Showing posts from August, 2015

Lathe Operations: Facing

Facing Operations Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes.

When a lathe cutting tool removes metal it applies considerable tangential (i.e. lateral or sideways) force to the workpiece. To safely perform a facing operation the end of the workpiece must be positioned close to the jaws of the chuck. The workpiece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end. Cutting Speeds

If you read many books on machining you will find a lot of information about the correct cutting speed for the movement of the cutting tool in relation to the workpiece. You must consider the rotational speed of the workpiece and the movement of the tool relative to the workpiece. Basically, the softer the metal the faster the cutting. D…

Understanding the Diferences between Heat Treatment, Annealing, and Tempering!

Heat Treatment, Annealing, and Tempering are three well-known methods for giving treatment to metals. Depending on the treatment used, a material may become more or less brittle, harder or softer, or stronger or weaker. The method chosen depends on the desired characteristics of the material. If steel is being treated, for instance, the designer may desire an end material with a high tensile strength but a relatively low degree of brittleness. In order to achieve the desired levels, the designer may make use of several treatments, alter the temperature at which the material is treated, or change the length of time over which the treatment is applied.
Heat Treatment
Tempering and annealing are actually two different types of heat treatment. Heat treatment is a process using the controlled application of heat to alter the physical and chemical properties of a material, and is generally used in metals. However, many different materials can be heat treated, including glasses. It is very com…

What is Off Setting the Tailstock?

The tailstock offset method is generally used to cut a taper when no taper attachment is available. This involves moving the tailstock centre out-of-line with the headstock centre. However, the amount that the tailstock may be offset is limited. This method will not permit steep tapers to be turned or standard tapers to be turned on the end of a long piece of work. Since the work will be essentially at an angle it will need to be machined between centres. Thus, a chuck or collet can not be used to hold the workpiece. Great care must be exercised in setting the tailstock back to centre when the taper turning operation is completed.

What is Compound Rest?

The compound rest is generally used for turning or boring short steep tapers, but it can also be used for longer, gradual tapers, providing the length of the taper does not exceed the distance the compound rest will move upon its slide. This method can be used with a high degree of accuracy, but is somewhat limited due to the lack of an automatic feed and the length of the taper being restricted to the movement of the slide.
The compound rest base is graduated in degrees and can be set at the required angle for taper turning or boring. With this method, it is necessary to know the included angle of the taper to be machined. An included angle is formed by and between two intersecting straight lines.)
The angle of the taper with the centerline is one-half the included angle and will be the angle that the compound rest is set for. For example: To true up a lathe center which has an included angle of 600 from parallel to the ways If the taper is given in taper inches per foot, the angle for…

Taper Turning Attachment

The taper attachment is used for turning and boring tapers. It is bolted to the back of the carriage saddle. In operation, it is connected to the cross-slide so that it moves the cross-slide laterally as the carriage moves longitudinally. This action causes the cutting tool to move at an angle to the axis of the workpiece to produce a taper.
The angle of the desired taper is set on the guide bar of the attachment, and the guide bar support is clamped to the lathe bed. Since the cross-slide is connected to a shoe that slides on this guide bar, the tool follows along a line that is parallel to the guide bar and hence at an angle to the workpiece axis corresponding to the desired taper.

Methods Of Taper Turning

In ordinary straight turning, the cutting tool moves along a line parallel to the axis of the work, causing the finished job to be the same diameter throughout. However, when cutting a taper, the tool moves at an angle to the axis of the work, producing a taper. Therefore, to turn a taper, the work must either be mounted in a lathe so that the axis upon which it turns is at an angle to the axis of the lathe, or cause the cutting tool to move at an angle to the axis of the lathe.
When the diameter of a piece changes uniformly, from one end to the other, the piece is said to be tapered. Taper turning as a machining operation is the gradual reduction in diameter from one part of a cylindrical workpiece to another part Tapers can be either external or internal. If a workpiece is tapered on the outside, it has anexternal taper; if it is tapered on the inside, it has an internal taper.

The method used for turning a taper depends on the degree, length, location of the taper (internal or extern…

What is the function of waterstops in joints of box culverts and drainage channels?

The principal function of waterstops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing watertightness to the drainage channel.
Besides, waterstops in drainage channels or box culverts can also serve two other purposes:
to avoid water contacting joints' dowel bars and causing corrosion. (ii) to avoid water seeping in from the underside of drainage channels or box culverts, thereby washing in soil particles and causing voids underneath these structures and finally leading to their failure. To serve the second purpose, obviously only one waterstop is required at any depth location.
To serve the first purpose, a waterstop has to be installed on top of dowel bars to prevent water from drainage channels from leaking through. On the other hand, a waterstop has to be provided below dowel bars to avoid underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using cor…

What is the difference between epoxy grout, cement grout and cement mortar?

Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent.
Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.
Cement mortar is normally a mixture of cement, wate…

What is the function of shear keys in the design of retaining walls?

In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows:
The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance.
On the other hand, friction between the wall base and the foundation soils is …

If on-site slump test fails, should engineers allow the contractor to continue the concreting works?

