Showing posts with label assignment. Show all posts
Showing posts with label assignment. Show all posts
Wednesday, January 5, 2011
Timber:
Defination of timber:
A single piece or squared stick of wood intended for building, or already framed; collectively, the larger pieces or sticks of wood, forming the framework of a house, ship, or other structure, in distinction from the covering or boarding.
Structure Of Wood:
The woods used for last-making grow from the outside - that is to say, the new wood is formed between the old wood and the bark. The newly produced wood is softer than the older wood, and in drying shrinks most. The old wood is termed heart-wood, and the new, sap-wood. Each season's growth of the tree may be distinguished by the markings of rings that in a section of a trunk would be concentric . These rings are termed annular rings, and the number of them gives the age of the tree. In the illustration, it will be noticed that there are other marks besides the concentric rings, and that they radiate from the centre, or pith, and form the radii to the circumference of the tree-section. These lines are termed medullary rays. As the timber dries or shrinks the direction of the shrinkage will be indicated by the annular rings, and in the "cracks" illustrated , it should be noticed that the shrinkage is greater nearer the bark than at the centre or pith. This fact will have an important bearing upon the way the wood is cut into lasts.
Spokes are the triangular pieces of wood, sawn from the trunks or boughs. They are usually either cut into lengths 36 to 40 ins., or into smaller sections about 12 ins. long. illustrates the former, and would provide timber for three lasts. There are three or four such wedgeshaped pieces to the length cut from the trunk, according to the size of the tree.
Seasoning the wood is an important operation in the preparing of the wood for last-making. The spokes are stacked in a dry, airy place, free from direct sunshine or heat, for a period of about two years at least. The imported blocks do not usually get such a lengthy drying, as they are supposed to be partially seasoned when received. With green wood, steaming is sometimes re-sorted to, being a shorter process and advocated because it prevents shrinkage. This steaming changes the colour of the wood, and makes it a little softer. The seasoning by exposure to the air seems to retain more of the nature of the wood.
Seasoning:
Wood Seasoning is the process of removal of moisture from Timber. This can also be termed as the drying process of timber. Fresh timber has a very high quantity of moisture and hence is not useful for use in construction or for manufacture of furniture.(or)Removal of moisture from wood to improve its serviceability, usually by air drying - drying by air exposure without artificial heat - or kiln drying - drying in a kiln with artificial heat. Plywood veneers are seasoned before lay-up and gluing into panels.
Air Seasoning :
The traditional method of seasoning timber was to stack it in air and let the heat of the atmosphere and the natural air movement around the stacked timber remove the moisture. The process has undergone a number of refinements over the years that have made it more efficient and reduced the quantity of wood that was damaged by drying too quickly near the ends in air seasoning.
The basic principle is to stack the timber so that plenty of air can circulate around each piece. The timber is stacked with wide spaces between each piece horizontally, and with strips of wood between each layer ensuring that there is a vertical separation too. Air can then circulate around and through the stack, to slowly remove moisture. In some cases, weights can be placed on top of the stacks to prevent warping of the timber as it dries.
Moisture loss from the side of the wood is at about the right rate not to cause collapse of the cells, but near the ends of the wood, the moisture loss can prove to be too fast. Often the ends are wrapped or painted to slow the moisture loss from the end grain.
While little additional energy needs to be supplied for this type of seasoning, the stacks of timber require a lot of land, represent a potential fire hazard, and the product is not able to be sold for a considerable time. The interest costs on holding stock for long periods can prove significant.
Air-drying of timber is really a more controlled facilitation of what happens to unseasoned sawn, timber, once it is placed into its “work” environment. The amount of drying that can occur is very much governed by the relative humidity of the drying environment and will often vary within individual boards as well as within the stack itself. The time taken for air-drying is a function of the thickness of the timber.
Air-drying is necessarily a slow process, particularly for hardwoods, typically taking 6 to 9 months to reach a moisture content in the range 20% to 25%.
Kiln Seasoning:
The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack .
Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:
Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air drying.
This means that if capital outlay is involved, this capital is just sitting there for a longer time when air drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive.
In addition, wood that is being air dried takes up space, which could also cost money.
In air drying, there is little control over the drying elements, so drying degrade cannot be controlled.
The temperatures employed in kiln drying typically kill all the fungi and insects in the wood if a maximum dry-bulb temperature of above 60 °C is used for the drying schedule. This is not guaranteed in air drying.
If air drying is done improperly (exposed to the sun), the rate of drying may be overly rapid in the dry summer months, causing cracking and splitting, and too slow during the cold winter months.
