Introduction

In this part of the Molex eGUIDE we will discover and learn the operations and procedures that go behind . As you learn you would appreciate the process of producing and finishing flat surfaces and you would find yourself better adapted to the processes and demands on work at the Molex tool room.

We will begin with a definition on grinding and then focus on surface grinding to understand machine types, machine know-how and the surface grinding process that is demonstrated on the job - to help you identify with best practices adopted at Molex.

This course is intended to serve as a guide to on the job demands and hence it is treated with a practical approach covering all facets of surface grinding as a process with the niceties of this grinding technology.

How would one define grinding?

Grinding is a process of fine metal cutting, achieved by the action of many small individual abrasive grains. These abrasive grains are formed into a grinding wheel that is rotated against the work piece at high speeds.

Grinding machines and grinding processes are essential to most manufacturing industries, and at Molex we have proven our expertise with very high precision work covering Surface Grinding, Profile grinding, CNC Surface grinding, CNC Jig Grinding and CNC Optical Profile Grinding.

Having understood the process of grinding, let's understand - What surface grinding is about?

Surface Grinding is the process of producing and finishing flat surfaces by means of a grinding machine employing a revolving abrasive wheel.

Machine Types

Surface grinding is performed on two different types of surface grinding machines that are differentiated by the position of the grinding wheel spindle in relation to the table that holds the work. The heart of any Precision Grinding machine is the Grinding wheel spindle & it's bearing.

The running true of the wheel is also essential to maintain steady pressure between the wheel & work. And yes, spindle positions can be horizontal or vertical.

Now lets get to understand the two machine types that are used for surface grinding.

You can identify the two machine types by the position of the grinding wheel spindle, and it's relation to the table holding the work. The spindle may be either horizontal or vertical. The types of wheel's used are determined by the spindle position and the working face of the wheel.

The machines are further categorized by reciprocating tables and others with rotary tables. Further they are also distinguished by their operation, either mechanical or hydraulic.

1 Having understood the basic differences between the two grinding machines, now we will see how they are classified based on the features described earlier.

Surface grinding machines are classified as:

> Horizontal spindle with reciprocating table.

> Horizontal spindle (segmental wheel) with reciprocating table

> Horizontal spindle with rotary table

> Vertical spindle with reciprocating table, and the Vertical spindle with rotary table

We will now try and understand a little more on these machines and how they work best for us.

The Horizontal Spindle Reciprocating machines are commonly used and is the preferred choice for the production of fine accurate surfaces on parts that have been previously machined in a shaper planner or machine or have been hardened.

The principal features of the machine include the bed, saddle, table, vertical column, wheel head, and the vertical adjustment mechanism, the driving mechanism and the feed mechanism.

Sometimes the Horizontal spindle reciprocating table machines use a segmental wheel, this machine is preferred to grind surfaces at right angles to the locating face. It is also suitable for roughing operations on castings and forgings.

The next machine type is the horizontal spindle rotary table machine that is used to produce fine accurate finishes. The rotary table enables a large number of parts that are moving continually beneath the wheel and that actually transverse across the table, hence this machine is suitable for production.

The Vertical Spindle reciprocating table machine is in many ways similar to the horizontal spindle machine in operation and construction except that the wheel spindle is vertical. It is particularly suitable for grinding previously un-machined castings or forgings. Some of the heavier machines using segmental wheels take cuts 2mm deep to rough out work that would otherwise be done in a planer mill or a lathe.

The Vertical Spindle Rotary Table machine, which is used much like the reciprocating table machine. The worktable rotates demanding no other feeding action, apart from the down feed of the wheel.

It is essential to know here that in both the horizontal and vertical spindle machines having either a reciprocating or rotary table there are two methods of grinding, they are:

> Grinding using the periphery of the wheel, and

> By using the side of the wheel, in which case the wheel is of cup form either of the solid or segmental type

You will now see at the various combinations of work movement and the spindle position.

Having so many choices of surface grinding machines, how does one go about choosing the right machine best suited for the job?

