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Trouble with tangent lofts

Question asked by James Dalessandro on Jun 15, 2015
Latest reply on Jun 17, 2015 by James Dalessandro

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Hi all, I am having some trouble lofting these profiles so that they stay tangent and the end result is a smooth loft with continuity between surfaces. I created these lofts by making a surface, flexing it so that it bends 180 degrees, and offsetting it. Then I added planes and profiles for my lofts. The bottom portion, where the curve meets the flat section, it stays tangent there. That is a smooth transition.

However, the top, where it goes from the curve to an angled up straight section has a little bump between surfaces. It doesn't meet smoothly. This is pointed at by the arrow. It is strange because in the top view it doesn't bump out on the sides. Only from a side view is the bump visible. It is especially weird because my surfaces I used as guide curves/centerlines for my lofts are smooth and tangent. How can I make this transition between lofts smooth and tangent like the bottom section?

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Jamil Snead Jun 15, 2015 11:37 PM
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It is hard to say for sure what is going on without seeing the actual part. Can you attach your file here? Here are some things you could check:

1. It may just be a graphics issue and not actual discontinuity in the part. In Document Properties > Image Quality move the slider at the top to a higher setting and see if it helps.
2. Edit the loft feature, right click on one one the profiles and select "Show All Connectors" or something like that. Then make sure the connection dots are in the same place in each profile sketch.

Also I just thought that maybe you can use a sweep instead of a loft if the profile doesn't change shape along the path. You might not even need a guide curve but you could try with and without.
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Alex Henry Jun 16, 2015 12:51 AM
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Hi James Dalessandro

There is an option that is meant to smooth out tangent faces available in both sweeps and lofts as you can see in the image below called "Merge tangent faces".

But I can understand if you are using the warp feature to get the shape you need the angle transition to the horizontal may not be tangent and it will be very hard to get it to do what you want it to do. I would suggest a different set-up for the path curve and use more controllable methods?

Like Jamil Snead - I look at your images and think a sweep would be more appropriate, based on my assumption that each one of the loft profiles is the same? The sweep will maintain the profile shape throughout the path without blowing out on the sides and you might find that fixes it - if not. Please post your model - people are always excited to see others' work and assist where they can ;-)

I've included a sweep example with a spiral which is not the path shape you have, but it demonstrates how the shape stays together.
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- sweep option.SLDPRT.zip121.1 KB
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- James Dalessandro @ Alex Henry on Jun 16, 2015 11:21 AM
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  Jamil Snead and Alex Henry - - So I am attaching the part file for you guys to look at. I took your advice and tried sweeping it, and it comes out about the same, maybe a little smoother but still with the bump at the top portion. The loft profiles are indeed the same, I was using loft because of the amount of control it gives. But I am willing to use any feature/command to create this as long as it comes out looking right. It is for a wooden stair handrail, so even a small amount of deviation is unacceptable as you will be able to feel it running your hand along it.
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  - smooth rail.SLDPRT.zip5.6 MB
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  - Jamil Snead @ James Dalessandro on Jun 17, 2015 12:16 AM
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    I have SW2014 so I can't open your part, but I modeled something similar. It's a tricky part. Even though the edge of the flexed surface should be smooth, the edge of the offset surface is not smooth, and that's what's messing up the continuity. I tried some various methods to get it smooth and kept running into issues. I finally got something that looks good which you can see in the attached part file. You can see that I created the path manually with a variable path helix and tangent lines coming off of it. Using your flexed edge may work just as well. The biggest limitation of my method I see is that I had to manually enter the twist amount into the sweep, so at the end of the sweep the bottom of the profile isn't perfectly horizontal.
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    HandRail JS.SLDPRT.zip273.6 KB
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    - James Dalessandro @ Jamil Snead on Jun 17, 2015 10:30 AM
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      Damn solidworks and their lack of backwards compatibility...
      
      I agree, it MUST be the offset surface edge that I am using as my guide curve. I created the outer surface and offset it in, and that surface does have a little bump where it transitions from curve to straight. Doing the opposite and creating the inner surface and offsetting outwards just creates a bump on the outer surface, albeit a bump in the opposite direction i.e. a dent. I wonder if there is a way to offset the curved surface and not have it have a bump...
      
      Your method had a pretty good outcome! However I cant be doing this for every handrail. I work at a stair design company, and modeling these wreath fittings (where the handrail turns and goes up) is becoming a big part of my job. Each one is different.
      
      If anyone has any ideas for how to create the surface without a bump, I'm all ears.
      
      I will keep trying in the meantime.
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      - Jeremiah Feist @ James Dalessandro on Jun 17, 2015 10:49 AM
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        Here is one possible way to get a smother path/guide
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        smooth path jpf.SLDPRT.zip69.2 KB
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        James Dalessandro @ Jeremiah Feist on Jun 17, 2015 10:56 AM
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        The problem comes from the offsetting of the surface. Look again at your inner surface where the curve meets the up angled straight section - the bump is still present.
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        Jeremiah Feist @ James Dalessandro on Jun 17, 2015 11:21 AM
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        I'm not seeing a bump...
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        James Dalessandro @ Jeremiah Feist on Jun 17, 2015 11:34 AM
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        That's because you're looking at it from above.
        
        Look at it from the front or back. It is very visible.
        
        Think of it like this: If you have a 90 degree corner that you fillet, and you offset that inwards, you will get to a point where the offset causes the fillet to return to a corner again. Because the radius of the fillet can't get any smaller at some point, it turns back into a corner.
        
        Here is a picture of the bump in your model.
        
        If I were to use this as a guide curve for my lofts, I will get the same bump. I need to have a smooth transition on the surface edge just like on the outer surface.
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