Updated by Prakash Patil, Sr. Manager, Grindwell Norton Ltd.
There are two main methods of metal cutting. Cutting performed using a single point tool and cutting performed using a multi-point cutting tool. We are well aware that while performing cutting using a multi-point cutting tool, if the material to be removed is of a high volume, then milling is performed; while if less material is to be removed and the surface finish is to be improved, then grinding is used. In this article, we are going to learn more details about the grinding wheel.
What is Grinding?
|Grinding is a machining process improving the surface finish of the job and producing small chips. The tool used for this process is the grinding wheel. It is a cutting tool in which millions of microscopic abrasive grains are bond together. Here, each abrasive grain acts like a spiky tool. As shown in the image, the abrasive grains are held together in the porous structure of the grinding wheel by a bonding substance (bond). When these grains come in contact with the surface to be cut, their sharp microscopic edges can cut the material on the surface. Like any other tool, the grains lose their sharpness and are to be regularly removed to allow fresh new grains to come forward. The abrasive grains are held together by a bond and while cutting, the grains get free from the bond.|
|As each grain starts taking a cut in the job, a chip starts forming out of it. It bends and ultimately breaks away from the job. As compared to other machining processes, the chip formed during grinding is quite smaller in size. Also since multiple grains are simultaneously performing this task, many microscopic size chips are formed and the action of material removal is quick.|| |
The grinding wheel comprises of following items:-
1. Abrasive grains - Cutting takes place due to them.2. Bond material - Due to which, abrasive grains are held together.
Apart from grains and bond, the porous construction of the wheel is an important part. The coolant entering deep into this during cutting process does not allow increase in the temperature of the wheel and the job.
The process of wheel wear:
In other machining processes, tool wear is a serious problem. But in grinding, that in itself is beneficial, because unless the wear of the wheel takes place, new sharp grains can’t be available for work.
|While the grains are taking a cut in the metal of the job, the grains get crushed and break down due to the force acting on the wheel. In grinding process, the process of breaking of abrasive grains & exposing sharp edges is called as friability. When the grains break down under pressure, the new edges for cutting emerge or are exposed. During this process, the binding material also gets removed and the grains which have become blunt get free and are removed. In this manner, due to the breaking, wear and the grains becoming free, the grinding wheel becomes a self sharpening multi-point cutting tool.|| |
Defects like ‘loading’ in which sometimes the grains and chips becoming free stick onto the grinding wheel or ‘glaze’ on the wheel surface which has become blunt due to heat, or the distortion in the shape of the wheel due to use arise in the grinding wheel. To repair these, the process of wheel dressing is followed.
Changing nature of the grinding wheel.
Approximately 60-70 years ago, manual or hydraulically operated grinding machines were in vogue. The quantitative and to a certain extent qualitative needs of that time were less as compared to those prevalent today, hence the wheels used in those days were suitable for the needs of that time. Brief information on the expected degree of quality of a job manufactured by grinding is given in the table below.
At present thumb rule which is being followed - “go coarser, go faster, go bigger’. For this purpose, large size grains are expected to be used. It is expected to attain the maximum possible wheel rotational speed or maximum possible cutting speed.
0.4 to 0.6 uRa
Quantitative need of the industry
Hardness of the job to be ground
58 to 64 HRC
Accuracy of the profile
profiles Tolerances were wide
Very stringent profiles tolerances
33- 45 M/S
Was not considered
Grinding cracks are absolutely not acceptable -Various Test Methods -Nital test, BN Value, Magnaflux, Fatigue Tests
Many changes have taken place in the grinding wheels to fulfill these needs.
- Grains: Aluminium Oxide and Silicon Carbide were used in the conventional grains. Now in their place, ceramic grains have been developed. New ceramic grades like Seeded Gel (SG), Targa Gel (TG), Norton Quantum (NQ), Cubitron (CU) are now in use. The new ceramic grains are made up of a number of small crystals. When the grain breaks, we get a number of small cutting points in a single grain since its crystals are also spiky. If the dressing of the grinding wheel is not carried out at the appropriate time, then due to the grains which have become blunt, the material on the surface of the job is not cut off; instead, the wheel is merely pressed on that surface. Due to the resultant friction, the temperature increases. Due to the increased temperature, the surface of the job becomes black as if it has burnt. Also it can develop grinding cracks (Grind Injury). While grinding surfaces at 90°, the possibility of this phenomenon taking place on the vertical face is high. To avoid this, it is necessary to ensure that only sharp grains will exist on the grinding wheel surface & coolant reach has to be ensured. In the wheels having the new ceramic grains, the surface of the wheel obtained after dressing has more sharp grains.
