Here is how centerless grinding works.
Here is how centerless grinding works.
According to Today's Machining World:
'how to make round bar rounder
'It seems everything is round at Boston Centerless, a provider of ground stock and grinding services in Woburn, MA. The radius on the front corner of the building gleams metallic in the morning sun. A disk-shaped roof supported by cylindrical columns shelters the main entrance. The lobby features curved walls and a circular reception desk. An alcove in the wall displays a metal sculpture inspired by centerless grinding'below, three cylinders representing a grinding wheel, regulating wheel and work piece; above, an artistic array of round, ground parts welded together.
'The manufacturing floor, however, is all business, straight walls and an expanse of floor holding dozens of grinding machines. Here, operators feed round bar into the machines and the bar comes out the other side, rounder than it went in.
The daily grind
'In a way, grinding is like any other machining process. It removes metal. But instead of having one, or two, or a few cutting edges, like a lathe cutting tool, a drill or an end mill, a grinding wheel has thousands of cutting edges; each grain of abrasive acts as a cutting tool.
'Centerless grinding, as the name implies, is a process that grinds the surface of a turned part or round bar of material, without the need to support the work piece on centers.
'For grinding round bar stock, a 'through grinding' process is used. A bar of material, typically twelve feet long, feeds into the grinding machine between a grinding wheel and a regulating wheel. A carbide-tipped strip of steel, the work blade, supports the bar and holds it at the correct height, above the centerline of the grinding and regulating wheels. This is critical to the roundness and diameter tolerances of the bar. Both wheels rotate. The regulating wheel is slanted up a little from the horizontal. As it turns, it pulls the bar past the grinding wheel, so no external feeding mechanism is required.
'Each abrasive grain on the surface of the grinding wheel takes a little bite of material every time it passes the bar being ground. This produces a lot of heat, of course, so the grinding area is flooded with a generous flow of coolant. The regulating wheel does not remove any material. It just presses the work piece against the grinding wheel and controls how fast the bar rotates and how fast it moves past the grinding wheel.
'A grinding wheel is made from grains of abrasive material bonded together into a cylindrical shape. Different grinding wheels have different sized grains of different abrasives, different bonding agents and different 'porosity,' which is the density or spacing of the abrasive grains and bonding material.
'The hardness or tensile strength of the material being ground determines which type of grinding wheel to use, explains Steven Tamasi, CEO of Boston Centerless. The company uses primarily silicon carbide and aluminum oxide abrasives. The grinding wheels used on many of the company's centerless grinding machines are 24 inches in diameter when new, and eight inches wide. The regulating wheels are the same width and 16 inches in diameter. Other machines use different-sized wheels.
'The operator first selects the right abrasive grinding wheel for the job, then installs the correct work blade. The work blade must hold the bar at the right height above the centerline of the wheels. The operator adjusts the inclination of the regulating wheel and shapes it to provide a full line of contact with the part. As the grinding wheel is used, it wears down and loses some abrasive grains from its surface. The operator must keep it dressed, or trued, in the correct shape and surface condition for optimum grinding.
Nearly perfect
'To produce straightened, ground bar, the first step is straightening. Small diameter bar is run through an automatic straightening machine, which passes the bar between two rollers. This may not straighten the bar completely, so after the operator measures the bar, he sometimes needs to straighten it further using a manual arbor press. Larger diameter bar that does not fit through the automatic straightener is straightened by hand. The operator uses a dial indicator to find where the bar is curved or bent and then applies pressure with a hydraulic ram to bring it in line.
'Straightness is measured in deviation over a certain length. A typical straightness tolerance for straightened, ground stock might be 0.005' per foot. The straightened bar then goes through a series of passes on the centerless grinders. Several passes may be needed. If stock is removed too aggressively, the bar can go out of round.
'A bar can lose its straightness after grinding, due to stresses being relieved, so straightness needs to be checked while the bar is being processed. Boston Centerless has two specially designed, mobile 12-foot-long straightness gages that operators can wheel to their location to measure bar coming off their machines.
