"Non planar face" that is actually quite flat problem...

[aptivaboy]

Good evening all,

I'm hoping that someone can help with this. I frequently design parts in Moment of Inspiration and then import them into Alibre at STEP, SAT, of 3DM files. They tend to import just fine, no missing faces or anything like that. However, when I try to extrude off of or into the part, I almost always get a non planar face error. The program can't seem to accept that a flat piece or a flat part of a piece is... Well, totally and completely flat. As a result, I can't extrude boss or extrude cut into the new part. Sweeps? Forget about it. I've tried using CAD Exchanger with no success concerning these issues; the non planar face issue persists. I've tried importing in different file formats and with different import settings, and the result is the same. Any ideas on how to correct this so I can actually edit the parts in question? They are pylons for a Trek starship for 3D printing.

Many thanks,

Bob

 

[JST]

How do you KNOW they are flat?

MOI says so?

Moi may not know.

Alibre uses a lot of decimal places internally, 19, if my memory serves me correctly. If the Alibre internal description of the part has ANY tiny deviations from flat, it may urp and refuse to consider that part flat, even though the NIST would write a paper on detecting errors at that virtually undetectable level.

Basically, if the Alibre description is not flat, then Alibre says it is not flat. You will never see why that determination is made. All you see is 6 places to the right of the decimal. But, I regularly see parts which measure (in 6 decimal places) as equal, yet ALibre says they are far enough "off" in length that they cannot line up. Obviously the error is somewhere "further down" than 6 decimal places.

I do not know at what decimal place the determination is made. I just know it is clearly beyond 6 decimal places. I'd LIKE to know that, although it may not make any difference in terms of using the program.

While I totally understand the reasons, it is worth remembering that the error represented is below the level of an error of 1mm in 1 km. That level of error, or more, exists in the majority of normal "precision" parts which fit together and work perfectly. Your automobile, if you have one, is full of such parts, and the errors do not seem to cause an issue.

So it is open to question as to just why we must continue to have mates, alignments and fits rejected due to errors which would, in the actual physical part being represented, be down in the area of a wavelength of light.

It is even more ridiculous if, as at least a couple of us do, we are working with parts which might be measured for cutting and drilling etc, to as fine a division as 1/32" (~~ 0,8 mm). And furthermore, measured with a tape measure, by eye.

 

[aptivaboy]

They're flat solids in MOI. So, a rectangle, or a square. In obj, step, sat, and 3dm, they show up as flat surfaces. This is also true when I import these designs into Fusion or my personal copy of Netfabb, for example. So yes, they're flat.

If its a decimal point issue, as you suggest, then is there a way to have the Alibre import options understand that a surface within point 000 whatever should be interpreted as a flat surface?

Many thanks,

Bob

 

[DavidJ]

A simple example complete with file(s) would let people here experiment and perhaps offer a solution.

You mention both solids and surfaces in the same sentence - are the models from MoI solid models or surface models? Remember that Alibre is primarily a solids modeller.

 

[bigseb]

You mention both solids and surfaces in the same sentence - are the models from MoI solid models or surface models? Remember that Alibre is primarily a solids modeller.

I reckon he means the 'faces' of a solid. And regardless of whether we're talking about solids or surfaces, planar means planar. Moi planar faces have zero height. Easy to check in Moi.

 

[JST]

I reckon he means the 'faces' of a solid. And regardless of whether we're talking about solids or surfaces, planar means planar. Moi planar faces have zero height. Easy to check in Moi.

Something with zero height in Moi may not show up in Alibre with zero height if the number of decimal places does not match.

For that matter, Alibre itself sees a difference in parts which measure exact;y the same (as far as Alibre lets you see).

Alibre is fussy about dimensions at or below the wavelength of light, apparently, so it would be easy for some mathematical artifact of a model conversion to pop up and cause an issue.

No, Alibre seems to have no ability to be set to a "degree of accuracy".

I have asked for that before, but have been screamed down by people howling that I want to dumb down Alibre or make it less accurate, or some such thing. I've never been able to determine exactly what they are trying to say. Whatever it is, they are wrong.

