Grinder Machine Buying Guide - TECHSPEX

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.

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 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.

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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

A considerable amount of time and investment goes into finding the right CNC solution to efficiently and effectively machine composites. The last thing manufacturers want is to invest in the wrong machine and be paying for that mistake for years to come. With little room for error, it is important to understand how different materials, machinery characteristics, and machinery options affect machining speed, precision, service, and overall production.

1.  Spindle HP & RPM - Consider high RPM, low torque

Material density plays a key role in determining the best spindle RPM, HP, and torque for an application. To aggressively machine hard materials, such as steel and Inconel® , some traditional machining centers typically are equipped with high torque, low RPM spindles that operate at a maximum of 12,000 RPM. When it comes to composites (such as foam, tooling board, or carbon fiber), a high torque, low RPM spindle is too slow to reach optimal chip load and thus results in inefficient (slower) composite machining. Instead, high RPM (18,000 - 24,000), low torque spindles are more efficient in reducing cycle time, lengthening tool life, and improving overall spindle reliability. For the best of both worlds (composite and nonferrous metal machining), consider a spindle capable of running at 20,000 - 24,000 RPM and higher feed rates for lighter-duty materials but also can operate in the 10,000 - 12,000 range for harder materials that need more torque. 

2.  Dust Containment / Collection - Improve employee safety and machine longevity

Machining composites often creates a large amount of dust and debris that can cause health problems to workers and damage to the machine. Some composite dusts can lead to lung damage if inhaled and some are electrically conductive, so they can damage machine circuits and cause spindle or machine wear at an increased rate. Thus, choosing a CNC solution with sufficient dust collection and properly sealed and covered components is imperative when working with composites. Some additional considerations for handling abrasive material machining include air knife systems, downdraft tables, and/or full enclosures.

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3.  3-Axis vs. 5-Axis - Depends on the part’s geometry

When machining complex 3D composite components, a 5-axis machine is more efficient than a 3-axis machine and provides a greater return on investment over time. With a 5-axis machine, multiple sides of a part can be machined without having to manually reposition material or swap out tooling for angled heads. A 3-axis machine can perform multi-face machining as well but at a much slower rate as it requires an operator to stop the machine and reposition the part after each operation. To achieve 5-sided milling, drilling, tapping, and/or sawing operations without a 5-axis machine or without having to manipulate parts, consider adding a 4th axis and utilize angled heads, known as aggregates.

4.  Rigidity -  For speed, tool life, and machine longevity

Due to the abrasive and unique structure of composites, it is worth investing in a CNC machine with fully reinforced structural integrity and rigidity. A sturdy design reduces vibrations and tool deflection to provide top acceleration/deceleration speeds, long tool life, and low maintenance and repair costs over the lifetime of the machine compared to less rigid, light-duty machinery. From an upfront cost perspective, a machine can be made cheaper by reducing the structure and quality of its components; however, it will fail to have the longevity of a better built machine and ultimately will result in a higher total cost of ownership.

5.  Work Envelope - Make sure it fits, and consider the benefits of larger tables

In addition to the type of composite material being machined, factor in its size, and select a work envelope accordingly. At a minimum, the work envelope needs to be larger than the largest part being machined. However, depending on production goals, buying a machine with twice the work envelope that supports pendulum processing, where materials are safely loaded/unloaded while the machine is still performing cutting operations on another part, will significantly speed up overall production time. Other advantages of large tables include batch processing which allows you to fixture multiple parts at one time and machine continuously without having to unload and reload.

Note - When searching for 5-axis machining solutions, be sure to consider the work envelope while the machine’s spindle is at 90 degrees.

6.  A Second Spindle (option) - Doubles the throughput

A second spindle is valuable when machining small or long composite parts at high volume. Adding a second spindle doubles the machine’s throughput without increasing its footprint. Depending on the machine manufacturer, take it a step further by adding up to 8+ spindles for maximum part production.

7.  Two Tables (option) - Continuous operation

Similar to a pallet changer for a traditional CNC machine center, a second table allows an operator to load composite material onto, or unload finished parts from, one of the tables while still machining on the other table, so production never has to stop. Additionally, on some machines, two independent tables can be electronically “locked” together to process extra-large parts.

Dual Process -  Combine a multi-spindle option with a multi-table option to unlock the ability to perform what is known as Dual Process machining. With this technology, one spindle performs an operation on one of the tables while the second spindle performs a completely separate operation on the other table, essentially turning one machine into two in the footprint of one machine.

8.  Return on Investment - Planning for the long haul

The cost of the CNC machine matters, but more importantly is the return on investment. A cheaper or lesser machine that fails to meet the unique challenges of machining composites and that has to be replaced after only a few years can cost a company more in the long run, not just in direct costs but in lack of part quality, production downtime, program delays, and added frustration. As John Ruskin says, “There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man’s lawful prey.” It is worth the time to work with experts in the field to ensure the machine being purchased is the very best for the specific composites manufacturing application.

9.  Support and Service - Questions to ask before buying

Asking the right questions provides an understanding of the service and support offered by an original equipment manufacturer (OEM) or dealer. Before making a purchase, some key questions to ask include:

• How many dedicated CNC technicians are there (not for other machinery)?
• Is there 24/7 support for emergencies?
• Is there remote login support?
• What is the lead time on replacement parts?
• What is the lead time on replacement spindles specifically (in the event of a crash)?
• At what point do machine parts become obsolete and are no longer made or carried?
• Where is the service department located?
• Where are spare machine parts manufactured and/or stored?

10.  American Made vs. Imports - Some food for thought

Buying an American-made CNC machine means:

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