In the high precision machining world, turning tools such as 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 precise machining is a definite advantage.
How does precision centerless grinding work?
Precision centerless grinding is one of several machining processes that use abrasive cutting to remove material from a part (workpiece). The process 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 in-feed grinding) or linear travel (for through-feed grinding)
- A larger abrasive grinding wheel
The beauty of precision centerless grinding is that the workpiece is held in place by the pressure of the rotating wheels. No fixturing is required, so the setup is simple and turnaround times are fast. And because the workpiece is rigidly supported, there is no deflection during the grinding operation.
But despite these and other advantages, precision 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 how it works.
What else is helpful to know about this somewhat mysterious process? 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. Precision centerless grinding picks up where metal CNC machining leaves off.
A downside of precision centerless grinding is, unlike machining small metal parts, you can’t have as many multiple axes operating on the workpieces. However, there are many small machined parts where the centerless 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 precision centerless grinding techniques.
In addition, there is no axial thrust on workpieces during the centerless process. That means it can be used to grind long pieces of brittle materials and parts that might otherwise be distorted.
2. The centerless grinding process is deceptively simple yet precise.
Since they owe much of their functionality to some basic principles of physics, centerless grinders don’t have a lot of moving parts. 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.
Precision centerless grinding is virtually continuous because, compared with grinding between centers, the loading time is small. So, long lengths can be ground continuously. Even large quantities of small parts can be automatically ground by means of various feeder attachments.
In addition, centerless grinders can perform consistently at high speeds. That makes the process a great choice for high-volume applications in aerospace, automotive, military, medical, and other industries.
3. Grinding methods differ in how parts are fed through the machine.
The primary difference between the two most commonly used methods of precision centerless grinding is in how the workpieces are fed through the machine.
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 through-feed 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, and the grinding and regulating wheels must be dressed to match the part’s desired profile cut.
With the in-feed method, 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.
Superabrasive and silicon carbide wheel materials are an advantage when you are centerless grinding extremely hard metals, for several reasons:
- The wheels are durable and maintain their sharpness longer.
- They have high thermal conductivity, maintaining their shape at high contact temperatures and 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. Angles affect success in precision centerless grinding.
The angles at which the centerless grinding 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 (1) in contact with the regulating wheel and (2) 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 go too far into the grinding zone. This can result 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. For example, when grinding with a 4” (101.6 mm) wide superabrasive wheel, the rest blade will generally work well at 30º.
But 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. In this case, changing the angle to 20º or 25º will reduce the pressure and eliminate the chatter on the part.
6. Keeping things cool is mandatory in precision centerless grinding.
Coolant is used in centerless grinding to not only keep the grinding wheel cool, but also remove heat from the zone where the workpiece contacts the grinding wheel.
Centerless grinding requires the use of correctly pressurized coolant to overcome the air barrier created between the grinding wheel and workpiece during the grinding process. This allows the coolant to flow in the space between the two.
The coolant step in centerless grinding is critical 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 has been around a long time, today’s machines are equipped with newer features that enhance performance.
To increase process efficiency and productivity, CNC programmable controls make 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
- Reduce setup times
- Accelerate loading and unloading to shorten cycle times
For example, the latest generation of centerless grinding machines remove the regulating wheel and replace it with stationary wire supports that have a bushing mode option. This option allows for intricately ground shapes and exotic dimensional features by performing similarly to the guide bushings on Swiss-style automatic lathes.
In addition, advances in software controls, direct drive motors, and 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. Rather, it is a skill often acquired from years of working 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 we continue to stay abreast of industry trends and customer demand by using the latest generation of equipment.
Why consider using precision centerless grinding?
In the right hands, centerless grinding is capable of producing a “machined surface” that a process such as turning simply cannot match — both as an Ra value and also on certain metals that are nearly impossible to turn with a cutting tool.
Even where turning is possible, it would never produce the precise material removal and resulting surface finish that a grinding wheel can achieve.
Almost 100 years after its inception, centerless grinding is still not as common as other metal fabrication methods. Yet, the unique qualities of a ground (vs. turned) finish combined with the innovations and variations available with centerless grinding allow it to produce metal parts that are irreplaceable for their applications.
How do you choose a precision grinding process? Learn more here.