollis

Good job - that is a challenging one for an intro. 

A few tips:

Your sketch 1 looks good; all black.  Sketch 2 is red due to being over-constrained.  I removed all but one of the 21.5 dimensions to fix that.  The arcs are tied to same center and have common end points so added dimensions cause error.   

Also, I put in construction lines for Coincident constraint on the vertices of arcs.  That makes the sketch black (fully constrained) and puts the vertices where you probably want them so the corner blend is correct. 

See copy with corrections here

rachel_hollis

Thank you for that Bruce, I wasn't overly worried but still tidies it up. The finished exercise is wonderful piece of work to do. here's the final item.

https://cad.onshape.com/documents/5e52be110a7a6769ba152ace/w/99dba5b325a59dd1674e5970/e/5bde45beb53f725d0a391c4e

baffour_osei

I just figured this out. When you go from a large circle to a small circle using the loft tool, the loft maintains its shape; however, when you go from a small circle to a large circle, the loft twists. I don't know the reason, but it does