what is a spot welder？

what is a spot welder?

Spot welding machines are divided into universal (universal) and specialized types according to their purpose; According to the number of welding points simultaneously, there are single point, double point, and multi-point types; According to the conductive method, there are single-sided and double-sided; According to the transmission mode of the pressurization mechanism, there are pedal type, electric motor cam type, pneumatic type, hydraulic type, and composite type (gas-liquid compression type); According to the characteristics of operation, there are non automation and automation; According to the installation method, there are fixed, mobile, or lightweight (suspended) types; According to the movement direction of the active electrode (usually the upper electrode) of the welding machine, there are vertical stroke (electrode moves in a straight line) and circular stroke; According to the supply mode of electricity, there are power frequency welding machines (using 50Hz AC power supply), pulse welding machines (DC pulse welding machines, energy storage welding machines, etc.), and variable frequency welding machines (such as low-frequency welding machines).
When the workpiece and electrode are fixed, the resistance of the workpiece depends on its electrical resistivity. Therefore, electrical resistivity is an important performance of the welded material. Metals with high electrical resistivity have poor conductivity (such as stainless steel), while metals with low electrical resistivity have good conductivity (such as aluminum alloy). Therefore, when spot welding stainless steel, heat generation is easy but heat dissipation is difficult, and when spot welding aluminum alloy, heat generation is difficult but heat dissipation is easy. When spot welding, the former can use a small current (several thousand amperes), while the latter must use a large current (tens of thousands of amperes). The electrical resistivity is not only determined by the type of metal, but also by the heat treatment state, processing method, and temperature of the metal.
In order to ensure the size of the weld nugget and the strength of the solder joint, welding time and welding current can complement each other within a certain range. In order to obtain a certain strength of solder joints, high current and short time (strong conditions, also known as hard specifications) can be used, or low current and long time (weak conditions, also known as soft specifications) can be used. The choice of hard or soft specifications depends on the performance, thickness, and power of the welding machine used. There is an upper and lower limit on the required current and time for metals with different performance and thickness, which is used as the standard.
The electrode pressure has a significant impact on the total resistance R between the two electrodes. As the electrode pressure increases, R significantly decreases, while the increase in welding current is not significant and cannot affect the decrease in heat generation caused by the decrease in R. Therefore, the strength of solder joints always decreases with increasing welding pressure. The solution is to increase the welding pressure while increasing the welding current.

Spot welding machines classification
Spot welding machines can be divided into universal (universal, specialized) spot welding machines according to their purpose;
According to the number of welding points simultaneously, it can be divided into single point, double point, and multi-point types;
Classified by conductivity method: unilateral, bilateral;
According to the transmission mode of the pressurization mechanism, it can be divided into: foot pedal type, electric motor cam type, pneumatic type, hydraulic type, and composite type (gas-liquid compression type);
Classified by operational characteristics: non automated, automated;
According to the installation method, it can be divided into fixed, mobile or portable (suspended) spot welding machines;
According to the movement direction of the active electrode (usually the upper electrode) of the welding machine, it can be divided into vertical stroke (the electrode moves in a straight line) and circular stroke.