This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.
The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) - (d), the contractor is not entitled to any claims of cost which is the main concern for most engineers. This is the contractual power given to the Engineer in case of any failure in tests required by the contract, even though some engineers argue that slump tests are not as important as other tests like compression test.
The second school of thought is to let the contractor to continue their concreting works and later on request the contractor to prove that the finished works comply with other contractual requirements e.g. compression test. This is based upon the belief that workability is mainly …

What are the functions of different components of a typical expansion joint?

In a typical expansion joint, it normally contains the following components: joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent.
Joint sealant: it seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement.
Joint filler: it is compressible so that the joint can expand freely without constraint. Someone may doubt that even without its presence, the joint can still expand freely. In fact, its presence is necessary because it serves the purpose of space occupation such that even if dirt and rubbish are intruded in the joint, there is no space left for their accommodation.
Dowel bar: This is a major component of the joint. It serves to guide the direction of movement of concrete expansion. Therefore, incorrect direction of placement of dowel bar will induce stresses in the joint during thermal expansion. On the other hand, it links the two adj…

What is the situation when an engineer will use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.
In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide …

What is fiddle-bow lathe?

Fiddle-Bow lathe was used mostly for small work, generally metal work, was called a "fiddle-bow lathe," on account of the method of driving it. In this lathe, which is shown in the figure below.

The same idea of propulsion is used, that of a cord passing around either the piece to be turned or a rotating part of the mechanism by which the piece was revolved. In this case, however, instead of the resistance of the flexible limb of a tree or of a "spring pole" acting to keep the driving cord taut, it is held in that condition by the flexible bow F, which is bent to the form shown by the driving cord D. The engraving is an exact reproduction of a lathe, the bed A of which was about twelve inches long and it had a capacity of about two inches swing, that was made by an older brother for the use of the author when he was nine years old, and in the use of which he became quite a boyish expert in turning wood and metals. The head-stock B, and rest C, were formed…

What are the basic principles of wood turning?

It must be noted, however, that although skill can only be acquired by practice, by "making shavings" as the saying goes, it is futile practising unless the basic principles are being applied. The basic principles are comprised of four main elements:

The choice of the correct tools

The use of properly sharpened tools

A good stance

The use of correct cutting techniques

Consideration must also be given to safety. Safe working habits should become habitual and are as much part of basic principles as the four points covered above. Unsafe practices may not prevent good turning but they may cut short a turner's career.
Much of this book is devoted to these basic principles. However, before he can practise the basic principles the prospective turner must provide himself with a certain amount of equipment. At the very minimum this will be a lathe, a grinding machine and a set of tools. He will also need somewhere to keep it and somewhere to work; usually, of course, these are the same…

How to develop wood turning skill?

In the case of woodturning there is a bit more to it than motor skills because, with the right attitude of mind, it is within the power of the individual to alter the shape of the learning curve. The key to skill is attitude. What does this mean? To begin with it means developing an understanding of the correct basic techniques. If the turner does not get the basics right then, however much he practises, he will not improve. In contrast he may develop a lot of bad practices which will be difficult to eradicate.
It is necessary to have a strong desire to learn and progress but at the same time one must have patience. It is no use the turner trying to make things which are way beyond his level ability, particularly in the early stages. On the other hand it is necessary for him to stretch himself with projects of steadily increasing difficulty. It may be a good idea for the learner to set himself a series of achievable goals.
Nevertheless, it has to be recognised that the hobby turner with…

How to make a curve?

Turning requires manual dexterity, visual judgement and the co-ordination of hand and eye. In this respect it is similar to games like tennis. Such activities require the development of what psychologists term 'motor skills'. The learning and development of these skills require relatively long periods of practice.
It has been said that it can take seven years, working full-time, for a turner with aptitude to reach the peak of his abilities and become fully skilled. But again this should not deter the beginner. What does 'fully skilled' mean? It means that the turner can perform all the operations with speed and accuracy. At the top level, for a professional needing to earn a living, speed is an important ingredient in skill.
This can be illustrated by the so called 'learning curve' which may be familiar to the reader. The general shape of the learning curve is illustrated in below Diagram.

It can be seen from this that typically, with continual practice, the indiv…

The art of woodturning

Woodturning is an art not a science. Each skilled practitioner has his own particular way of doing things. The reason for this is that wood unlike, say, metal or plastic, is not an homogeneous substance. No two pieces of wood are identical even when cut from adjacent positions in the tree. In contrast, consider a piece of steel to be used in a motor car. Numerous metallurgists, and other specialists, will have been employed in the production and testing of the steel, to ensure that it has the required characteristics, and that these will be consistent from one batch of material to another. This means that properties such as its granular structure, its hardness, its elasticity and its tensile strength will be the same for every sample.
Wood is not at all like that. Adjacent pieces will exhibit differences in such features as fibre structure, grain pattern, hardness and elasticity. As each unique work piece spins on the lathe and is traversed by the tool the turner has to make subtle adj…