A Grinding System

A Grinding System works on four factors working in tandem: Abrasive Product Factor, Machine Tool Factor, Work Material factor and Operational Factor.

All these factors working together at optimum efficiency results in parts with required Surface quality resulting into components performance at optimum cost.

Just consider: A grinding wheel lower in price may be giving more no of components but with a higher % rejection. Though the grinding cost per component may appear low, the total cost of such an operation is definitely high. Besides the cost of rejections, the cost of time spend in checking and segregating rejections, dressing cost – all of this considered together will show a much higher cost incurred in such grinding system. What is Total Cost of Grinding System?

In any Grinding System, the Total Cost is the summation of:

      Abrasive Cost = Wheel price / Parts per Wheel       Machine Cost = Machine hour rates / Parts per hour       Labor Cost = Labor hour rates / Parts per hour       Dressing Cost = Dresser Price / Parts produced per dresser      Downtime Cost = (Time of downtime) x (Machine + Labor hour

Rates) / Parts per downtime.       Energy Cost  = (Energy per hour) x Power tariff / Parts per hour.

 Apart from these quantifiable parameters, the Total cost also includes factors like    

      Rejection cost       WIP/in-process material on shop floor.       Abrasive Inventory carrying cost       Possible thru-put increase.

Most of our efforts on improvisation in a Grinding process are focused on improving one of the following –

      Parts per wheel       Parts per dress       Productivity etc.

It is very well possible that a Grinding wheel with higher initial price thereby higher Abrasive cost per component will still result in lower Total cost per component – which finally drives the price and margins for a product.

     Tools to optimize the Abrasive Cost:

        Field Instrumentation System

The technical output from FIS can be collected and analysed to arrive at the Economic aspects of Grinding. These economic aspects then helps us to arrive at the Economic output i.e Total Cost of Grinding system.

     How to calculate Cost per component?

At Grindwell Norton ltd. We have arrived at a table, which helps us to arrive at the cost per component considering all the aspects of the grinding system simultaneously. for more details please mail to: [email protected]

The Field Instrumentation System (FIS) is a tool developed by Saint Gobain Abrasive’s Higgins Grinding Technology Center (HGTC), Worcester to offer solutions to customers using the concept of ‘System Approach’. The System Approach utilizes all parts of the grinding system to influence the microscopic interactions of the process. It integrates science/engineering/management aspects of the process simultaneously thereby optimizing the outputs from a System – both Technical and Economical.

Today, maximizing System outputs is attempted thru Trial and Error approach – using only the Quality of components produced as the source of information. The FIS gives an insight into the ‘Process’ and hence, aids the user to adopt a more scientific approach to Process optimization. FIS Collects and analyses the macroscopic variables like power consumed, slide velocity, part size change, wheel behavior v/s time elapsed. This collection and analysis of macroscopic data helps to understand the microscopic interactions taking place at the grinding zone. Having established thisThese macroscopic variables can then be controlled to get the desired microscopic interaction and thereby desired performance from grinding system.

The FIS consists of hardware for measurement of power and slide position with respect to time and a software ( developed by HGTC ) to analyze these signals.

The FIS has been used and found beneficial for the following –

 1.Grinding Cycle Optimization - The FIS can be used to study the power drawn by the process at different Material removal rates (MRR). This can be used to find the forces in grinding – Tangential and Normal. Measuring deflections in the system ( using process with and without spark-out ) and using the forces in grinding, the System Stiffness can be calculated. The System Stiffness decides the part size repeatability and drives the Cpk values in an operation. Combining Stiffness values along with relation between power and MRR, new grinding cycles can be designed to yield –

a. Better Process Capability. b. Improved Productivity.

 2.Evaluation of New products – In any process, change of any input variable has an impact on the process, which is generally interpreted from the output part quality – in the absence of measurement of process. The FIS gives an insight into the effect of input variable on microscopic interactions in the process and hence, provides better insight on Real performance of any New product.

 3.Complaint Analysis – The FIS can be used to understand

a. Slide Position repeatability. b. Repeatability of feed rates ( rate of change of displacement ) c. Variations in grinding power d. Variations in Idle power

This helps in understanding of Non – performance of a product and can attribute the reasons to the right factor.

