Quality Applications

Renishaw's MP10 Machine Tool Probe Benefits Offers ±1 µm (0.00004 in.) repeatability Acts as an omni-directional switch, effective in the ±X, ±Y and +Z directions Full 360° optical machine interface allows probe to be remotely activated by machine control with spindle in any orientation www.renishaw.com

Machine Probe Helps Meet New Boeing Tolerances
Renishaw's MP10 Machine Tool Probe

Shops that machine aircraft parts have traditionally been asked to hold some of the tightest tolerances in the metalworking industry. Now, with its Accurate Fuselage Assembly program, Boeing has begun requiring ultra-tight tolerances that allow fuselage components to snap in place as Legos do. To meet these increased accuracy demands of AFA, Northrop Grumman's Applied Digital Technologies Group not only bought a new machine tool but also fitted it with a Renishaw MP10 machine tool probe for in-cycle gaging of critical assembly hole locations, reducing scrap and speeding throughput of the flight-critical components.

 "Our working tolerances used to be 0.03 in. or better on hole location," says Jeff Howard, ADT manager for Northrop Grumman's aerostructures business in Dallas. "To comply with Boeing's AFA program, we must now work with 0.01 in. true position, which is really half of 0.01 in."

 This and other tight specifications tested the limits of conventional sheet metal frame fabrication methods. The challenge for Northrop Grumman was to find a way to reduce the variability on formed sheet metal to a level comparable to that of computer numerically controlled machined parts.

 The parts in question, called "Z-frames," are arc sections of the frame that make up the barrel for the 747 fuselage. Typically 10 ft. to 18 ft. in length, they are made from roll-formed sheet metal that is subsequently stretch formed, then trimmed and drilled. They have a z-shaped cross section and are 0.063 in. thick.

 When Boeing reduced the design tolerance as a part of the AFA program, Northrop Grumman's existing frame supplier couldn't produce parts that would meet the specifications. After the forming and extruding processes, a typical sheetmetal part's dimensions varied widely. One day, parts may be on the high side of tolerance and the next day on the low side. "If you're trying to hold them in a fixture, the best way you can do that is hold them at nominal," says Matt Turner, a Northrop Grumman engineer. "For example, say we have a raw extrusion that has a 0.03 in. tolerance and we're trying to put a hole in the center with a 0.01 in. true position. If the part is on the high or low side, and you're holding one end at nominal, you're not going to cut a good part."

 As a result, Northrop Grumman decided to insource the frame fabrication and purchased a Komo three-axis router (28 in. long, 16 in. wide and Z of 24 in.) to produce the parts in compliance with AFA. On paper, the machine's tolerances show it has the accuracy to do the job. However, due to thermal changes and setup difficulties, the large sheet metal parts could never be fixtured in a way that allowed Northrop Grumman to accurately or repeatably drill the holes within specification.

 That's where the MP10 probe comes into play. Rather than setting up the part and probing the fixture, the fixtured part is measured with the probe before cutting, making machine axis compensations to put the part into the center of the tolerance band. "If the part is too wide or necked down, we can compensate," explains Turner. "There was no way we could do this through fixturing."

 With the new machine and Renishaw probe, Northrop Grumman has cut about 1,700 parts to specification. "We establish, before machining, that the part can be a good part after cutting," says Turner. "We don't waste time on a raw part that can't meet specifications."

 The MP10 touch-trigger probe acts as an omni-directional switch, effective in the ±X, ±Y and +Z directions. The probe is automatically spindle-loaded and then driven against the workpiece or fixture. Each contact generates a signal, relating the contact point to a common datum. Repeatability of the probe is ±0.00004 in. (±1 µm).

 Northrop Grumman uses two MP10 probes on the Komo router. One has a 2 mm stylus--smaller than traditional machine styli--which the company achieves by necking-down a coordinate measuring machine stylus. "Our smallest holes, No. 40s, have a diameter of 0.098 in., so we needed the 2 mm (0.05 in.) styli." Turner describes the holes as "determinant" holes used to bolt the parts together, while the rest of assembly is riveted together along mating surfaces.

 "We have parts with as many as 50 holes," says Turner. "Right now we're doing 100-percent inspection of the part with the probes, too, but we're planning to move to sampling as we firm up our statistical process control data and prove that we have a repeatable process. We're at a point of confidence where the post-machining probing will probably go away or be minimized," says Howard.

 A second MP10 probe with a larger stylus probes the outer contour of the barrel. The resultant data is sent electronically to AFA program engineers and can be used to analyze how any given part will mate up with other parts. The information is unique to individual parts, which are serialized with a work order number.

 "There is strong angularity on the outer contours, and to make sure we're not hitting the shank of the probe, we have to use the larger stylus," says Howard.

 Northrop Grumman is currently working on software to streamline the process. It will take real-time SPC data on the floor and give operators instant feedback. It will also transmit the data directly to the customer, all transparent to the operator.

 Using CATIA files, Howard's group has written a post processor that writes raw machine code and calls up the Renishaw macros. Parts take about 10-15 minutes to machine, and probing consumes about half of that time.

 "If you look at the entire process, we've made a quantum leap," says Howard. "We 'fit' the machine within the needed tolerance. Probing lets us locate the part and adjust the machine's axes to get right into the middle of the part's tolerance band."

 "The results in the next assembly at Northrop Grumman's Hawthorne, California, facility have been impressive," adds Turner. "Probing with the MP10 has eliminated assembly problems with the Z-frames and has helped us achieve Boeing's goal of a snap-together fuselage."