lack of precision in Creo Elements / Direct Modeling Express?

[vgillespie]

Hi Folks,

I am using Creo Elements / Direct Modeling Express. I like it a lot in some ways. But I find that I am spending quite a bit of time repeatedly fixing things that seem out of place or not quite in the right position. Parts that I thought I had snapped properly into place when I later look closely are not really in the right place. Parts that I thought had been put in place with faces at right angles to other parts, turn out later, when I look closely, to not really be at right angles. I will sometimes use the "mate" and/or "align" functions to make the corrections. That works but I am spending much time correcting these types of things. I used to use AutoCAD and I don't remember this problem with that program (or at least it was less common, to the best of my recollection). Do other users see this problem with Creo Elements / Direct Modeling Express? Is there anything that can be done about this?

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[FriedhelmK]

Hi Vincent,

I can not confirm these inaccuracies as you describe

If the result of the work is not right, I caused it myself

But a source for inaccuracies can be be the measuring.

By default is measured with an accuracy of six decimal places.

This can be to inaccurate, if you are using the measured values for movement or modeling.

Try this.

File Settings System Measure and switch Distance and Angle from 6 in all.

One more question.

How big are your models? It is advised to go not greater than a sphere of radius 100 meters.

You can go bigger, but then inaccuracies may be caused

[vgillespie]

Hi Friedhelm,

Thank you for your response. I just adjusted those things. (I went to File->Settings->System->Measure -- right?) The settings for distance and angle were 6 and 1, respectively (I think that represents the number of decimal places shown). I changed both to "all." But I think that this will not affect the accuracy of the snap system. I almost always use the snap function to position things because that should be the most accurate. I only rarely move things by entering a displacement value (well sometimes I do that but usually only when I know the exact amount of the displacement and it is a simple number, like 1/16 of an inch [0.0625] or 1/32 of an inch [0.03125] or 90 degrees). So I think that changing this setting probably will not fix the problem.

[FriedhelmK]

Hi Vincent,

So I think that changing this setting probably will not fix the problem.

I think so too.

But it is almost impossible without more information to see where it comes to your problem.

If you would upload an example, we could try to check it out.

Maybe two .pkg

One before, one after the misalignment

Another question.

Do you use maybe imported parts?

[vgillespie]

Hi FriedhelmK,

In answer to your question about imported parts: I started this drawing by importing a bunch of parts from AutoDesk Inventor. These were parts from a drawing that I had made. I have occasionally (once or twice) added more parts from Inventor (again, parts which I drew previously). I drew all these parts in AutoCAD and then transferred them to Creo Elements / Direct Modeling Express after saving them in Inventor (as STEP or IGES files). Also, I have sometimes imported drawings or parts from other Creo Elements / Direct Modeling Express drawings that I made.

It is not so easy to give you a sample file showing the problem because it only seems to happen when I do not know that it happened until weeks or months later. I will be working with a part which I placed in a position a long time ago (or weeks ago or maybe sometimes less time ago) and suddenly I find that the edges are not square with other parts or that it is not really in the place it is supposed to be. I have not seen this happen directly right before my eyes at the time that the error occurs. I only have learned that it happened at later dates. I do not see any specific action that I do which causes this.

Regards,

Vincent

[vgillespie]

Hi FriedhelmK,

In my prior post in response to you I stated that it would be difficult for me to include an attachment file to enable you to see for yourself the problem that I was talking about relating to a lack of precision in Creo Elements / Direct Modeling Express. I also posted a comment in response to Peter Kehoe's post saying that maybe the problem was simply that I wasn't being careful enough with the snapping functions, as he had suggested. But I have continued to see this problem as I worked.

I think I have been able to create a model so that you can reproduce and see the phenomenon for yourself. I have included a file as an attachment which will illustrate.

The file has a single part. The part has a base shape, you might say, of a flat rectangular bar. On one end there is a something like a three dimensional rectangle with four chamfered edges. On the other end there is a shape on one side of the flat rectangular bar only which has two prism shapes which extend towards the center of mass of the part. One of the prism shapes is 0.25 inches longer than the other. (For units I am using inches with a factor of 1 [as these parameters are set in the Systems Settings dialogue box under the Units tab [File->Settings->System->Units].)

Here is the problem I found: When I push the face of the longer prism shape so that it snaps to being parallel to the face of the other prism shape it does not come out exactly parallel.

Now, first, if you like, you can verify that the rectangular faces of the two prism shaped extensions are square (perpendicular to the lengthwise direction of the flat rectangular base shape) by measuring the distance between any two points on any of the three edges of the rectangular face of either of the two prism shaped extensions. (Measure from point to point and then choose "direction" and select a direction parallel to the length of the overall part. These distance measurements should all be zero.)

Ok. Next click on the face of the longer prism shaped extension so that the 3D Move image for moving a face pops up. Next grab an arrow end of the 3D Move image and push the face parallel to the length of the rectangular bar base shape in the direction towards making that face parallel with the face of the other (shorter) rectangular prism shape. Before this action is complete, without releasing the mouse button, move the pointer so that it snaps to the other rectangular face on the shorter prism shape; make sure that this other triangular face gets highlighted so that you are basically snapping one face to the position of the other one. (I am not sure if "snapping" is exactly the right word here.) Click the green circle with the check mark in it to accept the change.

