Can I omit Basic dimensions?

[bc23]

My drawings are created (& read) in 3D format (MBD). I read an article that basic dimensions can be omitted in MBD; the main advantage being shorter dimensioning time. I only know 2 situations where basic dimensions are required:
(a) for designer's information (to track changes or apply in calculations etc),
(b) for CMM application

Does anyone know of other conditions which would utilize basic dimensions?
Does ASME have any rules regarding omission of basic dimensions?
Note that since I have provided the drawing in MBD, any unknown dimension can be checked from the 3D.
Hope to understand what I'm missing.

 

[3DDave]

MBD isn't new. It's 30 years old and fails to be a substantial improvement at a large extra cost. It's a follow on to the "lights out factory" the DoD wanted to build where vast piles of raw materials, robots, and CNC equipment would be issued orders for on-demand parts which would be automatically produced without any people being involved. That's what STEP was intended to solve.

No doubt there are organizations that are sufficiently vertically integrated that they can use it, but I think they would have difficulty proving that it offers a significant cost savings except by focusing on the cost of plotters and paper.

The original basis for assuming it was cheaper was the supposition that using MBD would eliminate the need to create drawings, but the cost of drawings is mainly the cost of decisions about datum references, geometric characteristic tolerances, base topology, material, finish, and the analysis of the suitability for the function, plus all the decisions about the function. Making projected images onto 2D and displaying the requirements is a minor part of that.

One claim for savings is from those companies that fired their drafting group. In those companies the engineers would make rough models and somehow record many of the requirements and toss the models over the wall for drafters to make all the final dimensioning and tolerancing decisions as they made drawings. With no drawings, there's no need for the drafters and their salaries. Instead that effort is pushed back upstream to the engineers, who didn't make drawings and tend to do a bad job of it. The other cost saving is eliminating checkers, so when the task is back to engineering there's no one to be manage consistency. The software sales teams offer MBD as a solution.

"It is a parametric model" is not what comes out of STEP. The MBD workflow doesn't preserve the parametric model , unless you are on a completely integrated system down to QA/QC and the shop floor.

 

[SDETERS]

Why would a CMM program need Basic Dimensions? Are you using the 3D model to create your CMM program? How do you deal with the Basic dimensions that are rounded? For example, the model is 4.5654646 and the basic dimension says 4.565?

I am being curious at this point, and maybe a bit off topic.

Thanks.

 

[3DDave]

"For example, the model is 4.5654646 and the basic dimension says 4.565?"

That's a paddlin'.

 

[SDETERS]

This happens on models, especially die castings, or forgings, which have significant draft.

 

[greenimi]

MBD isn't new. It's 30 years old and fails to be a substantial improvement at a large extra cost. It's a follow on to the "lights out factory" the DoD wanted to build where vast piles of raw materials, robots, and CNC equipment would be issued orders for on-demand parts which would be automatically produced without any people being involved. That's what STEP was intended to solve.

No doubt there are organizations that are sufficiently vertically integrated that they can use it, but I think they would have difficulty proving that it offers a significant cost savings except by focusing on the cost of plotters and paper.

The original basis for assuming it was cheaper was the supposition that using MBD would eliminate the need to create drawings, but the cost of drawings is mainly the cost of decisions about datum references, geometric characteristic tolerances, base topology, material, finish, and the analysis of the suitability for the function, plus all the decisions about the function. Making projected images onto 2D and displaying the requirements is a minor part of that.

One claim for savings is from those companies that fired their drafting group. In those companies the engineers would make rough models and somehow record many of the requirements and toss the models over the wall for drafters to make all the final dimensioning and tolerancing decisions as they made drawings. With no drawings, there's no need for the drafters and their salaries. Instead that effort is pushed back upstream to the engineers, who didn't make drawings and tend to do a bad job of it. The other cost saving is eliminating checkers, so when the task is back to engineering there's no one to be manage consistency. The software sales teams offer MBD as a solution.

"It is a parametric model" is not what comes out of STEP. The MBD workflow doesn't preserve the parametric model , unless you are on a completely integrated system down to QA/QC and the shop floor.

3DDave,
I agree with you assessment. I just want to add that the transition to MBD will require a significant investment. I want to underline: significant.
The ROI is low, very low.
Just for selling software and consulting time with companies that will push other software / changing entirely native system to adapt theirs with no obvious benefits....
How can I say, I was fun of it….not anymore. Everything is fine until you get to the details of it and see that does not work….unless you pump another $$$$ …upgrades every 6-12 months, yearly maintenance costs.

We had this discussion before, this thread is just one of many examples

"I hate MBD". Why? - Drafting Standards, GD&T & Tolerance Analysis

"I hate MBD". This statement was made in another discussion about UOS all over profile application. As I have been pretty deeply involved in MBD for some time (drawing creation, proper CMM consumption of PMI characteristics, etc.), and personally see huge advantages of this methodology over...

www.eng-tips.com

 

[3DDave]

I would have to see an example to understand the difficulty of matching the model to a desired dimension because of draft.

The models I built drove the model geometry from the desired dimensions; there was no difference in the values.

