1. HOW TO DEAL WITH DRESSING RELATED PROBLEMS EFFECTIVELY?

a . WHY SHOULD A WHEEL BE DRESSED? 

Grinding is actually a cutting operation. The small cutting edges on the grains of the grinding wheel are like so many tiny teeth; they must be kept sharp in order to give a free cut. Function of a proper dresser is

• Re-sharpening these tiny cutting edges.

• Cleaning out the tiny spaces between the grains.

• Removing metallic dust and other clogged particles.

b. Proper truing and dressing of grinding wheels largely depends on

• Type of Dresser

• Dresser condition

• Dressing method

A rounded, blunt diamond cannot be expected to produce open texture on wheel surface Likewise it is not possible to produce a free cutting surface on wheel with a light dressing feed and slow traverse

The selection of the correct grinding wheel is of primary importance in the production of high grade work, but a close second is the selection of the proper dressing tool and its proper use.

c. Mantras for best Dressing & Truing results…………

1. WHAT is Dressing and Truing?

Dressing
It is the process of preparing the grinding wheel face. Dressing removes the blunt edges of abrasives and metal chips deposited in the wheel and presents a sharp grinding wheel to the component.

Truing
It is the process of making the grinding wheel face run true to the spindle or to put the required profile on the grinding wheel.

2. WHY Dressing and Truing?

The right preparation of the grinding wheel is the pre-requisite for the optimal grinding process.

Manufacturing of a true running grinding wheel with the necessary geometric form.
Activating of the necessary roughness of the grinding wheel.

3. Terminologies used in this chapter

ad = Depth of infeed of dressing tool (mm)
bd = Effective width dressing tool (mm)
ns = Grinding Wheel RPM
sd = Crossfeed rate of diamond dressing tool per grinding wheel RPM (mm/rev)
Ud = Overlap ratio (No.)
Vd = Crossfeed velocity of dressing tool (mm/min)

4. Effective Cutting width – bd

This dressing parameter designates the effective cutting width bd of a diamond tool at a certain depth of infeed ad. For the dressing tools as shown the effective width bd is a approximately as follows

Single point diamond                   Blade tool               Multipoint diamond*
0.5 to 1.0mm                            0.7 to 0.9mm         1.5 to 12mm

*in case of multipoint diamond tools with a measured bd> than 3mm take only 35% of measured bd.

5. Dressing with Single point Diamond ………….with pictures

Single point dressers are suitable for straight grinding wheels and simple profiles for small batch production of OD and ID grinding.
Clamping length as short as possible
Length b = max of 2 X a
Use drag angle of 10 to 15 degree
To avoid dulling of diamond, turn shaft by 90 degree from time to time.
Set diamond at drag angle up to 15 degree relative to cross axis

6. Dressing with Diamond Blade type………….with pictures

A modern universal dressing tool for profiling and straight dressing for large batch production. Diamond fliese tool features constant operating behavior throughout their service life. Guidelines Dress the wheel at full working surface speed. Always use blade tool in such as fashion that the diamond free section on the back of the blade touches the grinding wheel first. This will ensure a free cutting dressing operation. Apply ample amount of uninterrupted of grinding fluid while dressing as diamonds are very sensitive to heat.

7. Dressing with Multipoint Dressing tools ………….with pictures

A robust tool for straight dressing of peripheral and flat surfaces. One of their main advantages is their higher dressing federates.

Note
The crossfeed velocity vd should be between 620 and 1000mm/min

8. How to calculate the dressing crossfeed rates?

Overlay ratio ud
The illustration below shows an overlap ratio ud of 4. The overlap ratio indicates how often one point of the grinding wheel is covered by the effective width bd of the dressing tool. In other words, the dressing tool has moved across the wheel face, covering the distance of its own width in 4 wheel revolutions

Formula for crossfeed rate vd = bd.ns/ud
Guidelines for Ud
Rough grinding 2 to 3; Normal grinding 3 to 4; Finish grinding 4 to 6
example:
Wheel Dia, 500mm; Wheel speed 45m/s; wheel rpm 1700
Diamond used Fliese type Ud = 0.5mm appx.
For normal grinding considering Ud 3 the Dresser crossfeed rate comes to 283mm/min.
To achieve a superior surface finish and high metal removal rates, it is important to work with SMALL amounts of infeed ad (0.02 ~ 0.03mm)
In order to increase the wheel’s surface roughness, increase the crossfeed velocity Vd rather than increasing the depth of dressing infeed ad.
Higher crossfeed velocity = higher wheel surface roughness and vice versa.