It makes sense to me that one may want to use the results of all the calculations to a sensible number of decimal places..... Machining is commonly accurate to 0.0001 inch ( 0.0025mm), so there is little point to comparisons way down at 0.000000001 inch.

Alibre should do all the math as normal, and just use the results at a truncated choosable accuracy. So far the howling mob is winning this one, though.

 

[bigseb]

Something with zero height in Moi may not show up in Alibre with zero height if the number of decimal places does not match.

For that matter, Alibre itself sees a difference in parts which measure exact;y the same (as far as Alibre lets you see).

Alibre is fussy about dimensions at or below the wavelength of light, apparently, so it would be easy for some mathematical artifact of a model conversion to pop up and cause an issue.

No, Alibre seems to have no ability to be set to a "degree of accuracy".

I have asked for that before, but have been screamed down by people howling that I want to dumb down Alibre or make it less accurate, or some such thing. I've never been able to determine exactly what they are trying to say. Whatever it is, they are wrong.

It makes sense to me that one may want to use the results of all the calculations to a sensible number of decimal places..... Machining is commonly accurate to 0.0001 inch ( 0.0025mm), so there is little point to comparisons way down at 0.000000001 inch.

Alibre should do all the math as normal, and just use the results at a truncated choosable accuracy. So far the howling mob is winning this one, though.

Actually I agree with you on this. I recall Creo had this too. Really handy as many imported files had faults.

 

[DavidJ]

Sometimes apparently OK files actually contain faults - adjusting import options often resolves. Not obvious if this is the OPs case or not, but worth eliminating.

 

[Max]

I'm hoping that someone can help with this. I frequently design parts in Moment of Inspiration and then import them into Alibre at STEP, SAT, of 3DM files. They tend to import just fine, no missing faces or anything like that. However, when I try to extrude off of or into the part, I almost always get a non planar face error. The program can't seem to accept that a flat piece or a flat part of a piece is... Well, totally and completely flat. As a result, I can't extrude boss or extrude cut into the new part. Sweeps? Forget about it. I've tried using CAD Exchanger with no success concerning these issues; the non planar face issue persists. I've tried importing in different file formats and with different import settings, and the result is the same. Any ideas on how to correct this so I can actually edit the parts in question? They are pylons for a Trek starship for 3D printing.

If I understand you correctly, it's not that you can't extrude boss or cut etc. but rather that you are unable to select the faces in order to place a new sketch onto them - the resulting downstream feature doesn't matter if you can't create a sketch in the first place.

Please upload an example of a MoI file and the resulting file you are trying to import into Alibre Design.

For that matter, Alibre itself sees a difference in parts which measure exact;y the same (as far as Alibre lets you see).

Alibre is fussy about dimensions at or below the wavelength of light, apparently, so it would be easy for some mathematical artifact of a model conversion to pop up and cause an issue.

No, Alibre seems to have no ability to be set to a "degree of accuracy".

ACIS will treat “distances” less than 0.000001 as zero length. You can type in a value of 33.34365325. ACIS will accept this, no problem. If something else is 33.34365350, then, the two will be considered equal length. Also, if two things are less than 0.000001 units apart from each other, they are considered coincident. This is primarily in part modeling.

3D DCM, which handles 3D constraints, has similar but slightly different numbers. We are primarily at the mercy of the technology providers on what is possible here - it's not just a matter of "setting a degree of accuracy".

That being said, the import pipeline is different and has a few different considerations. There is no single global easy setting that a) exists nor b) is possible that will magically make everything work within some tolerance in all uses.

I do not believe it's a tolerance issue in OP's thread. People import MoI models all the time, and I've never heard of anyone having this issue. Leads me to believe something is amiss.

 

[JST]

ACIS will treat “distances” less than 0.000001 as zero length. You can type in a value of 33.34365325. ACIS will accept this, no problem. If something else is 33.34365350, then, the two will be considered equal length. Also, if two things are less than 0.000001 units apart from each other, they are considered coincident. This is primarily in part modeling.

...............................................

Regrettably, I think this is not actually true universally. Maybe it is supposed to be true, but it appears that is not true in all cases.

