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Dassault CATIA versus UG NX5 in automotive design and manufacturing

by Bob Sheaves

On May 3, 2010, Automotive News reported that Chrysler was to replace CATIA with Siemens’ NX to work better with Fiat. Bob Sheaves was part of the evaluation group when Chrysler chose CATIA around 1988.

CAD - CATIA Aries designChrysler has used CATIA since 1989, starting with the Jeep/Truck Engineering group (on the 1994 Ram). Before this, Chrysler had used manual drafting and its own program, Chrysler CADCAM, supported by internal UNIX and mainframe computer groups.

Chrysler’s CADCAM had been a great advance; design and engineering efforts were more repeatable and more accurate, with less “guesswork” on the projection of skewed views (views that are not parallel to the major drawing views, side, plan/top, front.) Chrysler claimed, with the Plymouth Scamp launch, that CAD had increased productivity by 3:1 over hand drawings.

Manual drafting is making a two-dimensional image of a three-dimensional object. It’s approximate, because one cannot apply mass or moment of inertia to paper. The more complex the object, the harder it is to project true views to ensure the representation is accurate. Manual drafting is slow, time consuming, and prone to errors.


Any CAD (computer aided design), CAE (computer aided engineering), or CAM (computer aided manufacturing) system — or, if you prefer, Product Lifecycle Management (PLM) — system is used for three reasons:

  1. CATIA allowed the Dodge Dakota team to conduct a full-plant computer simulation before the first tool was made; 600 experiments tested plant operations across 21.3 miles of the conveyer system, simulating 5,500 days of production.

    It increases accuracy, cuts rework, and reduces the technical risk.
  2. It lowers costs by minimizing lost effort and repeated effort.
  3. It provides an automatic engineering-program history to lower costs on future programs of the same type (in other words, PLM provides for continuity to not “reinvent the wheel”).

Defining the need

Siemens NX

CAD/CAM/CAE/PLM discussion

Dassault Systemes has grown through the Chrysler and Boeing relationships into, until a couple of years ago, the largest supplier of CAD/CAE/CAM and PLM software in the world. This has lead to the “fat, dumb, and happy syndrome” within Dassault, leading them to believe they could do no wrong; and they showed this arrogance heavily towards their customers.

cad wheel

CATIA v5, in particular, has had a checkered history, to be polite. Unstable and hampered in working with v4, early versions (up to version 5, release 15) proved to be extremely hard to implement, administer, and stabilize.

Siemens stepped into this gap, by purchasing UG from EDS (Ross Perot’s old company) and developing the NX family of software. NX was directly intended to supplant CATIA at the top of the heap.

CATIA v6 view  of BMW 3.5 liter V6 engine

NX was easier to install, administer, and more stable, but had no interconnection with any other system; data files (in this case, mostly Dassault) are always in a proprietary format which is unreadable by any other system. STEP was used to transfer data between systems, by converting to intermediate formats (e.g. International Graphics Exchange Standard) for interchange, just as people, long ago, might transfer a WordPerfect file to MacWrite through RTF, losing complex formatting along the way.

Both programs have the same precision in their visual display, but different kernel data used for wear calculations, manufacturing, etc.

File interchange usually works well with systems except CATIA, because of the extreme precision on the CATIA 3D data. CATIA, because of the 19-digit calculation precision in its core kernel (NX provides 11), is far more…stubborn…in creating Class A surfaces than anything else. The surface is either right or it is wrong, there is no middle ground for acceptability or tolerance.

CATIA v6 image for mechanical engineering

CATIA v6 allows users to integrate Functional Dimensioning & Tolerancing (FD&T) into 3D models; the information is carried upward to include relationships between the higher level assemblies, and to machinists and quality control people to establish the limits of acceptable variance. Carried through manufacturing modules of CATIA, such as KBE (Knowledge Based Engineering), this provides automatic parameter programming for DELMIA manufacturing using DNC, CNC, and NC processing, all without human intervention, with no manual APT or G-code programming of the machine tools.

This “stubbornness” can be a great thing (providing a stable and repeatable 3D model that is at the utmost accuracy) or a really, really bad thing when you try to work across various software platforms.