2 Having so many choices of surface grinding machines, how does one go about choosing the right machine best suited for the job?

The answer lies in understanding each machine type and the job its best suited for. These vital pointers could prove useful while choosing a machine:

> Speed of production

> Desired accuracy

> Finish required and the

> Amount of material to be removed

Now lets try and understand the various finishes obtained by using different types of machines:

> Surface produced by grinding with the periphery of a straight wheel with the work carried on a reciprocating table will produce fine parallel straight lines

> While a horizontal straight wheel with a rotary table will result in circular lines

> The rim of a cup wheel on either a horizontal or a vertical spindle reciprocating table machine would produce a finish that clearly shows concentric curved grinding marks and,

> The vertical spindle type of machine with a segmental wheel when grinding work on a rotary table produces a criss-cross pattern of circles

These patterns produced by the machine could depreciate in quality by several influencing factors, but they are fully under your control. If you notice depreciation of work finish, you should check for the spindle bearings slide adjustments, location and the stability of work piece. Also check for lubrication of all moving parts.

On machines designed for accurate production it is essential that all mechanical arrangements are checked and maintained, besides the wheel being properly dressed.

On the Job

Having understood the basics of grinding and having focused on surface grinding, let's begin the session that provides a hands- on experience of the machine and the job.

In the part of the tutorial we shall learn more on the machine used for surface grinding and get a good description of the various fixtures such as the optidress unit, optidress DRO, Vertical column, wheel cover, magnetic chuck, the horizontal side handle, cross fine feed, cross feed angle and the vertical feed handle and the fine feed.

The machine also has a optidress unit and accessories, magnetic chuck with ON/OFF handle, DRO of the machine slides, dust collector, coolant tank with motor and the hydraulic table movement adjusters.

Now lets move on to understanding how one goes about using this machine on the job. Once you have picked the drawing from the rack, begin by first studying the drawing, it is recommended that you make a hand sketch of needs to be ground following the study.

3 Having done this you can mount the appropriate grinding wheel on to the machine spindle. Ensure you have cleaned the machine bed thoroughly and dress the wheel to make it true to the spindle.

Then place the job on the cleaned magnetic bed with appropriate fixtures and ensure you support the job adequately. The support must be at least 75% of the job height. Having done this you can turn the magnet on, apply blue to the job surface and lower the grinding wheel closer to the job surface.

You must touch the job surface using the fine feed until the blue is ground, then proceed to make reset DRO to zero. Now use the fine feed to touch the surface of the job with the grinding wheel, set the DRO to zero again.

Now you can continue grinding as per the stock available to the drawing dimensions. On completion take the job out of the machine bed and clean it. You can now measure the job using the digital micrometer. Note here that if the job needs to be checked by a digital height gauge you can use the Trimos to do this. You can also use a profile projector to inspect jobs for accuracy.

How much to remove has been the biggest challenge in surface grinding, hence grinding allowance is of utmost importance as a work practice to operators.

Lets understand more - the amount of stock to be left on for grinding depends largely on the character of the work, whether it is long or short, stocky or thin the nature of the material; whether it is to be hardened the facilities for straightening and the nature of the previous machining, whether rough or smooth.

As surface grinding does not remove metal rapidly grinding allowances should be kept on the most economical limits. For ex, if a horizontal machine with a 600mm x 200mm table and a 180mm x 13mm grinding wheel were being used to grind a cast iron or mild steel plate flat and parallel, a grinding allowance of 0.13mm on each side could be sufficient to remove the tool marks, however if the material were thin and liable to wrap during heat treatment a grinding allowance of 0.4mm to 0.5mm might be necessary.

However, with heavy duty surface grinders having large cylindrical or segmental wheels little attention is given to grinding allowances because many castings without previous machining are ground to finished dimensions, removing up to 2mm or more in the process. Frequently grinding of forgings rolled bar stock and mild steel plate is preferred to milling planning or shaping. On such work it is possible to remove 0.13mm and more in each traverse so 2 or 4 mm may not be considered an excessive grinding allowance.

