Unit III

Grinding & Superfinishing

Question 1) Marking system for conventional grinding wheel?

Answer 1) The standard marking system for conventional abrasive wheel can be as follows:

51 A 60 K 5 V 05,

The number ‘51’ is manufacturer’s identification number indicating exact kind of abrasive used.

The letter ‘A’ denotes that the type of abrasive is aluminium oxide. In case of silicon carbide, the letter ‘C’ is used.

The number ‘60’ specifies the average grit size in inch mesh. For a very large size grit this number may be as small as 6 where as for a very fine grit the designated number may be as high as 600.

The letter ‘K’ denotes the hardness of the wheel, which means the amount of force required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can range between ‘A’ and ‘Z’, ‘A’ denoting the softest grade and ‘Z’ denoting the hardest one.

The number ‘5’ denotes the structure or porosity of the wheel. This number can assume any value between 1 to 20, ‘1’ indicating high porosity and ‘20’ indicating low porosity.

The letter code ‘V’ means that the bond material used is vitrified. The codes for other bond materials used in conventional abrasive wheels are B (resinoid), BF (resinoid reinforced), E(shellac), O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate)

The number ‘05’ is a wheel manufacturer’s identifier.

Question 2) Explain Types of abrasives?

Answer 2)

(a). Aluminum oxide

Aluminum oxide may have variation in properties arising out of differences in chemical composition and structure associated with the manufacturing process.

Pure Al2O3 grit with defect structure like voids leads to unusually sharp free cutting action with low strength and is advantageous in fine tool grinding operation, and heat sensitive operations on hard, ferrous materials.

Regular or brown aluminium oxide (doped with TiO2) possesses lower hardness and higher toughness than the white Al2O3 and is recommended heavy duty grinding to semi finishing.

Al2O3 alloyed with chromium oxide (<3%) is pink in colour.

Monocrystalline Al2O3 grits make a balance between hardness and toughness and are efficient in medium pressure heat sensitive operation on ferrous materials.

Microcrystalline Al2O3 grits of enhanced toughness are practically suitable for stock removal grinding. Al2O3 alloyed with zirconia also makes extremely tough grit mostly suitably for high pressure, high material removal grinding on ferrous material and are not recommended for precision grinding. Microcrystalline sintered Al2O3 grit is the latest development particularly known for its toughness and self-sharpening characteristics.

(b). Silicon carbide

Silicon carbide is harder than alumina but less tough. Silicon carbide is also inferior to Al2O3 because of its chemical reactivity with iron and steel.

Black carbide containing at least 95% SiC is less hard but tougher than green SiC and is efficient for grinding soft nonferrous materials.

Green silicon carbide contains at least 97% SiC. It is harder than black variety and is used for grinding cemented carbide.

(c). Diamond

Diamond grit is best suited for grinding cemented carbides, glass, sapphire, stone, granite, marble, concrete, oxide, non-oxide ceramic, fiber reinforced plastics, ferrite, graphite.

Natural diamond grit is characterized by its random shape, very sharp cutting edge and free cutting action and is exclusively used in metallic, electroplated and brazed bond.

Mon crystalline diamond grits are known for their strength and designed for particularly demanding application. These are also used in metallic, galvanic and brazed bond.

Polycrystalline diamond grits are more friable than monocrystalline one and found to be most suitable for grinding of cemented carbide with low pressure. These grits are used in resin bond.

(d). cBN (cubic boron nitride)

Diamond though hardest is not suitable for grinding ferrous materials because of its reactivity. In contrast, cBN the second hardest material, because of its chemical stability is the abrasive material of choice for efficient grinding of HSS, alloy steels, HSTR alloys.

Presently cBN grits are available as monocrystalline type with medium strength and blocky monocrystals with much higher strength. Medium strength crystals are more friable and used in resin bond for those applications where grinding force is not so high. High strength crystals are used with vitrified, electroplated or brazed bond where large grinding force is expected.

Microcrystalline cBN is known for its highest toughness and auto sharpening character and found to be best candidate for HEDG and abrasive milling. It can be used in all types of bond.

Question 3) Explain Vitrified bond?

Answer 3) Vitrified bond is suitable for high stock removal even at dry condition. It can also be safely used in wet grinding. It cannot be used where mechanical impact or thermal variations are like to occur. This bond is also not recommended for very high-speed grinding because of possible breakage of the bond under centrifugal force.

Question 4) Explain Resin bond?

Answer 4) Conventional abrasive resin bonded wheels are widely used for heavy duty grinding because of their ability to withstand shock load. This bond is also known for its vibration absorbing characteristics and finds its use with diamond and cBN in grinding of cemented carbide and steel respectively. Resin bond is not recommended with alkaline grinding fluid for a possible chemical attack leading to bond weakening. Fiberglass reinforced resin bond is used with cut off wheels which requires added strength under high speed operation.