The FIS has been in use in India for the last 4 years and has been found very beneficial at all accounts where it was used.

What Is Centerless Grinding?

Centerless grinding is an OD grinding process. It differs from other cylindrical processes in that the work piece is not mechanically constrained. It's the relationship among these three basic components—

Grinding wheel, Regulating wheel and

Work blade

Why Centerless Works

If the work piece rests on a flat work blade that is on center with the regulating and grinding wheels, the contact points form three sides of a square. As the part is ground in this setup, any high spot on the work piece will shift the work slightly on the blade, allowing the grinding wheel to cut a directly opposite low spot. Over time this setup will create three lobes on the work piece that may be dimensionally accurate but far from round.

Setting an angled work blade so it slopes toward the regulating wheel and supports the work piece centerline above the centerline of the regulating and grinding wheels is how the centerless operation is able to generate roundness. In this setup if a high spot comes in contact with either the blade or the regulating wheel, it does not create a directly opposite low spot because of the angle created between the centerlines of the wheels and work piece.

Instead of grinding a lobe shape in the work piece, the high spot is gradually reduced by the action of the grinding wheel. So rather than creating a low spot on the periphery of the work equal to the high spot, the grinding wheel generates a proportionally smaller low spot at its contact with the work piece.

The angle of the work blade helps keep the work piece in contact with and under the control of the slower rotating regulating wheel to resist any

tendency to "spin up" to the speed of the grinding wheel. In some cases, a spin-up can take a work piece from 850 rpm to near 60,000 rpm in the blink of an eye.

Setup Keys

Centers of grinding wheel & regulating wheel should be in the same line Work blade is inclined towards the regulating wheel. Angle should be in between 0 to 45 deg; (best is 30 deg)

steeper blade means faster rounding action, for large & longer work shallow blade angle is best.

Center Height of the job should be half the dia of job from centerline of grinding & regulating wheel and maximum 10 mm above the center

Too high – will produce chatter marks; Too low- will produce work out of roundness

Setting the centerline of a work piece on the centerlines of the grinding and regulating wheels will not produce rounding. In this setup, any high spot will create a diametrically opposite low spot. Over time, the work piece will look like the exaggerated shape shown at the bottom.

To enable the centerless grinding process to create rounding action on the work piece, it must be placed above the centerline of the wheels. In this setup, high spots no longer create diametrically opposed low spots. Instead, the high spots are ground and a gradual rounding of the work piece takes place.

Regulating wheel runs at a slower speed than grinding wheel. Best speed of regulating wheel is 30 RPM. It is used to control the speed of revolution & through feed of the work piece.

Best blade for resisting wear is tungsten carbide but this can scour the work when grinding with softer materials. In this case a cast iron blade should be used.

Kinds Of Centerless Grinding

The two most common centerless grinding techniques are infeed and thrufeed.

The work piece is placed between the wheels, and the grinding cycle begins. After spark-out, the finished part is removed and the next one goes in. Infeed centerless grinding is analogous to plunge grinding on a center-type machine in that the workpiece is static in the axial direction. For continuous, production-type centerless grinding, the thrufeed method is very effective. Thrufeed grinding is accomplished by passing the workpiece between the grinding and regulating wheel. Basically, raw material goes in one side of the grinding zone, and finished workpieces come out.

The work is driven axially between the wheels by inclination of the regulating wheel relative to the grinding wheel. Because all of the points to be ground on the workpiece contact all the points on the wheels, this centerless grinding method is best applied to straight cylindrical parts without shoulders or other interfering features.

In thrufeed grinding, the regulating wheel is inclined, creating a feed angle relative to the grinding wheel. This feed angle allows the regulating wheel to perform the dual purpose of rotating the workpiece against the grinding wheel and driving the workpiece across the face of the grinding wheel.

The regulating wheel can be swiveled relative to the grinding wheel. However, to do its job and use the full width of the grinding wheel, line contact between the workpiece, grinding wheel and regulating wheel must be m˜aintained. Therefore, the regulating wheel must be trued with a diamond block located on the truing attachment. The diamond block is a device that adjusts the diamond contact relative to the workpiece height above the grinding and regulating wheel centerline.

