Supporting Automated Processes
- Tool location and probing
- Ultrasonic knife cutting and trimming
- Stitching material
- Material slitting
- Nondestructive testing and inspection
Each time a mandrel or form is mounted on a machine, it may be necessary to probe a few locations on the form to determine its exact location. The information collected in this process will typically be used to adjust the datum of the machine's coordinate system, prior to laying material on the form. A probe, usually a ball of precisely known diameter, is mounted on the machine's head and a program drives it around a sequence of positions. At each probed location there could be a hole of known size, or a feature such as a ball or puck. Once the probe is close to the feature, a probing subroutine or macro is invoked to determine the feature's precise machine coordinates.
There are alternatives to an automated probing location process, but by comparison, they take much longer, and rely heavily both on human expertise and iterative methods.
Rather than using manual methods of locating a manufacturing tool within a work cell, or using fixed locaters, automated methods can be used to repeat this tedious process. Lasers and sensors can be used to accurately and repeatedly establish a location relationship between a tool’s form and the machine.
There are several benefits to cutting and trimming the part in its uncured “green” state, rather than waiting until the part is cured, in the case of thermoset materials. For example, certain manufacturing processes such as preparing the part for autoclave cure and post-cure machining benefit from trimming the part to near-net shape. The most common way to cut excess material away before cure is with an ultrasonic knife (USK).
While cutting can be done with shop floor technicians and hand-held knives, low repeatability, reliability, and accuracy result in most firms adopting an automated approach. Cutting is typically an add-on automation process and not the first process to be automated. As a result, it is more feasible for process integrators to add the cutting capability to the existing motion platform, the motion platform that also handled the main form of automation. In most cases, this is the same motion platform as the AFP or ATL process uses, either the same robot or the same gantry.
Cutting can be done in various methods, angles and orientations to produce the desired effect. Angled and straight cuts are both possible. Multiple layers of material can be cut at once, more so than by technician-led efforts, to increase efficiency through reduced processing time. Furthermore, ultrasonic cutting helps significantly reduce the “plowing” effect on the laminate, when compared to mechanical cutters. Lastly, other materials can be cut by easily changing the blade and frequency, such as open-cell, closed-cell, and honeycomb core.
Sharp knives vibrating at ultrasonic frequencies easily cut apart carbon and other types of fiber material. These trimming operations are done before the material has been cured and can reduce time spent processing the cured part downstream.
“In the 1980s researchers looked to textile composites as breakthrough technology. Supporters argued for new concepts which would use knitting, weaving, braiding and through-the-thickness stitching for reinforcement and use existing U.S. textiles manufacturing technology for cost efficiency.” (Source)
Since then, the composite stitching process has evolved to address several aspects of part stiffness as well as producibility. Stitching parts together in addition to curing layers together with thermoset resins increases the inter-laminar strength of the part and is typically used for relatively flat parts, such as wing skins. In addition, stitching can assist with manufacturability by helping to join different components, such as joining stringers to a wing skin. Pioneered largely by Boeing and NASA several decades ago, the technology has not gained nearly as much widespread use when compared to AFP, ATL, and other more common automated composites manufacturing processes. Still, the possibility of providing localized reinforcement only when needed is an attractive tool in the automated manufacturing toolbox, depending on the part and design requirements present. A 2001 study by the DLR in Germany found that “The influence of stitching in comparison to simple, unstitched fabric plaster reinforcements shows an improvement of 42%.” (Source)
Stitching processes are usually integrated into a share motion platform, oftentimes the same platform that AFP or ATL processes use. Modular end-effectors are shared between the same robot or gantry.
Stitching, effectively sewing, material together significantly increases the inter-laminar tensile strength, increasing part performance, and is typically used in conjunction with dry fiber material.
Instead of placing material at parent-roll width, which can commonly be found up to 5.5ft (1.7m) wide, many rolls of material are slit to smaller width. Process-dependent widths are critical parameters that affect machine specification and part design requirements. Accurate widths of material that are readily available is necessary to ensure operational capability. While many external companies offer slitting services, wait times for deliverables can be several months in the future, depending on many factors. To reduce supply chain risk and reduce overall costs, some companies choose to slit their material in-house instead of relying on vendors.
Bringing slitting services in-house helps reduce supply chain risk and ensures that the firm has the material widths they need, just when they need it. Instead of waiting months for a vendor to process their order, firms produce the spools of material just in time for manufacturing efforts. A component of the slitting process involves spooling the slit material onto smaller spools to be loaded directly onto the machine. This re-spooling capability not only allows the spooling of new material, but also facilitates the combining of partially used spools. Instead of starting a new part build with existing partial spools, which could require operators to stop mid-process to load more material, manufacturing operations can combine partially-spent spools into full spools. This process not only reduces non-value-added time (loading new material), but reduces waste through fully using partially spent material spools. Slitting processes are typically wholly independent of other systems (AFP, for example) and require dedicated hardware and floor space.
Unlike hand placed material, which uses wide rolls of material, many automated processes use more narrow pieces of material. Several commercial material providers offer slitting services to fit customer needs. However, in-house slitting operations provide further cost reduction benefit in the long run.
Automated non-destructive inspection processes help ensure complete inspection coverage, perform inspections faster, and have much higher repeatability when compared to manual inspection. These benefits and more help offset the high cost of purchasing the system, which must typically use purpose-built motion platforms, rather than leverage the material deposition motion platform like stitching, probing, and cutting can.
Systems typically involve two halves, one to spray water through which a frequency is emitted, while the other is on the opposite side of the part and “listens” for the frequency. Modulations or discrepancies help identify areas of concern which can consist of foreign objects, debris, pockets of resin rich/starved areas, and porosity, among others.
Rather than manual inspection techniques, automated nondestructive inspection can provide many benefits such as high inspection cell productivity, more-complete coverage of a complex inspection, defect detection, and faster inspection cycle times.