Discussions

Comparison between CAD based inspections

The table below reviews the differences in performance and functionality between software solutions (not necessarily limited to CMM inspection) with different level of integration. The comparison is based on the method of data exchange with the source CAD system. We look at three different levels of communication.

In the first column are applications using IGES for model data exchange. They are the most common ones and are usually limited to trimmed surfaces and wireframe geometry. This type of data translation provides low reliability and questionable accuracy. The data moves in one direction from the CAD system to the external CMM application. Any modifications to the CAD model require the translation process and programming to be repeated. STEP is not shown here, but it has many of the disadvantages of the IGES translation. The one good side of STEP is the support for solids. The main advantage of using IGES/STEP interface is that it works with different CAD systems.

Using common data formats (e.g. Parasolid in case of Unigraphics, ACIS for AutoCAD) is considerable improvement. Reliability, accuracy and performance are substantially better compared to IGES. Solids are supported. The cons are duplication of data, need to reprogram when the model is updated, no support for assemblies nor for embedded GD&T. It is important to distinguish between systems that use the format natively and the ones that only read this type of data. The latter still need to do an approximation.

An integrated solution works as a part of the CAD system. It doesn't read the part files directly, but rather uses the CAD API (Application Programming Interface) to access the model data. It provides complete integration between different manufacturing processes on top of a single CAD database. The only drawback is that it works with a specific system. However, it can fall back to the level of the other two types through Parasolid (in case of UG) or to any other CAD system through IGES. The advantages include single database, reduced time to complete the job, support for solids, assemblies and GD&T.

	External via IGES	External via Parasolid®	Integrated in UG
Prep Time	5 min - 5 days	5-20 min	1-2 min.
Eng. Changes	Prep Time Reprogram(1)	Prep Time Reprogram(1)	Automatic Update Or Reprogram
Data Duplication	Yes	Yes	No
Accuracy	Low	High(2)	High
Support for Solids	No	Yes(3)	Yes
Support for Assemblies	No	No	Yes
Support for Model Specific Attributes	No	No	Yes
Support for other CAD Systems	Yes	Yes	Yes(4)

The winners are shaded in cyan

1. Reprogramming in an external system may require redoing the whole program as this type of systems have limited CAD capabilities
2. Only if the external system is native Parasolid® system
3. Depends on the capabilities of the external system
4. Possible through IGES/STEP translation or if the system is Parasolid® based. Actually because Unigraphics® is a high-end system the chances for successful translation are better than in the other two cases. However all problems attributed to the other two remain.

 

 

Comparison between existing inspection processes

In the table below we look at different methods for dimensional inspection. This is a comparison between  conventional CMM in Manual and Programmed Mode as well as CAD based solutions for Off-line Programming and Part Verification. We farther subdivide the cases by looking at two different types of parts - the first one is relatively simple and the second one is more complex (see the footnotes). CAD based inspection is the only feasible way to inspect free-form surfaces. It provides high accuracy, repeatability and reproducibility. The Part Verification has the added benefit of being adaptive, much faster for first article inspection and well suited for manual DME's, such as manual CMM's, laser trackers, articulating arms, laser and optical devices.

	Conventional CMM Manual Operation	Conventional CMM Programmed Mode	CAD Based CMM Programming	CAD Based Part Verification
Inspect Prismatic Part (small/big)	1 hr / 2 days	3-4 hrs / 5-6 days	3-4 hrs / 3-4 days	30 min / 4-5 hrs
Inspect Free-Form Part (small/big)	N/A(1)	N/A(1)	3-4 hrs / 3-4 days	30 min / 4-5 hrs
Part Alignment	5-10 min	1-2 hrs	1-2 hrs	2-3 min
Applying Engineering Changes	New Drawings Needed	Modifications Needed	Modifications Needed	No penalty
Provide Multiple Views	Very Limited	Very Limited	Limited	Native
Use Manual CMM's	OK For Prismatic Parts	N/A	N/A	OK For All Parts
Adaptive	Yes	No	No	Yes
Repeat The Inspection	Same time as before(2)	Only execution time	Only execution time	Same time as before(3)
Repeatability	Average	Very High	Very High	High
Reproducibility	Low	Low	High	High
Chance of Errors	High	High	Low	Low
Personnel	High Requirements	High Requirements	High Requirements	Productive From Day 1

The winners are shaded in cyan

Small part is about 20 features (100 points)

Big part is about 200 features (1000 points)

Repeatability measures the variation in measuring the same part multiple times by the same inspector

Reproducibility measures the variations in measuring the same part by different inspectors

We assume that the CMM has sufficiently high repeatability

1. Defining and inspecting free-form surfaces is practically impossible
2. It is impossible to measure the same points
3. It is impossible to measure the same points unless a program is created. However, if a program exists the time is fraction of the time used the first time

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