Infeed centerless grinding is analogous to plunge grinding on a center-type machine. The work is placed on the blade, and the regulating wheel moves into a programmed feed to begin the grind. The work does not move axially in this type of centerless grinding.Thrufeed grinding is

generally applied to continuous, volume production applications. The wheels are dressed in such a way that the workpiece is driven through the cutting zone, across the face of the grinding wheel. Often a single pass is sufficient to finish the workpiece.

"A quick check to see if the thrufeed setup is running correctly," says Mr. Payne, "is to look at the wear line across the top of the workblade. This line should be parallel to the top of the blade. If it runs uphill, from front to back, there is too much diamond block offset. It's the opposite if the line runs downhill."

Is Centerless Grinding For You?

Traditionally, centerless grinding is found in shops involved in high volume production runs. That's still an important segment for the technology.

Increasingly, though, the advantages of centerless grinding, such as rapid rounding and accuracy, are finding application in shops that run relatively shorter job lot sizes. The enabler for this is the application of CNC and servomotor technology to the centerless grinding machine. With electromechanical actuation of slides, dressers and truing attachments, setup of the centerless machine is a much less daunting and time consuming exercise. Moreover, the ability to program grinding and regulating wheel contours using CNC actuated truing attachments eliminates the need for profile cams, which take time to manufacture and are usually less accurate. Centerless grinding is successfully applied to manufacture of parts ranging from hypodermic needles to bowling balls. The principles of the process remain the same regardless of the workpiece. Like most metalworking processes, centerless grinding is not magic, if you understand the fundamentals.

There are 6 factors which affect the choice of a grinding wheel specification.

1. Material to be ground and its hardness.2. Amount of stock removal and finish required.3. Whether the grinding is done wet or dry.4. Wheel speed.

The drawing above illustrates how the diamond block can be set to obtain true line contact on the regulating wheel. The drawing on the right illustrates a quick check to determine if the thrufeed setup is correct. The wear line on the workblade should be parallel to the top of the blade edge.

5. Area of grinding contact.6. Severity of the grinding operation.

AbrasivesVitrified grinding wheels: -

Vitreous glass is used to bind traditional abrasive grains of Aluminum Oxide, Ceramic Aluminum Oxide and Silicon Carbide) in a controlled porosity matrix.

Aluminum oxide: - Aluminium Oxide abrasives are well suited for

Steels and ferrous metalsCarbon SteelsAlloy SteelsHigh Speed SteelAnnealed Malleable IronWrought IronBronze

It is a bulky, hard grain and denoted by AExamples would include 38A, 86A, 57A, 64A, 25A etc. the numbers are the wheel manufacturer’s way of denoting different types of aluminum oxide.

The biggest factor separating different types of aluminum oxide is a property we call “friability.”

Friability: - is a measure of the grain’s ability to fracture and break down during use.

White aluminum oxide (typically a white wheel – think a manual tool and cutter wheel) is the most friable type of aluminum oxide (i.e. breaks down easiest).

A brown aluminum oxide (typically a gray wheel – think a bench grinder) is the least friable (i.e. the most durable).

(Note this is the durability of the grain, not the wheel. All vitrified wheels are basically made of glass and extremely fragile.)

Ceramic aluminum oxide (ceramic): -is a high performance version of aluminum oxide. Grain manufactures basically fire typical aluminum oxide grains to create an extremely hard resilient grain which has a property we refer to as “micro fracturing.” As the name suggests, each grain micro-fractures, allowing it to hold its shape and remain sharper for a longer period of time than typical aluminum oxide. This type of grain is sometimes referred to as “SG,” which is Saint-Gobain/Norton’s trade name for this material. Typically, grinding wheel manufactures call out this type of grain by depicting a “G” in the first position of the marking system. Generally the “G” will be accompanied by a digit that designates the percentage of ceramic in the wheel. Typical mixes are 10, 30 and 50% ceramic. The balance is usually a type of friable

aluminum oxide. There is also an extruded ceramic grain called “Targa” developed and patented by Saint- Gobain. It’s used in high Material Removal Rate (MRR) applications where coolant delivery and chip clearance are crucial. Ceramic should be used for materials that are difficult to grind due to hardness and/or toughness. Ceramic can also be used when you’re trying to extend wheel life, decrease cycle times by

increasing material removal rates, or increase the form holding capability of a process. Ceramic performs best when pushed because the grains need high unit pressure to perform the

microfracturing process. This is why ceramic does not work well when grinding soft materials like aluminum. The grains would never break down, which leads to dulling, rubbing, and ultimately loading of the wheel unless it was made exceedingly soft. On the other hand, if the wheel is too soft it will simply release the ceramic aluminum oxide grains and not allow the end user to fully utilize the higher performance grain.