spot welder working principle
The process of spot welding involves opening the cooling water; Clean the surface of the welded part, assemble it accurately, and send it between the upper and lower electrodes. Apply pressure to ensure good contact; Electrification causes the contact surface of two workpieces to be heated, resulting in local melting and the formation of a molten nucleus; Maintain pressure after power outage to allow the molten core to cool and solidify under pressure, forming solder joints; Remove pressure and remove the workpiece. Welding parameters such as welding current, electrode pressure, energization time, and electrode working surface size have a significant impact on welding quality.
A spot welding machine uses the high-temperature arc generated by the instantaneous short circuit between the positive and negative electrodes to melt the material being welded between the electrodes, in order to achieve their bonding. The structure of a spot welding machine is very simple, in other words, it is a high-power transformer that converts 220V AC power into low voltage, high current power supply, which can be either DC or AC. Welding transformers have their own characteristics, which are the sharp drop in voltage.
After the ignition of the welding rod, the voltage drops. In addition to the primary 220/380 voltage conversion, the secondary coil also has a tap change voltage, and an adjustable iron core is also used to adjust the working voltage of the welding machine. A welding machine is usually a high-power transformer made using the principle of inductance. The inductance will produce a huge voltage change when connected and disconnected, and the high-voltage arc generated by the instantaneous short circuit between the positive and negative poles is used to melt the solder on the welding rod. To achieve the purpose of combining them.
Spot welding is a resistance welding method in which a welded joint is assembled and pressed between two electrodes, and the base metal is melted by resistance heat to form a solder joint. Spot welding is commonly used for the connection of thin plates, such as aircraft skin, aviation engine chimney, and car cab shell. A spot welding machine and a welding transformer are spot welding appliances, with only one secondary circuit. The upper and lower electrodes and electrode arms are used for conducting welding current and transmitting power. When the cooling water passes through the transformer, electrode, and other parts to avoid heating during welding, the cooling water should be connected first, and then the power switch should be turned on. The quality of electrodes directly affects the welding process, welding quality, and productivity. Electrode materials are commonly made of copper, cadmium bronze, chromium bronze, etc; The shapes of electrodes are diverse, mainly determined by the shape of the weld. When installing electrodes, pay attention to keeping the surface of the upper and lower electrodes parallel; The electrode surface should be kept clean and often repaired with a sandcloth or file.

The welding cycle of spot welding and projection welding consists of four basic stages (spot welding process):
(1) Pre pressing stage - the electrode descends to the current connection stage, ensuring that the electrode is tightly pressed against the workpiece and there is appropriate pressure between the workpieces.
(2) Welding time - The welding current passes through the workpiece, generating heat to form a molten nucleus.
(3) Maintenance time - Cut off the welding current and continue to maintain the electrode pressure until the molten core solidifies to sufficient strength.
(4) Rest time - The electrode starts to lift until it starts to descend again, starting the next welding cycle.

In order to improve the performance of welded joints, it is sometimes necessary to add one or more of the following to the basic cycle:
(1) Increase the preload to eliminate the gap between thick workpieces and make them tightly fit.
(2) Using preheating pulses to improve the plasticity of the metal, making it easy for the workpiece to fit tightly and prevent splashing; This can ensure that multiple protrusions are in uniform contact with the flat plate before welding, ensuring consistent heating at each point.

Usage of spot welding machine:
1. When welding, the position of the electrode rod should be adjusted first, so that when the electrode is just pressed against the welded part, the electrode arms remain parallel to each other.
2. The selection of the number of stages for the current regulation switch can be based on the thickness and material of the solder joint. After power on, the power indicator light should be on, and the electrode pressure can be adjusted by adjusting the spring pressure nut to change its compression degree.
3. After completing the above adjustments, you can first turn on the cooling water and then turn on the power to prepare for welding. Procedure for welding process: Place the welding piece between two electrodes, press the foot pedal, and make the upper electrode contact the welding piece and apply pressure. When continuing to press the foot pedal, the power contact switch is turned on, and the transformer starts working. The secondary circuit is energized to heat the welding piece. When the foot pedal is released after welding for a certain period of time, the electrode rises, and the power is first cut off and then restored to its original state by the tension of the spring. The single point welding process is completed.
4. Preparation and assembly of welding parts: Before welding steel welding parts, all dirt, oil stains, oxide skin, and rust must be removed. For hot-rolled steel, it is best to first acid wash, sandblasting, or use grinding wheel to remove oxide skin at the welding site. Although uncleaned welded parts can be spot welded, it seriously reduces the service life of the electrodes, as well as reduces the production efficiency and quality of spot welding. For medium and low carbon steel with thin coating, direct welding can be applied.

In addition, users can refer to the following process data when using:
1. Welding time: When welding medium and low carbon steel, this welding machine can use strong standard welding method (instantaneous electrification) or weak standard welding method (long-term electrification). Strong standard welding method should be adopted in mass production, which can improve production efficiency, reduce electrical energy consumption, and reduce workpiece deformation.
2. Welding current: The welding current is determined by the size, thickness, and contact surface condition of the welded part. Usually, the higher the conductivity of a metal, the greater the electrode pressure, and the shorter the welding time should be. At this point, the required current density also increases.
3. Electrode pressure: The purpose of applying pressure to the solder joint by the electrode is to reduce the contact resistance at the solder joint and ensure the pressure required for the formation of the solder joint.