- Bond: The bonding material performs an important task to obtain consistency of profile and the required form of the profile. Vitrium bond (VT3 bond) is a newly developed bond which is stronger than the previous bonds and can withstand high speeds and can fulfill the need to hold grains for longer time in the grinding wheels which rotate at a high speed as per today’s requirement. This bond also provides chip clearance for better chip removal which reduced the bond or chip interactions with component.
- Porosity: Since the coolant enters the open spaces in the grinding wheel, the temperature of the process does not increase excessively. The more the quantity of coolant accommodated in the open spaces, the cooler will be the process. The heat carrying capacity of the wheel is required to be higher for the higher temperatures generated due to grinding carried out at high speeds. If the porosity is kept higher for this purpose, when the grinding wheel rotates at a high speed, there is a possibility that the strength of the grinding wheel may be less. Hence a correct balance of this is maintained. A vitrified porous bond VT3-10/12 (Vitrium), VTX (Vortex) was developed to cater to these needs.
At present progress is taking place towards the goal - “go coarser, go faster, go bigger’.
For this purpose, large size grains & wheels are expected to be used.
It is expected to attain the maximum possible wheel rotational speed or
maximum possible cutting speed. For example, cutting speeds ranging from 33 meters per second
to 45, 60, 80, up to 125 meters per second are now achievable.
Since the size of the wheel has also increased, a proportionately higher cutting speed is achieved
at the same R.P.M. The wheels being manufactured for this purpose are shown here in the pictures.
Examples of advantages achieved due to changes in the wheel specifications-
1) Change in the wheel grain:
Machine: Parishudh Cylindrical Grinding.
Wheel: Old type
Size: 05/S 700x85x304.8
Wheel Specification:- A60L5VCL 45M/S: Brown Aluminium Oxide wheel grade
Wheel: New-White Alo
Wheel Specification:- 38A60L8VT3 White Aluminium Oxide 38A
Job: Banjo Casing (Differential Casing)
Material:- EN.14B, Hardness:- 45-55 HRC
Here, grinding is performed on the bearing diameter of the Banjo.
- Instead of 3, 6 jobs are achieved after each dressing. That means the wheel life has increased.
- Cycle time came down from 270 seconds to 190 seconds.
- Cost per job reduced.
2) Change in the wheel design:
Machine:- Studer Cylindrical grinding machine
Single wheel, two settings.
Layered wheel made by combining two separate wheels having different specifications and grains.
Job: Turbocharger Shaft Fan Assembly
Material: Shaft: Hardened steel and Fan: Inconel 713C.
In this case, the turbocharger fan and the shaft behind are assembled together by friction welding. The hardness and material of these two parts are different. Hence instead of using two different grinding wheels having different properties in two separate grinding set-ups, a single layered wheel was used. A groove is formed by one of the layered wheels while the outside diameter of the fan is ground by the wheel having more width.
- Instead of 3500 jobs, now 4300 jobs are manufactured per wheel.
- Instead of 5, 7 jobs are achieved after each dressing.
- Efficiency increased since both the operations are performed in a single set-up instead of two set-ups.
3) Change in the bond material:-
Machine:- Parishudh Cylindrical grinding.
Wheel:- Old type
GNO 38A60I8VT3 Vitrium Bond.
Job:- Gear Shaft
Material: - Hard Steel
Hardness: - 58-62 HRC.
Since the wear of the wheel was non-uniform, a taper was forming on the shaft. Also after performing grinding on the shoulder, burn marks and grinding cracks could be seen on the job surface. As a solution for this, instead of VS3 bond, the advanced VT3 (Vitrium) bond was used.
- Instead of 5, 8 jobs are achieved after each dressing.
- The cost per piece got reduced by 15%.
- A consistency of job diameter has achieved and the formation of taper eliminated.
- Wheel life has improved and the problems of burn marks and cracks is eliminated.