Work Holding

Having understood the working of the machine its of vital importance to understand the various work holding methods and instruments involved with holding the job in the right position for surface grinding.

Most work on a surface grinder is held in position on a magnetic chuck, which may be of the electric, or non-electric type. On rotary table machines the magnetic chuck is circular whereas on the reciprocating table it is rectangular.

An electric chuck is an electro magnet operated by direct current from a power main, or from a small dynamo on the machine. The top surface of the chuck is divided into the north and south magnetic poles separated from each other by a non-magnetic metal. When the mains are switched on, the work is held by a force of the magnetic field that passes through the job, from one pole to the other. After grinding the chuck face is demagnetized by reversing the current through the chuck coils for a moment by means of a reversing switch. The switch must always be either 'ON' or 'OFF' not half way.

4 However, in the non-electric magnetic chuck, the top face of the chuck has a number of inserts (or inner poles) which are of similar polarity, while the enclosing frame forms the outer and opposite pole.

Inside the chuck and under the top face is a unit consisting of a number of permanent bar magnets and conductor bars that correspond to the number of inner poles in the chuck poles. When these magnets and the conductor bars are moved relative to the top plate of the chuck by means of a lever the work can be held or released as required.

The principle of the non-electric chuck can be understood in this animation in which permanent bar magnets are shown vertically between the top plate of the chuck and the bottom plate, which acts as a keeper. A hand lever slides the permanent magnets into positions that restrict the flow of magnetic lines of force - when in the off position and permits the flow of magnetic lines of force to pass through the work - when in the on position.

Having understood the importance of the magnetic chuck its good to understand how you need to prepare a magnetic chuck for your job. Before mounting the chuck on the machine table ensure that the underside of the chuck and the table are clean and free from burrs because an error of 0.03mm or more could occur. You can check parallelism by attaching a dial indicator to some rigid part of the machine and traversing the table by hand. If an error were to exist the chuck can be surface ground in position as seen in this animation.

There are also other aspects to holding work on the magnetic chuck - Only metals that can be attracted by a magnet can be held on the chuck and the holding force depends to a large extent on the shape and the section of the work.

In the case of small sections to be held, the grip will be light no matter what the intensity of the chuck magnets may be, as the grip is dependent on the number of magnetic lines of force that can pass through the section.

It is advisable to check the magnetic grip, and if pieces of small sections are to be held they should be placed lengthwise along the pole edges and bridging the non magnetic strip, in addition packing strips of mild steel may be placed between them to give extra support as seen here. In the case of very small pieces to be held for the job, you can use a thin plate of brass or non-magnetic material with cut outs to receive the pieces, which will be so spaced such that they can be located across the pole edges.

If you were to be working with awkward shapes, they can be secured by mounting them on mild steel parallel strips that will carry the magnetic field across the work. However the support must not cross the insulation strips because the work must form a bridge between the poles as shown here. We shall now move on to understand how one can set work parallel to the edge of the table. To enable work to be set up parallel, an alignment strip of adjustable height is bolted to the rectangular chuck and similar strips can be fitted to the end of the chuck to act as end stops for the work.

Magnetic blocks are especially useful here, they have special parallels consisting of alternate strips of steel and non-magnetic material held together by non-magnetic bolts or rivets. Magnetic blocks will support and hold work in any position provided their laminations are kept parallel to the chuck poles the work thus forming the bridge between the poles. They are particularly suitable for grinding thin work that must be held on the edge for a T or angle sections and may be machined out to permit triangular round or unusual shapes to be held as demonstrated here

5 We must have all heard about demagnetizing, but why is there a need?

Work held on the magnetic chuck becomes more or less permanently magnetized depending on the material from which it is made. High-speed steel and other alloy steels especially in their hardened state retain a considerable amount of magnetism and this could cause inconvenience if it were allowed to remain.

For ex a magnetized milling cutter would pick up the cuttings it produced and so clog the teeth while parts of machine using steel pins or tacks such as boot making machines would not function satisfactorily if they were magnetized.

