Extrude surface with taper - construction problem

This is my first time posting to the Forum, so I hope I can explain this correctly. I am trying to construct a model for a mold that looks something like the attached jpg (but with sharp corners). This is for a glass mold and needs to be tapered so the model can be extracted from the mold with as little friction as possible. I need a two inch extrusion with a 1 degree draft angle. The red piece and green piece need fit seamlessly together with the tips at the same height and the taper at all three points going in the same direction. I have used the solid extrude/taper command and tried flipping angles, flipping y axis, extruding from different starting points, but no matter what I do there is either 1) slight offset between the red piece and green piece when they are re- aligned 2) the taper for the red piece is in a different direction, or 3) tip of the red piece is slightly below the tips of the green piece. I’ve tried to do a wire cut but that fails because it cuts a straight path and there is a taper between the red and green piece. I have tried to extrude a curve with taper but again there is an offset where the red piece meets the green piece. This is very easy to make without a taper, but the taper seems to make it very difficult to get it exactly right.

construction2.3dm (247.8 KB)

You need to understand is that the extrude taper command is badly broken and needs repair but nobody is interested in fixing it so you will need to work around all its defects.

The only thing the extrude taper command does correctly is when the input is planar lines and arcs. Everything else it will usually just make a mess.

Your shape is composed of lines and and arcs so you can get good results if you follow the steps in the enclosed file.

ExtrudeTapered.3dm (165.5 KB)

Start with all the curves joined. This gets the direction of all curves going the same way. Then explode and use extrude taper while all curves are still selected. Then you need to extend those surfaces that need it so that they all fully intersect. Then trim the surfaces that intersect. The tangent curves don’t need trimming. After trimming join all surfaces and cap.

If the Extrude Tapered command was not broken, the way you tried to do it should have done all the work for you, but since the command is broken it doesn’t.

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Another way to build the objects is to create the wider side outline curve in top viewport, and then offset it inside by .035" - then move it 2.0" +z. Loft the two curves and cap the loft.

Thanks, guys. I have used a workaround for “manually” creating a taper by making a copy of the curve, then scaling based on the centroid location, and applying a loft. It seems like way too much work when the extrude/taper command was made to do just that. I appreciate the suggestions and will give them a try. Maybe V6 will show some upgrades?

Hi Scott - if you start the red peice with the end shape (i.e.offset) from the end position it should work out.

construction2_PG.3dm (167.2 KB)



Maybe I’m not understanding correctly. When I look at your file, the red piece is tapered in the wrong direction. It needs to have the same taper direction as the green tops. This is a requirement in order to remove the model from the mold (reduces friction). I think the basic problem is that the bottom of the red piece gets tapered downward and the top mating point of the green piece gets tapered upward. I need these two surfaces to be on an even, aligned plane.


Hi Scott - sorry - yes, I see - I was thinking only about the fit between the pieces.