Frequently Asked Questions
Questions about Manufacturing Data Standards
What is RS274D (also known as ISO 6983)?
RS274D is the data standard currently used to tell CNC systems how to make a part. An RS274D file contains a list of instructions called G-codes. Each code tells the CNC machine where to move the cutting tool next. If the CNC executes all of the instructions correctly, then the part is made. Each code is very primitive and an RS27D file needs to contain hundreds of thousands of codes to make a part. The CNC is not given any information about what it is making or why the instructions have to be executed in the given order. Therefore, no optimizations can be made on the control.
The RS274D standard is also limited in functionality, and CNC vendors have invariably extended it to include newer features in their controls such as spline interpolation. These "dialects" are not standardized, so programs written for one vendor's CNC are unlikely run on another vendor's CNC.
What is a Post Processor?
To handle the wide variety of RS274D dialects (over 5,000), post processors are used to automatically convert tool path data generated by CAM to G-codes specific to a CNC's RS274D dialect. In some cases this is done by the CAM system internally, in others the CAM system generates APT "cutter location" data that is converted to G-codes by a third party post processor. Post processing is not a value-added step, and no new capabilities are added. STEP-NC eliminates the requirements for post processors, because STEP-NC has no dialects.
What is IGES?
IGES is a standard for transferring drawing information between CAD systems and between customers and suppliers. When a customer wants a job shop to make a part, it sends a description of the part as a drawing in an IGES file. The job shop then reads that IGES file into a Computer Aided Manufacturing (CAM) system and an operator uses it to generate the RS274D code necessary to make the part on a specific machine tool. It can take quite a long time for the operator to convert the drawing into RS274D codes and if anything changes the whole process has to start again. The people who developed IGES used the lessons learned to develop an international standard for the complete life cycle of a product and called it STEP. There will be no more versions of IGES after the current one.
What is AP-238?
AP-238 is the official ISO STEP number for the STEP-NC standard. In AP-238 the information required to control a machine is linked to the information created by CAD and CAM systems to create a truly independent, fully documented CNC control file for the first time.
What is ISO 14649?
ISO 14649 is the machining data model underlying AP-238. The authors of ISO 14649 assessed the state of the art in CNC, and developed an object-oriented data model based on the concepts of features and working steps that reflected today's high-performance machining needs. AP-238 takes this data model and brings it into the suite of STEP standards, so that CNCs can be fully integrated with CAD, CAM, CAPP and other CA- applications.
What is AP-203 edition 2?
AP-203 Edition 2 is a recently released new version of the AP-203 standard for exchanging 3D geometry between CAD systems. The new edition extends the old version to include Geometric Dimensions & Tolerances (GD&T) data. The new data has been designed to meet the requirements of design and manufacturing so the same model has also been incorporated into AP-238 (and the other STEP Manufacturing AP's). When this model is fully implemented, smart algorithms in the CAM and CNC systems will be able to compute the best speeds and feeds for some operations automatically.
What are the other STEP Manufacturing standards?
There are several. The most well known is AP-224. This was the first standard to describe machining features for milling and turning. The same features have been adopted by the other STEP manufacturing standards including AP-238. Other standards in the STEP Manufacturing suite include AP-219 for CMM inspection data, AP-240 for process planning data and AP-223 for casting and forging data.
Why is AP-238 special?
AP-238 defines a new interface for existing systems. With the exception of conformance class 4, it does not require new capabilities to be added to existing systems. Instead it defines a new standard for existing information that is already in CAM systems so that the information can be passed into the CNC systems to make them easier to program and operate.