Hence, demagnetizers are used to remove this magnetism and are of two types, in the platen type the entire face of the work must be slowly passed over the plate and in the aperture type the work is passed rapidly through the aperture.

An alternating current is used to energize the coils of the powerful laminated magnets reversing their polarity rapidly normally 100 times per second.

We have now understood the methodology to hold magnetic parts on the magnetic chuck, but how would you handle non-magnetic parts?

Non-magnetic pieces can be held on a magnetic chuck by placing steel blocks or strips around them. The steel blocks are firmly held on the chuck and prevent the pieces from moving. Placing blocks of steel or cast iron in them can hold thin non-magnetic box like shapes. The magnetic attraction of the blocks will hold the pieces if their thickness does not exceed 2 mm. If this method is to be used check the firmness of the grip and ensure you do not take risks.

We shall also try and understand how a job with a definite shape and size can be held. When the shape the size or the material is such that the work cannot be held on the magnetic chuck it may be clamped directly to the table.

There are also vices available to clamp a job and are normally distinguished as - the plain vice which is used to hold rectangular work when a magnetic chuck is not available or when the material is non magnetic. The swivel vice which is used when the work must be held at an angle in the horizontal plane and the universal vice which permits the work to be set at any angle in the horizontal plane and up to 45° in either direction in the vertical plane.

Besides vices, Angle plates are often used on surface grinders to hold work that must be ground square on adjacent faces. They are usually made of cast iron in sizes varying from 75 to 150 mm and are ground on all sides and faces square within 0.003 mm. Pads are machined on the back of each main face for clamping purposes, and the work is usually clamped with either C clamps or toolmakers clamps

In surface grinding we also use other fixtures, just as with milling suitable fixtures may be used for holding work but they are confined to use on production machines alone like the magnetic chuck, punch former, Vee block, L angle block, square block and the grinding vice. There are also other specially designed fixtures. A correctly designed magnetic fixture often provides a satisfactory solution to many work holding problems. To ensure efficient holding, it may be necessary to design the fixture so that the normal arrangement of the magnetic poles of the chuck are modified in order to induce higher flux densities into the work pieces or to apply the magnetic forces at more effective points that would otherwise be impossible.

In addition when the components are of complex shape and do not incorporate flat contact surfaces it is necessary to provide specially shaped pole pieces. It may also be noted that the presentation of a work piece at an angle for any type of machining operation is often facilitated by the use of a suitable magnetic fixture.

6 Surface grinding is used to grind various surfaces, like plane, angular and square surfaces that demand special know-how to ensure accuracy on the job. We shall now discover good practices that make working with different job types accurate and perfect. In the case of Plane surfaces where the opposite sides are parallel and adjacent sides are square with a hardened parallel strip on a horizontal spindle machine with a reciprocating table the work procedure would demand that you mount the magnetic chuck on the machine table and check for parallelism.

Then select and mount the wheel (38A46HVBE would be suitable for this kind of work), ensure you true and dress the wheel with a diamond trueing tool mounted on the magnetic chuck. Check the material for straightness (if warped straighten under a press unless it is hardened tool steel which should be straightened during heat treatment) If only warped 0.02 to 0.05 mm grinding can proceed.

Ensure that the work surface and chuck are clean and free from burrs, place the work piece lengthwise along the chuck and if it is warped place shims of paper under the hollow places to prevent magnetism from pulling a section flat against the chuck. Thin warped sections ground without packing will return to their original shape when magnetism is released An alternative method is to be place a vice on the magnetic chuck hold the work in the vice and grind one side and then place this side on the magnetic chuck.