Ok. So now the two faces should be parallel. They should be EXACTLY parallel and they should be both located at the exact same position along the length of the rectangular bar which forms the base shape of this part. But when I do it (when I follow the above steps) the two faces are NOT exactly in the same place.

When I measure the edge to edge distance between the edge created by the hypotenuse of the triangular face of what was the shorter prism shape (which should not have been altered by you if you followed the above directions) and the nearest parallel edge towards the other end of the part the distance should read out as 1.09375. (That is the reading I get.) But when I read out the analogous measurement for the edge created by the hypotenuse of the triangular face of the other prism shape, the one we just adjusted (if you followed the above directions), the reading I get is 1.0937501444. (i.e., the distance between that edge and the nearest parallel edge towards the other end of the part comes out to that long number.) Those two readings should not be different but they are. (Make sure the displayed number of decimal places for distance is set to "all." [File->Settings->System->Measure])

So this is an example of what I am speaking about regarding an apparent lack of precision with the snapping system in Creo Elements / Direct Modeling Express. Note that this is just one example of how this problem occurs. I think it also occurs with various other functions in this program (besides pushing a face to being parallel with another face and snapping to the position of the other face).

So let me know if you can reproduce the problem by following my instructions. If you can, then you can see the problem I have been speaking of. If you can't then something else might be going on. If you can replicate the problem, what are your thoughts? A CAD program should not have inaccuracies like this.

Thanks in advance,

Vincent

[FriedhelmK]

Hi Vincent

(Un)Fortunately I can not reproduce the described behavior.

The only conspicuous is the resolution of the part, but that should not be a Problem.

[vgillespie]

Hi FriedhelmK,

Well thanks for trying. I don't know why you would not get the error in doing the same thing as me. It appears from the pictures you provided that you properly followed the steps I described.

I changed the part resolution to a lower value, to 1.0E-2mm from 6.0E-3mm, but the error was still there when I repeated the process.

Are you using the free version of the program (as I am) or the paid version?

Vincent

[vgillespie]

Hi FriedhelmK,

Did you set the number of decimal places for measuring distance to "all?" That makes a difference here. I just set the number of decimal places to six, which is what the default setting was as I recall, and now I get the same readings as you. Please check what setting you had here and if it was set to six or less, then please try again with this setting set to "all."

Regards,

Vincent

[FriedhelmK]

Hi Vincent

> 6.0E-3mm

Low resolutions are typical for imported parts.

To reduce the resolution brings anything.

This makes the part only less accurate.

> Are you using the free version of the program (as I am) or the paid version?

Have both available. For the test I used Express 6 the free version.

>Did you set the number of decimal places for measuring distance to "all?"

Yes, this is my personal default. Who gave you the tip ?

[vgillespie]

Hi FriedhelmK,

I know it was you who gave me the tip. But I thought that nonetheless you might have left the value for the number of decimal places shown at the default setting of six. As I wrote earlier, when I set that value to six I got the exact same output values as you, so that would have explained the difference in your output and mine. But if you say your value is set to "all" I will take your word for it.

But this is frustrating. I have a second computer with Creo Elements / Direct Modeling Express installed on it and I repeated the process there and got the same values as what I get on my main computer. What am I supposed to think here? This may be some kind of bug in the program.

Vincent

[PeterKehoe]

I have seen users have issues similar to what you have described; usually the reason has to do with not being careful with catching, snapping, and selection. Just because the cursor is over a particular point, vertex, edge, or face when you click with the left mouse button, it doesn't always mean that it will catch to that point, snap to or select that vertex, edge, or face. The software will almost always highlight points, vertices, edges, and faces if they are being caught, snapped to, or selected; so watch for that as you are modeling.

Usually I hear the opposite problem with Creo Elements/Direct Modeling-it is too precise! I will seen lots of parts that are designed in other CAD software (as solids) that cannot be imported into Creo Elements/Direct Modeling as solids because the model was not modeled precisely enough for Creo Elements/Direct Modeling. Some software can assume that two faces are connected even though there is a gap of 0.1mm, but Creo Elements/Direct Modeling requires gaps to be at most 0.01mm.

[vgillespie]

Hi Peter,

Thank you for your comment/post. It is possible that you are correct about what has been happening (although I am not certain). Maybe I just have not been being careful enough in snapping to objects and points. I will try to pay attention to that and see if that is the problem.

Thanks,

Vincent

[vgillespie]

Hi Peter,

In my prior post in response to you I said that maybe the problem I have been speaking of (regarding a possible lack of precision in Creo Elements / Direct Modeling Express) amounted to just me not being careful enough in snapping to objects and parts, as you suggested. However, the problem has persisted. I think I have found a way to reproduce the problem and I have attached a file so that others can reproduce it and see for themselves what I am talking about. See my most recent post in response to FriedhelmK, above; and see the attached file I included with that post.

Regards,

Vincent