Round and rounder
'One's first impression can be that nothing happens here; round bar stock comes into receiving, and round bar stock goes out from shipping. Customers know the difference, though, and are willing to pay extra for these straighter, rounder, precise-diameter bars. Straightened, ground bar can cost considerably more than the same material straight from the mill. Can it save money in the manufacturing process? Yes, says Tamasi.
'Most Boston Centerless customers use the straightened, ground stock in Swiss-type screw machines, he says. A straighter bar can prevent vibration and allow the machine to be run faster. A precise-diameter, very round bar can eliminate chatter and tool breakage, allow tighter machining tolerances and prevent bushings from seizing.
'Tamasi points out that typically the cost of the raw material is only about 10 percent of the cost of a screw machine part, so in many cases the benefits of using ground bar may easily outweigh the additional cost.
'And some shops find it cost effective buy bar stock ground to the diameter they need for their finished parts.
Ground diameter as final O.D.
'Brookfield Engineering Laboratories, a manufacturer of viscosity measurement instruments in Middleboro, MA, has bought ground bar stock for many, many years, according to machine shop manager John Romanuck. The shop uses several different diameters of ground stock, from 1/16' to 3/4', in stainless steel and brass. It also uses regular mill-run stock.
'For parts that are visible to the customer or have special tolerance requirements, the company orders the bar stock ground to size and uses the ground surface for the outside diameter of the finished part.
'One application is an instrument stand. Brookfield uses stainless steel special ground to a diameter of 0.623', the final size of the part. 'This is advantageous because we have a ground finish [that looks good],' Romanuck says. 'It's less expensive than if we went for a larger diameter and turned it down and then had it ground].'
'Brookfield sells spindle assemblies that consist of a shaft press-fit into a disk. It would be quite difficult to turn the 4-or-5-inch shafts to 1/8' diameter at the tight tolerances required. So it makes sense to start with the stock ground to size, Romanuck says.
'His company buys a lot of bar in 1/8' to 5/32' diameter, plus or minus half a thousandth, Romanuck says. 'This eliminates all O.D. work.' With only some lathe operations, such as turning the ends or cutting grooves, the parts are finished.
'Brookfield also uses straightened, ground stock for close-tolerance work on Swiss-type screw machines. With mill-run stock, the diameter varies too much and 'we wouldn't be able to hold the tolerances we need,' Romanuck says. 'The overall quality of the ground stock is a thousand percent better than anything you could get from the mill.'
Through the mill
'Mill run bars might hold tolerances of two or three thousandths on the diameter, and half that on roundness. Boston Centerless routinely holds tolerances of two ten-thousandths of an inch (0.') on bar diameter and 50 millionths (0.') on roundness.
'With such tolerances, the bars come out with a smooth, fine surface finish; the tighter the tolerance, the smoother the bar's surface will be. If needed, Boston Centerless can produce an even smoother surface finish, as fine as 8 micro inches, on the ground bar.
'A grinding house can work with customers to help them determine what tolerances they need on straightness, diameter and roundness for their particular parts and processes. The grinding company can also work with the mills to obtain the material needed, including any special requirements. Also, a grinding company that buys metal in quantity can provide stock to customers who might need an amount of material that is less than a mill's minimum order.
'Sometimes the mill will refer a customer to a grinding house. 'If there is a product that Carpenter is not suited to produce directly,' a referral to Boston Centerless may be in order, says Michael Speers, market manager for medical products at Carpenter Technology Corporation, Reading, PA, a supplier of specialty alloys, including stainless steel and titanium.
'Round bar is not all the same. Straightened, ground bar held to close diameter and roundness tolerances is available in many different materials and it can help Swiss-type screw machines produce tight tolerances at maximum speeds, while protecting the machine and the tools from damage.'
Original Source
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.
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:
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.
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.
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.
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.
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 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.
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.
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:
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.
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:
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.
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.