9. How to calculate depth of infeed of dressing tool ?

The amount of loading, glazing and wheel wear between the 2 dressing cycles will depend on the amount of stock, available coolant, wheel speed and other process parameters.
Hence, Ideally there will not be a fixed dressing depth for a wheel, machine or Application.
As a thumb rule ad on radius is selected between 0.02 ~ 0.04mm depending on Finish or super finish requirements.

10. How important is the Coolant requirement for Dressing?

Diamond is the most pure form of Carbon.
Heat will lead to Oxidation.
High heat will mean quick Oxidation and faster wear.

Coolant of at-least 5 – 7.5 lpm to be provided at the point of dressing.

2. HOW TO SELECT THE RIGHT GRINDING WHEEL??

1. FACTORS AFFECTING GRINDING WHEEL SELECTION

Grinding wheel is a form of Cutting Tool, the Abrasive, Grit Size, Grade and Bond type should be correctly selected to fit the particular job. There are 6 factors for selection:

• Material to be ground and its hardness

• Amount of stock removal and finish required

• Whether the grinding is done wet or dry

• Wheel speed

• Area of grinding contact

• Severity of the grinding operation

Considering first the material to be ground and its hardness, which affects the choice of Abrasive, Grit-size and Grade.

2. Choice for Abrasives

Aluminium Oxide abrasives are well suited for steels and ferrous metals
Silicon Carbide abrasives are ideal for grinding Cast Iron, nonferrous metals and nonmetallic materials.

3. Grit Size

Fine grit-size works best in hard brittle material.
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 grits wheel can not penetrate as deeply into the hard work0piece without burning it.
On soft ductile materials, however, the larger grains penetrate easily and provide the necessary chip clearance to minimize wheel loading (fig. 3) and heat generation.

4. Grade

The harness of the material to be ground also affects the choice of the wheel grade or hardness. A harder grade can be used on soft, easily penetrated materials than on hard materials that 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.

5. Amount of stock removal and finish required

The second factor in selecting the correct wheel is the amount of stock to be removed and the finish required. These affect the choice of grit size and bond.
Grit-size: As a rule, coarser grit is selected for fast-cutting action and fine grit where a high finish is required.
Bond: Vitrified bonded wheels are generally used for fast-cutting action and commercial finish. Resinoid, Rubber and Shellac bonded wheels produce the highest finish.

6. Is the operation wet or dry

Generally 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 the heat generated in grinding.

7. Wheel Speed

The speed at which the grinding wheel is to be operated often dictates the type of bond. Vetrified 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 or 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 minutes must never be exceeded.

8. Area of Grinding Contact

The area of grinding contact between the wheel and the work effect the choice of grit-size and grade.
Grit-Size: A coarser grit is required when the areas is relatively large to provide adequate chip clearance between the abrasive grains. As the area of grinding contact becomes smaller and the unit pressure tending to break down the wheel-face becomes greater, finer grit wheels should be used.
Grade: On large areas of contact, soft grade wheel provides normal breakdown of the grit, ensuring a continuous free-cutting action. On the other hand, a harder grade is needed to withstand the increasingly higher unit pressure, as the area of contact becomes smaller.

9. Severity of the Grinding Wheel

This affects the abrasive and grade.
Abrasive: A tougher abrasive is required for grinding operation like snagging, for light grinding operations an intermediate abrasive is used for grinding jobs of average severity
Grade: The severity of the grinding operation also influences the ‘grade’. Hard grades provide durable wheels for rough grinding such as snagging, while medium and soft grade wheels are generally used for less severe precision grinding operations."

Conclusion
Selecting the right grinding wheel for a specific application, if properly understood, is not difficult. In actual practice, where rate of production as well as accuracy and surface finish are important, the grinding wheels must be selected with the utmost care.
Sometimes, the first selection of the wheel may not turn out appropriate and modifications in grade or grit-size, or both, may be necessary. In such cases, a correct specification could be established after conducting trials in two or three different grits and arriving at the optimum grade.
The skills of the operator also play an important role in the correct use of a grinding wheel. A skilled operator can make a wheel act one grade softer or harder by varying the other parameters like in work speed, traverse, feed, etc.

3. FOR TROUBLE FREE GRINDING, READ THIS

1. Why you lose productivity, accuracy and end up with higher cost of grinding?

Grinding process depends to a considerable extent on the correct choice of process parameters. The advantages of a good machine and a correct wheel can be lost by operating them under unfavorable conditions

2. Mantras to guide you in selection of Cutting parameters

Wheel wear in grinding is quite small and the cutting efficiency tends to gradually decrease. This means that the wheels should be dressed periodically to restore cutting efficiency and accuracy.

