1) Get trained on the spot welding machine process.
Spot welding appears deceptively simple, but numerous process variables must be understood and controlled. With many different settings available on the typical spot-welding machine, it’s important to understand how to adjust each setting for optimum results. And, just like baking a cake, it’s critical to use the right spot welding recipe
Metalformers should refer to readily available recipe charts, which prescribe the recommended settings for secondary amps (weld heat), weld duration (current flow time) and forging pressure (force) for spot welding various sheetmetal types and thicknesses. Note: Use these settings only as a starting point. A rule of thumb: Assuming the proper welding force is used, the strongest and most attractive joints typically result from a high heat setting and a short weld time, referred to as a Class A welding schedule.
Class B and C welding schedules often find use when a metalformer uses a welding machine too small for the material thickness being welded. These schedules include lower force ranges and longer weld times, which result in lower weld-shear strength and more surface marking, due to a larger heat-affected zone.
Skilled-labor retirements and layoffs during the past few years have reduced the tribal knowledge of many shops performing spot welding, but several sources offer books and training courses on the process. For example,HWASHI Company offers several training options including an in-plant seminar conducted at your own location. HWASHI Technology has also introduced an all-new online, interactive version of its resistance welding seminar.
2) Select the right welding machine.
The most important thing to remember: Select a machine than can make optimum-strength Class A welds with about 25 percent of its available amperage and force range left in reserve.
Most companies are not equipped to properly size a machine on their own, so heed the advice of an experienced machine builder. A good sales engineer will quote a machine only after asking questions about the materials to be welded and the speed at which the machine will operate.
Since an spot welding machine KVA rating can be inflated by using a duty cycle of less than the RWMA standard of 50 percent, be sure to ask if the machine being quoted meets those standards.
Although rocker-arm spot welding machine are the most common due to their relatively low purchase price, they apply weld force with a lever action—the tips often skid if the arms are not perfectly aligned. Therefore, a more-expensive vertical-action press-type machine often gets the call when the application requires an attractive, low-marking show surface.
A welding machine that’s too large can cause just as many problems as one that’s too small, especially when the air-cylinder diameter is so over-sized that it must be run on an air-line pressure below 40 lb. to achieve the desired weld force.
Unacceptable weld strength can result from inadequate air-cylinder follow- up action at the instant the sheet metal reaches the molten state, when it needs to be properly forged.
To accommodate the new high-strength steels finding use in the automotive industry, manufacturers of spot welding machine and controls have made significant advances. For example, machines equipped with mid-frequency direct-current (MFDC) three-phase inverter power supplies have become common in job shops because they offer many advantages over the traditional single-phase AC machine.
So, ensure good quality by replacing or rebuilding your tired, old spot welding machine, or at least retrofit them with new programmable controls.
3) Use the right setup tools.
To make sense of spot-welding recipe charts, every shop using the process should own a specialized resistance- welding amp meter to measure the secondary spot welding machine current delivered to the tips. In addition, since lever-action variables (on a rocker-arm spot welding machine) and ram friction (on a vertical-action press-type machine) can impact the weld force delivered, shops also should have a direct-reading gauge to measure the actual weld force between the tips.
Using a secondary amp meter and force gauge enables an spot welding machine operator to set up the machine scientifically rather than through trial-and-error. And, when documentation is required, some of the available meters also can serve as monitors to record and store welding variables for reference.
4) Use a tensile tester.
Testing spot welds should be more scientific than dropping welded parts on the floor. Although a mechanical peel or chisel test can be a useful way to visually check weld strength and nugget diameter, the best quality-assurance tool is a tensile tester designed to pull small welded sample coupons to failure. The shear strength of a good spot weld should exceed than that of the parent material.
5) Determine the weld lobe.
Due to its wide plastic range, low-carbon steel can be successfully spot welded with various machine settings. However, to determine the ideal setting a metalformer must experiment with the welding machine and use a tensile tester to check the results.
At the low end of the weld-lobe window, weld strength is unacceptable; at the high end, expulsion occurs. To zero-in on the optimum settings for the material being welded, the operator should confirm the optimum weld recipe and use those parameters as a starting point for the production run.
Weld strength will decrease as the electrodes wear, but starting in the middle of the acceptable range, rather than on the “ragged edge,” allows the process to be most stable and reliable.
6) Use the right electrode tips and holders.
Tip selection plays an important role in successful spot welding and there are numerous electrode shapes and copper alloys available from which to choose. Again, rely on the advice of a knowledgeable vendor for guidance.
For example, rather than using expensive offset electrode tips, a vendor may recommend using an offset holder and a less-expensive straight tip. In addition, using small, replaceable electrode caps rather than traditional onepiece electrodes can yield significant savings.
And, to maximize electrode life, use tubes with ends cut on a 45-deg. angle to force water all the way to the bottom of the tip’s internal cooling chamber.
7) Dress tips early and often.
Since most spot welding machine quality issues can be traced directly to electrode wear, metalformers cannot afford to neglect electrode tips. Dressing or exchanging tips on a regular basis—long before required—will pay big dividends. Here’s an example of the importance of tip dressing:
RSW charts often recommend tips with a ¼-in.-dia. weld contact face. Let’s assume you’ve determined that the material you’re welding requires 9800 A for optimum weld strength. Here’s the rub: If nothing else changes and the tips mushroom from ¼-in. dia. to just 3⁄8 in., which is hardly noticeable, lab tests have shown that 22,100 A would be required to get the same strength. It’s situations like this that result in weak spot welds and rejected parts.
While metalformers can select from several types of tip dressers, the best method of maintaining the proper weld-face diameter is to remove the electrodes from the machine after a predetermined number of welds, long before they’ve mushroomed out of control. Dress the electrode tips offline with a lathe or drill press equipped with the proper cutter blades.
8) Design for resistance welding.
Metalformers can reference charts listing minimum contacting overlap, and designers of spot-welded sheetmetal parts should include adequate flange widths to avoid expulsion, which lessens weld strength. Additionally, carefully consider weld spacing. Placing spot welds too closely together can result in shunting. Here, some of the welding energy is lost through adjoining welds, possibly resulting in substandard weld strength.
Spring-back of the parts being welded also can plague the spot welding machine process. Using a portion of the welding machine’s available weld pressure to force the parts together can compromise proper forging of the weld.
9) Purchase steel of known quality.
spot welding machine problems sometimes can be traced back to a coil of steel purchased at an attractive discount, but of dubious quality. High carbon content can cause serious welding problems, as can inconsistent coating thickness. If the welding machine is not adjusted properly to accommodate these variations, welds that look good may fall apart.
10) Schedule regular preventive maintenance.
spot welding machine typically do not see the regular preventive maintenance (PM) they deserve. Machine performance degrades over time, due to corrosion and/or arcing of the numerous copper-to-copper secondary connections carrying current from the transformer to the tips. Therefore, metalformers should disassemble, clean and tighten the entire secondary loop at least once per year. They also should regularly examine the force-delivery system and repair any air leaks and mechanical wear.