A resistance welding process variation in which spot or projection welds are made by manually applying force to one electrode and using the workpiece or a support as the other electrode.
In push welding, sometimes called poke welding, pressure is applied manually to one electrode only. It differs from spot welding in that only one electrode is used in direct contact with the spot to be welded. The other electrode is clamped to any part of the metal in much the same manner that the workpiece is connected to the power supply in arc welding. This element makes it possible to weld in places that would be inaccessible for machine-made spot welds. In poke welding, the moveable electrode is operated by hand and placed at the point to be welded. When sufficient pressure is exerted on the workpiece, a pressure switch initiates the weld current at a preset value. Usually the welding machine controls the time of current flow to a set value.
An instrument capable of indicating temperatures higher than a mercurial thermometer will indicate. There are several types of pyrometers, including the
following:
Optical Pyrometer. There are several variations of optical pyrometers which use a lens system to focus the radiation from the heated object to be measured on the sensor of the instrument. One type focuses the radiation from the object to be measured on a thermopile, which produces a small voltage proportional to the body temperature. The voltage is converted to temperature and is indicated on a calibrated meter.
Another type of optical pyrometer measures the temperature of a heated object to be measured by comparing the color, or redness, of the heated object with a wire heated in the instrument by an electric current. This current converted to temperature is indicated on a calibrated meter.
Photoelectric Pyrometer. This type measures the voltage from a photronic or photoelectric cell, which receives radiation from the heated object. The voltage is converted to temperature and is indicated on a calibrated meter.
Electrical Resistance Pyrometer. This type uses a Wheatstone bridge to measure the resistance (which varies with temperature) of a fixed length of platinum wire which is exposed to the temperature to be measured. It is useful to 2400°C (4350°F).
Thermocouple Pyrometer. Thermocouple pyrometers make use of the small voltage produced when two different metals which are in contact are heated. Wires of the two different metals are welded to form a hot junction and a voltage is measured by a sensitive voltage measuring instrument at the other ends of the wires at room temperature. Several standard metal combinations are available, and temperature-voltage
values have been determined and published for each. The more common thermocouple metal combinations are: chromel-alumel; platinum-platinum 10% rhodium; platinum-platinum 13 % rhodium; iron-Constantan; and copper-Constantan.
Mechanical Pyrometer. A mechanical pyrometer utilizes the differential expansion of two different metals when heated to actuate a pointer by means of gears or levers. These are not accurate over 538°C (1000°F).