I have MANY times run into parts which are the same distance between hole centerlines as measured to 6 places by Alibre, and which are KNOWN to have the holes parallel, but which fail alignment. You can measure them and Alibre claims they are the same distance apart, zero angle between centerlines, but they will not align.

Next time, if I remember and have time, maybe I should send it in.

 

[Max]

That's a constraint issue which is a 3D DCM issue which has similar but different tolerances than acis. Do send an example though if anything for clarity.

 

[JST]

OK.

If it is any help before I find an example, the issue often comes up on "computed" dimensions, such as diagonal parts in bolted truss bracing. The diagonal can come out to ANY dimension as far as numbers after the decimal. The number may be a repeating decimal that goes out infinitely and so is not directly bound by 6 place input limits.

You may be able to manufacture your own examples without waiting for my input. But I will send in what I find.

I was under the impression (from earlier discussions of this issue) that Alibre works to 19 places (presumably 19 places floating point) internally. I was under the further impression from those discussions that the decisions were made at a number of decimal places beyond the 6 visible ones.

Regardless, in many cases any accuracy past 0.0001" or even 0.001" is irrelevant, and it would be, as the other poster mentioned, quite useful to be able to use dimensions rounded to some selected number of places in the decision process for constraints etc.

The problem is really quite irritating when some workaround has to be used to fix a part in place due to a failure of the constraint. Without doing a bunch of "side-track" calculations and/or investigation, one cannot be certain that the problem is in Alibre, as opposed to some "user error" such as a non-parallel bore, etc.

 

[aptivaboy]

"If I understand you correctly, it's not that you can't extrude boss or cut etc. but rather that you are unable to select the faces in order to place a new sketch onto them - the resulting downstream feature doesn't matter if you can't create a sketch in the first place."

Yes and no. Sometimes I can select the face but when I try to extrude, Alibre won't do so. At other times, Alibre won't allow me to select a face, at all. It seems to often work when the shape or object created in MOI is really simple. However, the more complex the design becomes, it becomes far less likely that I'll be able to edit or manipulate the design in Alibre.

Thank you everyone for your suggestions and offers of help. Here is the design, a pylon for a Star Trek starship that I've been asked to design. Literally everything else works except the pylon. I've highlighted one of the faces that I've been trying to extrude, or along which sweep. In MOI and Netfabb, it shows as a flat surface, but as a non-planar face in Alibre. There are others, but I thought this large surface would be the easiest to point out.

Many thanks,

Bob

 

[aptivaboy]

PS. Apologies if I'm missing something obvious. I'm self taught and I do CAD work on the side as a diversion and for a few extra bucks. My real job is teaching high school history and economics, so its likely that the mistake is simply something rather obvious and dumb on my end. Thank you again!

Bob

 

[HaroldL]

Alibre will let you select surfaces if they are flat, curved or whatever. I was going to suggest creating a 3 point plane by selecting 3 points from the surface you want to work on.
I did that then added a couple of extrude cuts (holes) to see how they interacted with the surface. Well instead of making holes they mad bosses. I have no idea why they did that. This is something that Support may want to have a look at.

Here's how I added the 3 point plane:


And here's what the part looks like after I added two extrude cuts: The second one is on Plane <4> which is offset from the first plane I added, Plane<2>


If someone can explain that please do so.

 

[Max]

Perform a check part. That happens typically when there is bad geometry. If that's the case, "all bets are off" with Acis.

 

[bigseb]

I opened your part in Moi. There were several errors that popped up beyond the planar thing.

The planar face: It isn't (according to Moi) a planar face. If it were I should be able to delete it, select the boundary edges, hit planar and the surface should be reconstructed. It wasn't so that means the original surface wasn't planar.

The were a lot of errors that would prevent successful import into AD: a lot of open edges ( I recommend running the 'open edges' script regularly during design to detect and eliminate them) and a fair bit of 3D geometry that is either wrong or superfluous. And your main model isn't a solid. That cause big headaches with AD.

Fortunately Moi is pretty good at this sort of thing so you'll need to spend time either fixing it or (my first choice) remodelling it accurately.

I didn't import your attached 3dm file in AD because of the faults in the file. This is not (at least not yet ) Alibre's fault.