3. LOSS OF CUTTING EFFICIENCY

What is Cutting Efficiency?
The cutting efficiency of a wheel is its ability to remove material at a sufficiently fast rate without causing problems of burns, cracks or excessive deflections

Why cutting efficiency goes down?
Rapid loss of cutting efficiency is due to the wheel glazing

How to recognize loss of cutting efficiency?
Quite often, loss of cutting efficiency is accompanied by the appearance of chatter marks on the job surface, after grinding a few pieces.

How to maintain Cutting efficiency?
The process of abrasive fracture and exposure of fresh cutting edges automatically while grinding maintains the cutting efficiency of a wheel:
This can be achieved by:
Increasing feeds rates, depth-of-cut etc.
Note: In extreme cases it may be necessary to use a softer wheel and simultaneously use a coarser grit size if finish is not a criterion.

4. LOSS OF FORM AND ACCURACY

What is loss of Form and accuracy?
When grinding profiled jobs, the wheel is dressed to the required form. This form is gradually distorted due to uneven wear and leads to form errors on the job.

How to eliminate the Form error?
Grinding wheel necessitates redressing. But frequent dressing also leads in poor wheel life.

How to overcome the problem of poor wheel life?
Reducing the grinding allowances and the severity of the operation can minimize frequency of dressing.

5. DRESSING

Why Poor wheel life?
Wheel wear in dressing is substantially more than wheel wears in grinding. Most operators have a tendency to dress the wheel more than it is necessary and this only results in reduced wheel life.

What is the correct depth of cut for dressing?
It is recommended to remove only about 0.05 mm redial depth in dressing by taking two or three cuts 0.02 – 0.03 mm depth after the diamonds dressing tool touches the wheel.

What is the importance of dressing operation and how it influence accuracies?
An important aspect of the dressing operation is that it can be modified, to alter the cutting efficiency of the wheel and surface finish on the job.
Coarse dressing with a diamond traverse rate of 400 – 500 mm/min. will result in wheels with fast cutting action and rough finish, while fine dressing with a traverse rate of 100-200 mm/min. will result in a better finish.

It is also possible to make number of diamond passes at rates of 50 – 100 mm /min. without in feed to produce a wheel – condition giving a very fine finish. The latter approach is particularly useful when only a few special parts have to be made to the best surface finish in a tool room or maintenance shop.

Why BALANCING of Wheels?
Proper balancing of the grinding wheel is an essential prerequisite if good results are to be obtained in precision grinding. This is because an unbalanced wheel rotating at high speeds causes severe vibrations of the spindle and leads to chatter marks on the job and damage to spindle bearings

6. SURFACE FINISH

What is wrong about the widespread belief that jobs should always be ground to a very fine finish?

This is totally unwarranted and can be compared to a statement that all parts should be ground to a tolerance of say 5 microns. Such arbitrary job specifications only create difficulties during manufacturing without contributing to the functional quality of the parts.

It is extremely important to specify surface finish in quantitative terms i.e. in micron Ra or micro inches CLA, so that a proper evaluation can be made on the relevant instruments at least on a sampling basis.

What are the problems to achieve desired surface finish and their remedies?
Quite often, problems are encountered on the shop floor in achieving the desired finish. This parameter is no doubt considered when choosing the wheel specification but the end result is dependant on the operating conditions. The factors affecting surface finish are discussed below.

a . Spark-out
During grinding, the entire system is subjected to deformations under the action of the cutting forces. Thus for example, the job deflects considerably in cylindrical grinding and wheel spindle deflects appreciably in internal grinding. Even if wheel infeed is stopped at a given moment, material removal continues due to a gradual reduction in the deflections of various parts. This process is called “spark-out”
As the deflections are quite small, the corresponding depth-of-cut during spark-out is also small and gradually reduces to zero. This explains why spark-out is extremely effective in improving surface finish.

b . Dressing
The influence of dressing has been discussed earlier. A typical study showed that a 46-grit cylindrical grinding wheel produced a surface finish of 30 micro inches CLA when dressed at traverse rate of 500-mm/min. and reduction in traverse rate to 100 mm/min resulted in a 12 micron CLA finish.

c . Cutting Parameters
Beside the depth-of-cut, surface finish is also influenced by the other cutting parameters. The finish can be improved somewhat by reducing the job speed and reducing the traverse feed

d . Cutting Fluids
The influence of cutting fluids on surface finish is relatively small and in general fluids with greater lubricating action impart a somewhat better finish. The more important point is to ensure proper filtration of the fluid because suspended particles of abrasive and metal can caused deep scratches. Isolated scratch marks are a sure sign of dirty fluids. The remedy in such cases is to clean the tank and use magnetic separators at frequent intervals.