Using a ceramic wheel requires high quality diamonds in your truing device. Ceramic has the reputation of prematurely wearing diamond tools. That’s true under certain circumstances, but a high quality diamond tool or roll will greatly reduce this problem.

Another point to remember is that due to the sharpness of the grain, ceramic will typically leave a deeper scratch than aluminum oxide grains of the same size. Therefore it is sometimes necessary to decrease grit size by one to reach required finishes versus aluminum oxide. Remember the following when considering ceramic wheels for your grinding operation:

o Horsepower and machine stiffness is critical to the performance of the graino As a general rule, decrease dress depth by 50% when switching from aluminum oxide

wheelso Due to sharpness of grain, it’s sometimes necessary to move to a finer grit size vs.

aluminum oxide to reach required surface finishes Pay attention to total process cost rather than wheel cost with ceramic grain wheels

Zirconia Alumina: -It is used for rough grinding operation, like cutoff operation.

Silicon Carbide (SiC): - Grey Iron

Chilled IronBrassSoft BronzeAluminiumStoneRubberCast ironNon-ferrous metals Non-metallic materials.

Traditionally, before diamond-grinding wheels were readily available, silicon carbide wheels were the wheels of choice for grinding carbide-cutting tools.

Wheels containing silicon carbide grain are almost always either black or green in color and will be typically called out in the marking system by the letter “C.” Another way to easily identify silicon carbide is simply by looking at the wheel. Wheels containing silicon carbide will “sparkle” when tilted to the light.

A number of factors make silicon carbide unique. Silicon carbide is extremely hard and sharp, like ceramic, but is also very friable. This property allows silicon carbide to perform well on both extremely hard materials (due to its sharpness) and extremely soft materials (due to its friability, or “re-sharpening” property). It is quite a versatile grain in environments where the material is either too hard for aluminum oxide to penetrate or too soft to create an environment for aluminum oxide grains to breakdown.

Grit Size:

Grit size is the second position in the grinding wheel marking system, and will typically range between 46 and 220 on vitrified bonded abrasive wheels. (There are uses for grits down to 4 and up to over 600 in various bonded abrasive applications.) The lower the grit size number, the larger the grain size. The chart below depicts this relationship:

A GritGrit size has a direct effect on the rate at which the material is removed as well as the scratch

depth that remains afterwards. The first things to consider when selecting a grit size are the required surface finish and any radiuses that need to be ground in the part. The chart below is a good reference, but it’s just a starting point. These factors can vary depending on dressing and grinding parameters.

As a rule, coarser grit is selected for fast-cutting action and fine grit where a highfinish is required.

Grain Penetration

It’s also important to understand grain penetration when considering grit size. In a given operation, the smaller the grit size the higher the unit pressure per grain, as illustrated below:S

izeAvage Particle Siz

This helps understand the next crucial point: Many users mistakenly move to a courser grit size when burn is present on a ground component, but that is the opposite of what they should do! Moving to a finer grain can help alleviate a burn. Burns are often caused by rubbing of the grain on the ground surface. By moving to a smaller grit size you create more unit pressure on the grain, therefore allowing for better grain penetration into the material.

A relatively fine grit-size works best on hard and brittle material. A coarser grit capable of taking heavier cuts can be used advantageously on soft and

ductile materials. To explain, on hard materials the increased number of cutting points, on the face of a moderately fine grit wheel (Fig. 1) will remove stock faster than the fewer

cutting points presented by a coarser wheel (Fig. 2). The larger abrasive grains in a coarser grit wheel can not penetrate as deeply into the hard work-piece without burning it.On soft ductile materials, however, the larger grains penetrate easily and provide the necessary chip clearance to minimise wheel loading (Fig. 3) and heat generation.