Question 5) Explain Cutting action?

Answer 5) Grinding wheel cutting action is done by different types of particles called abrasive particles like granite, bonding materials etc.. When the wheel rotates this particles used to comes in contact with the work piece and removes the metal from work. some of the important thing in this grinding wheel is, some of the particles used in grinding wheel has self-sharpening action, this can sharp there cutting particles at the time of cutting process. an make them self particles sharp.

At some times on the cutting material gets brakes or get some cracks this is due to resistance offered by the work piece. By this we can get the new cutting points to make further operation.

There are some factors affecting the wheel selection:

Speed of the wheel may causes affects.

Some of the operation conditions.

Area of contact between the wheel and work.

Hardness of material.

Question 6) Dressing of super abrasive wheel?

Answer 6) Dressing of the super abrasive wheel is commonly done with soft conventional abrasive vitrified stick, which relieves the bond without affecting the super abrasive grits.

However, modern technique like electrochemical dressing has been successfully used in metal bonded super abrasive wheel. The wheel acts like an anode while a cathode plate is placed in front of the wheel working surface to allow electrochemical dissolution.

Electro discharge dressing is another alternative route for dressing metal bonded super abrasive wheel. In this case a dielectric medium is used in place of an electrolyte.

Touch-dressing, a new concept differs from conventional dressing in that bond material is not relieved. In contrast the dressing depth is precisely controlled in micron level to obtain better uniformity of grit height resulting in improvement of workpiece surface finish.

Machining Time required for cylindrical grinding (T) = Length of cut x Number of cuts Feed/rev x R.P.M

Length of cut = length of tapper + over travel

Question 7) Surface Grinding Machine Working?

Answer 7) A Surface Grinding Machine uses rotating abrasive wheel that removes the material from the surface of the workpiece to create a flat surface with a high surface finish.

The grinding wheel revolves on the spindle and the workpiece is mounted on a reciprocating table.

The reciprocating table moves in forward or backward direction and the workpiece is adjusted according to the grinding wheel position.

When the power supply is provided and suitable speed is given to the grinding wheel, the grinding wheel rotates on the surface of the workpiece so as to remove the material from the surface of the workpiece till high accuracy is obtained.

The Aluminum oxide, silicon carbide, diamond, and cubic boron nitride (CBN) are the four commonly used abrasive materials for the surface of the grinding wheels.

Question 8) Explain external cylindrical grinding?

Answer 8) External cylindrical grinding is used for the production of cylindrical or tapered workpiece, such as the grinding of shafts, axles and spindles as used in the general machine tool, automotive and aerospace industries. The circumference of the grinding wheel is used to remove material from the circumference of the workpiece. This can be done in the radial (plunge grinding) or axial (traverse grinding) modes. To allow for greater accuracy in clamping, workpiece are usually mounted between centres. Multiple idle strokes (spark-outs) are used to improve the form accuracy and surface quality.

Question 9) Explain Internal cylindrical grinding?

Answer 9) Internal cylindrical grinding is primarily used for machining cylindrical or tapered bores. During internal cylindrical grinding, the longitudinal feed movement is typically carried out by the grinding wheel, with the radial infeed movement during internal cylindrical grinding handled by the wheel head or the workhead, depending on the design of the machine. Basically, the same kinematic relationships apply as with external cylindrical grinding. The contact area between grinding wheel and workpiece is however considerably larger, which makes the removal of chips and the cooling of the process more difficult

Question 10) what is Polishing?

Answer 10) A process to generate a reflective surface

Normally, the polish is generated by using a fine-micron or sub-micron abrasive particle in combination with a liquid. Polishing is a “wet” process.

Often the polishing process utilizes a pad to contain the abrasive, so polishing may not be a “loose abrasive process.” The pad is softer than the part.

Very little material is  removed during the polishing process, normally measured in microns

The surface finish of the work-piece to be polished must be of a high quality prior to the polishing process taking place, so the pre-polishing process is often a “lapped” surface.

Question 11) How Does Polishing Work?

Answer 11) Polishing often uses a polishing pad and water-base slurry to generate the reflective or clear surface

An unblemished, scratch-free surface finish is critical on polished surfaces. To generate the required finishes, the polishing slurries are often caustic. As such, the polishing systems may feature stainless steel exposed components such as the hardware, rings and plates.

Further to the above, some polishing applications also require thorough water rinsing during the end of the processing cycle, in order to remove the polishing media so it does not “stain” the surface. This is another reason why stainless steel is required. The polishing pads are usually grouped into either “soft” pad or “hard” pad categories.