security of spot welding machine
1. For on-site use, there should be a rain proof, moisture-proof, and sun proof machine shed, and corresponding fire-fighting equipment should be installed.
Within a 10m range of the welding site, flammable and explosive materials such as oil, wood, oxygen cylinders, acetylene generators, etc. shall not be stacked.
3. Welding operations and cooperation personnel must wear labor protection equipment according to regulations. And safety measures must be taken to prevent accidents such as electric shock, high-altitude falls, gas poisoning and fires.
4. The secondary tap connection copper plate should be tightly pressed, and the terminal post should have a gasket. Before closing, the wiring nuts, bolts, and other components should be thoroughly inspected and confirmed to be intact, complete, and free from looseness or damage. There is a protective cover at the terminal post.
5. Before use, it is necessary to check and confirm that the wiring of the primary and secondary wires is correct, the input voltage complies with the nameplate regulations of the welding machine, and know the type and scope of welding current of the spot welding machine. After connecting the power, it is strictly prohibited to come into contact with the live parts of the primary circuit. Protective covers must be installed at the primary and secondary wiring points.
6. When moving the spot welding machine, the power should be cut off and the welding machine should not be moved by dragging the cable. When there is a sudden power outage during welding, the power should be immediately cut off.
7. When welding non-ferrous metals such as copper, aluminum, zinc, tin, and lead, it must be done in a well ventilated area, and welding personnel should wear gas masks or breathing filters.
8. When multiple spot welding machines are used in a centralized manner, they should be connected separately to the three-phase power supply network to balance the three-phase load. The grounding devices of multiple welding machines should be connected separately from the grounding electrode and should not be connected in series.
9. Welding is strictly prohibited on pressure pipelines, containers containing flammable and explosive materials, and load-bearing components during operation.
10. When welding preheating components, baffles should be installed to isolate the radiation heat emitted by the preheating components.

spot welding machine Installation and maintenance
Welding machines must be properly grounded before use to ensure personal safety. Before using the welding machine, a 500V megohmmeter should be used to test the insulation resistance between the high voltage side of the welding machine and the casing, which should not be less than 2.5 megohms before power can be applied. Cut off the power supply before opening the box for inspection during maintenance. The welding machine should be filled with water before welding, and it is strictly prohibited to work without water. The cooling water should ensure the supply of industrial water at 5-30 ℃ under the inlet pressure of 0.15-0.2MPa. After the winter welding machine is completed, compressed air should be used to blow off the water in the pipeline to prevent freezing and cracking of the water pipe.
Welding machine leads should not be too thin or too long. The voltage drop during welding should not exceed 5% of the initial voltage, and the initial voltage should not deviate from the power supply voltage by ± 10%. When operating a welding machine, gloves, aprons, and protective goggles should be worn to prevent sparks from flying out and causing burns. The sliding part should be well lubricated, and metal splashes should be removed after use. After 24 hours of use, the screws of each component of the new welding machine should be tightened once, especially paying attention to the connection screws between the copper flexible joint and the electrode. After use, the oxide between the electrode rod and the electrode arm should be regularly removed to ensure good contact.
If the AC contactor is found to be improperly engaged during the use of the welding machine, it indicates that the grid voltage is too low. The user should first solve the power supply problem, and only use it after the power supply is normal. It should be pointed out that if there are quality issues with the main components of the newly purchased welding machine within half a month, a new welding machine or the main components can be replaced. The main body of the welding machine is partially covered by a one-year warranty, and long-term maintenance services are provided. In general, after notifying the manufacturer, the service will be provided within three to seven days based on the distance traveled. Welding machine damage caused by user reasons is not covered by the warranty. Vulnerable and consumable parts are not covered by the warranty.
Due to the fact that the contact area of the electrode determines the current density, and the electrical resistivity and thermal conductivity of the electrode material are related to the generation and dissipation of heat, the shape and material of the electrode have a significant impact on the formation of the melt nucleus. As the electrode tip deforms and wears, the contact area increases, and the strength of the solder joint will decrease. The oxide, dirt, oil, and other impurities on the surface of the workpiece increase the contact resistance. An excessively thick oxide layer can even prevent current from passing through. Local conduction, due to excessive current density, can cause splashing and surface burning. The presence of oxide layers can also affect the uneven heating of various solder joints, causing fluctuations in welding quality. Therefore, thoroughly cleaning the surface of the workpiece is a necessary condition to ensure the acquisition of high-quality joints.

