8 Principles of Precision Centerless Grinding

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.

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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.

Grinding is an abrasive machining process capable of achieving tolerances and surface finishes unattainable by any other process. When dimensional accuracy is unobtainable with milling, turning or electrical discharge machining (EDM), or when tolerances below ±0. inch are required, grinding steps in. Grinding can repeatedly deliver accuracy as tight as ±0. inch and do so repeatedly and reliably under proper conditions. Only honing can produce bore sizing tolerances below that which grinding can deliver.

If you want to learn more, please visit our website CNC shaft turning machine.

 Automotive, aerospace, medical, machine tools, die/mold, energy, tooling and general products are but a few industries that utilize grinding daily. The type of grinding machines available in the market vary by design, based upon the specific parts or components being produced.

Grinding Machine types include: surface grinders, cylindrical, tool and cutter grinders, thread, gear, and cam and crankshaft grinders. Grinding machines can be further divided by the type of grinding they perform, such as surface, form, ID, OD, thread, plunge, centerless and through-feed grinding. Although manually operated toolroom grinders are still available, full CNC machines are now the norm, largely because of their high productivity and capability for unattended operation.

Two Main Reasons to Grind: Accuracy and Surface Finish

In addition to high accuracy, surface finish is a primary reason for using grinding. Typically, a milling machine can produce a surface finish of around 32 microinch Ra and a lathe can produce a surface finish of around 16 microinch Ra. Grinding is required for a surface finish of 16 micro inch Ra and below. In fact, grinding can produce a super finish of 8 microinch Ra and below, and in some cases achieve a 2-microinch Ra, considered to be a micro finish. Super finishes are accomplished using two different, fine-grit abrasive wheels, as well as a polishing wheel when necessary.

When grinding for accuracy or surface finish, the amount of material left to remove after machining is usually somewhere around 0.010 inch. The finer the surface finish required, the finer the wheel grit or polishing wheel needed. The cycle time to achieve the finished part size also becomes longer. Ideally, the least amount of material should be left after machining to provide just enough stock for the grinding operation to clean up to finish size. This approach will provide the optimum cycle time for the grinding operation.

Abrasive Grinding Basics

Grinding wheels are available in a multitude of sizes, diameters, thicknesses, grit sizes and bonds. Abrasives are measured in grit or particle size, and range from 8-24 grit (coarse), 30-60 (medium), 70-180 (fine) and 220-1,200 (very fine). Coarser grades are used where relatively high volumes of material must be removed. Finer grades are generally used after a coarser grade to produce a higher surface finish.

Grinding wheels are made from a variety of abrasive materials including silicon carbide (generally used for non-ferrous metals); aluminum oxide (used for ferrous high-tensile-strength alloys and wood; diamond (used for ceramic grinding or final polishing); and cubic boron nitride (generally used for steel alloys).

Abrasives can be further classified as bonded, coated or metal-bonded. Bonded abrasives consist of abrasive grits that have been mixed with binders and then pressed into the shape of a wheel. They are fired at a high temperature to form a glassy matrix, commonly known as vitrified abrasives. Coated abrasives are made of abrasive grits bonded with resin and/or glue to flexible substrates such as paper or fiber. This method is most often used for belts, sheets and flap disks. Metal-bonded abrasives, most notably diamond, are held together in a metal matrix in the form of a precision wheel. The metal matrix is designed to wear away to expose the abrasive media.

Types of Wheel Dressers for Grinding Operations

During the grinding process, the abrasive wheel can wear, become dull, lose its profile form or “load up” as swarf or chips stick to the abrasive. Then, rather than cutting, the abrasive wheel begins rubbing the workpiece. This condition creates heat and reduces the effectiveness of the wheel. When the wheel loads up, chattering will occur, and the workpiece surface finish will be affected. Cycle times will increase. At this point, the wheel must be “dressed” to sharpen the wheel, thereby removing any material lodged on its surface and returning the wheel to its proper form, as well as bringing fresh abrasive grit to the surface.

Many types of wheel dressers are utilized in grinding. Most common is a single-point, static, on-board diamond dresser that sits in a block, usually positioned on the machine’s headstock or tailstock. The face of the grinding wheel is passed over this single-point diamond and a small quantity of the abrasive wheel is removed to sharpen it. Two or three diamond blocks can be used to dress the face, sides and form of the wheel.