 

[drawoh]

I have done testing this past year with MBD. I have sent 3D PDF's with STEP 242 files. I found out majority of our vendors don't want it. They are used to 2D dwgs, they still want PDF's of the 2D dwgs along with STEP files.
If I try to force the issue, and train them how to use it, they say the price will go up. Weird.

Ctopher,

The STEP 242 models provide dimension information. What do the 3D PDFs do? If I have a drawing with dimensions, I can measure with scales, calipers, and micrometers. I can set up gauge blocks and dial indicators on the granite block, and I can design tooling and get someone to check my drawings.

I am not familiar with coordinate measuring machines (CMMs). Are they easy to program to work with a 3D model?

 

[ctopher]

drawoh,
A 3D PDF has additional notes, material/finish callout, revision block, etc. It also shows the different views of the MBD model, and has STEP 242 attached. It can be used by planners/buyers/sales for quoting.
It's more of a document that can be filed away for future reference.
The STEP 242 is for mfg and insp.

 

[greenimi]

The STEP 242 is for mfg and insp.

Yes, only if those parties can use it (the STEP242) at its full capacity.
I found how that the vasty majority of OUR supplier do not care about the PMI's in the STEP242 at all.
They cannot use them anyway. Waste of time.
Actually is good and usefull in order to create the 3Dpdf. But if the vendor does not like the 3DPDF and the fancy way to see the PMI's is still doubtfull of a good value added.

 

[ctopher]

Yes, only if those parties can use it (the STEP242) at its full capacity.
I found how that the vasty majority of OUR supplier do not care about the PMI's in the STEP242 at all.
They cannot use them anyway. Waste of time.
Actually is good and usefull in order to create the 3Dpdf. But if the vendor does not like the 3DPDF and the fancy way to see the PMI's is still doubtfull of a good value added.

I agree. I have tested it, some vendors either can't see the STEP 242 info, or now how to use it. Same as the 3D PDF. Some machine shops in Asia don't use files/dwgs at all. They program CNC data directly into the machine. Although they have the dwgs and models to work from. Strange.

 

[3DDave]

Any PDF can have any other file embedded/attached, including a 2D drawing carrying the native model file and/or STEP 242 file or any other file that the user thinks is useful to add.

If the majority of work coming in is 2D drawings at what point will a Y14.41 model help? It just adds to the overhead of producing parts by having a more complex process that is infrequently practiced.

There are sure to be some shops that specialize in 3D data transfers to making parts and I would expect them to be possibly more expensive and maybe offer a quicker turnaround, but so much affects turnaround that might get lost in the noise.

 

[greenimi]

Question for the people with MBD knowledge:

Can you “measure” basic dimensions from the 3D PDF?

I know the 3D PDF format can be very fancy and if you click on a FCF then the associated feature will be highlighted along with the applicable datums. So, if the 3D PDF is recognizing features (features = meaning in the Y14.5 terminology a surface that you can touch) then again, my question is: can you use some sort of measuring tool that is providing a measurement value from one feature to another? I mean can basic dimensions be measured from the 3D PDF?

Anyone encountered such option to get the basic dimensions?

 

[SDETERS]

"@greenimi Yes, I can take measurements with the 3DPDF using Acrobat, I do not know about the free Adobe Reader. Siemens uses a program called JT2GO, which allows for measurements and offers a much more robust user interface when using 3D PMI.

I have not had much luck with the highlighting of PMI and related Geometry on a 3DPDF. To be fair I have not tried much lately..


On the 3DPDF in Acrobat you can right click go tools, and there is a 3D Measurment tool.

 

[ctopher]

I ran a test again yesterday with a vendor. There are still issues with them reading 3D PDF and STEP 242 files.
The 3D pdf has no dims, the STEP 242 shows dims in eDrawings, but the dims are huge and bunched together.

 

[bc23]

We use JT-PDF which requires JT2Go viewer. This can measure 3D features very roughly. (can measure point to point, but not plane to plane). It is really meant for general purpose usage. For detailed purposes, user needs a dedicated 3D software. It can also display adjustable cross sections.
For PMI & related surfaces, JT2Go will show it when you select the dimension or specification.
My main beef with JT2Go is that it doesn't show attachments. I have to open the JTPDF with Adobe to extract any attachments.

 

[SDETERS]

"For your 3D PDF, is there a way to toggle between Model Views? Usually, PMI is placed into different model views. In NX, this seems to work well. Is there a way to change or export all of your model views without altering any settings during the 3D PDF export? Just some random thoughts."

 

[ctopher]

SDETERS, yes it can

 

[SDETERS]

Thanks!

 

[greenimi]

SDETERS, yes it can

How accurate is the measurement from 3DPDF? Otherwise I am asking if you measure a basic dimension from the native CAD and the sameone from the 3D PDF how far apart are they? I know I put a over generalized question and I might get a ove generalized answer

By the same token, how far apart are the same measurements from the 3D CAD native model to the AP242 STEP file?
Any "significant" measurement errors? Any discrepancies?

 

[SDETERS]

The 3DPDF Measurement is not very user-friendly. A couple of small measurements I took, seemed to be consistent with the 3D model. 3D PDF can only go out to 6 decimal places. The PDF and cad matched out to 6 decimal places. For what it is worth. These are simple measurements like a diameter or simple distance. The measurement tool is not on par with the cad measurement tool.