What is STEP?
In design and manufacturing, many systems are used to manage technical product data. Each system has its own data formats so the same information has to be entered multiple times into multiple systems leading to redundancy and errors. The problem is not unique to manufacturing but more acute because design data is complex and 3D leading to increased scope for errors and misunderstandings between operators. The National Institute of Standards has estimated that data incompatibility is a 90 billion dollar problem for manufacturing industry [1].
Over the years many solutions have been proposed. The most successful have been standards for data exchange. The first ones were national and focused on geometric data exchange. They included SET in France, VDAFS in Germany and the Initial Graphics Exchange Specification (IGES) in the USA. Later a grand unifying effort was started under the International Standards Organization (ISO) to produce one International Standard for all aspects of technical product data and named STEP for the Standard for Product Model Data [2]. The types of systems that use STEP are shown in Fig. 1.
Nearly every major CAD/CAM system now contains a module to read and write data defined by one of the STEP Application Protocols (AP's). In the USA the most commonly implemented protocol is called AP-203. This protocol is used to exchange data describing designs represented as solid models and assemblies of solid models. In Europe a very similar protocol called AP-214 performs the same function.
STEP Application Protocols
A list of the STEP Application Protocols (AP) as of June 2004 is given in Fig.2. The ability to support many protocols within one framework is one of the key strengths of STEP. All the protocols are all built on the same set of Integrate Resources (IR's) so they all use the same definitions for the same information. For example, AP-203 and AP-214 use the same definitions for three dimensional geometry, assembly data and basic product information. Therefore CAD vendors can support both with one piece of code.
Part 201 | Explicit Drafting |
Part 202 | Associative Drafting |
Part 203 | Configuration Controlled Design |
Part 204 | Mechanical Design Using Boundary Representation |
Part 205 | Mechanical Design Using Surface Representation |
Part 206 | Mechanical Design Using Wireframe Representation |
Part 207 | Sheet Metal Dies and Blocks |
Part 208 | Life Cycle Product Change Process |
Part 209 | Design Through Analysis of Composite and Metallic Structures |
Part 210 | Electronic Printed Circuit Assembly, Design and Manufacturing |
Part 211 | Electronics Test Diagnostics and Remanufacture |
Part 212 | Electrotechnical Plants |
Part 213 | Numerical Control Process Plans for Machined Parts |
Part 214 | Core Data for Automotive Mechanical Design Processes |
Part 215 | Ship Arrangement |
Part 216 | Ship Molded Forms |
Part 217 | Ship Piping |
Part 218 | Ship Structures |
Part 219 | Dimensional Inspection Process Planning for CMMs |
Part 220 | Printed Circuit Assembly Manufacturing Planning |
Part 221 | Functional Data and Schematic Representation for Process Plans |
Part 222 | Design Engineering to Manufacturing for Composite Structures |
Part 223 | Exchange of Design and Manufacturing DPD for Composites |
Part 224 | Mechanical Product Definition for Process Planning |
Part 225 | Structural Building Elements Using Explicit Shape Rep |
Part 226 | Shipbuilding Mechanical Systems |
Part 227 | Plant Spatial Configuration |
Part 228 | Building Services |
Part 229 | Design and Manufacturing Information for Forged Parts |
Part 230 | Building Structure frame steelwork |
Part 231 | Process Engineering Data |
Part 232 | Technical Data Packaging |
Part 233 | Systems Engineering Data Representation |
Part 234 | Ship Operational logs, records and messages |
Part 235 | Materials Information for products |
Part 236 | Furniture product and project |
Part 237 | Computational Fluid Dynamics |
Part 238 | Integrated CNC Machining |
Part 239 | Product Life Cycle Support |
Part 240 | Process Planning |
Each Application Protocol includes a scope describing its purpose, an activity diagram describing the functions that an engineer needs to perform within that scope, and an Application Requirement Model describing the information requirements of those activities. These information requirements are then mapped into the common set of Integrated Resources and the result is a data exchange standard for the activities within the scope.
The ultimate goal is for STEP to cover the entire life cycle, from conceptual design to final disposal, for all kinds of products. However, it will be a number of years before this goal is reached. The most tangible advantage of STEP to users today is the ability to exchange design data as solid models and assemblies of solid models. Other data exchange standards, such as the newer versions of IGES, also support the exchange of solid models, but less well.
