Aerospace parts manufacturers face numerous machining and finishing challenges, one being the introduction of new alloyed materials. To increase fuel efficiency and produce lighter planes, incorporating materials that are lighter, stronger, and can better manage heat is crucial. While these alloys have amazing characteristics, they can sometimes be difficult to process, resulting in the need for grinding expertise and the latest technologies. Without optimizing the grinding process, parts made with these new alloys may have poor surface quality, internal metallurgical damage, increased part cycle times, and higher manufacturing costs.
Another challenge is bottlenecks on the manufacturing floor. Some customers use wire electric discharge machining (EDM) to carve profiles and shapes into various aerospace components. While EDM is effective for parts with faces that are tough to access, grinding may be a better option for many operations. Wire EDM machining is initially less expensive than grinding, however, using EDM can take a long time to complete a part. Grinding is much faster in removing material, and while it may be more expensive upfront, the benefits of grinding – saving production time, unclogging bottlenecks, and having a smooth-running line – almost always offset the cost and result in a substantially more efficient process.
Grinding vs. machining
Aerospace components generally have very low Ra surface finish requirements and tighter dimensional tolerances, as well as precise complex shapes and forms. Grinding is much better at producing these precision parts and holding necessary shape and dimensional tolerances due to the way the material is removed. During grinding and machining, the material ground off is removed as chips. In grinding, much smaller chips are created which allows for more precise shapes and smoother surface finishes, while machining produces significantly larger chip formations.
Due to the larger chips and aggressive cuts of material, traditionally, machining has generated higher material removal rates (MMR) than grinding. With newer grinding technology, this isn’t necessarily true. For example, new Norton TQX grain technology has been able to hit Q’ (Q- prime specific material removal rate) values of >3in3/min/in in creepfeed grinding of aerospace components, which is usually the max Q’ achieved with ceramic grain bonded wheels. These values rival those of machining processes. Figure A (pg. 30) shows the improvement of TQX versus other ceramic grains. With the other ceramic grains, as Q’ is increased, G-ratio (the volume of material removed from the work per unit volume of wheel wear) declines and eventually bottoms out at about 3.5in3/min/in. Alternatively, TQX can increase G-ratio with increased Q’ up to 2.5in3/min/in and maintain this high G-ratio at Q’s over 2.5in3/min/in, but in this case, TQX reaches a Q’ of 5.5in3/min/in. This new technology challenges machining processes, producing parts with improved quality and eliminating the need for further finishing processes.
How materials affect grinding
Different material types affect grinding processes, depending on the material hardness, variety of material properties (especially newer alloys), and behavior of the material once it heats up (among other operational factors). This means one grinding wheel type or specification may not work for all materials or it may not work for various material types within similar applications. It depends on many factors, so consult with a grinding expert to determine grinding wheel requirements based on material type, and how to consolidate specifications and wheel sizes to keep inventory levels low.
One issue often seen with some newer alloys and more exotic materials, typically used for turbine blades, is they become gummy or act soft upon grinding, especially prior to heat treatment. Examples include Rene 108 and Inconel. Grinding gummy materials can load up a grinding wheel, which will result in heat generation, burn, poor surface quality, and the need for additional dressing. In this scenario, generally a softer grade, porous grinding wheel is needed with a friable abrasive that’ll fracture a bit easier and produce new, sharp cutting points to continue grinding through the soft material.
Additive manufacturing (AM) for aerospace components adds simplicity to making complex parts, including those with intricate internal shapes and channels. It also gives the ability to produce parts with different metals or materials that weren’t in use before due to difficulty in grinding, machining, etc. With AM growing, grinding is still relevant. For example, an abrasive cut-off wheel can separate 3D-printed parts from their build plate. Afterward, a grinding wheel can grind down this metal plate, flattening it, removing any additive material, and ensuring the appropriate surface finish so the plate can be reused for the next part.
Abrasives are also used in AM finishing steps such as improving dimensional tolerances and surface quality. Although the growing use of AM overall will decrease the need for creepfeed grinding of aerospace components, new requirements will move toward deburring and finish grinding. Parts with precision features will require finish grinding regardless of how they are initially produced. The main difference with finishing 3D-printed parts compared with finishing traditionally made parts may just be that additively manufactured parts need less stock removed to get to the final geometry and surface finish.