Where Machining Ends, Centerless Grinding Begins
In the machining world, turning gets all the attention. Lathes and mills are the flashy stars — in fact, they ARE machining to most people. However, the centerless grinding process is also very important. Although it has a smaller cast of players and comparatively fewer people know much about it, if you have the ability to do precision centerless grinding in addition to machining, you have a definite advantage.
Let’s Turn To The Centerless Grinding Process.
While the downside of the centerless grinding process is you cannot have as many multiple axes operating on the workpieces, there are many machine parts where the process addresses the limitations of machining in terms of dimensions, materials, and surface finishes. Hence, where machining ends, centerless grinding begins. For instance, you can take a part that is out of round from a turning machine and achieve roundness through centerless grinding. In addition, there is no axial thrust on the workpieces during the process, which means it can be used to grind long pieces of brittle materials and parts that might otherwise be distorted.
At this point, you might think we’d delve into the types of products you can make with the centerless grinder, which has been around for almost a century. However, since a lot of people really struggle with the fundamentals of the process and its unique advantages, we will turn to some basic principles of precision centerless grinding — things it is helpful (and we hope, interesting) to know about this mature and yet still somewhat mysterious process.
1. The Centerless Grinding Process Is Deceptively Simple Yet Precise.
Centerless grinders don’t have a lot of moving parts and instead owe much of their functionality to some basic principles of physics. That makes centerless grinding a relatively simple process that’s ideal for finishing the outside diameter of small cylindrical metal parts requiring a tight tolerance. The grinders are also capable of performing consistently at high speeds, making the process great for high-volume applications in aerospace, automotive, military, medical, and other industries.
The centerless grinding process involves the part being supported on a workpiece rest blade that sits between two rotating cylinders: a regulating wheel, which controls the rotational speed and feed rate (for the in-feed grinding method) or linear travel (for through-feed grinding) of the part, and a larger abrasive grinding wheel. The workpiece is held in place by the pressure of the rotating wheels, with no fixturing required — a factor that simplifies setup and makes for fast turnaround times.
Centerless grinding is virtually continuous because, compared with grinding between centers, the loading time is small. Long lengths are ground continuously or even large quantities of small parts can be automatically ground by means of various feeder attachments. Because the workpiece is rigidly supported, there is no deflection during the grinding operation.
2. There’s A Choice Between Through-Feed And In-Feed.
As we mentioned above, there are two methods used in the centerless grinding process. Through-feed grinding is typically used for parts with consistent roundness across the length of the part. In this method, the workpiece travels along the rest blade between the two wheels. Driven by a slight angle applied to the regulating wheel relative to the grinding wheel, through-feed basically “squeezes” the workpiece across the grinding wheel and out the other side.
The in-feed method (also called plunge grinding) is used to grind cylindrical parts with notches or complex shapes, such as gear shafts. Here, the workpiece rest blade needs to be tooled to match the shape of the part; the grinding and regulating wheels must be dressed to match the part’s desired profile cut. The regulating wheel spins the part at one speed while pushing it towards the grinding wheel, which is spinning at a faster speed; the greater the difference in speeds, the faster the removal rate.
3. The Wheel’s The Thing.
A key factor in the centerless grinding process is the choice of the grinding wheel. It must be suited to both the metal from which your parts are made and the surface finish you want to achieve. The wheels come in different grain types and grit sizes, often using superabrasive materials such as polycrystalline diamond and cubic boron nitride, and are available in different diameters and widths/thicknesses.
Superabrasive and silicon carbide wheel materials are an advantage when grinding very hard metals, because the wheels themselves are durable and maintain their sharpness longer; they also have high thermal conductivity, maintaining their shape at high contact temperatures and at high rotational speeds. In addition, less time is required for the dressing cycle, and wheel life is much longer than that of wheels made of materials such as aluminum oxide abrasives.
4. Roundness Depends On Angles.
In the centerless grinding process, the angles at which the wheels contact the part are critical to achieving the proper roundness and tolerance. Generally, the centers of the regulating and grinding wheels are set at the same height on the machine, and the center of the workpiece is situated higher. However, if the workpiece is set too high, it may exhibit chatter; if the workpiece is set too low, it may be out of round.
The goal is to keep the part in contact with the regulating wheel and rotating at a slower speed while the faster, larger abrasive grinding wheel applies the force that creates the precise roundness of the part. Using the correct wheel angles helps to ensure that the entire surface of the grinding wheel is in use. In addition, if the angle of the regulating wheel is too acute, it can cause the workpiece to enter too far into the grinding zone, resulting in uneven wear, tapering, and reduced wheel life. If the regulating wheel is too close to parallel with the grinding wheel, it can cause the parts to stall between the wheels — or, worse case scenario, cause a workpiece/wheel crash.
The angle of the workpiece rest blade is also critical to the centerless grinding process. For example, when grinding with a 4” (101.6 mm) wide superabrasive wheel, the rest blade will generally work well at 30º. However, with a wheel width of 6” (152.4 mm) or 8” (203.2 mm), that same angle may generate too much pressure toward the grinding wheel and cause chatter. Changing the angle to 20º or 25º will reduce the pressure and eliminate the chatter on the part.
5. The Pressure Is On To Be Cool.
Coolant is used in the centerless grinding process to not only keep the grinding wheel cool, but also remove heat from the zone where the workpiece makes contact with the grinding wheel. Accomplishing proper cooling requires the use of correctly pressurized coolant to overcome the air barrier created between the grinding wheel and workpiece during the grinding process, allowing the coolant to flow in the space between the two.
This step is critically important to preventing grinding heat from returning to the workpiece or the grinding wheel. Otherwise, it can be difficult to hold tolerances for roundness and straightness, and thermal damage can even cause the grinding wheel to blister and crack.
6. You Can Teach An “Old” Process New Tricks.
Although centerless grinding is a mature process, today’s grinding machines are equipped with newer features such as CNC programmable controls, which increase process efficiency and productivity by making it even easier to set up and change the equipment from one job to the next. Other newer technologies that are reducing setup times and enhancing already tight tolerance capabilities include direct drive motors and robotic loading/unloading of workpieces.
7. It’s Rare To Do It All — But We Do!
A lot of machine shops may have a hand grinder or a manual surface grinder available somewhere in-house. But it is truly unusual to have full through-feed and in-feed centerless grinding capabilities in the same facility, as we do here at Metal Cutting. In fact, our full-service precision operations also offer CNC surface grinding, double disk grinding, lapping, and other processes, in addition to our extensive precision cut-off expertise. Our equipment and skilled machinists are capable of generating specific surface diameter finishes and creating highly customized ends, tapers, and diameters. That makes us a smart choice for the centerless grinding process, as well as parts cut-off, machining, and other techniques that may be required for your application.
For tips on how to choose the best supplier for your metal fabrication needs, download our free guide 7 Secrets to Choosing a New Contract Partner: Technical Guide to Outsourcing Your Precision Metal Fabrication.
Source: http://metalcutting.com/centerless-grinding-process/