Most workpieces used in aerospace and medical industries are complex alloys and have very close tolerances and as such the sizes, shapes, and forms of the part have to be at its best for a flawless functionality of the assembly. In order to achieve all of this the most advanced CNC machines are used and often times abrasive CNC machines are necessary to hold the closest tolerances.
Abrasive machining is a process of material removal using abrasives such as aluminum oxide, silicon carbide, resin bond and many other abrasives either natural or synthetic. The process involves a gradual removal of material from a workpiece additionally incorporating high-pressure equipment. Common abrasive processes are Grinding, Honing, Sanding, Polishing, Buffing, Lapping, abrasive waterjetting, Sand Blasting and Glass Blasting.
CNC machines have their own limitations of holding close tolerances consistently, and when producing large volumes, it’s sometimes easier to add stock on parts where close tolerances are required and finish all other dimensions using an external process, and this will reduce CNC machine down time and frequent change of the consumable tools and streamline the process, which is why in many cases whenever there is a requirement for good surface finish and/or close tolerances such as flatness, roundness, etc., abrasive machining process is adopted.
Traditional / Conventional CNC machining process involves metal cutting using machine tools that turn and/or mill workpieces resulting in a large chip removal process - in many cases these process yield ‘average’ surface finishes... certainly not those able to be held when utilizing an abrasive machining process. An abrasive machining process involves forcing abrasive particles onto workpiece so that those abrasive particles remove tiny portions of material from the workpiece, which helps the surface finish look far superior to that generated from CNC machining involving cutting tools. This basic same concept is used in grinding, honing, sanding, polishing, buffing, lapping, superfinishing, buffing and abrasive waterjet cutting machines.
Grinding - Grinding is the most common abrasive cutting machine process. Grinding is done using a grinding wheel or abrasive belt on a grinding machine. On a round part an outer diameter (OD) can be ground using a centerless grinder (either by utilizing a through-feed grinder or an end-feed grinder) and inner diameters (ID) can be ground using internal grinding machines. Flat surfaces can be ground using surface grinders. An OD / ID grinding process is used when holding close tolerances on the size (and/or roundness) of both an OD and and ID and surface grinders are used when holding tight tolerances of flat parts. The hardness of he grinding wheel / grinding disc is measured by grit, and grinding wheels usually have a range between 24 and 100 grit. Depending on the size and quality of the surface finish requirement different grinding wheels and//or wheel diameters are available that when properly chosen can be achieve the desired results.
Grinding can be done using traditional grinding machines or on cnc grinders nd be inclusive of automation, allowing some cnc grinding to be preformed unmanned.
Honing - Honing is process that is used to control the size and form of an ID surface. Honing uses a cutting tool called honing stone which is composed of abrasive grains bound together with an adhesive. Honing is slightly different than grinding as it can be characterized as a self-truing grinding process. Like grinding, honing is a process where material removal is done gradually. Honing is often times a slower process than grinding, and the heat and pressures are lower, which means honing typically utilizes better size control. Honing may take longer than some grinding operations, and honing can be done either vertically or horizontally.
Honing can be done using traditional grinders or on cnc honing machines and if a process is setup with the proper automation, cnc honing can be done unmanned.
Sanding - Sanding is a finish process to make the surface of the machined parts look uniform and remove any imperfections and/or to improve corrosion resistance. Sanding is usually performed by high-speed sanding discs. There are many sanding discs available in the market and sanding discs come in various sizes and shapes most of which are Resin bond fiber discs.
Polishing - Polishing is also again a finishing process where material removal is very minimal. This process involves the multiple applications of finer and higher grit abrasives along with a suitable polishing compound to yield the desired finish. Electropolishing is another process that creates smooth, corrosion-resistant surface that increases the life for many critical aerospace and medical parts..
Buffing - Buffing is performed to remove burrs and enhance surface finishes. Buffing is performed using the aid of stationary polishers and die grinders and can be additionally supported by automation using robots. Buffing using abrasive wheels is most commonly alongside machines that are processing parts and done so prior to the part moving to next operation, and are used to remove small burrs and sharp edges on the parts which cannot be obtained from tumbling.
Lapping - Lapping is used for material removal from flat parts. Lapping is a process involving two surfaces having an abrasive between them. Here the part is ‘sandwiched’ between two abrasive wheels, allowing both sides of the workpiece to be addressed. Lapping uses loose abrasive instead of the more commonly bonded abrasives used in grinding wheels. Lapping is often used to obtain specific surface roughness and it is often used to generate mirror-like surface finishes.
Abrasive Waterjet Cutting - Waterjet cutting is simply an accelerated erosion process - highly pressurized water is sent through nozzle into a mixing chamber which creates a vacuum and draws garnet sand into the stream where the mixture is then sent through a high pressure nozzle and towards a workpiece for cutting. The sand particles cut through the material literally tearing the metal away.
Waterjet cutting machine contains three main components
Other components includes abrasive hopper and catcher tank - the abrasive hopper will provide a measured flow of granular abrasive to the nozzle. The catcher tank is filled with water to dissipate the energy from the waterjet cutting machine after it has cut through the workpiece.
There are significant benefits of abrasive waterjet cutting compared to traditional / conventional machining, some of which are :
Because of its wide range of benefits, waterjet cutting machines have been proven to be the very versatile and safe, and it’s ability to cut thicker and a wider range of materials is additionally why it has become one of the favorite cutting machine options for manufacturing industries utilizing harder workpieces.
Sand / Glass Blasting - These processes are used to etch the surface using an abrasive media. Different abrasive materials are available that range from fine to extremely coarse - the coarser the grit, the faster it will ‘blast’ and define the texture. Some of the abrasive media used are Sand, Garnet, Aluminum Oxide, Silicon Carbide dust.
There are some other tools / machines like abrasive cut-off, cutting disc, cutting wheels, abrasive disc which are used for various metal cutting machines / deburring machines.
Abrasive machining can help achieve a wide variety of surface characteristics on any workpiece made from any number of metal. They operate using less consumable tooling than those used on machine tools that cut material, and as such their productivity is significantly higher for maintaining tight tolerances and excellent surface finishes.