STEP-NC interface on a CNC, showing product shape and color-coded tolerance state
STEP-NC is a machine tool control language that extends the ISO 10303 STEP standards with the machining model in ISO 14649, adding geometric dimension and tolerance data for inspection, and the STEP PDM model for integration into the wider enterprise. The combined result has been standardized as ISO 10303-238 (also known as AP238).
STEP-NC was designed to replace ISO 6983/RS274D G-codes with a modern, associative communications protocol that connects computer numerical controlled (CNC) process data to a product description of the part being machined.
A STEP-NC program can use the full range of geometric constructs from the STEP standard to communicate device-independent toolpaths to the CNC. It can provide CAM operational descriptions and STEP CAD geometry
to the CNC so workpieces, stock, fixtures and cutting tool shapes can
be visualized and analyzed in the context of the toolpaths. STEP GD&T information can also be added to enable quality measurement on the control, and CAM-independent volume removal features
may be added to facilitate regeneration and modification of the
toolpaths before or during machining for closed loop manufacturing.
Motivation
Impeller machined using STEP-NC
Input to a CNC in the ISO 6983/RS274D G-code control language is
often machine-specific and limited to axis motion commands. The machine
tool is given little or no information about the desired result of the
machining.
STEP-NC allows more information about the machining process to be
sent to the machine control and adds new information about the product
being machined. This "Smart Data for Smart Machining" enables applications such as the following:
- Toolpath descriptions that are portable and independent of machine geometry.
- Visual process, to show toolpaths in context of the machine and workpiece, and eliminate drawings.
- On-Machine Simulation, to check for gouges, machine interference and other undesired behavior.
- Simplified Inspection, with linked tolerances, on-machine probes and inspection workplans tied to part tolerances.
- Feed and Speed Optimization, using tolerances, cross section information, sensor data.
- Associativity so feedback can be sent from manufacturing back to design.
Capabilities
Overview of STEP-NC process model
STEP-NC can communicate a complete machining process description to a
machine tool control or between manufacturing software applications.
The information handled by STEP-NC can be divided into the following
general categories. The standard handles technology-specific parameters
for milling and turning, and extensions for other technologies under development (see Future Work).
STEP-NC can exchange the explicit toolpath descriptions in use today,
and add part, stock, and fixture geometry, a description of the tools,
geometric dimensions and tolerances, and PDM information. A STEP-NC file
is difficult to edit by hand because it contains geometry descriptions
but for large programs the file size can be smaller because STEP-NC uses
a compressed XML format instead of ASCII codes.
History
STEP-NC is not the first attempt at providing better quality information to a CNC. The EIA 494 Basic Control Language (BCL)
defined a control language that was portable and had toolpaths
independent of machine geometry, but did not contain any of the other
product model information found in STEP-NC.
The core of STEP-NC is the ISO 14649 model for CNC control developed
by European ESPRIT and IMS
STEP-NC projects begun in 1999. These were led by Siemens with
contributions from RWTH Aachen University and the University of
Stuttgart in Germany, Komatsu and FANUC in Japan, Heidenhain in Switzerland, and the Pohang University of Science and Technology in Korea. Models for the control of CNC milling and turning machines were published in 2005, and draft models exist for EDM and contour cutting.
Integration of the CNC model into STEP to produce ISO 10303-238 was done in the United States, under the NIST ATP
Model Driven Intelligent Control of Manufacturing project, led by STEP
Tools, Inc. with an industrial review board (IRB) consisting of Fortune
500 companies, CAD and CAM software developers, machine tool
manufacturers, job shops and industry experts. STEP-NC AP238 was published in 2007.
STEP-NC Crown Wheel
In 2005 the OMAC STEP-NC Working Group hosted an AP238 testing forum
in Orlando to demonstrate 5-axis parts machined using AP238 CC1 machine
independent toolpaths. Four CAD/CAM systems produced AP238 machining
programs for milling a 5-axis test part (an NAS 979
circle/diamond/square with an inverted NAS 979 cone test in the center).
Each run on a pair of CNCs configured for completely different machine
geometries (AB tool tilt vs. BC table tilt). In addition, Boeing cut parts on a variety of machines at their Tulsa facility and a machine at NIST in Gaithersburg.
