Challenges and Opportunities in Automated Airframe Drilling
Drilling holes for fasteners for airframes accounts for 85% of the quality issues and 80% of the lost time due to injuries in airframe assembly. This is according to noted expert Dr. George ‘Nick’ Bullen, an SME Technical Fellow. He recently published a book capturing his decades of knowledge in developing automated airframe fastening and assembly. He offered more insights in this exclusive interview with AeroDef.
Nick Bullen retired as Principal Engineer and Technical Fellow from Northrop Grumman Corporation at the end of 2010. He also founded the International Aerospace Automation Consortium and is still active in that organization. His credentials and expertise in automated airframe assembly is solid-- he holds 16 patents for technology innovations related to manufacturing, mechanization, robotics, and robotics control software. You can find his latest book “Automated/Mechanized Drilling and Countersinking of Airframes” at http://books.sae.org/book-r-416/.
He was kind enough to give some time discussing automation problems in this exclusive interview with AeroDef:
Why are holes so important in airframe assembly?
Airframes are unlike any other vehicle. They are subjected to severe twists, strains, and stresses along with wide variations in temperatures. They climb to cruising altitudes where the air temperature may be -40 or -50 degrees F after sitting on the tarmac in the hot sun where their skins may have been raised to over 200 degrees F. The best way to hold structures and substructures together under those conditions is with drilled holes filled with fasteners and screws.
These are not simple holes, however. They have severe quality requirements, everything from critical edge distance, hole diameter, to straightness of the hole. For example, the centerline placement of the hole in relationship to the substructure cannot vary more than +/- 0.010 of an inch with a diameter tolerance of - 0.0000/+.0015”.
What happens when they fail?
One bad hole can cause catastrophic failure. That was true a few years ago when a Hawaiian Airlines 737 had an in-air emergency. The top part of the skin, literally, came off. That was caused by a zipper effect when a single hole failed.
Why try and automate?
Besides controlling quality, the economics are important as well. 65% of the cost of an aircraft is in its air frame, of which 65% is in assembly. Of that - I know this sounds funny -but 65% of the assembly is drilling and countersinking holes. So 27% of the total cost of most airframes is drilling and countersinking. In just the tail section of the F-18 fighter, we at Northrop Grumman estimated the cost at 40 to 45% for drilling alone.
There must have been many challenges in automating the process early on?
You could not take standard machine technology and do this because of the variation in the assembly and the dissimilarity of the materials that you were using to put the airframe together. Dissimilar materials were important too, with titanium, aluminum, and carbon fiber all used in airframes.
Adjusting for the human factor was vital because mechanics would work [and adapt] to the variation. They could drill pilot holes with drills much smaller than the final hole to see if there were problems with edge quality and delamination.
Automation needed to do it in one shot, perfectly. So, first we had to figure out ways to drill holes perfectly. We figured out ways to do “peck” drilling and other precision methods [as a precursor] to automation.
How have you seen automation progress in airframe drilling?
The first phases of automation involved putting plates for mounting lead screws directly on to monument style fixed assembly jigs. The second phase was purpose-built automated drilling machines. They were in-line machines, where you slid the airframe structure into these dedicated machines designed to drill a particular section of a particular aircraft.
Now we have progressed to the third stage, using small light-weight articulated arm robots that fit into assembly cells. These are similar to what you see in automotive assembly lines, but highly accurate with precision guidance from lasers and other devices.
Is automation always the solution for lower cost drilling and assembly?
Not always. I like to phrase it in terms of three Don’ts - Needs, Affordability, and Complexity.
Sometimes, you simply do not need it. I visited a fly-away factory for a military aircraft and found that the labor rates were so low--and the production volumes so low--they really were not going to gain anything by investing in automation.
Sometimes it sounds really cool to do automation, but the requirement just is not there. You see that today with drones. We looked at Global Hawk and saw such low volumes that the return-on-investment just was not there. It would end-up costing more per hole after investing in automation than doing it by hand.
Finally, there are some structures that are so complex, with small interior spaces and trying to apply adequate drill forces, that automation simply will not work.
There are tasks that people do better than machines, at least in aerospace right now.