STEP led the way with three dimensional data exchange by organizing an implementation forum for the CAD vendors so that they could continually improve the quality of the solid model data exchanges. The history of this success is relatively interesting because it show that the initial reluctance of vendors to implement user-defined standards can be overcome with enough perseverance.
At first, in 1996, there was a significant body of opinion that solid model geometry data could not be exchanged between systems using a neutral standard. However, in 1997 Ford, Allied Signal and STEP Tools, Inc. demonstrated the first successful data exchange of 3D geometry using STEP. Once this basic capability had been demonstrated a pilot project, called AeroSTEP, was organized by Boeing and its Aircraft engine vendors to test the first translators by exchanging data about where an engine fits onto the airframe. This project started out by exchanging simple faceted models but eventually demonstrated the exchange of models with great complexity.
The AeroSTEP project made it clear that STEP data exchange of solid model data was both feasible and valuable. As a result, vendor neutral implementation forums were formed in Europe, the Far East and the USA and the quality of the translators was raised to the level that allowed anyone, including ordinary users in small organizations, to use STEP for data exchange of solid models after about 2001.
STEP for Geometric Dimensioning and Tolerancing
Manufacturing needs more than a geometric model to make a part. There are many additional specifications required but most important is a description of the required tolerances because this will drive the selection of the manufacturing process and the manufacturing tools used to manufacture the part.
A model of Geometric Tolerance and Dimension (GD&T) information is being added to STEP as part of an upgrade to AP-203 called Edition 2. Several iterations have already been made with significant feedback about functionality from the CAD vendors. The initial GD&T models were developed for several different AP's with different scopes so each was slightly different. A new harmonized model for all the known GD&T requirements was produced in September 2004.
The new model is highly integrated with the existing model for geometry because the GD&T data qualifies geometric items already in the geometry model with tolerances and datums that may also be in the geometry model. The necessary data sharing has been achieved by developing a highly intricate data model with many inter-twined data definitions. This makes implementing the GD&T model almost as challenging as implementing the original geometry models that were very object oriented with many inheritance relationships.
Consequently, there are questions as to whether or not the new model can be implemented so early implementation projects are being formed to show that such implementation is feasible and valuable. The history of the original AP-203 implementation project is relevant here. It can be predicted that if the first experiments are successful then an industry segment that really needs the new efficiency will work with the vendors to pilot the new capability to a level where it can be used for complex products, and then implementation forums will take it to a level where all users including small job shops can take advantage of the new capability.
STEP for CNC Machining
The STEP-NC AP238 standard is the result of a ten year international effort to replace the RS274D (ISO 6983) M and G code standard with a modern associative language that connects the CAD design data used to determine the machining requirements for an operation with the CAM process data that solves those requirements. STEP-NC builds on the previous ten year effort to develop STEP data standards for CAD data and uses the modern geometric constructs in STEP to define device independent tool paths, and CAM independent volume removal features.
You can find more information about the development and deployment on the STEP-NC Standard Home Page and on the pages for our STEP-NC Write, and STEP-NC Machine products.
Background
STEP-NC defines a CNC part program as a series of operations that remove material defined by features. The features supported include holes, slots, pockets and volumes defined by 3D surfaces. Each operation contributes to the manufacture of a feature by defining the volume of material to be removed, the tolerances, the type of tool required and some basic characteristics such as whether this is a roughing or finishing operation. The operations are then sequenced into a work plan that converts the stock into the final part. The work plan may be sophisticated and include conditional operations that depend on the results of probing operations, and it may be divided into sub-plans to be executed concurrently on machines that have multiple cutting heads.
A key feature of STEP-NC AP-238 programs is that they are machine and organization independent. If a machine has the underlying capabilities (axes, table size etc), then a STEP-NC "compiler" should be able convert the part program into a sequence of tool movements for that machine. If a CNC has a Tool Cutter Programming (TCP) interface then the tool movements can be executed directly without converting to axis movements. This has two significant consequences for industry.
- If parts can always be rapidly manufactured from an AP-238 description, then there is no longer a requirement to keep copies of those parts in the inventory. A recent study for the UK Navy estimates that at least $4M can be saved for one depot (and up to $640M can be saved for the entire UK Navy) if the depot can store its spare parts as electronic product data instead of as physical items.
- If parts can be made independently of the axis codes, then the same CNC program can be run on many machines. This allows a part program to be made once and run anywhere. Another study has shown that a mid sized machine shop could save as much as $0.5M per year in reduced CAM costs, less waste, and greater throughput if it received reliable machine independent CNC data from its customers.
