Join Veejay Gahir for an in-depth discussion in this video Bi-rail sweep surfaces, part of Alias Essential Training.
- In this video we're going to move on to the Bi-Rail Option in the Rail Surface. So let's open up Rail Surface. And we're going to click on Reset. And in this case we're going to use 1 Generation Curve with 2 Rail Curves. And you'll notice at the top now the naming convention has changed from monorail to birail. So let's zoom into this first scenario. And the first thing I want to mention here is that there can't be any gaps between your Generation Curve and the Rail. Now, this is obviously a very large gap. But the gap could be as small as .1 or .05 of a millimeter.
And let's see what happens here. So Alias is asking us to select Generation Curve. This is our Generation Curve. Now it's selecting the primary rail curve and then the secondary rail curve. And Alias comes back with an error saying that there's basically a gap between the entities. So let's have a look what it's telling us. Basically it's saying that Generation Curve and the secondary rail must intersect. Right now we have a 4.2 millimeter gap. Well, that's way too big. So that's the first thing to note. I'm just going to go ahead and delete that. Let's go back to that command.
So we're going to use two rails. So 1 Generation Curve, primary rail, secondary rail, and as a result we get it's a 2 by 1 Span, 3 by 6 Degree surface. And again, as we mentioned before, the reason why we have a Span in here is because this curve is actually a 2 Span curve. And let's put on the edit points for that. Let's take a look at this curve at the back as well. That's also a 2 Span curve. And we're going to put the edit points on that as well.
So let's pick that surface. And we're going to delete that. Let's go back into the rail command. Now, the next option we're going to take a look at is with 2 Generation Curves and 2 Rail Curves. So Generation 1, Generation 2, Rail 1, Rail 2. And as we mentioned before, the complexity is high because this Generation 2 Curve has the most complex DNA of all the information that we've used. So if we wanted to reduce this complexity, we would have to reduce the Generation 2 Curve.
And let's do that very quickly. Let's pick that arc, and we can take that back down to 1 Span, check the Deviation is 0, and the surface is a lot more simpler now. Let's go ahead and accept that. Let's take that rail surface and we'll delete that again. Let's open this up. Now we're going to go to 2+, so Generation 1 Generation 2, Generation 3, now spacebar to tell Alias that we are done with the Generation Curve selection portion.
And now it says, well, where's your Rail 1, and that's this one here. This is our Rail 2. So now we have a 2 Span by 1 Span, 3 by 9 Degree surface. So let's undo this. Now I want to show you a scenario that can happen very easily. And it can lead to some very undesirable results. So we've got these two curves selected. And we've got edit points here and here. And we've also got a profile curve that's attached to those edit points. So let's go ahead, change those back to single.
Check the deviation, right now it's .067. And what I want to do now is I'm going to go ahead and insert an edit point right about here. And I'm going to insert an edit point on this one right about here like that. So I've inserted these edit points with a gap that we can see. But the gap here between this edit point and the end of this curve in fact could be 10,000th of a millimeter. So it could be very, very small. But just for clarity's sake, I've opened up the gap so we can see the end result here.
So let's go ahead and create the surface. So with 2+ we're going to use Generation 1, Generation 2, Generation 3, spacebar, first rail, second rail. And now what I'm going to do is I'm going to just simply move this surface. I'm going to lose construction history, and that's fine. So the end result here is a surface that has three Spans that are grouped together very close like that. And that's not a good situation in surfacing.
And the reason for that is that when Alias was creating this rail, it was taking this profile. And as it was sweeping along, the first thing that it hit was this edit point here so it created a span. Then it hit the profile, and it created another span. And then it hit this edit point here and created another span. So we actually have three spans in this region grouped very closely together. So again, just be aware that if you do end up with a surface like this, the most likely reason is that you've got edit points or profiles very close to each other and that can cause this situation.
So let's move on to this scenario here. In fact, let's just select this information, and let's press I for invisible. So let's shade this now. We've been looking at curves and profiles. But now we can actually use the edge of a surface as our rail curve. Let's go ahead and switch off the grid in this case. Now, we're using periodic entities here. And this is again not a good idea. And I'm going to show you what can happen if we do this. So let's go back into our Bi-Rail. And we're going to pick 2 Generation Curves and 2 Rail Curves.
So we're going to pick this as our first Generation Curve and this one as our second Generation Curve. This is our rail, and this is our rail here. So again, it's adopting the complexity of these generation curves, which is quite a complex curve. But you'll see that the surface is generated along the top, which is where we've selected those generation curves. So that's a predictable result. Let's undo that. I'm going to pick now on the lower portion of this curve, lower portion of this curve, Rail 1 and Rail 2.
And you'll see now that the surface has been created below. Again, in any CAD system it's not a good idea to work with these type of entities. It would be a good idea to split the entity at those two points here so that we're working with just half of that circle. So let's undo that. If I pick at the top here, and then I pick at the bottom for the generation curves, and then I pick my rails, you'll notice that we have a very strange result. So now it becomes very sensitive as to where I'm picking on those generation curves.
So just be aware if you get a result like this, you have to be a little bit more careful where you pick on your generation curves. So let's undo that, and let's go to 2+. And we can pick top, top, top, spacebar to accept. Rail 1, Rail 2, and that's the result that we get. Now, because we've chosen a surface for our Rail 1 and Rail 2, we have another option. So we can change Rail 1 and Rail 2 to give us tangency continuity. So tangency continuity is green right now, so that means we've achieved tangency.
So that's another benefit of using a surface edge. Now, these are very simple surface edges. If I light up the control points, you can see that they're actually natural edges. They're not trimmed edges. Let's take a look at what happens when you use a trimmed edge. So let's select this data here. I for invisible. And let's move over to this one here. Now, before we get started on this, let's take a look at these two surfaces. If I bring up the control points, you can see that in the side view they've been trimmed with this S-shaped curve.
So that's a fairly complex trim. And if I go into the top view, you'll notice that those two surfaces are also curved like so. So they're actually quite complex raw edges that we're going to be using for our rails. So let's have a look at what happens here. So we're going to use 2 Generation Curves, 2 Rail Curves. And I'm going to pick, this is a Generation 1, Generation 2, Rail 1, Rail 2. Now, you can see that this is a very complex surface. Terminology-wise we can call it a complex surface.
We can call it heavy surface. But basically it's a very, very undesirable surface. The reason for this is that we were using a raw edge as our rail. This is not good practice. So Alias now is trying to run this generation curve down this raw edge and at the same time it's trying to maintain tangency. And at the same time it's trying to orient the profile normal to this edge as well. So we're asking Alias to do a lot. And whenever you ask the system to do so much, you end up with a complex situation.
Now, sometimes it's unavoidable to use a raw edge like this. So what we're going to do is we have an option to rebuild this edge. So internally during this command, Alias will try to rebuild this edge, simplify it, and execute the command to simplify the surface. And to make this rebuild work, we have to click on rebuild for our Rail 1. And you'll see that when I do this, we go from 63 Spans down to 32. And if I click on Rail 2 rebuild, it goes down to 20.
Now, again, this is not the best result. But it's gone from 63 down to 20 just by using rebuild. So that's where you would use rebuild if you're using a raw edge like this.
- Manipulating views and entities
- Working with layers
- Creating curves
- Sweeping, extruding, revolving, offsetting, and blending surfaces
- Modifying geometry
- Moving, scaling, flipping, and rotating objects
- Trimming curves and surfaces
- Creating copies of objects
- Aligning, combining, and splitting objects
- Analyzing geometry
- Shading models