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 From:  steve (STEEVE)
2047.1 
Hi

I have a model in both stp and igs formats which is to be used in a prototyping machine.

If I import say the igs file into Moi, make some changes and export back to igs, will the data still be of the same type.

With my limited understanding, what I'm thinking is:

Alibre or Solidworks create solids as opposed to surfaces. Is Moi also a solid modeler? And when it writes a file in igs format will the objects still be solids?

Hope my question is clear.
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 From:  Michael Gibson
2047.2 In reply to 2047.1 
Hi steve,

> Alibre or Solidworks create solids as opposed to surfaces.

This is true, but the way that solid modeling programs define a solid is by a set of surfaces that touch each other along common edges to form a "watertight" skin that encloses a volume.

So surfaces are one of the elements that make up a solid model.


> Is Moi also a solid modeler?

Yes - MoI can work on both surfaces and solids, which is now the most common method. Long ago "solid modelers" like SolidWorks would focus their user interface on only working with solids, but these days they typically also allow working on surfaces individually or solids in combination with one another.

That's also the way that MoI works - in MoI you can work on solids which are made up of surfaces joined at common edges, or you can also work on an individual surface level and then use the Edit/Join command to glue them together to make a solid.


There is a lot of misconception out there that the solids defined in a "solid modeler" are somehow "more solid" than ones defined by a set of surfaces. But actually the system used by 99.9% of solid modelers is the "boundary representation" method which is using surfaces that are connected together at edges to make a solid volume.


> And when it writes a file in igs format will the objects still be solids?

Essentially yes. But the details are a bit complex.

Typically igs format does not contain full solids but instead all the surfaces that make up the solid, and then programs that want to work on it as a solid will join those together to make a solid. So yes, the final result at the end will be a solid.

STEP format is similar, but contains not just surfaces but also the connection information in between them, so that stores a complete solid in a STEP file. It can be good to use this method instead of IGES because it means there is one less step for a receiving program to do - it does not have to go through the process of figuring out which edges to glue together like has to be done with IGES format transfers.

So I would probably recommend trying STEP, but like I mentioned, the end result of the IGES transfer will also be a solid at the end as well.


The major differences between file formats are actually not so much whether they contain surface or solids in them, but rather if they contain "NURBS surfaces", or "Polygons" in them.

Both STEP and IGES are NURBS formats, which mean they transfer true surface data using mathematically curved surface sheets for the definition of the objects. Like a sphere is a true sphere surface.

Polygon-based file formats are what are a lot different, where they store no smooth surface data at all and instead have a large number of small planar facets to make up an object. So with a polygon format a sphere is not defined as a sphere but instead there are a lot of flat pieces arranged to be close in shape to a sphere.

Typically to transfer data to another CAD program you want to send that as NURBS because whether it is NURBS surfaces or NURBS solids, the true smooth surfaces will go over.

However, polygon formats are used pretty frequently for different kinds of CAM tasks as well, like STL is a polygon format which is typically used for sending data more directly to a rapid prototyping machine.


> If I import say the igs file into Moi, make some changes and export
> back to igs, will the data still be of the same type.

Yes - the data in the igs file will still be the same kind of NURBS surface data.


Let me know if you need any more clarification on this stuff, I know there are a lot of details here.


- Michael
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 From:  Michael T. (MICTU_UTCIM)
2047.3 
Hi Steve and Michael G.,

Great break down Michael on the STEP vs. IGES!

Steve, Alibre does not allow surface manipulation like other so-called solid modellers like SolidWorks. When you import IGES or STEP it will try to stitch the surfaces together as a "solid" or you can turn that off for individual surfaces. Also, Alibre will direct import 3dm files.

What I do is to use MoI 3D to create most of freeform surface stuff, bring it into Alibre as reference geometry. I then us it to extrude to etc., and add things like draft to it in Alibre. Actually adding draft to my part(s) and making 2D drawing layouts is pretty much all I use Alibre for currently. Everything else I have been doing in MoI 3D.

Also Alibre and Pro-E import STEP and IGES to specific tolerances. This can make them show up as surface models instead of solid models. You can change the model tolerances to generate solids from surfaces most of the time, but it can effect the "down-the-road" usage of the model such as with manufacturing software (CAM programs etc.)

Michael T.
Michael Tuttle a.k.a. mictu http://www.coroflot.com/fish317537
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 From:  steve (STEEVE)
2047.4 In reply to 2047.2 
Hi Michael G

Thanks for that clarification. I will keep it on hand so that I can refer to it from time to time as I digest the subtleties!

I can see now that a cube for example would only truly be "solid" if its data referenced every point on its surfaces AND also every point within the body of the cube!!

I can also see now that the distinction I was really grappling with was between polygon modelers and NURBS modelers. Very clear now. Thanks.