The significant advantages of conventional kiln drying include higher throughput and better control of the final moisture content. Conventional kiln and solar drying both enable wood to be dried to any moisture content regardless of weather conditions. For most large-scale drying operations solar and conventional kiln drying are more efficient than air drying.
Compartment-type kilns are most commonly used in timber companies. A compartment kiln is filled with a static batch of timber through which air is circulated. In these types of kiln, the timber remains stationary. The drying conditions are successively varied from time to time in such a way that the kilns provide control over the entire charge of timber being dried. This drying method is well suited to the needs of timber companies, which have to dry timbers of varied species and thickness, including refractory hardwoods that are more liable than other species to check and split.
The main elements of kiln drying are described below:
( a) Construction materials: The kiln chambers are generally built of brick masonry, or hollow cement-concrete slabs. Sheet metal or prefabricated aluminium in a double-walled construction with sandwiched thermal insulation, such as glass wool or polyurethane foams, are materials that are also used in some modern kilns. Some of the elements used in kiln construction. However, brick masonry chambers, with lime and (mortar) plaster on the inside and painted with impermeable coatings, are used widely and have been found to be satisfactory for many applications.
( b) Heating: Heating is usually carried out by steam heat exchangers and pipes of various configurations (e.g. plain, or finned (transverse or longitudinal) tubes) or by large flue pipes through which hot gases from a wood burning furnace are passed. Only occasionally is electricity or gas employed for heating.
( c) Humidification: Humidification is commonly accomplished by introducing live steam into the kiln through a steam spray pipe. In order to limit and control the humidity of the air when large quantities of moisture are being rapidly evaporated from the timber, there is normally a provision for ventilation of the chamber in all types of kilns.
( d) Air circulation: Air circulation is the means for carrying the heat to and the moisture away from all parts of a load. Forced circulation kilns are most common, where the air is circulated by means of fans or blowers, which may be installed outside the kiln chamber (external fan kiln) or inside it (internal fan kiln). Throughout the process, it is necessary to keep close control of the moisture content using a moisture meter system in order to reduce over-drying and allow operators to know when to pull the charge. Preferably, this in-kiln moisture meter will have an auto-shutoff feature.
Solar Seasoning:
Solar seasoning offers a compromise between the low energy requirement of air drying and the speed of kiln drying. A number of different configurations have been tried, including a double skinned inflatable kiln. Because the energy input is variable, the kilns often have effective insulation to hold the heat inside at night time. Some kilns developed overseas can generate temperatures of over 40C inside even though the temperature outside is – 20C.
Solar-drying, which can take nearly twice the time required for kiln-drying, is well suited to high-grade applications such as furniture. Solar-drying is really a slower and gentler form of kiln drying. Some seasoned hardwood producers use air-drying down to fibre saturation point (FSP) prior to stacking the timber in the solar kiln.
Chemical Seasoning:
Chemical seasoning, which is sometimes used for very high value applications such as rifle butts, golf club heads and carvings, where the timber is required to be completely free of surface checks. The process involves soaking the green timber (as soon as it is cut) in a hydroscopic chemical for about a day, which slows down the rate of moisture loss and minimises the risk of inducing the internal stresses which cause checking to occur during the subsequent air-drying.
What is LEATHE Machine? its working properties.working principle.
0 comments Posted by org at 12:43 PM
LATHE MACHINE
DEFINATION:
A lathe is a machine tool which spins the workpiece to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.
Introduction:
Lathes were developed as early as the 15th century and were known as "bow"
lathes. The operator rotated the workpiece by drawing a bow back and forth,either by hand or with the use of a foot treadle. Next came Bessons lathe
in 1568, which was driven by a cord passing over a pulley above the machine.
This in turn drove two other pulleys on the same shaft which rotated the
workpiece and a crude, wooden lead screw, which in turn allowed the operator
to remove metal from the piece being machined. The screw cutting lathe
originates in the 17th century. Development and advancements have continued
and today we have sophisticated computerized controlled lathes.
Lathes have allowed man to reshape, machine and manufacture many precision
cylindrical components made of various types of metal, wood, plastics, and
other materials. Without the lathe, man would still be trying to produce
cylindrical components.
Boring:
Boring is an operation to enlarge and finish holes accurately. This may be done on a lathe or a milling machine. Boring is a machine operation in which the work is in contact with a single point tool.
A work piece may be held in a 3, 4, or 6 jaw chuck and collets.
Broaching:
Broaching is an operBroaching can be done on both internal and external surfaces.ation that completes the cutting in one stroke or cut. The teeth of a broaching tool are equally spaced so that as the tool advances into the workpiece, each tooth removes a specified amount of metal.
