Unit - 4
Carpentry
Q1) What is the difference between Wood and Timber?
A1) Wood
Wood is available in nature in the form of trees. Useful part of the tree is cut and is used for carpentry work. Trunk is most useful part of the tree.
Timber
The wood obtained from fully grown trees is cut and is prepared for engineering purposes. This wood is known as Timber.
When the tree is in a living one, the timber is called stationary Timber, after felling rough Timber and after sawing into suitable market sizes, it is known as converted Timber.
Q2) Write the advantages of timber?
A2) 1) Easy to work on it
2) Lighter in weight
3) Low cost
4) Good response to polishing and painting
5) Very suitable for doors, windows and furniture work.
6) Soundproof in nature.
7) Good strength
Q3) What are the Characteristics of soft and hard Woods?
A3)
Softwood | Hardwood |
It is light in colour. | It is dark in colour. |
It is lighter in weight. | It is heavier in weight. |
It has straight fibres and fine texture. | Its fibres are quite close and compact. |
It has good tensile resistance but poor shear resistance. | It has good tensile and shear resistance. |
Relatively less durable. | It is some durable. |
Catches fire easily. | Does not catch fire easily. |
Easy to work. | Difficult to work. |
It is a resinous wood. | It is non resinous wood. |
Q4) Define Plywood and its Classification?
A4) It is laminated and compound wood made up of plies and veneers. The plies are made of thin layers of word glued together under pressure.
Classification of plywood:
(a) Ply sheets: Thickness is less than 3 mm and consists of an odd number of plies.
(b) Multiple boards: These are more than 3 mm in thickness and consists of more than three layers of veneers (pila).
(c) Composite boards: These are made from several plies of wood with one or two layers of asbestos fibre or some other light emulating material.
(d) Blackboards: It consists of a core of wood less than 25 mm thick and glued between two faces plies under pressure.
Advantage:
1) Higher strength
2) Shrinkage and expansion are negligible.
3) Does not split while nails are hammered into it.
4) Economical
5) Used for household and decorative purposes economically.
Q5) What do you mean by Seasoning of wood?
A5) The main objective of seasoning is to reduce the moisture content in the wood to the extent it is desirable so as to make it suitable for various purposes. Moisture's presence will render the wood unsuitable.
Advantages of seasoning
1) Durability and strength are increased.
2) Workability is increased.
3) Fatigue of worker is reduced due to decrease in weight.
4) Distortion is minimized.
5) Surface finish is improved.
6) Resistance to fire is increased.
Seasoning methods:
1. Natural seasoning: In this method, the bulbs of timber are stacked in a shed such that they are not directly exposed to Sun and rain water free circulation of air taken place through them. It is kept for 6 months to 2 years. due to the circulation of free through the state the excess moisture content in the world is evaporated and the wood gets dried.
2. Water seasoning: This process consists of immersion of timber balks in flowing water for a period of 2 to 3 weeks point during this time the flowing water drives away the sap of the wood with it. The timber is then taken out of the water and air seasoned in the usual way. This method takes relative Li less time than the former but the strength of the wood is reduced.
3. Kiln seasoning: This process is the quickest of all the commonly used processes for wood seasoning. In this Timber balks are stacked over large trolleys which are, then driven into hot chamber, or ovens called Kilns. Inside these Kilns balks are allowed to remain under controlled conditions of temperature and humidity for nearly a fortnight or so. hot here or dry steam is pushed into the chamber whereas the temperature raises gradually. In this case, seasoning rate is faster but quality is inferior.
Q6) What are the Defects occur in wood?
A6) Knots: The impressions left behind by the broken branches later appear as knots. This offers difficulty in working and reduces the strength of the wood. When the separation of branches or limbs takes place before the tree is cut, the knots formed are called Dead knots.
If separation occurs at affecting of tree comedy knots formed are known as live knots.
The knots are also classified according to their shape:
1. Circular knot
2. Oval knot
3. Irregular knot
4. Line knot
5. Symmetrical knot
6. Group knot
2. Shakes: When tree is not cut even after attaining full maturity coma the cohesion amongst the wood grains is lost due to evaporation of gums, moisture, resins and oil set. also burning of tissues and shrinkage of inferior parts take place, which cause radial or circular ruptures in tissues and create cavities which are known as shakes.
Following are types of shakes:
- Heart shakes: A small cavity is formed at the centre of the tree.
- Ring shakes: these shapes are formed on the structural rings of the tree.
- Twisted shakes: these are continuous shakes in twisted form.
- Peripheral shakes: these appear on the outer part of the tree, that is bark. It grows towards the centre of the trunk.
3.Ring gall: It is an uneven broken part of the tree. It generally affairs where branches are cut or irregularly broken.
4. Distortion: Due to uneven moisture inside to word the shape of wood is distorted. It is caused by improper seasoning.
Growth of wood
The tree consists of mainly three parts vig, roots, trunk (including branches) and leaves. In spring season, the roots of the tree suck the requisite food from the soil in the form of sap, which is a dilute solution of mineral salts. This sap rises to leaves through the cells to give food to them. Under direct sun, the leaves release moisture from sap and in turn absorb carbon dioxide and form dense sap. This dense sap descends down word in autumn and gets deposited under the bark and form a hard layer. This process repeats a number of times.
Q7) Describe the different type of growth of trees?
A7) The growth of trees is of two types:
1. Exogenous or outward growing: In this, growth occurs outward from the centre adding almost concentric layer of fresh word every year, known as annual rings. (Mango, Shisham, oak)
2. Endogenous or inward growing: Here, trees grow inward, i.e., every fresh layer of sap wood is added inside instead of outside. Example: bamboo, coconut etc.
Q8) Describe in brief detail about the Structure of wood?
A8) The structure of word is shown in figure. The main features illustrated are as follows:
Q9) What do you mean by conversion of timber?
A9) The process of cutting the board into marketable forms from Timber logs is called conversion of wood. Suitable shrinkage allowances are kept while sawing the wood.
Types:
(1) Flat or ordinary sawing: In this process, the timber log is cut into a number of boards by taking various parallel saw cuts. It is the simplest and cheapest method of sawing.
(2) Tangential sawing: In this method, the sawing takes place in such a way that the width of the boards are tangential to the annual rings (shown in figure). The wood sawn through this method is seasoned quickly and sawing wastage is also less but again it is likely to wrap as flat sawn wood.
(3) Quarter or radial sawing: In this method, the timber logs are so sawn that the width of the sawn boards fall along the medullary rays, i.e., dirham radially across the section of the log. The word obtained from quarter sawing is very suitable for all sort of quality wood work viz cabinet making, decoration and framework etc.
Q10) What are the different criteria for good Qualities of Timber?
A10) The following qualities are expected of a good Timber.
