Design and Fabrication of a Portable High-Current Resistance Spot-Welding Machine using Upcycled Components

Design and Fabrication of a Portable High-Current Resistance Spot-Welding Machine using Upcycled Components

Bagul Adit Manojkumar

Student, Department of Mechanical Engineering, Birla Vishvakarma Mahavidyalaya (Engineering College), Vallabh Vidyanagar, Gujarat, India.

Shah Dhruv Brijen

Student, Department of Mechanical Engineering, Birla Vishvakarma Mahavidyalaya (Engineering College), Vallabh Vidyanagar, Gujarat, India.

Prajapati Vraj Pareshkumar

Student, Department of Mechanical Engineering, Birla Vishvakarma Mahavidyalaya (Engineering College), Vallabh Vidyanagar, Gujarat, India.

Jainikkumar Kirtikumar Patel

Assistant Professor, Department of Mechanical Engineering, Birla Vishvakarma Mahavidyalaya (Engineering College), Vallabh Vidyanagar, Gujarat, India.

Popat Dhairya Nishant

Student, Department of Mechanical Engineering, Birla Vishvakarma Mahavidyalaya (Engineering College), Vallabh Vidyanagar, Gujarat, India.

Abstract – This paper presents the design, fabrication, and performance analysis of a low-cost, portable resistance spot-welding (RSW) machine. Conventional industrial spot welders are highly efficient but are predominantly stationary and capital-intensive, creating a functional gap for small-scale workshops and remote repair tasks. To address this mobility and cost constraint, a discarded Microwave Oven Transformer (MOT) was safely extracted and modified into a step-down transformer. By replacing the high-voltage secondary coil with heavy-gauge copper cabling, the system successfully outputs a high-current, low-voltage electrical profile of approximately 2300 A at

2.5 V. The architecture integrates a spring-loaded mechanical lever to apply consistent forging pressure and an active cooling system to mitigate thermal overload. Empirical testing demonstrated the system’s capability to successfully forge structural weld nuggets in 2 mm Mild Steel sheets within a 5-second weld cycle. By utilizing repurposed electronic waste, the total fabrication cost was reduced by 70% to 90% compared to commercial units, offering an accessible, plug-and-play manufacturing solution without sacrificing fundamental welding efficiency.

Keywords – resistance spot welding; microwave oven transformer; portable fabrication; joule heating; upcycled manufacturing

inefficient and carries a severe risk of burning straight

1. INTRODUCTION

   Resistance spot welding (RSW) remains one of the most reliable and fundamental manufacturing methods for fusing thin metal sheets. The technique operates on the principle of electrical resistance, specifically Joule heating; when a massive electrical current is forced through clamped metal surfaces, it generates intense, localized heat that melts the faying surfaces into a solid weld nugget. However, while traditional industrial RSW machines are incredibly efficient, they are also notoriously bulky, completely stationary, and require massive capital investment.

   This high cost of entry essentially locks independent fabrication workshops, hobbyists, and educational laboratories out of utilizing high-quality spot welding technologies. Furthermore, because industrial resistance welders are tied to fixed factory floors, they cannot be deployed for remote or on-site repair jobs. While operators sometimes attempt to use portable arc welders as a substitute for joining thin metals, arc welding often proves highly

   through thin-gauge materials. Therefore, there is a critical functional gap in the market for a joining technology that is both highly mobile and accessible.

   To bridge this divide, researchers and independent fabricators have increasingly explored low-cost alternatives, most notably the upcycling of electronic waste like Microwave Oven Transformers (MOT). Standard MOTs naturally operate as step-up transformers, but previous academic studies have demonstrated that by stripping away the high-voltage secondary coil and replacing it with just a few turns of heavy-gauge copper cable, the unit can be successfully converted into a high-current step-down transformer. While the core physics of this processgoverned mathematically by Joules Law of Heating (H=I2Rt)are well established, building a reliable portable system introduces significant functional hurdles. A review of existing literature highlights that maintaining consistent electrode contact pressure to prevent metal expulsion, alongside managing the severe thermal degradation of the

   transformer windings in air-cooled environments, are the primary causes of failure in portable setups.

   Addressing these specific engineering gaps, this paper details the design, fabrication, and evaluation of a highly portable, cost-effective resistance spot welder built around a repurposed MOT. The primary objective of this study is to engineer a compact system that successfully generates high-amperage welding currents while integrating critical safety and performance features. By incorporating an active thermal cooling system to prevent overheating and a customized spring-loaded lever mechanism to guarantee uniform forging pressure, this work aims to provide a sustainable, reliable welding solution that bridges the divide between heavy industrial power and small-scale accessibility.