Because CATIA has more functionality built into and integrated with it, CATIA costs more and is harder to learn, but it is more robust. NX is far easier to learn, has many third party applications, is less expensive, but is sloppy in ultimate accuracy and repeatability.

Clarification: Dassault CATIA, Siemens NX, and accuracy

The Toledo plant pictured below is as real, mathematically, as the final physical plant in Ohio. If you were to view it in a “tank” (a room for viewing hologram data) you would see and walk through the plant exactly is if you were in the physical plant (and remember this is from a few years ago). All the line, mechanisms, human motion studies are figured out long before you pay to build anything.

2000 CATIA diagram

Accuracy has been a point of contention since CATIA was first written in the late 1970s. With CATIA, a model is always designed to nominal, just like on the drafting board. The issue arrives in the definition of “accuracy.” Let's take two examples....

In the first, we are going to build a house of 2x4s. A 2x4 can vary considerably in the end measurements, because of non-uniform drying, warp, etc. The variance can be mathematically described as GD&T (Geometric Dimension & Tolerances), but the 3D model must represent the nominal. In this case, physical clearances between each 2x4 are loose. Accuracy of 19 places is not required.

3D capability

In the second, we look at a diesel fuel injector with tolerances of machining at 1/10000 inch. Since you will machine the part directly from the 3D math data, accuracy is more important. The model of tolerance stackups between close fitting parts must be as accurate as possible so your NC machine’s model for predicting wear follows the correct path; and interchangable parts all work as designed when machined.

That leads to a question: how accurate do we need to be, without adding excessive cost? If we change from CATIA to NX, how much data will be lost? (Remember, CATIA v5 is Chrysler Engineering, in all its facets... )

CME CATIA PLM software with RFLP design method

(In CATIA v5 and v6, you can set the coarseness of the solid model representation on the fly, from a 0.030mm sag up to 10mm sag value. The sag is for graphics only; it doesn't change the actual model, only the display of the math behind the screen. )

How they choice was made in 1989

Erik Latranyi wrote: “If you are using a cutting tool, for every part you make, that tool wears down a little bit. If you machine the next part exactly like the first, with the CNC motions exactly as the first part....that second part will be slightly larger than the first. CATIA takes tool wear into account.”

Iaccoca said “fix it,” and that was what was going to happen. The top 12 or so design engineers were assigned training in all the other programs, to carry out the evaluation with the Technical Computer Center — which was responsible to Engineering, not the IT group.

Test problems were assigned; this was a fake pilot program that was carried out in each package. Evaluation criteria were:

  1. Speed to learn (how fast a person could be trained to be productive, how much on the job training was required to reinforce the training)
  2. Speed to perform (the time to accomplish the same design task between all systems)
  3. How much analysis could be done concurrently with the design (the capability to be used in the "no rework" process direction)
  4. How much interoperability existed between all the different software and the translated accuracy differences, as well as interfacing with the various supplier systems and processes
  5. How can we change the design and engineering process to minimize time to complete and take advantage of the built in software modules for varios procedures (such as vehicle dynamics simulation, FEA, etc.)

Plymouth Scamp CAD drawings

The group evaluated the internal systems of Chrysler (CADCAM), Ford (TBGS), and GM (CGS), and off-the-shelf products such as SDR and Pro Engineer, seeking the highest gain from the lowest effort. CATIA was already used by AMC, which Chrysler had acquired in 1987, so there were in-house experts already; and some existing vehicles which would not need to be completely changed.

The switch required Chrysler’s reference point to be moved forward by one meter, to avoid negative numbers, but also cleaned up the mess of different regional design standards; Chrysler spent almost $2 billion over five years to convert everything, from product to process, to CATIA. The T300 (1994 Dodge Ram pickup) was the first new vehicle on CATIA with the new standards.

1994 dodge ram

These measurements led to a complete rethinking of how Chrysler designed and built cars.....things like SCORE, integrated manufacturing, etc that were the hallmark of Chrysler in the early 1990s.

With the SCORE program, guys like me set up the suppliers to match Chrysler hardware and software, trained the supplier employees, and assisted with on the job training. If you were a T1 supplier, this was not negotiable, until Eaton came in and forced the suppliers to eat everything and do it on their own. This was the beginning of the end.......and another story entirely.

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