Lasers Prove Fast and Accurate for Fiat (and Ram) Van Production

Here’s how the use of laser scanners on the ends of horizontal measuring machines are allowing the Fiat Doblò (and the Ram ProMaster City, which is based on the Fiat) to be manufactured with accuracy.

One of the types of vehicles that is becoming increasingly popular in the U.S. but which has been something of a stalwart among tradespeople in Europe for quite some time is the light commercial vehicle. For some reason, Turkey is a popular place to build these vehicles. The previous-generation Ford Transit Connect was produced at the Ford Ososan Kocaeli Plant in that country. The current-generation Fiat Doblò, which comes to the U.S. as the Ram ProMaster City, is produced by FIAT-Tofaş in a manufacturing facility in Bursa, Turkey. (The Ram is upfitted at the Chrysler Group Transformation Center in Baltimore, incidentally.)

At the plant they are using two Hexagon ( DEA BRAVO horizontal-arm coordinate measuring machines (CMMs) that have a continuous-wrist CW43 interface. They have been retrofitted with Nikon Metrology ( XC65Dx-LS Cross Scanners and DMIS-based CAMIO multi-sensor metrology software.

One of the interesting attributes of the laser scanner is that it scans the part with not a single line of visible laser light but with three stripes, thereby quickly acquiring complete 3D geometry of the features being measured.

Another feature of the Nikon laser system is that it has “smart laser intensity adaptation,” which means that there is automatic, real-time adjustment of sensors settings so that it is able to work on surfaces that might otherwise need to be given a matt spray.

One important process characteristic of the non-contact probe systems used on the arms of the CMMs is that there is a comparatively long stand-off distance—as much as 170 mm in this case—so that there is the ability to reach features of the body-in-white (BIW) that otherwise would be difficult to access, particularly were touch probes to be used in the application.

The system is being used at the Bursa plant for inspecting individual car panels as well as the entire sheet metal structure after all panels have been welded in place.

Before implementing the laser scanning system quality engineers at FIAT-Tofaş wanted to be sure that it would meet the requirements as well as touch probes.

So they ran a number of assessments. For example, they performed comparative benchmarks in accordance with ISO 10360 acceptance and re-verification tests for CMMs. Three criteria were investigated: probing error (MPEP), length measurement error (MPEE) and tactile scanning error (MPETHP).

In the worst case, MPEP was 7 microns after five scans on a ceramic sphere and in one of the zones, 1-micron error was recorded. The catalog value is 15 microns, so all results were acceptable. Five MPEE measurements of a ball bar in seven diagonal positions yielded sigma values under 7 microns. As scanning is done from different angles, the 25-mm sphere needs to be measured at five angular positions. Deviation was found to be 11 microns, within the 12 microns stated in the catalog. All three error values were at least equivalent to those achieved with tactile probing.

Then they tested the system with a Doblò side panel, inspecting its contours and geometrical features with both a touch probe and then the laser scanner. The touch probe results were used as nominal. And the laser scanner showed that it met the requirements (e.g., when tolerance bands of <±0.2 mm are required for surface measurements).

According to Özgür Ogur, Diagnosis Measurement Lab Leader at Tofaş, “Laser scanning fulfills our accuracy requirements and there are no significant differences in accuracy between tactile and scanning. We require 0.2-mm uncertainty on a 5.5-meter diagonal and as the inspection device has to resolve to one-tenth of the tolerance band, the scanners need to measure down to 20 microns. Measurements have proven that the scanners achieve this requirement in line with their listed specifications.”

What’s more, they’ve found that the process is significantly quicker than the conventional method: “Inspection is now twice as fast as when we used touch probes,” Ogur said.

They’ve also determined that by using the scanners on the set of CMMs rather than using them on two portable CMMs, the measuring can be done in a way that is more accurate and repeatable. What’s more, whereas the portable devices would require two operators to manipulate the portable arms, using the DEA BRAVO arms, the procedures are performed automatically.

Ogur noted, “The savings are significant when using scanners instead of tactile probes for feature and surface inspections of BIWs on our CMM.”

The Doblò, incidentally, is a two-time International Van of the Year award winner, and more than 1.3 million units have been sold.