According to Today's Machining World:
'how to make round bar rounder
'It seems everything is round at Boston Centerless, a provider of ground stock and grinding services in Woburn, MA. The radius on the front corner of the building gleams metallic in the morning sun. A disk-shaped roof supported by cylindrical columns shelters the main entrance. The lobby features curved walls and a circular reception desk. An alcove in the wall displays a metal sculpture inspired by centerless grinding'below, three cylinders representing a grinding wheel, regulating wheel and work piece; above, an artistic array of round, ground parts welded together.
'The manufacturing floor, however, is all business, straight walls and an expanse of floor holding dozens of grinding machines. Here, operators feed round bar into the machines and the bar comes out the other side, rounder than it went in.
The daily grind
'In a way, grinding is like any other machining process. It removes metal. But instead of having one, or two, or a few cutting edges, like a lathe cutting tool, a drill or an end mill, a grinding wheel has thousands of cutting edges; each grain of abrasive acts as a cutting tool.
'Centerless grinding, as the name implies, is a process that grinds the surface of a turned part or round bar of material, without the need to support the work piece on centers.
'For grinding round bar stock, a 'through grinding' process is used. A bar of material, typically twelve feet long, feeds into the grinding machine between a grinding wheel and a regulating wheel. A carbide-tipped strip of steel, the work blade, supports the bar and holds it at the correct height, above the centerline of the grinding and regulating wheels. This is critical to the roundness and diameter tolerances of the bar. Both wheels rotate. The regulating wheel is slanted up a little from the horizontal. As it turns, it pulls the bar past the grinding wheel, so no external feeding mechanism is required.
'Each abrasive grain on the surface of the grinding wheel takes a little bite of material every time it passes the bar being ground. This produces a lot of heat, of course, so the grinding area is flooded with a generous flow of coolant. The regulating wheel does not remove any material. It just presses the work piece against the grinding wheel and controls how fast the bar rotates and how fast it moves past the grinding wheel.
'A grinding wheel is made from grains of abrasive material bonded together into a cylindrical shape. Different grinding wheels have different sized grains of different abrasives, different bonding agents and different 'porosity,' which is the density or spacing of the abrasive grains and bonding material.
'The hardness or tensile strength of the material being ground determines which type of grinding wheel to use, explains Steven Tamasi, CEO of Boston Centerless. The company uses primarily silicon carbide and aluminum oxide abrasives. The grinding wheels used on many of the company's centerless grinding machines are 24 inches in diameter when new, and eight inches wide. The regulating wheels are the same width and 16 inches in diameter. Other machines use different-sized wheels.
'The operator first selects the right abrasive grinding wheel for the job, then installs the correct work blade. The work blade must hold the bar at the right height above the centerline of the wheels. The operator adjusts the inclination of the regulating wheel and shapes it to provide a full line of contact with the part. As the grinding wheel is used, it wears down and loses some abrasive grains from its surface. The operator must keep it dressed, or trued, in the correct shape and surface condition for optimum grinding.
Nearly perfect
'To produce straightened, ground bar, the first step is straightening. Small diameter bar is run through an automatic straightening machine, which passes the bar between two rollers. This may not straighten the bar completely, so after the operator measures the bar, he sometimes needs to straighten it further using a manual arbor press. Larger diameter bar that does not fit through the automatic straightener is straightened by hand. The operator uses a dial indicator to find where the bar is curved or bent and then applies pressure with a hydraulic ram to bring it in line.
'Straightness is measured in deviation over a certain length. A typical straightness tolerance for straightened, ground stock might be 0.005' per foot. The straightened bar then goes through a series of passes on the centerless grinders. Several passes may be needed. If stock is removed too aggressively, the bar can go out of round.
'A bar can lose its straightness after grinding, due to stresses being relieved, so straightness needs to be checked while the bar is being processed. Boston Centerless has two specially designed, mobile 12-foot-long straightness gages that operators can wheel to their location to measure bar coming off their machines.