spot welding machine Troubleshooting
1. The welding machine does not work when stepping on the foot pedal, and the power indicator light does not light up:
a. Check if the power supply voltage is normal; Check if the control system is functioning properly.
b. Check if the foot switch contacts, AC contactor contacts, and tap shifter switches are in good contact or burned out.
2. The power indicator light is on, and the workpiece is tightly pressed and not welded:
a. Check if the foot pedal travel is in place and if the foot switch is in good contact.
b. Check if the pressure bar spring screws are properly adjusted.
3. Unexpectedly splashing during welding:
a. Check if the electrode head is severely oxidized.
b. Check if the welded workpiece is severely corroded and has poor contact.
c. Check if the adjustment switch is set too high.
d. Check if the electrode pressure is too low and if the welding procedure is correct.
4. Severe indentation and extrusion of solder joints:
a. Check if the current is too high.
b. Check for unevenness in the welded workpiece.
c. Check if the electrode pressure is too high, and if the shape and cross-section of the electrode head are appropriate.
5. Insufficient strength of welded workpiece:
a. Check if the electrode pressure is too low and if the electrode rod is securely fastened.
b. Check if the welding energy is too low and if the welded workpiece is severely corroded, resulting in poor contact of the welding points.
c. Check for excessive oxidation between the electrode head and electrode rod, as well as between the electrode rod and electrode arm.
d. Check if the cross-section of the electrode head has increased due to wear, resulting in a decrease in welding energy.
e. Check if the electrode and copper soft joint and bonding surface are severely oxidized.
6. Abnormal sound from AC contactor during welding:
a. Check if the incoming voltage of the AC contactor is 300 volts lower than its own release voltage during welding.
b. Check if the power lead is too thin or too long, causing a significant voltage drop in the circuit.
c. Check if the network voltage is too low to function properly.
d. Check if there is a short circuit in the main transformer, causing excessive current.
7. Welding machine overheats:
a. Check if the insulation resistance between the electrode holder and the body is poor, causing a local short circuit.
b. Check if the inlet pressure, water flow rate, and water supply temperature are appropriate, and check if the water system is blocked by dirt, which may cause overheating of the electrode arm, electrode rod, and electrode head due to poor cooling.
c. Check if the copper flexible joint, electrode arm, electrode rod, and electrode head contact surface are severely oxidized, causing an increase in contact resistance and severe heating.
d. Check if the cross-section of the electrode head has increased too much due to wear, causing the welding machine to overload and heat up.
e. Check if the welding thickness and load duration exceed the standard, causing the welding machine to overload and generate heat [1]

spot welding has the following advantages:
1. When the molten core is formed, it is always surrounded by a plastic ring, isolating the molten metal from the air, and the metallurgical process is simple.
2. Due to short heating time and concentrated heat, the heat affected zone is small, and the deformation and stress are also small. Usually, there is no need to arrange correction and heat treatment procedures after welding.
3. No need for filler metals such as welding rods and wires, as well as welding materials such as oxygen, acetylene, and argon, resulting in low welding costs.
4. Easy to operate, easy to achieve mechanization and automation, improving working conditions.
5. High productivity, no noise or harmful gases. In mass production, it can be assembled together with other manufacturing processes on the assembly line. However, flash welding requires isolation due to spark splashing.

spot welding has the following drawbacks:
1. Lack of reliable non-destructive testing methods, welding quality can only be inspected through destructive testing of process specimens and welded components, as well as various monitoring technologies to ensure.
2. The lap joint of point and seam welding not only increases the weight of the component, but also results in lower tensile and fatigue strength of the joint due to the formation of an angle around the fusion nucleus between the two plates.
3. The high power, mechanization, and automation of the equipment increase the cost of the equipment. Maintenance is difficult, and the commonly used high-power single-phase AC welding machine is not conducive to the normal operation of the power grid, requiring separate power distribution.
The main resistance welding methods include spot welding, seam welding, projection welding, and butt welding.