Alibre Loft Challenge

[jwknecht]

Alibre Loft Challenge

For those that have some time, please take a look at the attached. It was lofted in another software with ease. I can't get Alibre to Loft it. I am able to get all but leave some in the center missing, then I cannot fill it in properly. And, believe it or not, the imported IGES file ends up smaller than my native Alibre Loft.

I don't want to give any clues on my approach, because that might set you down my same path. I am looking for thinking outside of the box I am in.

Following the loft, I will be doing a shell deleting the bottom (flat) face. That is not a problem.

 

[MilesH]

Hi John,

It seemed to work for me.

 

[MilesH]

Hmmm. I see there is a lack of tangency at the mirroring points with my solution....

 

[MilesH]

I tried "overshooting" the lofted quadrant in order to be able to cut it back to a more tangential termination, but I failed to get this loft to build - just a load of specious error mesages.... Hope we get more robust lofting with ACIS 16....

 

[jwknecht]

I think I have successfully changed my thought process for this one. Instead of lofting from a slice of the pie to another slice, I am thinking that a small rotation of a spline at the top to create the top portion. Follow this with a loft or sweep to get the bottom portion, then a loft to connect the 2. I think this will work.

 

[larthen]

Well you just need to make a loft to a single node with proper tangency on. Works as a dream. Took about 30sec to get it.

 

[MilesH]

Hey, that's cheating :lol: I thought the challenge was to match the geometry exactly.. Mind you, I think you could get it pretty close by tweaking the tangent magnitudes...

Actually, it looks as if the original is lofted to a circle and node... could be just something to do with the iges file, though.

 

[jwknecht]

I did not know that you could loft to a node. Very helpful indeed. Miles is right that you did not match the shape exactly, but I can work on that.

 

[jwknecht]

OK. I worked some more. I got real close the shape I wanted. Was pretty happy. Then, my shape kept getting wierd. Checked the part, it had errors. I checked the sketch, no errors. Redid the base sketch to make is more simple, got a shape I thought I could live with. Wierd results. Sometimes after the shell, an error. Sometimes not. Changed a radius on the simple sketch to get more the look I wanted, resulting in an error upon check part.

I am thinking that there are some issues with the robustness of the Alibre modeler now. I don't know if this is due to 9.1. I have not done much lofting in Alibre so I can't judge if it is better or worse than before 9.1.

Anyway, I may :cry: stick with the imported IGES as the basis for this project.

 

[swertel]

The dome feature is definitely one where I'd look into a modeler that has surfacing capabilities. This is easily defined by a cross curve surface.

1) Extrude a square to your height.
2) Add fillet features on the corners.
3) Create two sketches on planes normal to each other. The sketches will be identical so link them to the same variables in the equation editor.
4) **Create a cross curve surface** with the two sketches.
5) Replace face, boolean subtract, or whatever tool is used to cut the surface into the solid extrusion.

DONE!

But this is a very difficult task for AD. Perhaps the Rhino plug-in? All the more reason I think AD needs at least some basic surface features.

--Scott

 

[larthen]

Hey cheating was not my intention :lol:
I know that you may tweak it a bit more to get it more precise, but I thought that precision is not the main point in this case. Just wanted to show a handy method, which as it has appeared, not everyone knew about. :wink: I guess when dealing with outer shape design the precision is not always the key, but the ease of creating good looking things.

 

[larthen]

But of course to make it really precise we would need an option of lofting to a point with a guide curve, which is now disabled. With this option on and running fine there would be no need of additional surface capabilities in Alibre. Well, maybe joining surfaces or capping holes... But you can live without them. :wink:

 

[MilesH]

Hehe, only kidding Maciej

I realised you were just quickly demonstrating how easy it was that way.

I guess we were pursuing different objectives. I didn't go down your route because I though it would be too difficult to match the geometry exactly, but after playing around with your model a bit I got it quite close.

It only took me a few minutes to model my first solution to this. It's a pity that guide-curve lofting is a bit weak at the moment - I've had endless problems with the way the loft resolves onto a profile.

I'm not sure I agree with Scott, on this - suggesting surfaces as a solution is a bit of a cop-out i.m.o. I'd much rather we had improved lofting than devoting development resources to implementing surfacing tools.

 

[MilesH]

We were writing in tandem

 

[swertel]

Interestingly enough, the first thing I tried was Larthen's idea of a loft to point with guide curve. (I've used loft to point before, but never needed the guide curve.)

I set up all my sketch geometry and began the loft command only to find the same dismay at not being able to use a guide curve when lofting to a point. Actually, the command works fine but errors out once you click OK.

That's when my mind completely changes gears to find a new solution and surfacing came to mind.

 

[jwknecht]

Attached is as close as I could get. The surpressed features are soley for the purpose of making the solid to make a shape for the projected sketch elements in the base sketch. This seemed to get rid of many of the errors. The extrude cut cylinder just before the shell gets rid of a vertice error that was happening after the shell.

I am still not using this because I get 6 edge errors if I check the part at the highest setting. And, the surface quality of the imported IGES is much better in appearance. It is nice to know that I could come close and could have used this had I not another alternative.

Note: The scale of this is larger than the other.

 

[MilesH]

I experimented with lofting from the given base profile to a very small circle at the apex using 4 guide curves, one projected from each sectional quadrant of John's sample. Whilst I managed to get the loft to build, the resultant shape didn't even approximately follow the guide curves :cry:

 

[MilesH]

Ok, I think I've cracked this... :shock: .... Talk about Byzantine..... mostly because of having to work from imported geometry, though...

Anyway, the Alibre method would be simpler than the one used in IDEAS, all other things being equal, and doesn't require any fancy surfacing commands :wink:

The pic shows my loft superimposed on John's original.

 

[jwknecht]

Miles,

How did you create the splines?

Edit: Never mind. I think I see what you did.

 

[MilesH]

I projected the 2 edge splines from the original then traced over both with a single spline and deleted the 2 original ones.

Is IDEAS unable to do a loft when the profiles partialy coincide?