Waterjets & Airframes
A look at machining aircraft composite structures with abrasive waterjets.
By Mohamed Hashish
Senior Vice President of Technology
Flow International Corp.
Abrasive Waterjets (AWJ) are commonly used now for trimming of composite structures. Composite Machining Centers (CMC) may also include routing and diamond drilling tools. The use of organic matrix composites (OMC), especially carbon-fiber-based composites for aerostructures have been rising steadily since the 1960s. Carbon composites were first used on military airframes and now are extensively used on commercial aircraft.
Most revolutionary is the use of composites on the Boeing 787 and Airbus A350XWB which will contain 50% composite structure by weight and 90% by volume. In comparison, the 777, which entered service over 10 years ago, contains only 10% composite structure by weight.
Airframe components made out of carbon composites range in size from relatively large such as the wing box and fuselage to small parts such as clips and doors. The growing use of composite materials on primary aircraft structures requires technology offering lower manufacturing and tooling costs, and reduced potential for product defects. Also, more complex 3-D geometries, small and large-size parts, as well as hard to access geometries need to be machined.
Traditionally, conventional cutting tools—hand-held diamond or carbide-tipped routers, bandsaws, cutoff saws and abrasive wheels—were used to cut composites. However, due to the composition and fiber orientation of advanced composites, these traditional cutting tools can damage the composites either by overheating, or by leaving frayed or delaminated edges. Other machining technologies such as laser, ultrasonic machining, and EDM have limited applications.
The AWJ technology offers several advantages over conventionalmachining methods which can resolve many of the above problems. Among these advantages are:
-No distortion due to limited jet forces and its micromachining action;
-No heat affected zones;
-Higher cutting speeds than routers;
-Reduced fixturing and tooling;
-No subsequent processes are needed;
-No splintering or fraying edges;
-Process automation and multi-operations are possible;
What follows is a summary of some AWJ composite machining applications.
Composite trimming, shape cutting and drilling are the most commonly used AWJ applications. However, the AWJ process has been successfully demonstrated for turning and milling.
Significant work has been done on the use of waterjets and abrasive-waterjets for composite parts trimming. This trimming is required as parts are typically oversized due to the composite manufacturing process. For AWJ trimming of composites, a six-axis machine is used to manipulate the jet and also to position the catcher under the jet.
The standard AWJ cutting head’s water body is designed with enough length to provide high-coherency waterjets while the lower sections for abrasive entrainment, mixing and accelerations are designed for producing high-efficiency AWJs. Vacuum assist has been critical for piercing holes in composites. It ensures that abrasives are in the mixing chamber before firing the waterjets. Delamitation does not occur when piercing composites using this approach.
When cutting composites with an AWJ, the surface finish and surface integrity results are important. It is known that there are three phenomena associated with AWJ cutting that must be considered in trimming and shape cutting. The first is that the jet is deflected opposite to the direction of the motion. This means that the exit of the jet from the material lags behind the point at the top of the material where the jet enters. The distance the exit lags the entrance is typically called the trail-back, lag, or drag.
For trimming, the location of the catcher must be considered to accommodate for the jet trailback angles (~ 10–20º) of deflection. An additional degree of freedom may be needed to automatically position the catcher relative to the jet. Also, the mouth of the catcher must be wide enough to accommodate jet spreading.
The second phenomenon is that the width of the jet varies along the cut from top to bottom. This difference in width is typically called the taper of the cut. A taper can be either positive or negative, that is, the width at the exit of the cut may either be smaller or larger than the width at the top. Typically, the kerf width at the exit side is smaller than that at the entry at practical cutting speeds. For trimming of straight sections with minimal thickness variation, a fixed taper angle can be included in mounting so that cut taper is compensated for.
The third phenomenon is related to the surface finish of the cut. Similar to other beam-like tools, striations (waviness) will form along a cut surface, especially near the exit and at relatively high cutting speeds. The surface roughness when stria-tions are eliminated is only a function of the abrasive particle size. It is typically specified for composite cutting that surfaces should be better than 10 µm.
While the majority of trimming applications addresses relatively thin sections (less than 19-mm), there are several thick-section (up to 76-mm) cutting applications near the wing-to-fuselage joining areas. For thick sections (~76 mm), the taper and bow (surface curvature) are important parameters. For example, a 600 MPa jet produces less taper than a 380 MPa cut for the same orifice size (more power is used at 600 MPa). For the same taper, the cutting speed for the 600 MPa jet is substantially higher than the case of 380 MPa jet. The main criterion for selecting the final parameters is to meet the 10-µm requirement as a maximum allowable surface finish. The cut taper, though related to surface finish, is not used for selecting the cutting parameters as the taper angle can be compensated for.
Shape-Cutting and Drilling
Shape cutting may require piercing of a starting hole. Some systems use a mechanical drill to pre-drill the starting holes; but most commonly now, AWJ is used to drill these holes as part of the shape cutting sequence. In this case, vacuum-assist nozzles are used to mitigate risks of delamination. The Raytheon Premier 1 was the first all-composite fuselage aircraft built. An AWJ mounted on a five-axis column type machine was used to shape cut the opening in the fuselage. For these types of applications, special jet catchers are used to catch and contain the spent jet.
Delamination is the main mode of failure when drilling materials such as graphite epoxy with waterjets. This problem is resolved when vacuum assist and proper parameters are used. Also, with proper process timing, holes can be drilled rapidly. While not as fast as high-speed drills in relatively thin materials, the holes are typically of high integrity and good surface finish. The pierced hole size can be controlled by selecting the process parameters and the dwell time. The larger the dwell time, the larger the final hole size at the exit.
Accurate control over the dwell time is needed to control the hole size within certain tolerance limits. Another interesting observation is that the drilling time is improved by reducing the water flow rate and increasing the abrasive flow rate into the hole.
Drilling deeper holes may require pressure ramping and/or trepanning. Pressure ramping is needed because the return flow affects the jet momentum and the drilling speed. Also, the impact of the jet at high pressure may cause a shock wave impact that may result in delamination. Trepanning is used to provide an improved and larger escape path for the return flow. The drilling time can be significantly reduced (by 300%, for example) with these techniques.
Hybrid Waterjet Systems
In order to trim and route composites, both waterjets and solid tool routers have been incorporated on special hybrid systems. In these systems, two five-axis masts are used: one for the AWJ and another for the router. The AWJ is used to trim the part using an end effector. A small catcher is provided on a sixth axis for catching the exit jet and directing the waste outside of the wrist area to the collection tank. The router is used to drill and countersink the required holes or trim some critical areas not easy to address with the AWJ.
This system provides significant advantage in minimizing setup time. Machines range in size from 10 to 150' (3–46 m) in length with width up to 50' (15.2 m). Probing and linear scales are used for obtaining the required accuracy.
Advances in the AWJ process, tools, and machinery has made AWJ a mainstream tool for composite machining. Today, all airframe manufacturers use AWJ for trimming composites. However, fixturing and probing still need improvement. New software and off-line programming have also been critical to the acceptance of the AWJ systems in the aircraft industry. ✈