(The transformation from nurbs to polygon can be clearly witnessed of course, when Moi does an export to obj etc.)


And Michael T

Really good news that Moi models can so easily be imported into Alibre for 2d layouts etc.

I will start experimenting with switching stitching on and off and also adjusting import tolerances for igs and step files.

steve
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 From:  Michael Gibson
2047.5 In reply to 2047.4 
Hi Steve, I'm glad the STEP/IGES/Polygon/NURBS explanation helped!

Please let me know if you need more information or if anything was not clear.

Yes, I think that the really major difference to be aware of is between formats that hold Polygon data and ones that contain NURBS data - that is actually a much larger fundamental difference in the way that the object data is structured.


> I can see now that a cube for example would only truly be
> "solid" if its data referenced every point on its surfaces AND
> also every point within the body of the cube!!

From a purely mathematical standpoint, any method that partitions space into different regions with an "inside" and and "outside" portion is a solid.

A lot of people seem to get the feeling that a solid defined by a connected skin of surfaces is not a "real" solid, but mathematically it is just as real as any other method.

In older systems there used to be a different method used called "Constructive Solid Geometry", or CSG - where solids were defined by a kind of tree structure that combined basic elements like spheres, boxes, and cylinders. That method is not very good for objects that have more sculptured surfaces that are not one of those simple primitive shapes. So it has been a while now that nearly all solid modeling systems use the "boundary representation" method instead which focuses on that connected skin of surfaces. It is a lot more flexible.

Anyway, that is probably more detail than necessary - just to sum it up - MoI uses the same method of representing a solid as those "solid modeling" systems, so a solid that you create in MoI can be exported as a solid to Alibre or SolidWorks (if you use one of those NURBS-based formats). MoI just does not force you to only work with solids, you can work with surfaces as well.

- Michael
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 From:  steve (STEEVE)
2047.6 In reply to 2047.5 
Thanks Michael

Getting clearer all the time. I think, the more one understands the inner workings of the software, the better modeler one can be.

Part of my background is in crystallography so my concept of a solid is tainted (?) with ideas of a space bounded by connected smooth faces and packed full of atoms!
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 From:  steve (STEEVE)
2047.7 In reply to 2047.3 
Hi Michael T.

I have noticed that when exporting from MoI to Alibre (as stp), Alibre does not appear to recognise circles, I mean for the purpose of dimensioning radii. Is this true? Perhaps they are just seen as curves as opposed to circles??

Steve
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 From:  Michael T. (MICTU_UTCIM)
2047.8 In reply to 2047.7 
Hi Steve,

I think that's right. I'll check it out. I think any indirect file transfer type breaks entities down that way, i.e. curves versus circles, lines etc.

I'll let you know if I find anything else out.

Michael T.
Michael Tuttle a.k.a. mictu http://www.coroflot.com/fish317537
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 From:  Michael Gibson
2047.9 In reply to 2047.7 
Hi Steve - yeah it looks like currently when MoI writes the step file it writes the circle as a spline curve type.

Often times if it is important for a receiving application to handle circles in a special way, they can check to see if the spline curve is the standard kind of spline curve that makes a circle, and flag it as being a circle primitive at that point. But it sounds like Alibre is not doing that kind of shape detection in it.

Could you please post an example file with a circle in it saved out from Alibre so I can see how it prefers to have circles structured in the STEP file? I'm not sure yet if I will be able to do much about that but it may be possible for me to adjust that.

- Michael
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 From:  Michael Gibson
2047.10 In reply to 2047.7 
Hi Steve, can you give the attached file a try in Alibre and see if it can recognize it any better or not?

This one is still written as a spline curve, however it has a flag set on it that tags it as a "CIRCULAR_ARC" type form.

If that seems to work better and helps Alibre to recognize it as a circle, that could be an easy adjustment for me to make.

However, it may not do the trick because Alibre may want a full "circle primitive" definition instead. Currently the library that I'm using for exporting to STEP format does not handle that so that one would take a while longer.

- Michael
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 From:  steve (STEEVE)
2047.11 In reply to 2047.9 
Thanks for that Michael T. It does seem to be the case.

Michael G, I could not get Alibre to see that test_circle.stp file at all. I tried various import options but nothing showed up on the display.

And here is an stp file with lots of circles, saved out of Alibre.

Good luck!

Steve
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 From:  Michael Gibson
2047.12 In reply to 2047.11 
Hi Steve,

> Michael G, I could not get Alibre to see that test_circle.stp file
> at all. I tried various import options but nothing showed up
> on the display.

That one was a circle curve, when you mentioned circles previously I was thinking you were talking about a circle curve like you would draw with the Draw curve / Circles / Center command in MoI.

But from your example it looks like you are more interested in circles that are edges of solids.