Drilling:
Drilling is an economical way of removing large amounts of metal to create semi-precision round hole or cavity. Drilling allows a person to make holes through boards, metals, and other materials.
Used for last removal of stock on preparation for other operations like boring, reaming, or tapping.
Drill press:
A machine designed to hold drill bits which will produce cylindrical holes. Used for producing cylindrical holes, as well as reaming, boring, counter-boring, counter-sinking, honing, lapping and tapping
There are three major types:
- Sensitive drill (light drilling)
- Upright drill (heavy duty drilling)
- Radial arm drill press (large, heavy workpieces)
Facing:Facing is a lathe operation in which the cutting tool removes metal from the end of the workpiece or a shoulder. Facing is a machine operation where the work is rotated against a single point tool. A workpiece may be held in a 3, 4, or 6 jaw chuck, collets or a faceplate.
Grinding:
Grinding is an operation in which the cutting is done by the use of abrasive particles. Grinding processes remove very small chips in very large numbers by cutting the action of many small individual abrasive grains. The abrasive grains are formed into a grinding wheel.Very smooth surfaces can be accomplished by the use of the proper grinding wheel.
Honing:Honing is an internal cutting technique that uses abrasives on a rotating tool to produce extremely accurate holes that require a very smooth finish. Similar to lapping where abrasive sticks are mounted in a rotating tool.
Capable of accuracies of less than 1/10,000th of one inch.
Non-precision grinding:
Non-precision grinding is a cutting technique used when the grinding does not need to be accurate.
Non-precision grinding is done when accuracy is not important. Non-precision grinding is a free-hand operation done on a pedestal or bench grinder.
Precision grinding:
Precision grinding is a cutting technique used when close tolerances and very smooth finishes are required. Precision grinding as often used as a finish machining process. Precision grinding allows very small amounts of material to be removed from a workpiece. This is extremely useful in acquiring smooth fnishes.
Reaming:
Reaming is a sizing operation that removes a small amount of metal from a hole already drilled. The reamer is the tool used for this operation.
Shaping:
Shaping is an operation used to produce flat surfaces. When the cutting tool reciprocates while the work is fed towards the tool removing material on each stroke, it is called shaping.
Single point threading:Single point threading is a lathe operation used for creating internal / external precision machined threads with a single point tool. Uses a special cutting tool that reflects the thread form.
There are over 100 thread forms.
Tapping:
Tapping is the process of cutting a thread inside a hole so that a cap screw or bolt can be threaded into the hole. Also, it is used to make threads on nuts. Tapping is done with a tool called a "Tap".
Tapping may be done by:
- hand
- lathe machine
- milling machine
- tapping machine
Turning:
Turning is a lathe operation in which the cutting tool removes metal from the outside diameter of a workpieceA single point tool is used for turning. A workpiece may be held in a 3, 4, or 6 jaw chuck, collets or may also be held between centers.
Wood
Defination of wood:
Wood is an organic material, a natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists compression. In the strict sense wood is produced as secondary xylem in the stems of trees (and other woody plants),or The secondary xylem of trees and shrubs, lying beneath the bark and consisting largely of cellulose and lignin. the hard fibrous lignified substance under the bark of trees ,
Types of Wood:
There are two types of wood.
1. Soft wood
2. Hard wood
Characteristics Of Soft wood:
Softwood is from an evergreen or coniferous (cone-bearing) tree. Common varieties are pine, fir, spruce, hemlock, cedar and redwood. These woods are mostly used in the home construction industry. Cedar and redwood are excellent choices for outdoor projects, while pine is often used for "Early American Country Style" furniture.
Pine and most other softwoods will absorb and lose moisture more than hardwoods so are not as stable. Purchase the lumber at least two weeks before starting your project and keep it indoors. Place stickers (small pieces of scrap wood) between the boards to permit good air circulation around each piece of stock. This will allow the wood to reach an equilibrium with the indoor environment which reduces the likelihood of dramatic wood movement after a project is complete.
You will generally find only two grades at most suppliers: select and No. 2 common. The common grade allows tight, solid knots in the face of the board. For furniture applications the select grade is the better choice as it is free from most knots, though tight pin knots and small resin pockets are allowed.
You can achieve a more uniform stained surface by applying a wood conditioner to pine first, it's a good idea to run a test first on an inconspicuous part of your project to make sure you're happy with the results.
You will generally find only two grades at most suppliers: select and No. 2 common. The common grade allows tight, solid knots in the face of the board. For furniture applications the select grade is the better choice as it is free from most knots, though tight pin knots and small resin pockets are allowed.
You can achieve a more uniform stained surface by applying a wood conditioner to pine first, it's a good idea to run a test first on an inconspicuous part of your project to make sure you're happy with the results.