In June 2006, a live 5-axis STEP-NC machining demonstration was
hosted by Airbus at the Université Paul Sabatier Laboratoire de Génie
mécanique in Toulouse. Further machining and measurement demonstrations were conducted in Ibusuki Japan in 2007.
On March 10–12, 2008, the STEP Manufacturing team (ISO TC184 SC4
WG3 T24) met in Sandviken and Stockholm, Sweden to demonstrate use of
STEP-NC for feed and speed optimization, high-speed machining,
tolerance-driven tool compensation and traceability. The participants in
the demonstrations included Airbus/Univ. Bordeaux, Boeing, Eurostep,
KTH Royal Institute of Technology, NIST, Sandvik Coromant, Scania, STEP Tools, and Univ. of Vigo.
On October 1–2, 2008, the STEP Manufacturing team met at the
Connecticut Center for Advanced Technology, in Hartford, Connecticut to
demonstrate closed-loop machining,
feed optimization, and measurement using STEP-NC. The highlight of the
meeting was the live 5-axis machining of a titanium impeller.
Participants in the machining demonstration and other activities
included Boeing, Connecticut Center for Advanced Technology, Concepts
NRec, DMG, KTH Royal Institute of Technology, Mitutoyo, NIST, Sandvik
Coromant, Scania, Siemens, and STEP Tools.
These participants and others continue to hold STEP-NC
international implementation and testing events on a roughly six-month
cycle. The demonstrations in 2009 focused on machining a Mold part at
multiple sites from the same AP238 data including one part machined on a
FANUC-developed STEP-NC control. At a meeting in Seattle the parts were
then measured for accuracy using a CMM probe and a laser scanner.
STEP-NC machining on an Okuma CNC at IMTS 2014.
In the first half of 2010, the testing activity focused on tool wear
management and machining a part in multiple setups with multiple
alternate machining plans for 3, 4 and 5-axis machining. The new test
part was a gear box that must be machined on all six sides. The tool
wear and consequent machine loads were predicted from the STEP-NC data
and verified using a dynamometer.
In the second half of 2010, the testing forum applied STEP-NC to set
up compensation with on-machine measurement of part and fixture datums
using a FaroArm portable measurement device.
In 2012, the testing focused on machine tool accuracy
calculations, culminating in a demonstration in June at the KTH
production engineering labs in Stockholm. The test case milled a forged
blank for a Crown Wheel Gear on an older Mazak VQC 20. Accuracy data
from the machine was combined with tool engagement information from the
STEP-NC to predict the deflections, which were tested against actual
machining results.
In 2014, CAM data exchange using STEP-NC was shown at IMTS 2014
with daily machining demonstrations hosted by Okuma. A base machining
process for a mold part was created by Boeing and then sent to Sandvik
and ISCAR for optimization, producing a STEP-NC description containing
all three process options. All machining was done in titanium and a
range of CAM software was used, with all results captured as STEP-NC.
At IMTS 2018, a team consisting of Airbus, Boeing, DMG MORI, Hyundai WIA, Renishaw, and Mitutoyo demonstrated Digital Twin manufacturing by combining STEP-NC model and process data with MTConnect machine tool status and Quality Information Format (QIF) metrology results.
Future work
STEP-NC plasma cutting
Work continues within the ISO standard committees to extend STEP-NC
to new technologies and to incorporate refinements discovered during
use. Process models for new technologies are usually produced by the
ISO TC184/SC1/WG7 committee. Models for Wire & Sink EDM and contour cutting of wood or stone are under investigation.
Work on extending and integrating STEP-NC with the manufacturing
enterprise takes place in the ISO TC184/SC4/WG3/T24 STEP Manufacturing
Team.
This group also works on extensions and refinements discovered during
testing. A series of traceability extensions have been proposed for
linking STEP-NC machining programs with sensor feedback and machine
state information during execution.
The National Shipbuilding Research Program (NSRP) has also hosted
work to implement a prototype that connects a shipyard design system to
a plate cutting using STEP-NC. This work involved extending STEP-NC to steel plate cutting and marking using lasers and plasma torches.
A second edition of AP238 is being prepared for model-based
integrated manufacturing, with geometry, tolerance, and kinematics
improvements first introduced by AP242.