Fig. 3 shows how design data is communicated to manufacturing in current practice. Design creates the specification for a product as a 3D model. Detailing decides the manufacturing requirements for the product by making a drawing. Path planning generates tools paths. Manufacturing controls production. The job of design is performed using a CAD (Computer Aided Design) system, the job of detailing is performed using a drawing CADD (Computer Aided Design Draughting) system, the job of path planning is performed using a CAM (Computer Aided Manufacturing) system, and job of manufacturing is controlled using a CNC system. In many cases the CAD, CADD and CAM functions are combined into a single integrated CAD/CAM system but in all cases the CNC function is performed by a separate system.
Information can be lost in the pipeline because incomplete data is sent from the CAD to the CAM, because fixes to the geometry are made in the CAM and not communicated back to the CAD, because only the surface data is communicated to the post, and most of all, because the RS274D standard only allows axis movement data to be communicated to the control. This means that no adjustments can be made on the control in response to changes in the available tooling, the control cannot optimize the machining process for the capabilities of the selected machine, and the operator cannot rely on software in the control to check the safety of the set-up and the program.
In the new method enterprises can continue to use their existing systems for CAD, CADD and CAM, but the end result is sent to the CNC as a STEP-NC AP-238 file instead of an RS274D file. Fig. 4 shows the modified pipeline. The change is small because no systems need to change only the interfaces, but the advantages are significant:
- The AP-238 file can make developing a CNC part program more efficient because the programmer only has to describe the tasks to be performed on the machine and not the tool motions necessary to achieve those tasks.
- The AP-238 file allows a CNC to optimize and check a part program for the tooling available at the time of manufacturing instead of having it fixed at the time of planning.
- The AP-238 file reduces the requirement for drawings on the shop floor and it allows manufacturing to send requests for changes back to design by annotating the original full fidelity design information.
- The AP-238 file makes manufacturing data portable between machines and organizations and allows a part to be made on any machine with sufficient resources (axes, table size etc).
The deployment of AP-238 may take a different path to that of AP-203 because the economic benefits are quite significant. There has been considerable early testing of AP-238 by industry as a new interface for defining machine independent tool paths. This is the concept originally defined for BCL (RS494). However, BCL did not have a means to display the design geometry and tolerances of the part, or the features being manufactured in each operation. There is strong reason to believe that if this data is added then AP-238 CNC machine independent files can be used as the basis for contracting work between customers and suppliers and if this is so then the deployment of AP-238 may be driven by stronger economic forces than those that lead to the implementation of AP-203.
Future of STEP
Despite the many successes of STEP there is still a question in users minds about the speed of its development and deployment [5]. Many critics point out correctly that the XML standards for e-commerce are being developed much more quickly.
Fundamentally, product model data is different to other kinds of e-commerce data such as invoices, receipts, etc. The traditional method for communicating product model information is to make a drawing and the traditional method to communicate an invoice is to make a form. When you make a drawing or 3D model you need to define information with many subtle and complex relationships and this makes the STEP data exchange problem more difficult.
An XML data format is being developed for STEP but the STEP architecture requires the information requirements of an Application Protocol to be mapped into the common set of Integrated Resources. This allows all of the protocols to share the same information and is essential if all of the interfaces shown in Fig. 4 are going to share and reuse the same set of data. However, the sharing necessarily divides the original data into multiple entities that are not so easy to understand in XML or any other format. This is disappointing because one of the attractions of XML is that is self-documenting (at least for programmers and domain experts). Therefore, a new level of documentation is required in the STEP data to show how the information requirements have been mapped. The required structures are currently in development and it is anticipated that STEP will have a self-documenting XML format in the very near future [5].
The real issue that stops faster STEP deployment is the commitment of those with the resources necessary to define the standards. The government does not like to pick solutions for industry, and industry does not like to fund the development of solutions that can also be used by their competitors. Consequently, much work only gets funded in situations of clear and desperate need such as when the high cost of manufacturing is causing excessive job losses.
The Internet and the World Wide Web broke through this cycle when "killer" applications made the benefits of the new infrastructure clear and compelling for all users. AP-203 made STEP useful by allowing solid models to be exchanged between design systems. AP-238 will make STEP compelling for some users by allowing them to machine parts more efficiently. However, like the early Internet there will be alternatives that are considered more reliable by other users. The killer application that makes STEP ubiquitous has yet to be identified.