You will find that softwoods are sold in standard thickness and widths, for example a 1 X 4 will be 3/4" thick and 3 1/2" wide similar to construction materials. The material will usually be priced per lineal foot and the price will increase accordingly for the wider boards.
Panels made from strips glued together and sold as shelving are also available, these range from 12 to 24 inches wide and 2 to 8 feet in length.
Characteristics Of Hard wood:
Hardwood lumber comes from deciduous trees, the ones that shed their leaves annually. Popular domestic species are oak, maple, cherry, birch, walnut, ash and poplar. Of these common native hardwoods, only red oak and poplar are usually stocked in home centers and lumberyards, the others have to be obtained from speciality stores. The material stocked at home centers and lumberyards is usually sold in similar dimensions to softwood and by the lineal foot as well.
At speciality stores the thickness of hardwood lumber is specified in quarters of an inch, measured when the wood is in a rough state. The thinnest stock is 4/4, representing 1 in., and the thickest usually available is 16/4, representing 4 in. Rather than being milled to specified dimensions, like pine, hardwoods are sold in random widths and lengths.
Hardwoods are sold by the board foot, which is defined as a square foot of rough lumber that is 1 in. thick. If a board is thicker than 1 in., the dealer multiplies the square footage by the thickness to arrive at the sale price. An 8/4 board will therefore cost twice as much as a 4/4 board of the same size.
Working with hardwoods is quite different from working with pine, you cannot drive a screw through hardwood lumber without first boring a pilot hole. Cutting and planing hardwOak and ash, are known as open-grain woods. These species have alternating areas of relatively porous and dense wood, when stained the open-grain areas absorb the color readily while the harder areas are more resistant. This accentuates the grain patterns, creating a dramatic effect.
Cherry, maple and birch are closed-grain woods, demonstrating a more uniform texture throughout a board. Poplar is also a closed-grain wood, but its color ranges from a beige to olive green, and often has purple highlights thrown into the mix. Because of this unusual coloration, it is rarely used if a furniture piece is going to have a clear finish. This wood is best when stained or even painted. Poplar, being less expensive, is also a good choice for framing hardwood projects.oods requires extremely sharp tools.
Cherry, maple and birch are closed-grain woods, demonstrating a more uniform texture throughout a board. Poplar is also a closed-grain wood, but its color ranges from a beige to olive green, and often has purple highlights thrown into the mix. Because of this unusual coloration, it is rarely used if a furniture piece is going to have a clear finish. This wood is best when stained or even painted. Poplar, being less expensive, is also a good choice for framing hardwood projects.oods requires extremely sharp tools.
Characteristics Of Good Timber:
The quality of timber depends greatly upon the treatment the tree has received, the time of felling, and, above all, on the nature of the soil in which it has grown.
These branches of the subject do not fall within the province of the engineer or builder, and will not here be entered upon; it will be sufficient to point out some of the characteristics by which good timber may be known.
Good timber should be from the heart of a sound tree - the sap being entirely removed, the wood uniform in substance, straight in fibre, free from large or dead knots, flaws, shakes, or blemishes of any kind.
If freshly cut it should smell sweet; "the surface should not be woolly, or clog the teeth of the saw," but should be firm and bright, with a silky lustre when planed; a disagreeable smell betokens decay, and "a dull chalky appearance is a sign of bad timber."
The annual rings should be regular in form; sudden swells are caused by rind-galls; closeness and narrowness of the layers indicate slowness of growth, and are generally signs of strength. When the rings are porous and open, the wood is weak, and often decayed.
The colour of good timber should be uniform throughout; when it is blotchy, or varies much in colour from the heart outwards or becomes pale suddenly towards the limit of the sapwood, it is probably diseased.
Among coloured timbers darkness of colour is said by Rankine to be in general a sign of strength and durability.
Good timber is sonorous when struck. A dull heavy sound betokens decay within. Among specimens of the same timber, the heavier are generally the stronger.
Timber intended for use in important work should of course be free from the defects mentioned in page 388. The knots should not be large or numerous, and on no account should they be loose.
The worst position for large knots is when they are near the centre of the balk required, and more especially when they are so situated as to form a ring round the balk at one or more points.
The sap should be entirely removed. According to Mr. Laslett, however, the heart of trees having the most sapwood is generally stronger and better in quality than the heart of trees of the same species that have but little sapwood.
The strongest part of the tree is generally that which contains the last-formed rings of heartwood, so that the strongest scantlings are obtained by removing no more rings than those containing the sap.
Timber that is thoroughly dry weighs less than when it was green. it is also harder, and consequently more difficult to work.
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