For reading the curve into Alibre, I'm not sure how that works exactly, but you may need to do some command like "import sketch" or something like that to read curve data in (rather than solids).


Just one other test to make sure I understand what is happening, do you also get the same problem with the additional cylinder_test.stp attached here?

And can you create a minimal cylinder object like that (with nothing else) in Alibre and post that here for me to compare with?

I think that the STEP library that I am using does not have this function in it to write out circle entities instead of spline entities, so there may not be anything that I can do about that immediately. But I have sent an e-mail to the author of the step library to ask if he can include that function in the future.

Thanks,
- Michael
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 From:  Michael T. (MICTU_UTCIM)
2047.13 In reply to 2047.12 
Hi Steve and Michael G.

Here is a PDF file from Alibre's design guide regarding import and export for Alibre Design 10.

Hope it helps. You can download Alibre's complete user guides from the web site.

I don't want to clutter up Michael G.s forum :-)

Michael T.
Michael Tuttle a.k.a. mictu http://www.coroflot.com/fish317537

EDITED: 4 Nov 2010 by MICTU_UTCIM

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 From:  Anis
2047.14 In reply to 2047.13 
Hi All....

Below are the test result :
Image Attachments:
Size: 21.7 KB, Downloaded: 51 times, Dimensions: 628x471px
Size: 29.5 KB, Downloaded: 38 times, Dimensions: 733x559px
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 From:  Michael Gibson
2047.15 In reply to 2047.13 
Hi Michael T, thanks for posting that - unfortunately it doesn't seem to cover technical details like what kinds of entities are recognized as circles...

When you import into Alibre, there are a variety of import options including things like "Heal solid", you might try enabling those options to see if that will make Alibre recognize the circles or not.

- Michael
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 From:  steve (STEEVE)
2047.16 In reply to 2047.15 
Hi All

Michael still no go with your cylinder_test file.

Just to clarify, my interest here is in being able to take a model from MoI into Alibre and use the "create drawing" feature with dimensioning, annotation, etc.

Anis' result is the same as mine. In Alibre's drawing window, the dimensioning utility should recognise the circle and dimension it as it does in SoidWorks. But it doesn't happen. It is the same in the 3d window of Alibre...one should be able to dimension circles there too. Lines are no problem .

I tried quite a few import options without success.

Here is a cylinder object made in Alibre. There are options to export 3 diff types of stp file, I just picked one.

In MoI when one makes a cylinder by extrusion, it is customary to delete the original circle, whereas in Alibre the original circle remains as an editable object. So I also tried extruding a cylinder in MoI and not deleting the circle. But Alibre would have none of that and refused to even open the file.

I will keep playing around as and when I have time. But, Michael, please don't loose sleep over it!!
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 From:  Michael Gibson
2047.17 In reply to 2047.16 
Hi Steve, I've talked to the author of the STEP library that I'm using and he says that it should not be very difficult to enable an option to write out circles as "circle" entities instead of as more generic "spline" entities. I'm not exactly sure when that will be complete but it is somewhere in the queue now.

However, he also mentioned that there is an option for this that can be enabled currently for IGES export.

I've attached a test file that sets that option.

Can you please see if this cylinder_test.igs file works better for dimensioning in Alibre?

If so then that is one thing that will be easy for me to set up for the next v2 beta, and that could be an option to get it done until the STEP equivalent is ready.


> So I also tried extruding a cylinder in MoI and not deleting the
> circle. But Alibre would have none of that and refused to even
> open the file.

That seems to be a particular quirk of Alibre - other programs seem to deal with curves present in a STEP file as well as solids... If they don't want to read in "stand alone" type curve data, they should just skip them instead of failing the whole import. I think that one is something that they will need to fix up in Alibre.

- Michael
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 From:  steve (STEEVE)
2047.18 In reply to 2047.17 
OK great news, Michael

With your igs test file it is possible to dimension circles in 2d drawing mode.

Also, in 3d mode, Alibre recognizes the cylinder edge as a circle.

So that seems like the solution. I don't think it really matters to many people whether they export from MoI as stp or igs.

Thanks:)


EDITED: 14 Oct 2008 by STEEVE

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 From:  Michael Gibson
2047.19 In reply to 2047.18 
Hi Steve, that is great news!

So for the next beta the IGES export will be tuned up to do this by default - it will export lines, circles, and arcs as those specific kinds of entities in the IGES file instead of as generic splines.

If it causes any problems (not likely but just in case), it will be possible to turn that off and return to the old behavior by editing an entry in the moi.ini file:
[NURBS Export]
WriteAnalytics=y

Later on at some point this will happen for STEP as well.

- Michael
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 From:  steve (STEEVE)
2047.20 In reply to 2047.19 
Thanks again Michael, that will certainly be a valuable feature!
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