QMT Features: March 2008
On-machine metrology
Global manufacturers are getting serious about on-machine metrology asĀ  real time inspection on CNC machine tools delivers big productivity gains.

On-Machine Metrology Software for CNC machines allows manufacturers to readily
create true metrology programs for their spindle probes to aid in part setup, in-process
checks or on-machine inspections. PC-DMIS NC (shown here) is currently being
used with variety of CNC equipment:
vertical mills, horizontal mills, lathes, water-jets (including dual head systems)
and multi-tasking machines.

Perhaps the greatest barrier to integrating on-machine gaging with CNC machining has been the inherent difficulty of creating meaningful data captures and feedback loops using conventional G and M codes.

With the advent of true, on-machine metrology software that barrier is gone.  These products allow programmers to develop inspection routines that will run on a wide variety of CNC machines from a single software platform.  With these they can make short work of generating programs for setting up and/or inspecting parts on their machine tools. 

This consideration has not been lost on many of the world’s leading manufacturers. More and more of them are investing heavily in machine tools equipped with one or more variations of the on-machine gauging software. They are moving with deliberate speed to integrate this technology into selected manufacturing processes. Here is why.

On machine advantages
True on-machine metrology software, unlike the probing utilities provided by the CNC equipment manufacturers and spindle probe vendors, allows users to perform powerful alignment, data collection, analysis and reporting functions without having to move the parts to a CMM.  Significant features of on-machine metrology software include:

CAD Based Programming. This provides for the fast programming of routines that automatically evaluate critical part geometries, align work pieces and calculate work and tool offsets. For this sort of on-machine metrology, the software allows for the integration of a model of the machine with a model of the part.  This makes it possible to devise measurement routines that both quickly locate their parts in the machine space and automatically take the measurements needed for the task at hand.  In addition, users can perform all of their programming off-line, eliminating the need to use precious machine time for developing metrology programs.

Sophisticated probe handling. Many NC machine tool probe calibration routines provide for probe calibration measurements in single X, Y or Z directions only.  Because of this, probe lobing errors increase making it difficult to measure parts in three dimensions rapidly and correctly.  True metrology software, however, calibrates probes using a sphere.  Because of this they are better equipped to take advantage of newer more accurate spindle probes.  They can also manage probes on an articulating wrist, allowing measurements to be taken anywhere within the 3D measurement envelope.  This is particularly important when 3D part features are to be evaluated or when 3D part features or tooling balls are used to locate fixtures or freestanding parts on the machine tool.

Parametric programming. Using true metrology software, users can easily create new programs from existing ones.  This is especially useful for programming families of parts since generating a new program is simply a matter of changing values in a parametric table.  So, programs can be written once and used many times over for similar parts with a minimum amount of effort.

Integrated statistics and reporting. True metrology software packages provide statistical and reporting capabilities that are powerful, flexible, easy to understand and easy to distribute.  Users of on-machine metrology software, get these capabilities by definition.  This gives them instantaneous, actionable feedback on the integrity of the manufacturing process and a set of SPC tools for monitoring and improving their processes.  It is even possible to use this functionality to modify cutting programs based upon measurement results.

Data history. Users have the option of saving their measurement data along with their measurement programs.  This means that they can, if they have the data needed to support it, perform after-the-fact-analyses of parts or part features.  For example, if there is a perceived issue with a feature’s circularity, they can check this simply by processing the data in the database.  They do not have to remeasure anything.  This historical data analysis capability can dramatically shorten the time spent trouble-shooting process problems. 

Certified algorithms. Measurement algorithms identical to those used on coordinate measuring machines capture and analyse data more accurately.

Two on-machine approaches
Based on specific end-user requirements, metrology software developers have taken one of two approaches to implementing on machine gaging on CNC equipment. The first is batch-oriented and does all of its evaluation and analysis on a workstation without impacting cutting operations.  Users create measurement routines on an off-line programming station.  Then, CNC-specific postprocessors translate these routines into the G and M codes that will drive the spindle probe and capture data.  During the probing processes, the measurement data is sent back to an evaluation engine for analysis and reporting.  Because of this architecture a single, server-based, metrology system can capture and analyse data from many CNC machines on a single network.

The other approach is to run the on-machine metrology software (once it has been developed off-line) on a PC independent of the CNC machine’s controller. With this approach, the PC assumes total control of the machine during its measurement, data collection and analytical processes. This means that operators can do almost anything on a CNC with a spindle probe that they can do with a coordinate measuring machine.  The drawback is that, while the software is processing the data, the machine tool is not cutting.

These two approaches typify Wilcox Associates’ PC-DMIS NC Server and PC-DMIS NCi (interactive) software respectively. PC-DMIS NC Server was offered first. The rationale for putting software on a server was to maximize machine-cutting up-time by making measurement routines as unobtrusive as possible and to provide the end user with the economy of allowing a single software seat to regulate measurement processes on multiple machine tools.

Wilcox introduced the interactive software about two years later to satisfy the need of some users who require on-machine measurement programs capable of automatically adjusting themselves on the fly based on measurement results (e.g. Iterative alignments of parts or fixtures, logical program branching, relative measurements, etc.) In either case, users can now obtain fast but sophisticated measurement results on their machine tools, which, contrary to decades of conventional wisdom, can save them lots of time and money.

Global Implementation
World-wide, users of PC-DMIS NC systems of both types are currently setting up and measuring parts on all sorts of machines in a wide range of industries.  The industries include aerospace manufacturing, aircraft engine components, automotive racing, blade manufacturing, composites manufacturing, mouldmaking, and water and wastewater treatment system parts. 

The biggest driver of PC-DMIS NC installations has been the need to locate large flexible composite parts for aerospace applications in milling machines and waterjet cutting systems.

The software dramatically improves the up time of expensive waterjet cutters, sometimes by as much as 50%, by accurately locating parts for cutting.
Real-time inspection data delivered by on machine metrology systems relieves the burden on precision and ultra-precision CMMs in monitoring higher volume parts manufacturing operations. This leads directly to greater overall manufacturing enterprise productivity (parts shipped) without sacrificing part quality.l

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