Round and rounder
'One's first impression can be that nothing happens here; round bar stock comes into receiving, and round bar stock goes out from shipping. Customers know the difference, though, and are willing to pay extra for these straighter, rounder, precise-diameter bars. Straightened, ground bar can cost considerably more than the same material straight from the mill. Can it save money in the manufacturing process? Yes, says Tamasi.
'Most Boston Centerless customers use the straightened, ground stock in Swiss-type screw machines, he says. A straighter bar can prevent vibration and allow the machine to be run faster. A precise-diameter, very round bar can eliminate chatter and tool breakage, allow tighter machining tolerances and prevent bushings from seizing.
'Tamasi points out that typically the cost of the raw material is only about 10 percent of the cost of a screw machine part, so in many cases the benefits of using ground bar may easily outweigh the additional cost.
'And some shops find it cost effective buy bar stock ground to the diameter they need for their finished parts.
Ground diameter as final O.D.
'Brookfield Engineering Laboratories, a manufacturer of viscosity measurement instruments in Middleboro, MA, has bought ground bar stock for many, many years, according to machine shop manager John Romanuck. The shop uses several different diameters of ground stock, from 1/16' to 3/4', in stainless steel and brass. It also uses regular mill-run stock.
'For parts that are visible to the customer or have special tolerance requirements, the company orders the bar stock ground to size and uses the ground surface for the outside diameter of the finished part.
'One application is an instrument stand. Brookfield uses stainless steel special ground to a diameter of 0.623', the final size of the part. 'This is advantageous because we have a ground finish [that looks good],' Romanuck says. 'It's less expensive than if we went for a larger diameter and turned it down and then had it ground].'
'Brookfield sells spindle assemblies that consist of a shaft press-fit into a disk. It would be quite difficult to turn the 4-or-5-inch shafts to 1/8' diameter at the tight tolerances required. So it makes sense to start with the stock ground to size, Romanuck says.
'His company buys a lot of bar in 1/8' to 5/32' diameter, plus or minus half a thousandth, Romanuck says. 'This eliminates all O.D. work.' With only some lathe operations, such as turning the ends or cutting grooves, the parts are finished.
'Brookfield also uses straightened, ground stock for close-tolerance work on Swiss-type screw machines. With mill-run stock, the diameter varies too much and 'we wouldn't be able to hold the tolerances we need,' Romanuck says. 'The overall quality of the ground stock is a thousand percent better than anything you could get from the mill.'
Through the mill
'Mill run bars might hold tolerances of two or three thousandths on the diameter, and half that on roundness. Boston Centerless routinely holds tolerances of two ten-thousandths of an inch (0.') on bar diameter and 50 millionths (0.') on roundness.
'With such tolerances, the bars come out with a smooth, fine surface finish; the tighter the tolerance, the smoother the bar's surface will be. If needed, Boston Centerless can produce an even smoother surface finish, as fine as 8 micro inches, on the ground bar.
'A grinding house can work with customers to help them determine what tolerances they need on straightness, diameter and roundness for their particular parts and processes. The grinding company can also work with the mills to obtain the material needed, including any special requirements. Also, a grinding company that buys metal in quantity can provide stock to customers who might need an amount of material that is less than a mill's minimum order.
'Sometimes the mill will refer a customer to a grinding house. 'If there is a product that Carpenter is not suited to produce directly,' a referral to Boston Centerless may be in order, says Michael Speers, market manager for medical products at Carpenter Technology Corporation, Reading, PA, a supplier of specialty alloys, including stainless steel and titanium.
'Round bar is not all the same. Straightened, ground bar held to close diameter and roundness tolerances is available in many different materials and it can help Swiss-type screw machines produce tight tolerances at maximum speeds, while protecting the machine and the tools from damage.'
Original Source
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.
Are you interested in learning more about china turning shaft companies? Contact us today to secure an expert consultation!
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:
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.
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.
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.
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.
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 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.
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.
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:
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.
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:
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.
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.
For more principle of lathe machineinformation, please contact us. We will provide professional answers.