Upgrading and/or Retrofitting Your CMM Machine
The frame of the average CMM has not changed much during the last 35 years. The mechanical structure, in most cases, retained its dimensional integrity. Motors, scales, drive systems, and bearings, if maintained properly, will last a long time. Besides, all these items are easily replaced or repaired at minimal cost.
The major technical advances in coordinate measuring machines are the peripheral technologies:
- Software
- Probing
- Controls
Software has changed dramatically over the past five years and is still undergoing radical changes, as the CMM manufacturers incorporate new sensing (probing) products. The big players in the CMM software business all incorporate CAD (Computer Aided Design) capabilities and complete off-line part measurement sim ulation.
Major software products are: PC-DMIS (Wilcox and associates), Calypso (Zeiss), and Polyworks (Innovmetrics). These softwares hold the lion's share of the North American market; however, there are many other niche software products to choose from, notably Quindos, Metrolog, Cosmos, and Modus.
As scanning becomes more popular, so does the requirement to handle copious amounts of data, and to present that data in a rational format, enabling engineers and inspectors the ability to make decisions based on graphical presentation formats such as colour maps and 3-D part simulations.
Installing a new software package on your existing CMM can be a cost-effective way of providing today's technology in your measurement process without spending a fortune on a new machine. Many software products have built-in interfaces to work directly with your older controller, reducing costs even more!
Specialized software products are also available, enabling gear and blade measurement, Statistical Process Control, and various other specialized outputs that are specific to certain industries and processes.
With the increasing use of the CMM for in-process measurement, there is now the ability to monitor temperature, CMM air pressure, and crash detection and report it in to your cell phone.
Probing. When considering an upgrade to your CMM, it might be wise to look at the business end of your machine, the probing system.
Motorized Indexing Heads. Upgrading from a fixed or manual probe head to an automatic one can change the operation of your machine substantially. Programming the machine can be accomplished much faster, as during a "teach" session the probe can index into any position on the part without changing probe setups. This enables faster part inspection, without operator intervention during the measurement cycle, and no lengthy probe changes.
Modular probes consist of a main body and a stylus module; most have kinematic mounts for quick and precise disconnecting and reconnecting.
Their big advantage is crash protection, such as when a bump happens; the module disconnects, saving the main body and the probe head from massive damage. The metrology advantage is ingenious, as stylus modules have optimized metrology for different lengths of styli. Select the correct module for the stylus length you are using and the error is minimized. In the past, the probe consisted of a body only and adjustment for different lengths of stylus was achieved by adjusting the spring tension of the probe, a poor metrology choice that resulted in large probing errors.
Perhaps the most popular probing upgrade, a scanning probe gives additional capability to the older CMM machine. This upgrade works best on newer frames, because older machines do not have the smoother dynamics newer systems have or the more precise positioning required for a scanning probe. If in doubt, it's best to call in an expert who has retrofitted similar machines in the past.
For profiles and form deviations, scanning probes provide more accurate data and save time, as they collect thousands of points per second. They also can operate in single touch mode for optimum flexibility.
Controls
The heart of all coordinate measuring machines is the controller, it is by far the most vulnerable component on a coordinate measuring machine, with a limited lifespan as it is subjected to obsolescence because of its electronic nature as it is essentially a computer.
Controllers in the late 70s and 80s were essentially mainframe computers, such as Digital’s PDP series. However, later most manufacturers purchased their CMM controllers from specialised control manufacturers such as Deva and Pantec.
During the 1990s, most CMM controllers consisted of 5 to 10 boards, consisting of individual boards for the following: Power supply/servo amplifier/motor distribution/CPU or motherboard/hard drive/ encoder signal boards and PWM board. Each board costing in some cases as much as the original controller. New controls have between two and four boards making service and replacement much more affordable.
New controls enable the modern coordinate measuring machine to do many varied tasks and allows implementation of new probing and software:
- Circular interpolation. This allows the coordinate measuring machine to move about the component to be measured in circular motion rather than by a stop and go motion about the part. This speeds up the time required to measure the part, but also preserves accuracy and enhances lifespan of the coordinate measuring machine.
- Error map. The dimensional integrity of the machine resides within the controls error map, this map is populated during a calibration and provides the best possible volumetric accuracy and repeatability to the system.
- This allows only authorised software and the dimensional information to be accepted by the controls. It provides security and maintains integrity of the system.
- Scanning probes. The ability to use analogue and digital scanning probes and collect the necessary amounts of data associated with these probes.
- Stylus and probe changers.
- Laser probes. Provides for the collection of copious amounts of data to be then transferred to the coordinate measuring machines on-board computer.
- CCD camera systems.
The lifespan of most controls is quite a bit shorter than the lifespan of the coordinate measuring machine and its other components, most controls have a useful life of no more than 20 years. Generally speaking, replacement boards are usually only available for 10 years from new, although in many cases reconditioned boards are available.
It is highly recommended when doing a thorough upgrade on an older machine to replace the CMM controller as this then opens the door to all the modern benefits provided by probing and software.