Rotary dressing is now becoming a popular method. A rotary wheel dresser is coated with hundreds of diamonds. It is often used in creep-feed grinding applications. Many manufacturers have found rotary dressing to be superior to single-point or cluster dressing for processes that require high part production and/or close part tolerance. With the introduction of vitrified superabrasive grinding wheels, rotary dressing has become a necessity.

A swing dresser is yet another type of dresser that is used for large form wheels which require deeper and longer dressing travel.

Off-line dressers are used primarily to sharpen the wheel away from the machine while using an optical comparator to verify form profiles. Some grinding machines use wire EDM to dress metal-bonded grinding wheels still mounted in the grinder.

Grinding Machine Construction

On a surface grinder, workpieces are most often held with a magnetic chuck, vacuum chuck or special fixtures bolted directly to the table. For cylindrical grinding, the workpiece is normally held between centers, in a collet, with a three- or four-jaw chuck or on special fixtures. Tool and cutter grinders most often use precision collets.

Counteracting Vibration and Friction for Consistency

To consistently produce part accuracy of 0. inch and below, with super finishes under 16 microinches, grinding machines must be designed to control vibration and thermal growth. Machine bases are often constructed from granite or special epoxies to minimize thermal expansion and vibration. Any vibration in the machine will directly affect the surface finish of the part.

Naturally, grinding wheels create friction, which in turn creates heat. Heat from the workpiece may be transferred to the machine. Grinding heads, motors, drives, tailstocks, electronics and other moving components also create heat, which can influence the accuracy of the machine.

Stability and Temperature Control

The latest machine designs provide stability and consistent dimensional accuracy by controlling the temperature of the various machine components. By circulating chilled, filtered coolant through the machine’s workhead, wheelhead, tailstock, and wheel dresser, each component of the machine is more likely to expand from heat at the same rate. Some machine designs also use fluid-cooled drives to ensure that any thermal growth is consistent throughout the entire machine.

The same chillers used in the coolant filtration system also flood the grinding wheel to control the thermal growth during operation. As an added measure, grinders are often placed in a thermally stable, temperature-controlled shop environment.

In-Process Gaging for Closed-Loop Grinding

Closed- loop, in-process gaging is an option for measuring diameters and other features such as length during the machining cycle. For cylindrical grinding, electronic probes or gage heads may be mounted on the table, on slides or in the indexing turret to access the part being measured. In-process gaging of multiple diameters or dimensions can be accomplished using multiple gage heads or multiple slides, all using the same gage readout.

By using a precision ring gage of a known size, gage fingers can detect the precise workpiece diameter, then touch the top and bottom of the part diameter to feed results to the control system to command the machine to stop or to continue grinding until the exact diameter size is achieved.

For tool and cutting grinders, closed-loop gaging is quite remarkable because of the complicated geometry of most cutting tools. Grinding flutes and complex surfaces, such as the helixes on cutting tools that can vary widely by design, require equally sophisticated, odd-shaped touch probe styli to access the tool surface. Worn cutters can easily be restored by using the gage to instruct the machine when to retract during the regrinding cycle.

Latest Developments in Industrial Grinding Technology

Advancements in grinder design are producing high-precision, high-output, exceptionally fast grinders. The operation is becoming more automated, and the skill level of the experienced grinding operator is being embedded in the CNC control so that almost any machine operator can produce consistent, accurate parts.

Backlash-free, direct-drive linear motors are replacing ballscrews. Linear drives enable the machine to move exceptionally quickly, perform precise contouring and provide vibration damping in all infeed axes, thus resulting in better grinding performance, better surface finish and greater precision.

Likewise, there is a move toward faster, integral spindle drive motors with high-frequency air bearings capable of running at between 80,000 and 120,000 rpm while maintaining a constant torque curve throughout the speed range. Elsewhere in machine design, direct-drive motors are replacing drive belts to gain better machine control, higher speed and better precision.

