# Thread: Loft guide rails and constraining 2D splines

1. ## Loft guide rails and constraining 2D splines

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I want to make a loft for which one of the guide rails is a projection of a 2D spline plus 2 tangent straight lines on either end of the spline. The loft fails to work and it tells me the cross sections do not intersect the guide rail. I know that the each cross section intersects one of the three segments that make up my guide rail.

So to begin, it seems that the loft command requires that the guide rails be one continuous line/spline. Is this correct? I cannot find a polyline command or any other way to join the spline with the lines. Is there any method to do this?

So, I'm trying to accommodate the software and draw one spline that matches the previous compound curve. On the ends of the spline I need a straight segment between the last two points. I planned to activate both handles and set one parallel to a construction line, then the other parallel to the first handle, then set the end handle's curvature to flat. However, it never gets quite flat.

Also, as long as I have one handle active I cannot fully constrain the spline. On that handle I have a coincident constraint to a defined geometry, the slope is constrained parallel to defined geometry, and I also defined the length of the handle and the radius of the curvature (although I would rather not constrain these last two). It tells me I need four additional dimensions, but I cannot imagine what they would be. If I use the sketch doctor it tries to add coincident constraints but each one is rejected with an over constrained warning.

If I deactivate the handles it will say it's fully constrained. But if I experiment and change some of the underlying construction dimensions the parallel constraints I placed do not hold. I gather the handles must be active for constraints to work on them, though I could not find this mentioned explicitly in the help literature.

All I really want to do is constrain the slope of 4 of the handles; the radii and handle lengths I would rather have Inventor calculate or adjust intuitively via mouse drag. But even if I relent on that and constrain everything I can think of it still asks for more.

-Robert

2. Originally Posted by Robs92XJ
I know that the each cross section intersects one of the three segments that make up my guide rail.
I'll bet not. How did you connect them?

So to begin, it seems that the loft command requires that the guide rails be one continuous line/spline. Is this correct?
No. Only requirement is tangency.

-Robert
Have you gone through the tutorials in my signature?
Attach your file here (roll up the EOP and zip before attaching).

3. JD,

Thanks for responding. I am glad to hear I was mistaken about needing a continuous line for a guide rail.

I say I know the cross-sections intersect because I had to spend a lot of time searching here to make it so. The original sketch is drawn on the X-Y plane, but the geometry has to be projected onto a plane perpendicular to the XZ plane but angled very slightly to the XY (This loft will have a flat bottom lying on that angled plane).

The cross sections for the loft are on planes parallel to the YZ axis. There are vertical construction lines in the original sketch defining the height of each cross section. These lines are bound by two curves, an arc on top and a line-spline-line on the bottom. The two curves will be projected onto the angled plane to serve as guide rails for the loft.

The cross sectional planes are located based on these vertical lines. I needed to project the vertical construction lines from the original sketch onto the angled plane, then project that into the appropriate cross sectional plane. I also projected the two guiderails onto the angled plane.

However, the angle of the plane caused the projections to move very slightly with respect to the X axis, so that the vertical lines no longer lay in the cross sectional planes. This was not obvious because the angle is so slight, but if you zoomed in enough you could see it. The ends of the guiderails did not touch the front and back cross sections once they were projected into their cross sectional planes

Anyways, thanks to this site I found I had to make a 3D sketch on the angled plane, which enabled me to project the original sketch geometry (including the two guide rail curves) orthogonal to the XY plane, then project that 3D sketch onto a 2D sketch in the angled plane. Doing this, everything appears to line up visually no matter how far I zoom. All of my sketches are fully constrained according to Inventor.

I will try to post the file tonight.

4. I think one problem may be with how the spline was constrained (or not constrained) in the original sketch.

I activated each endpoint handle and set a parallel constraint to the adjoining line. That should ensure tangency between the line and the end of the spline. However once I did this I would deactivate the handles, assuming the constraints were set. Now it looks tangent, but I'm not sure it's actually constrained with the handles deactivated. I have a feeling if I changed some of the underlying geometry the tangency would not hold.

The reason I deactivated the handles was that if they were active I could not find enough constraints to add so that Inventor considered the spline fully constrained, as I mentioned in my original post. If I deactivated the handles it would say fully constrained again.

Supposing my suspicion is correct and there is no actual tangency constraint, would this give a cross-section/guiderail failure to intersect error when lofting using that line-spline-line as a guide rail?

5. Need the actual file to make any evaluation.

6. Fair enough. I've attached the file. It is a knife with a tapered tang through the handle section. What I am trying to loft is the contoured handle scales. Once you roll down the EOP you should see where I am with the loft. The loft should go through the 3 semi-elliptical cross sections with the arc on top and line-spline-line on bottom as guide rails.

The guide rails and vertical lines visible on the "scale face" sketch are projected from the 2D sketch "tang profile" (by way of an intermediary 3d sketch). In going back and looking over the tang profile sketch, I realize I had applied tangent constraints to the spline where it touches the lines. It actually would not let me activate any handles unless I deleted these tangent constraints, which is fine with me. My other comments regarding constraining spline handles were from when I was trying to create a new spline to incorporate the straight segments on either end. But it seems my original spline was constrained properly (tangency) to allow it to serve as a guide rail with the two straight lines. (Edit: the spline in the attached part is the original, not my subsequent attempts)

I just can't figure out how to make that lower guide rail work. If I leave out that guide rail it will loft, but obviously not with the proper shape.

Any help would be appreciated.

Thanks- Robert

7. So, has anyone tried to make a loft where one guiderail was multiple segments? I cannot find a tutorial where this was done, it's always an arc or a single spline.

Is there a special way to select the individual segments during the loft command? I tried holding -ctrl while selecting each but this didn't seem to have any effect.

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There is more involved with this part than that. It will be a week or so till I can get back to this problem. Bump it if I don't remember to get back to it (or is someone else doesn't jump in here with the solution). You will have to become familiar with surface modeling to finish this part.

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