2. METHODOLOGY AND SYSTEM ARCHITECTURE

   1. Electrical Modification and Power Conversion

      The core of the welding apparatus relies on the strategic modification of a standard Microwave Oven Transformer (MOT). In its factory state, an MOT operates as a step-up transformer designed to generate thousands of volts. To achieve the specific electro-mechanical profile required for resistance spot welding, the transformer had to be converted into a step-down unit. This was accomplished by carefully extracting the original high-voltage secondary coil using hand tools, ensuring the primary coil remained completely intact and undamaged. The secondary winding was then replaced with 1.5 to 2 loops of heavy-gauge, insulated copper cabling. When standard 230 V alternating current (AC) is supplied to the primary coil, this modified secondary coil fundamentally alters the outputdropping the voltage down to a highly safe range (approximately 2.5 V) while proportionally amplifying the current to extreme, welding-capable levels.

   2. Circuit Design and Thermal Management

      The systems architecture bifurcates the incoming AC power into two parallel sub-circuits to maximize both operator safety and machine longevity. The first sub-circuit is hardwired directly to a rotary cooling fan. This active cooling system continuously dissipates heat from the transformer coils as soon as the machine is powered on, which is a critical feature for mitigating thermal overload during repeated short-circuiting weld cycles.

      To ensure precise control, the primary welding circuit incorporates a dual-switch safety mechanism. Power is initially routed through a primary ON/OFF switch, which arms the system and illuminates a visual LED indicator on the chassis. From this indicator, the current passes to a momentary contact trigger switch, repurposed from an angle grinder. Mounted directly on the operating lever, this component functions as a dead-man’s switch. It grants the

      operator instantaneous manual control over the duration of the current burst, allowing for precise timing adjustments to prevent accidental burn-through on thin metal sheets. The complete structural and mechanical design of the portable spot-welding prototype is detailed in the CAD models shown.

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   3. Mechanical Assembly and Electromechanical Interface

   To house the electrical components securely without risking conductivity or grounding issues, the main chassis and base platform were fabricated using highly durable, lightweight High-Density Moisture Resistant (HDMR) sheets. The entire assembly is suspended slightly by PVC base studs to tolerate minor operational vibrations.

   The physical welding interface consists of two solid copper-alloy rods, shaped into pointed electrodes using a grinding wheel. This truncated cone geometry was selected to maximize and localize current density directly at the weld joint. These electrodes are connected to the heavy-gauge copper output cables via customized aluminum clamps. The lower electrode remains stationary, while the upper electrode is manipulated via a spring-loaded driving bar mechanism. This lever system allows the operator to manually apply consistent, heavy forging pressure to the overlapping metal sheets. When the trigger is engaged under this clamping pressure, the extreme current encounters a bottleneck of high electrical resistance at the faying surfaces of the steel workpieces, instantly generating the required localized heat to form a permanent weld nugget.

3. MATHEMATICAL MODELING AND CALCULATIONS

   To validate the system’s operational viability, a theoretical thermal and electrical model was established to compare the input power against the heat required to successfully melt 2 mm mild steel (MS).

   1. Input Power and Energy Requirements

      The system draws from a standard single-phase alternating current power supply. Assuming a primary voltage (VP) of 230 V and an estimated primary current (IP) of 25 A under load, the total input power (Pin) is calculated as:

      Pin = VP × IP ..(1)

      This yields a total power input of 5750 W. To determine if this power is sufficient, the theoretical heat energy (Q) required to bring the faying surfaces of the MS sheet to a melting point must be calculated. The specific heat capacity (C) of mild steel is approximately 450 J/kg°C, the density () is 7850 kg/m3, and the required change in temperature (T) to reach the melting point is approximately

      1500 °C. Based on the target weld nugget volume, yielding an estimated mass (m) of 0.00066 kg, the heat requirement is:

      Q = m × C × T ..(2)

      According to (2), the theoretical energy strictly required to melt the nugget is 445.5 Joules.

   2. System Efficiency and Secondary Output.

   Empirical testing of this upcycled prototype determined that an optimal weld duration (t) of 5 seconds was required to achieve a secure joint in 2 mm MS sheets. The total energy delivered (H) by the machine over this timeframe is:

   H = Pin × t ..(3)

   Operating at 5750 W for 5 seconds delivers 28,750 Joules of energy. Consequently, the thermal efficiency () of the system can be evaluated as the ratio of theoretical heat required to the total energy delivered:

   systems. “The complete operational parameters and performance benchmarks of the prototype, compared against standard commercial and industrial units, are summarized in

   Table 1.