Auto-balancing of the grinding wheel while the spindle is running is another notable development. It uses a sensor that disperses weight to balance the wheel automatically to adjust for uneven distribution of wheel mass. Grinding wheels can sometimes become unbalanced because oil or coolant becomes trapped in portions of the wheel, whereas a balanced grinding wheel provides higher cutting rates, reduced cycle times and finer surface finishes.

Control units on today’s grinders have more options and more automatic functions to assist the operator. For example, auto dressing with compensation enables the machine to go right back into the cut after dressing. CNC units with touchscreen capability and teach functions enable the operator to skip typing in data manually.

Common Applications for Grinding Machines

Common automotive applications for OD and ID grinding include brake cylinders, brake pistons, hydraulic steering pistons, selector shafts, spline and gear shafts, connecting rods, camshafts, and crank shafts.

Precision grinding of outside shaft diameters provides near-perfect fit between gears, bearings and other mating components. OD grinding of these components enhances concentricity of the shaft to its centerline while ensuring that accompanying diameters are concentric to one another. Offset ODs for non-concentric diameters, such as crank pin journals and cam lobes, are also precision ground. For this application, special crank and camshaft grinders are required. They can be programmed to grind both on-center and offset diameters on the same shaft. Likewise, ID grinding is required for precise fitting of brake cylinders, connecting rods and other applications.

The medical industry uses grinding to produce surgical drills, dental drill bits, hip stems, hip balls, hip sockets, femoral knee joints and needles.

The aerospace industry is known for workpiece materials that are tough to machine with conventional cutting tools and processes. These high-strength, high-temperature materials enable components to survive in the severe environment of aerospace engines. However, the same attributes that make these materials difficult to machine are also likely to make them suitable for grinding. Turbine rings, turbine shafts, and inner and outer rings are a few of the aerospace components which are commonly precision ground.

Note that when milling or turning with conventional machines and tooling, part tolerances and surface quality are degraded as tooling inserts wear. In contrast, a grinding wheel can be dressed frequently to keep the cutting edges of the abrasive sharp and the shape of the wheel constant, thus resulting in a consistent finish and close dimensional accuracy.

Machine tool manufacturers use ground components for spindles, linear guideways, ballscrews, Hirth couplings in indexers and rotary tables, roller bearings, cams, racks, valve spools, and pistons.

The die/mold industry uses grinding to produce thread dies, stamping dies, press brake tools, draw dies, thread rolling dies and mold inserts, along with many other die and mold components.

The tooling industry that supports the die/mold and machine tool industries uses precision grinding to produce three- and four-jaw chucks, profile inserts, step drills, drill points, reamers, taps, ring gages and collets. ISO and HSK adapters and shanks for toolholding also require grinding.

If you want to achieve a tight part tolerance and a fine surface finish while consuming less production time and lower operator involvement, now is the time to look at the latest in grinding technology.

What Are Abrasive Bonds?

A bonding material or medium holds the abrasive grit within the grinding wheel and provides bulk strength. Open space or porosity is intentionally left within the wheel to enhance coolant delivery and release chips. Other fillers may be included, depending on the wheel’s application and type of abrasive. Bonds are generally classified as organic, vitrified or metal. Each type offers application-specific benefits.

Organic or resin bonds can withstand harsh grinding conditions such as vibration and high side forces. Organic bonds are particularly suited for increased stock removal in rough applications such as steel conditioning or abrasive cutoff operations. These bonds are also beneficial for precision grinding of ultra-hard materials such as diamond or ceramics.

Vitrified bonds provide excellent dressability and free-cutting behavior when precision grinding ferrous materials such as hardened steel or nickel-based alloys. Vitrified bonds are specifically designed to provide extremely strong adhesion to cubic boron nitride (cBN) grains through a chemical reaction, thus enabling an excellent ratio of stock removal to wheel wear.

Metal bonds offer excellent wear resistance and form-holding ability. They can range from single-layer, plated products to multi-layered wheels that can be made extremely strong and dense. Metal-bonded wheels can be too tough to dress effectively. However, newer wheels with a brittle metal bond can be dressed in a manner similar to vitrified wheels and have the same beneficial free-cutting grinding behavior.

From “Bond Selection for Production Grinding,” by Robin Bright of Norton Abrasives