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Line 184: − + + + + + + + + + + − + − + + + + + + + + + + + + + + + + + + + + − ! Reinforced resin+ − + + + + + + + + + + + + + + + − − === Grain Types === +
Boyer Schultz 6-12 Surface Grinder (view source)
Revision as of 18:06, 6 August 2018
, 18:06, 6 August 2018→Grinding Wheel Types: more info on wheel types
== Grinding Wheel Types ==
== Grinding Wheel Types ==
<blockquote>Every grinding wheel is marked by the manufacturer with a stencil or a small tag. The manufacturers have worked out a standard system of markings.</blockquote>
[[File:Surface-grinding-wheel-markings.jpg]]
=== Wheel Shape ===
[[File:Surface-grinder-heel-shapes.jpg]]
{|class="wikitable"
! Shape
! Type nos.
! Description
|-
! Straight
| 1, 5, 7
| Straight wheels are commonly applied to internal, cylindrical, horizontal spindle, surface, tool, and offhand grinding and snagging. The recesses in type numbers 5 and 7 accommodate mounting flanges. Type number 1 wheels from 0.006" to 1/8" thick are used for cutting off stock and slotting.
|-
! Cylinder
| 2
| Cylinder wheels may be arranged for grinding on either the periphery or side of the wheel.
|-
! Tapered
| 4
| Tapered wheels take tapered safety flanges to keep pieces from flying if the wheel is broken while snagging.
|-
! Straight cup
| 6
| The straight cup wheel is used primarily for surface grinding, but can also be used for offhand grinding of flat surfaces. Plain or beveled faces are available.
|-
! Flaring
| 11
| The flaring cup wheel is commonly used for tool grinding. With a resinoid bond, it is useful for snagging. Its face may be plain or beveled.
|-
! Dish
| 12
| The chief use of the dish wheel is in tool work. Its thin edge can be inserted into narrow places, and it is convenient for grinding the faces of form-relieved milling cutters and broaches.
|-
! Saucer
| 13
| The saucer wheel is also known as a saw gummer because it is used for sharpening saws.
|}
=== Grain Size/Grit ===
=== Grain Size/Grit ===
<blockquote>Abrasive grains are selected according to the mesh of a sieve through which they are sorted. For example, grain number 40 indicates that the abrasive grain passes through a sieve having approximately 40 meshes to the linear inch. A grinding wheel is designated coarse, medium, or fine according to the size of the individual abrasive grains making up the wheel.
The softer and more ductile the material, the coarser the grain size. The larger the amount of stock to be removed, the coarser the grain size. The finer the finish desired, the finer the grain size.</blockquote>
{|class="wikitable"
{|class="wikitable"
=== Grade ===
=== Grade ===
<blockquote>The grade of a grinding wheel designates the hardness of the bonded material. A soft wheel is one on which the cutting particles break away rapidly while a hard wheel is one on which the bond successfully opposes this breaking away of the abrasive grain.
Most wheels are graded according to hardness by a letter system. Most manufacturers of grinding abrasive wheels use a letter code ranging from A (very soft) to Z (very hard). Vitrified and silicate bonds usually range from very soft to very hard, shellac and resinoid bonds usually range from very soft to hard, and rubber bonds are limited to the medium to hard range.
The grade of hardness should be selected as carefully as the grain size. A grinding abrasive wheel that is too soft will wear away too rapidly, the abrasive grain will be discarded from the wheel before its useful life is realized. On the other hand, if the wheel is too hard for the job, the abrasive particles will become dull because the bond will not release the abrasive grain, and the wheel’s efficiency will be impaired.
The harder the material, the softer the wheel. The smaller the arc of contact, the harder the grade should be. The arc of contact is the arc, measured along the periphery of the wheel, that is in contact with the work at any instance. It follows that the larger the grinding wheel, the greater the arc of contact and, therefore, a softer wheel can be used. The higher the work speed with relation to the wheel speed, the milder the grinding action and the harder the grade should be.</blockquote>
{|class="wikitable"
{|class="wikitable"
|}
|}
=== Bond ===
=== Bonding Material ===
<blockquote>The abrasive particles in a grinding wheel are held in place by the bonding agent. The percentage of bond in the wheel determines, to a great extent, the “hardness” or “grade” of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. “Hard” wheels retain the cutting grains longer, while “soft” wheels release the grains quickly. If a grinding wheel is “too hard” for the job, it will glaze because the bond prevents dulled abrasive particles from being released so new grains can be exposed for cutting. Besides controlling hardness and holding the abrasive, the bond also provides the proper safety factor at running speed. It holds the wheel together while centrifugal force is trying to tear it apart. The most common bonds used in grinding wheels are vitrified, silicate, shellac, resinoid, and rubber.</blockquote>
{|class="wikitable"
{|class="wikitable"
! Bond type
! Bond type
! Symbol
! Symbol
! Description
|-
|-
! Vitrified
! Vitrified
| V
| V
| The majority of grinding wheels have a vitrified bond. Vitrified bonded wheels are unaffected by heat or cold and are made in a greater range of hardness than any other bond. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels.
|-
! Silicate
| S
| Silicate bond releases the abrasive grains more readily than vitrified bond. Silicate bonded wheels are well suited for grinding where heat must be kept to a minimum, such as grinding edged cutting tools. It is not suited for heavy-duty grinding. Thin cutoff wheels are sometimes made with a shellac bond because it provides fast cool cutting.
|-
|-
! Resin
! Resin
| B
| B
| Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings), which operate at 9,500 SFPM. It is also used in cutoff wheels.
|-
|-
! Resin bonded
! Rubber
| B2
| R
| In rubber-bonded wheels, pure rubber is mixed with sulfur. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs. Most abrasive cutoff machine wheels have a rubber bond.
|}
=== Abrasive structure ===
[[File:Surface-grinding-wheel-structure.jpg]]
<blockquote>Bond strength of a grinding wheel is not wholly dependent upon the grade of hardness but depends equally on the structure of the wheel, that is, the spacing of the grain or its density. The structure or spacing is measured in number of grains per cubic inch of wheel volume.
The softer, tougher, and more ductile the material, the wider the grain spacing. The finer the finish desired, the closer, or more dense, the grain spacing should be.</blockquote>
=== Abrasive material ===
<blockquote>Most grinding wheels are made of silicon carbide or aluminum oxide, both of which are artificial (manufactured) abrasives. Silicon carbide is extremely hard but brittle. Aluminum oxide is slightly softer but is tougher than silicon carbide. It dulls more quickly, but it does not fracture easily therefore it is better suited for grinding materials of relatively high tensile strength. The tensile strength of material to be ground is the main factor in the selection of the abrasive to be used.</blockquote>
{|class="wikitable"
|+ Appropriate abrasives per work material
! Silicon carbide
! Aluminum oxide
|-
|-
|
| BF
* Gray and chilled iron
* Brass and soft bronze
* Aluminum and copper
* Marble and other stone
* Rubber and leather
* Very hard alloys
* Cemented carbides
* Unannealed malleable iron
|
* Carbon steels
* Alloy steels
* High speed steels
* Annealed malleable iron
* Wrought iron
* Hard bronzes
|}
|}
{|class="wikitable"
{|class="wikitable"
|+ Abrasive material subtypes
! Type / Composition
! Type / Composition
! Symbol
! Symbol