   Table 1

   = (

   ) × 100 ..(4)

   This results in a system efficiency of approximately

4. RESULTS AND DISCUSSION

   1.55%. While seemingly low, an efficiency range of 1% to 2% is highly standard for portable, air-cooled spot-welding fabrications, which must dissipate massive amounts of excess heat into the surrounding electrodes and chassis due to the lack of industrial water-chilling.

   Finally, the output profile of the modified transformer can be determined. Measuring the secondary voltage (VS) at the copper electrodes yields approximately 2.5

5. Applying the power relation, the secondary welding current (IS) is:

   1. Electrical and Mechanical Performance

      The primary objective of scaling down high-current resistance welding into a highly mobile format was successfully achieved. The modification of the MOT effectively converted the high-voltage input into a high-current, low-voltage output profile. Yielding approximately 2300 A at 2.5 V, the system provided the precise electrical footprint required for standard RSW operations without tripping standard workshop circuit breakers.

      Practical trials confirmed the machine’s operational

      IS =

      ..(5)

      This confirms that the step-down modification

      readiness. By leveraging the spring-driven lever to exert steady downward force, operators were able to produce robust weld nuggets on 2 mm mild steel (MS) test plates.

      successfully outputs roughly 2300 A of current, providing the extreme amperage required for Joule heating.

      C. Performance Benchmarking

      To contextualize the viability of this prototype, its operational parameters were benchmarked against commercial portable units and heavy-duty stationary industrial machines. While industrial welders operate at 25 kW to over 50 kW to deliver ultra-fast welds under 0.5 seconds, they require exorbitant capital investment and dedicated three-phase power supplies. The fabricated MOT welder operates on a standard 230 V single-phase supply, drawing roughly 5.75 kW. By sacrificing processing speedrequiring a 5-second weld time compared to a commercial portable unit’s 1-to-2 secondsthe upcycled model successfully delivers comparable forging power at a fraction of the cost, utilizing active air cooling and a manual lever to mimic the pneumatic pressure of expensive industrial

      Although massive industrial machines can complete this fusion almost instantly, the fabricated device successfully balanced thermal penetration with manual clamping force over an ideal 5-second activation window. Subsequent visual inspection of the joined MS-to-MS and MS-to-Stainless Steel (SS) workpieces confirmed strong, uniform metal fusion with negligible surface spatter or expulsion.

   2. Safety and Operational Stability

Managing heat accumulation is one of the most prominent hurdles when designing compact welding equipment. To combat this, the active cooling fan performed exceptionally well during live tests. By constantly circulating air and expelling trapped heat from the enclosure, the setup effectively shielded the transformer coils from catastrophic thermal damage even during back-to-back welding sequences.

Additionally, the control scheme was designed to prioritize operator safety and reduce mistakes. A bright LED panel light offered immediate feedback regarding the system’s live status, lowering the chance of unintentional firing. Furthermore, using a repurposed angle grinder switch as a responsive dead-man’s trigger gave users direct, split-second authority over the power delivery, helping them cut off the current exactly when needed to avoid burning through the thin metal. In the end, this condensed design successfully converted a traditionally stationary, factory-bound process into an accessible, user-friendly tool perfect for tight workspaces.

V. CONCLUSION

This study successfully demonstrated that high-amperage resistance spot welding, a process traditionally reserved for heavy manufacturing, can be effectively adapted into a highly accessible, low-cost, and mobile framework. By strategically upcycling a discarded microwave oven transformer, the fabricated equipment reliably generated a welding output of roughly 2300 A at 2.5 V, proving fully capable of fusing 2 mm mild steel plates within an optimal 5-second timeframe.

Ultimately, this project highlights a sustainable approach to small-scale fabrication. By utilizing repurposed electronic waste and standard hardware, the financial barrier to entry was reduced by 70% to 90% when compared to commercial alternatives. Furthermore, th integration of forced-air cooling and a spring-actuated trigger ensured that the system remained safe, thermally stable, and user-friendly over repeated cycles, entirely eliminating the need for permanent facility installations or specialized power grids.

While the current prototype meets all primary functional objectives, several design iterations could elevate its performance for professional environments. Implementing automated digital timing controlssuch as a microcontroller or a 555-timer circuit paired with a solid-state relaywould eliminate human timing errors and deliver exact millisecond precision. Mechanically, replacing the manual lever with a compact pneumatic cylinder would guarantee perfectly uniform forging pressure. Finally, incorporating a TRIAC-based dimmer circuit for variable power output and exploring closed-loop water-cooling for the electrodes would allow the system to seamlessly handle thinner metal foils and higher-volume production workloads.

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