Today’s Grinding Technology

By Barry Rogers

 

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 ±0.0002 inch are required, grinding steps in. Grinding can repeatedly deliver accuracy as tight as ±0.00003 inch and do so repeatedly and reliably under proper conditions. Only honing can produce bore sizing tolerances below that which grinding can deliver.

grinding stock photo

 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. 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.

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.

An Abrasive Process

Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool. A grinding wheel consist of hard, sharp-edged particles, so that as the wheel spins, each particle acts like a single-point cutting tool.

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.

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 high 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 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 which 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.

Wheel Dressing

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.

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.

To consistently produce part accuracy of 0.0002 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.

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.

Multi-Spindle Configurations

Although most machines are designed with a single-spindle grinding head, universal machines, or what might be called multi-tasking machines, are designed with multiple grinding heads and multiple spindles. Depending on the requirements, machines can be equipped with up to four grinding spindles and a measuring probe.

Universal grinding machines provide high-precision machining for internal, external and face grinding of large or small workpieces. Multiple grinding wheels can be used (each with a different form, abrasive grit size or bond) to rough grind, finish grind, and polish—all in one setup. For example, multiple ODs can be ground, then the machine can switch to a different spindle by using a turret and precision Hirth coupling to auto index 0 to 180 degrees, thereby enabling an ID spindle to grind internal features. Another spindle might then be used to finish-grind the part face.

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, requires equally sophisticated, odd-shaped touch probes 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

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.

Applications

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.

 

 


ABOUT THE AUTHOR

Barry RogersBarry Rogers recently retired as global director of sales for a major machine tool OEM. Prior positions include director of global sales and marketing for Sunnen Products, and national sales and market manager for Renishaw North America. He also has served as general manager of Cincinnati Milacron’s LK CMM division in Detroit, Michigan. Barry recently started Alpha Strategies, a Chicago-based consulting firm he serves as president. alphastrategiesconsulting.com