Background

Introduction

Manufacturing and development projects of an increasingly high technology kind form an important and integral part in the working lives of all engineers at various stages in our careers.

The handling of such projects usually include different stages such as design, planning, costing, manufacture and assembling of components, testing, documentation, and final evaluation. A good engineer, particularly in the role of a project leader, must therefore have a sound appreciation of all of these elements.

All project works, done in groups, will certainly involve other people. Thus, it will develop our ability to co-operate, participate and discuss with others. In doing this, we will learn how to exercise tact and diplomacy. Abilities such as these become essential in the 'real world' when performing our 'professional activities'.

Manufacturing project work in the Industrial Centre is intended to emphasize, and give experience in, all of these project stages. The projects are so structured that we can bring our previous practical and technical knowledge and experience together into one coherent activity. Also it also gives us a chance to work together as a team, and have the opportunity to exercise our managerial and decision making abilities.

Furthermore, the final analysis and presentation of such manufacturing project works will give us some confidence and competence to undertake industrial type work of the level us will operate at in the future.

This training module is designed to give us a good appreciation on the skill of industrial manufacturing. Emphasis is not placed on us to become a skilful fitter within such a short period of training, but rather to let us understand the uses of different equipments to create different components. Nevertheless, on completion of the training and through the hands-on practice given, we will acquire some of the basic skills and techniques involved with these hand processes.

To get the maximum benefit from the training, it is essential that we use every opportunity to consolidate what we observe and to interact between our self and the staff member in charge of our training. This is self-motivated and the drive must come from us.

 

Project Background

This program is called Manufacturing Projects, which aims to develop the skills and knowledge of students in product innovation and related technologies. The Program included about 30 days. We have seven people in a group. We need to learn how to communicate with our group mates and maintain group cooperation.

The IC operates 5 days a week from 0830 to 1700. Normal project days in the summer are from Monday to Thursday. In these project days, we have to follow the IC timetable and sign in/out from our supervisor's office. Friday is not a normal project day but we may at our discretion to work at our own pace. We should observe the IC rules related to timekeeping during the timetabled project periods.

During the timetabled project periods (i.e. Monday to Thursday), facilities will be available for manufacturing activities. We are requested to plan our work carefully and make our booking.

 

Principles behind a toaster

To begin our project, we bought a toaster to be our reference. It was because we wanted to study the inner structure of the toaster. So that, we could have a clear idea how we developed our toaster in this project.
Therefore, we bought the Tefal toaster. The name was Express and the model number is TT3561. Below was the photo of the toaster.


express_25_main
The main function of this toaster was:

- It could control the darkness of the surface of toasts

- It had a residue tray

We found that these two functions were two necessary functions. So, we decided that our design of the toaster should also have these two functions.

We disassemble it to study the inner-structure. Here were the photos of the parts.


DSC01250  This was the heat wires.

View Image  This was the IC board.

These two were the main parts of the toaster.

A toaster uses infrared radiation to heat a piece of bread. When you put your bread in and see the coils glow red, the coils are producing infrared radiation. The radiation gently dries and chars the surface of the bread.
The most common way for a toaster to create the infrared radiation is to use nichrome wire wrapped back and forth across a mica sheet, like this:

principle

Nichrome wire is an alloy of nickel and chromium. It has two features that make it a good producer of heat:

-Nichrome wire has a fairly high electrical resistance compared to something like copper wire, so even a short length of it has enough resistance to get quite hot.

-The nichrome alloy does not oxidize when heated. Iron wire would rust very quickly at the temperatures seen in a toaster.

The very simplest toaster would have two mica sheets wrapped in nichrome wire, and they would be spaced to form a slot about an inch (2.5 cm) wide. The nichrome wires would connect directly to a plug.
For the toast to be poped out, it needs a spring-loaded tray. This keeps you from having to turn the toaster upside down.  ­The photo below shows you one of the slots of a typical toaster:

spring loaded tray

Two mica/nichrome sheets line either side of the slot. A metal holder rides up and down in the slot to raise and lower the bread. Many toasters include a pair of grates on either side of the slot. The grates press against the bread and center it. Two metal springs get pushed when the holder nears the bottom of the slot, and they pull the grates inward. The holders in each slot are connected to the handle that you depress to lower the bread into the toaster, as shown below:

spring loaded tray1

When you push the handle down, three things have to happen:

  1. Mechanism needs to hold the handle down to keep the toast inside the toaster for a period of time.
  2. Power needs to be applied to the nichrome wires.
  3. Timer needs to release the holder at the proper time so the toast pops up.

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Plastic plate attached to toast-lowering lever, plastic wedge (on left) that switches on the power

When the bar is lowered, the metal tab contacts the electromagnet.


The plastic bar presses into a pair of contacts on the circuit board to apply power to the nichrome wires, and the piece of metal gets attracted to an electromagnet to hold the toast down. You can see both the contacts (copper strips on the right) and the electromagnet (green block on the left) below:


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Toaster circuit card showing electrical contacts

Toaster circuit board with contacts engaged

Here is how the whole mechanism works:

-When you push down on the handle, the plastic bar presses against the contacts and applies power to the circuit board.

-120V power runs directly through the contacts to the nichrome wires to start toasting the bread.

-A simple circuit made up of transistors, resistors and capacitors turns on and supplies power to the electromagnet.

-The electromagnet attracts the piece of metal on the handle, holding the bread in the toaster.

-The simple circuit acts as a timer. A capacitor charges through a resistor, and when it reaches a certain voltage it cuts off the power to the electromagnet. The spring immediately pulls the two slices of bread up.

In the process, the plastic bar rises and cuts off power to the toaster.