Here is a tip on packing warped work - while the machine is stopped adjust the reversing trips to the length and position of the work. Start the machine and check the table traverse and adjust it if necessary so that the center of the wheel just clears each end of the work as shown in this demonstration

Stroke length

To get the correct length and position of stroke the following steps would prove useful:

Set the wheel and the table in motion so that approximately 3mm of the wheel face is covering the work but not touching it. Then, lower the wheel head until the wheel almost contacts the work and then continue lowering 0.03mm at each stroke until sparks indicate contact - if there is a high spot on the work make contact on this part. Set the cross feed to give approximately 2mm movement and set the depth of cut to less than 0.1mm and start the cut using an automatic feed. The depth of cut will depend on the strength of the work the finish required and the amount of metal to be removed. The depth of the finishing cut will vary from 0.005 to 0.05mm. Now remove the work from the magnetic chuck and check for flatness with a straight edge or and feeler gauge. Ensure you clean the surface of the chuck and the work and replace the work with the grounds-side facing down in approximately the same position. Lower the wheel head and grind to size - remove and check the work for parallelism using a micrometer.

Sometimes internal stresses are released causing further warping the material. Here, it might be necessary to regrind the first ground surface.

While grinding work square, begin with clamping the work onto a suitably sized angle plate as shown in this demonstration, by using a G or toolmakers clamp with the face to be ground in the correct position.

Adjust the length and position of the stroke and grind as before. After cleaning up the surface, check with an accurate square from the finished face after removing burrs with an oilstone. Now place the job on the table and grind the fourth side and then check with a micrometer.

7 Specialized grinding

Now lets understand the methods used to grind angular work. There are two methods of grinding angular work using a surface grinder: In the first, the work is set to the required angle and ground using a plain wheel. In the other method the wheel is trued to the required angle thereby reproducing the shape on the work.

Work can be set in many different ways to suite the required angle, the universal vice is bolted to the table and the graduated swiveling base permits the work held in the jaws to be set to any angle in the horizontal plane. A graduated pivot also permits the work to be up to any angle between 0° and 45° in either direction in the vertical plane.

The adjustable angle plate comprises of a tilting table mounted on a rotating base, which accurately graduates from 0° to 90° in each direction.The top of the table is accurately ground and is provided with 15mm T slots to enable the work to be clamped on to the table. The table can be set to any angle from 0° to 90° by means of a built in protractor.

Adjustable magnetic chucks are also used, and are distinguished as the tilting type, which consists of a magnetic chuck pivoted at one end and a bolt at the other end passes through a slotted bracket to which the table may be clamped by a thumb screw at the desired angle between 0° to 15°. The second adjustable magnetic chuck is the duplex or universal type, which can be swiveled 90° either side of the horizontal and tilted up to approximately 15 degree.

It is useful here that we also understand how one can true the wheel to achieve a required angle for the job. Angular grinding is done by trueing the wheel to the required angle. This method reduces the life of the wheel unless it is reversed for the particular work piece on a production basis. Several methods are available for trueing the angle of the wheel. Whatever method is used the diamond must be set on the centerline of the wheel to achieve the correct trued angle.

The angular surface may be obtained by using an angle-trueing fixture, by brining the diamond in contact with the corner of the wheel that is hand fed along the fixture slide. The depth of cut is obtained by using the table hand cross fed.

It is important when grinding an angular surface that the edge of the work is parallel to the longitudinal direction on the work. It may be necessary to put a parallel strip between the back rail of the magnetic chuck and the work to ensure correct location

Over the next few minutes we shall try and understand a little on checking of angle tapers, and briefly touch upon some types of specialized surface grinding.

There are several methods available to check angles of a flat taper, you can use a vernier protractor which is accurate enough for most work as it is capable of measuring differences of 5 minutes in an angle; or use rollers in conjunction with gauge blocks and a micrometer; or a sine bar in conjunction with gauge blocks or vernier height gauge or dial indicator.

Surface grinding has been used to grind recesses, making a groove or slot. A key way in a shaft may be ground using a horizontal spindle grinder as shown in the video. The grinding of clearance slots extending slightly below the bottom surface and beyond the side surfaces is greatly assist in this operation as they enable the recessed wheel to be traversed sideways without contacting the sides of the groove.

Grinding the sides of a groove presents great challenges, but it may be finished with a saucer wheel T section wheel a recessed wheel or a disc wheel suitably dressed with an undercut or a narrow edge. Correct alignment of the spindle is essential if the keyway is to be centralized along it.

8