Grade:Grade is the measurement of the relative hardness of the grinding wheel. Manufacturers

create different grades by varying the ratio of bond to abrasive grain in the wheel. Grade is called out by a letter in the 3rd position of the marking system. The letter “A” would be the softest grade and the letter “Z” would be the hardest. By adjusting the grade, you allow the wheel to release grains at an increased or decreased rate. Typical precision grinding operations using vitreous bonded wheels will usually use a grade between D and M. The grade of the wheel affects its material removal rate, as a softer wheel offers the constant presence of new sharp grains and the release of older dull grains. Grade also affects the form holding capability of the wheel. It is difficult to hold a tight part form with a soft wheel grade. The chart below covers these points and more:

A harder grade can be used on soft, easily penetrated materials than on hard materials which naturally tend to dull the wheel faster. However, the softer grade wheel releases the dulled grains more readily, enabling the new, sharp grains lying under it to do the work.

Structure:The structure of a grinding wheel is a measurement of its porosity, or the amount of air

relative to other components. Structure is represented by a number that generally ranges from 4 to 30. The lower the number, the less porous the wheel. A 4, 5 or 6 structure would be considered a very dense wheel. At the other end of the spectrum, 24 – 30 structure wheels are very porous. Manufacturers typically induce structures over an 8 by putting an additional ingredient into the mix that will burn off during firing and leave small hollows in the wheel. Porosity can be “engineered” into the construction of a Targa wheel, which has an extruded ceramic grain.

High porosity leads to better coolant delivery into the cut plus better removal of the swarf from the grind zone, so porous wheels are typically used in surface and creepfeed grinding applications. The downside of a porous wheel is shorter wheel life.

Grade/Structure ShiftsStructure can affect wheel life positively or negatively. This creates a problem when

adjusting more than one wheel parameter at a time. In general, shifting the structure by three points is equivalent to one letter grade. So if you move from an “E” grade “18” structure to an “F” grade in a “21” structure you will have two wheels which act the same as regards hardness. The below chart demonstrates this fact:

The bond type is called out by the 5th position in the marking system. The vitrified call-out letter is almost always a “V,” while the others vary depending on the

grinding wheel manufacturer. One of the most important things to remember when using vitrified grinding wheels is that

they are extremely fragile. The manufacturing process is very similar to that of china dinner plates. It is a glass bond fired in a kiln at high temperatures (approximately 2300° F) for a set time. As the wheel size increases, so does the “burn time,” or length of time in the kiln.

Vitrified bonds wear by pressure breaking the bond posts in the grinding wheel. The majority of vitrified grinding wheel wear comes from the truing process. This is why an increase in parts per dress is directly related to an increase in wheel life. Other key points to remember about vitrified bonds versus other bonds:

• Excellent resistance to abrasion from the part• Good form holding ability• Free cutting bond (porosity)• Excellent ability to draw coolant into grind zone• “Self sharpening” bond (releasing of dull abrasive grains)

Vitrified bonded wheels are generally used for fast-cutting action and commercial finish. Resinoid, Rubber and Shellac bonded wheels produce the highest finish

Bond Modification:Abrasive Grain Grit Size Grade StructureBond Type Bond Modification5SG 60 – L 8 V HP

The 6th and last position in the wheel marking system is what we call the bond modification. This is basically the wheel manufacture’s way to internally identify the differences in the bond system. It designates, among other things, bond traits, speed limitations, and specialty bonds for specific abrasives. And wheel manufacturers generally do not release the meanings of each modification.

Whether the grinding is done wet or dryGenerally for precision grinding, coolant is necessary. However, in some cases (e.g. Tool

Regrinding) the process may be dry, in which case a softer grade wheel may be necessary.Whereas for wet grinding, a one grade harder wheel can be used as the coolant reduces theheat generated in grinding.

WHEEL SPEED The speed at which the grinding wheel is to be operated often dictates the type of

bond. Vitrified Bonded wheels should not be used at peripheral speeds over 33 meters

per second except for specially designed wheels. Standard organic bonded wheels (resinoid, rubber and shellac) are used in most

applications of over 35 meters upto 45 meters per second, and specially designed wheels for speeds upto 80 meters per second.

The speed at which a grinding wheel revolves is important. Too slow a speed means wastage of abrasive without much useful work achieved, whereas an excessive speed may result in a hard grinding action and may introduce the danger of breakage. Hence the safe operating speed marked on the wheel or blotter, in revolutions per minute must never be exceeded.

As a general rule, it is best to operate a grinding wheel at somewhere near the speed recommended for a certain grinding operation. Some of the more common operating speed guidelines for particular types of grinding wheels are listed below, in surface meters per second. A mild abrasive such as 86A, 32A or 38A is best

ConclusionThe grinding wheel is a unique part of the grinding process. Abrasive cost is typically only

2-4% of the entire process cost. But that 2-4% can affect more than 15% of your process cost by improving cycle times, improving quality, and combining operations. When you look at the grinding process as a system you can permanently improve it, leading to meaningful savings: The type of savings that will help you compete in the global economy.

Surface Grinder

Different Grinding Processes

Grinding: IntroductionGrinding is a finishing process used to improve surface finish, abrade hard materials, and tighten the tolerance on flat and cylindrical surfaces by removing a small amount of material. Information in this section is organized according to the subcategory links in the menu bar to the left.  In grinding, an abrasive material rubs against the metal part and removes tiny pieces of material. The abrasive material is typically on the surface of a wheel or belt and abrades material in a way similar to sanding. On a microscopic scale, the chip

formation in grinding is the same as that found in other machining processes. The abrasive action of grinding generates excessive heat so that flooding of the cutting area with fluid is necessary.

Reasons for GrindingReasons for grinding are:

1. The material is too hard to be machined economically. (The material may have been hardened in order to produce a low-wear finish, such as that in a bearing raceway.) 

2. Tolerances required preclude machining. Grinding can produce flatness tolerances of less than ±0.0025 mm (±0.0001 in) on a 127 x 127 mm (5 x 5 in) steel surface if the surface is adequately supported.

Machining removes excessive material.

Flat Surface GrindingThe figure below illustrates a typical flat surface grinding machine.

The figure above is of the horizontal spindle, reciprocating table type, as detailed below.

Other Types of Flat Surface GrindingOther types of flat surface grinding machine configurations are shown below.

Surface Grinding Design Guidelines1. As with other machining operations, set up changes should be minimized.

This is possible if all flat ground surfaces are parallel and on the same side of the part.  

2. Magnetic chucks are typically used for holding down parts during surface grinding. If the part can be designed to be compatible with a magnetic chuck, convenience and throughput increase. Use of a magnetic chuck requires a ferrous (magnetic) material and a flat seating surface opposite the ground surface.  

3. Ground surfaces should be situated as high up above obstructions as possible, as shown below.

Ground surfaces should be as continuous as possible and well supported since the grinding wheel will be pressing down upon them.

Centered GrindingGrinding for surfaces of rotation (axially symmetric surfaces) can be either centered or centerless. Centered grinding involves fixturing the part on a spindle axis as it is ground, as illustrated below.

This configuration can be compared to fixturing a part on a lathe with or without a tail stock. The abrasive material is on a grinding wheel that rotates in a direction such that rolling or sliding contact occurs where the wheel and work piece touch. Centered grinding is accurate and stable, but set-up takes time and through-put suffers.

Centered Grinding Part Design: External1. Undercuts should be avoided.

2. For inside corners, the best practice is to machine a relief at the corner prior to grinding so that a sharp ninety-degree male object can be placed in the corner. This is illustrated below. 

For internal holes with sharp corners, the figure below illustrates recommended practice.

3. Plunge grinding is a centered form of grinding in which the wheel "plunges" radially into the part. The four figures below illustrate good plunge-grinding design practices. 

a. The figure below shows how inside radii merging with cylindrical surfaces are to be avoided.

b. The illustration below shows how tapers and angular surfaces should be replaced by straight surfaces if possible.

The illustration below shows how deep, narrow grooves are to be avoided. The tool required has a thin, tall protrusion that breaks and wears easily and is not rigid.

c. The illustration below shows how complex surfaces are best avoided for plunge-ground parts.

Centered Grinding Part Design: Internal1. Undercuts should be avoided.

 2. Radii should be the same in order to simplify wheel dressing and minimize

tool changes. 

3. Hole depth to diameter aspect ratio should be minimized. This is for the same reasons that bored holes are not too deep. Avoiding length to diameter ratios of more than six is a good practice. 

4. If possible, blind holes should be avoided. Blind holes restrict the flow of coolant.Circumferential interruptions should be avoided. Even axial interruptions should be avoided.

5. An option to consider is a hardened and ground liner. This option must be weighed against the tolerance build up and extra logistics involved.

Adequate access for coolant flow should be provided. This can be difficult since the coolant shed by the rotating grinding wheel provides an effective "curtain" which prevents coolant exchange

Three Main Types of GrindingCenterless grinding is similar to centered grinding except that there is no spindle. This allows high through-put since parts can be quickly inserted and removed from the process. There are three main types of centerless grinding: 1. Through-feed grinding.

2. In-feed grinding.

3. End-feed grinding.

Through-Feed GrindingIn through-feed grinding, the part rotates between the grinding wheel and a regulating wheel as shown below.

 For through-feed grinding, one or both wheels of the centerless grinding machine are canted out of the horizontal plane, as shown below. This imparts a horizontal velocity component to the work piece, so that outside feed mechanisms are not necessary.

The grinding wheel is canted with respect to the other two axes so that a component of its surface velocity pushes the part in the direction shown below. This auto feeding characteristic is useful for rapidly processing many parts in quick sequence.  Because of the axial movement, through-feed parts can only have right circular cylindrical ground surfaces. The wheel cannot be dressed to grind more complex shapes. Below are parts produced with the through-feed centerless grinding process. As can be seen from the quantities produced, through-feed grinding is primarily a mass-production process because of its high throughput.

In-Feed GrindingIn-feed grinding differs from through-feed grinding in that the part is not fed axially so that the ground surface does not need to be a right circular cylinder. The grinding wheel can be dressed to accomodate the part. Once the work piece part is in place, the grinding wheel is fed in radially. Because of the set up time involved for each part, in-feed griding does not have the high throughput of through-feed grinding. In-feed grinding is illustrated below.

End-Feed GrindingIn end-feed grinding, the part moves in axially between the grinding wheels, stops for grinding, and then moves out again. The wheel can be dressed to form more complex shapes, but the part can only get progressively smaller in diameter. End-

feed grinding is illustrated below.

Centerless Grinding Guidelines1. The largest diameter of the workpiece should have the ground surface, if

possible. This allows through-feed grinding. 

2. The axial length of a centerless-ground workpiece should be at least equal to the diameter. Short workpieces are more susceptible to surfaces that deviate from right circular cylindricity. 

3. Radii should be as uniform as possible in order to simplify wheel dressing and/or set up changes. 

4. For a flat at the end of a shaft, it is preferable to incorporate a matching flat on the opposite side of the shaft. This will prevent a high spot from forming opposite the flat. Alternatively, the flat can be brought inboard so that the end is a complete cylinder, as shown below in the right-hand view.  

Holes with diameter to depth ratios of over four should be avoided unless widening of the mouth of the hole can be accomodated, as illustrated below.

5. As the following diagram shows, internal grinding should allow for as large a diameter tool support as possible. This illustration shows a hole with an entrance that is smaller than that of the ground area.

When face-grinding turned surfaces, ground undercuts should be avoided. As the figure below shows, face undercuts require specially-dressed grinding wheels that

are expensive to maintain and replace.

If at all possible, blind holes should be avoided. If a blind hole must be implemented, the middle two geometries of the following diagram can help with grinding.

Some examples of dimensioned centerless-ground parts are shown in centerless grinding

Grinding Wheel Overall ShapesIllustrated below are some common grinding wheel forms. Of course, any shape is possible, but these shapes have evolved because of their utility and robust nature.

6. The figure below illustrates how ends of in-feed ground parts need to be terminated for satisfactory results. The included angle of the pointy end should be less than 120 degrees. If in-feed grinding is not used, ends of cylindrical parts should not be ground.

Grinding Wheel DressingsBelow are shown some typical grinding wheel dressing shapes. The dimensions are recommended for grinding accuracy and wheel longevity.