What is helpful to know about this somewhat mysterious process?
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 ability to do precision centerless grinding in addition to machining is a definite advantage.
Despite that fact, centerless grinding has fewer practitioners than machining. And although the centerless grinder has been around for almost a century, a lot of people struggle with the fundamentals of the process and its unique advantages.
So, let’s take a look at 8 basic principles of centerless grinding — things it is helpful (and we hope, interesting) to know about this mature and yet still somewhat unfamiliar process.
1. Centerless grinding picks up where machining leaves off.
A downside of centerless grinding is you can’t have as many multiple axes operating on the workpieces. However, there are many parts where the process addresses the limitations of machining in terms of dimensions, materials, and surface finishes.
That’s why we like to say that where machining ends, the centerless grinding process begins. For instance, if you have a part that is out of round from a turning machine and the part’s diameter is too small or its center is impossible to mount, you can achieve roundness through centerless grinding techniques.
In addition, there is no axial thrust on the workpieces during the centerless process, which means it can be used to grind long pieces of brittle materials and parts that might otherwise be distorted.
2. It’s 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.
Centerless grinding involves the part being supported on a workpiece rest blade that sits between two rotating cylinders:
A regulating wheel, which controls the part’s rotational speed and feed rate (for the in-feed method) or linear travel (for the through-feed method)
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. Because the workpiece is rigidly supported, there is no deflection during the grinding operation.
Centerless grinding is virtually continuous because, compared with grinding between centers, the loading time is small. Long lengths can be ground continuously and even large quantities of small parts can be automatically ground by means of various feeder attachments.
In addition, centerless grinders are capable of performing consistently at high speeds. That makes the process a great choice for high-volume applications in aerospace, automotive, military, medical, and other industries.
3. Methods differ in how parts are fed through the machine.
The primary difference between these two most commonly used methods of centerless grinding is in how the workpieces are fed through the centerless grinding machines.
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, the method basically “squeezes” the workpiece across the grinding wheel and out the other side.
In-feed grinding — 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.
4. The choice of grinding wheel is critical.
Another key factor in centerless grinding is the choice of grinding wheel. It must be suited to both the metal from which the parts are made and the surface finish you want to achieve.
In addition to being available in different diameters and widths/thicknesses, centerless grinding wheels come in different grain types and grit sizes, often using superabrasive materials such as polycrystalline diamond and cubic boron nitride.
These superabrasive and silicon carbide wheel materials are an advantage when centerless grinding very hard metals, for several reasons:
The wheels themselves are durable and maintain their sharpness longer.
They have high thermal conductivity, maintaining their shape at high contact temperatures and at high rotational speeds.
Less time is required for the dressing cycle.
Wheel life is much longer than that of wheels made of materials such as aluminum oxide abrasives.
5. Roundness depends on angles.
There are a number of centerless grinding tips related to angles. The first is, 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.
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, worst case scenario, cause a workpiece/wheel crash.
The angle of the workpiece rest blade is also critical to centerless grinding. 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.
6. Keeping things cool is mandatory.
Coolant is used in centerless grinding 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 in centerless grinding is critically important to preventing 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.
7. You can teach an “old” process new tricks.
Although centerless grinding is a mature process, today’s grinding machines are equipped with newer features that enhance performance. For example, CNC programmable controls 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 are making it possible to produce previously impossible ground shapes, dimensions, and tolerances, while also reducing setup times and accelerating loading and unloading to shorten cycle times of centerless grinding.
These innovations include the latest generation of machines that remove the regulating wheel and replace it with stationary wire supports that have the option of bushing mode. This option allows for intricate ground shapes and exotic dimensional features by performing similarly to the guide bushings on Swiss-style automatic lathes.
In addition, advances such as ever-more capable software controls, direct drive motors, and even robotic loading/unloading of workpieces allow the simple concept of centerless grinding to make complex parts that were previously unthinkable.
8. Experience is part of the centerless grinding skill set.
The centerless process is usually not taught, but rather is a skill acquired from years of being in the portion of industry that supplies centerless grinding services to customers.
So, to get the best results, you'll want a partner that:
Considers centerless grinding important enough to develop an expertise despite its niche demand
Has continued to grow with the industry instead of relying on the original machines from decades ago
For example, from the beginning Metal Cutting has been augmenting our cutting capabilities with centerless grinding for the production of glass to metal seal parts. More than 50 years later, we still perform centerless grinding virtually every day and continue to stay abreast of industry trends and customer demand using the latest generation of equipment.
In the right hands, centerless grinding is capable of producing a "machined surface" that a process such as turning is simply not able to match — both as an Ra value and also on certain metals that are nearly impossible to turn with a cutting tool.
Even if turning is possible, it would never result in the precise material removal and resulting surface finish that a grinding wheel can achieve.
The unique qualities of a ground (vs. turned) finish combined with the innovations and variations that are now available with centerless grinding are what produce metal parts that, while not as commonly required as other metal fabrication methods, are irreplaceable for their applications.