With the current closed loop manufacturing trend, the Coordinate Measuring Machine (CMM) is finding a new role as a flexible production gauge adjacent to the machine tool or production process. With the advent of Industry 4.0, the CMM remains a highly accurate, flexible measuring device capable of delivering to the required production gauging ‘Gauge Repeatability and Reproducibility’ results.
Taking the CMM out of the quality laboratory and closer to the point of production is nothing new; pioneers in flexible cell manufacturing such as General Electric were doing this way back in the early 90’s. In those days the standard bridge configuration of air bearing CMM was placed inside an air-conditioned enclosure, with the production operator loading and unloading parts through an opening door. CMM enclosures still remain common place today for larger part shop-floor applications.
Shop-hardened CMMs have emerged over the past decade, whereby the CMM air-bearings are replaced with mechanical linear guide-ways, and CMM structures manufactured from cast-iron, polymer concrete or granite in an attempt to create a thermally inert device. A small CMM footprint has also been demanded for shop-floor CMMs; interestingly the industry has returned to the early cantilever design principle, pioneered by early industry participants Ferranti and Sheffield, as its compact foot-print offers the most effective measuring volume.
Shop-floor machines typically no longer use air-bearings due to the issues associated with contamination from air-borne particulates in machine-shops and hostile manufacturing environments. The developed volumetric error compensation techniques has significantly enhanced the accuracy of cantilever configuration machines, which are now much closer to moving bridge machine accuracy. In addition thermal compensation techniques are common place to overcome temperature issues associated with shop-floor measurements.
While temperature variation remains the enemy of accurate measurement many CMM manufacturers are now specifying machines uncertainties at different temperature bandwidths. For example Hexagon Manufacturing Intelligence specify their Tigo SF shop-floor machine with 3 temperature range bandwidths, temperature gradients and ISO 10360 accuracies:
18 ÷ 22 °C Temperature Range | 1 °C/hour – 2 °C/24hours | E0,MPE = 2.2 + L/300
16 ÷ 26 °C Temperature Range | 1 °C/hour – 5 °C/24hours | E0,MPE = 2.5 + L/250
15 ÷ 30 °C Temperature Range | 2 °C/hour – 5 °C/24hours | E0,MPE = 2.7 + L/200
This new generation of shop-floor CMMs have also introduced tactile scanning to the production floor allowing critical feature form the be accurately measured and trends monitored. Tactile scanning probes have dramatically reduced in size over past years allowing their use on compact shop-floor CMMs. In the early days of shop-floor CMMs they were predominately touch-probe measurement only with more accurate scanning part analysis still taking place in a centralized quality laboratory.
To aid the CMM to migrate to the shop-floor the traditional CMM software has been masked from the user by the incorporation of a shop-floor graphical user interface providing a program-launch facility to the operator eliminating any need to enter the more complex programming software intricacies of typical CMM software’s. One such solution offer is INSPECT, part of the Single Touch Interface solution offered for PC-DMIS.
The uptake of optical sensors has also migrated to the production CMM allowing the faster measurement of part topology and the generation of point cloud data allowing rapid feature size and position extraction for direct comparison against CAD data. Tool changers are common place on shop-floor CMM allowing for sensor changing and automated part inspection without operator intervention.
With the open architecture of the cantilever design of CMM automatic part loading using an industrial robot, or part transportation system, is now practical allowing the complete unattended running of the CMM inspection cell.
Closed loop feedback with CNC machine-tool offset information generation has also been pioneered whereby gauging results can be interpreted real-time allowing offset values to be updated to the CNC controller, enabling true automated process control compensating for common causes of process instability such as example tool wear and thermal drift.
The development of new innovative CMM products with mass market appeal for shop-floor applications, has also seen the emergence of a generation of new mechanical structures into the market utilizing parallel kinematic constraint mechanisms. These structures are less expensive compared to traditional tactile scanning CMMs. The Renishaw Equator is marketed as a comparator gauge measuring against a master part that has been previously measured on a high accuracy CMM. The idea of comparative gauging is a ‘throw-back’ to the early days of production gauging with regular gauge remastering removing the issues of temperature variation from gauging results while producing CMM style data rather than just Go-No-Go data.
Aberlink offers its Xtreme CMM shop-floor model which also utilizes a parallel kinematic constraint mechanism. Two models sizes of machine offer a 300 and 500mm cylindrical measuring volume with a quoted accuracy E0,MPE = 3.0 + 4L/1000.
Factories of the Future foresee off-line, near-line and in-line metrology cells. The current CMM configurations are unsuitable for most in-line measuring applications. Mitutoyo has introduced a measuring station for integration into automated production lines. The Mach Ko-Ga-Me product has a measuring range of 120mm2 x 80mm with a quoted accuracy of E0,MPE = 3 + 7.2 L/1000 and is faster than a traditional CMM by 40%. The unit can be used with touch-probes and tactile scanning probes. The Ko-Ga-Me main application is integration within bespoke automation.
Many of the original shop-floor CMMs had typical measuring volumes of 500 x 400 x 400mm which satisfy a large portion of the potential market but too small for many larger part measuring applications. Wenzel has recently introduced its SF 87 shop-floor model CMM with a measuring volume of 800 x 700 x 700mm complete with sensor change rack and suitable for larger part production gauging.
It’s apparent that as manufacturing transitions over the coming decade the CMM will adapt to new roles and CMM builders are accepting the new challenges being demanded by users by introducing a plethora of new measuring solutions, software, sensors and hardware to suit.
The CMM market continues to grow and the current shop-floor migration trend will ensure the increasing penetration of tactile and multi-sensor measuring solutions demanded by high accuracy applications.
For more information on shop-floor CMMs: