Tack Welding Tips from NASCAR to Your Shop

A good tack weld comes from a compromise between heat and penetration.

Hendrick Motorsports has won ten NASCAR championships in the last ten years and currently fields teams in the sanctioning body’s top two divisions: the NASCAR Sprint Cup Series and Nationwide Series. The team attributes their success at winning to qualified drivers such as Jimmie Johnson and Jeff Gordon, but also to the skills of a talented fabrication team who custom design each of their championship Chevrolets.

As teams work within the Car of Today (COT) fabrication regulations, tack welding plays a crucial role now more than ever before. Every car needs to fit within a new interlocking template to be legal to race. This new emphasis on driver safety put an end to the highly twisted and aerodynamically raked bodies of the past. The car’s skin is built from numerous complex panels that all come together, giving it the required COT shape. The tack welds that shape a single panel can make a difference when tested in the template, so team fabricators must master the techniques of good tacking.

Tips for making a good tack

It's important to get the tack hot enough and flat enough to not burn through the material, yet maintain a low profile to save time spent on grinding. Jeff Dixon, Team Fabricator with Hendrick Motorsports has laid a few thousand tack welds on panels with the team’s Millermatic® 180 with Auto-Set™. He has some useful advice that can make a difference in your own tack welds.

“The secret for a good tack weld is good heat, good penetration and good wire flow,” says Dixon.

Jeff Dixon, team fabricator with Hendrick Motorsports uses a Millermatic 180 with Auto-Set to tack the rear quarter panel on this Sprint Cup car.

Setting up your machine – more voltage, less wire than normal

A good tack weld comes from achieving a compromise between heat and penetration, according to Dixon. This is obtained by using more voltage and less wire than you would on a normal weld.

Even though most 24-gauge material is welded at number 1 or 2 on the voltage dial, a hotter voltage setting like 3 or 4 is more ideal for tack welding. The higher the voltage, the wetter the puddle will be, leading to a flat tack. This is known as “wet-out”. A higher voltage also gives you a more positive arc start, which is an ideal characteristic for most tack welds since they only take one second to make.

The wire speed should be set a little lower, around 3 or 3-1/2, anymore and you’ll risk burning through. For tacking 24-gauge stainless panels, Dixon prefers to set the Millermatic’s Voltage to 4, and the wire speed just under 5.

For tack welding .023 diameter wire works great because it burns fast and doesn’t leave a big ball on the end of the wire. In most cases if there is a small ball, the higher voltage setting will burn it off just fine during the next tack.

In most race shops, teams have welders designated for a specific job. They recognize that a 180-amp machine has a “sweet spot” for thinner material and a larger unit (200 amps and above) is more ideal for thicker material.

This regulation Car of Today (COT) template must perfectly fit the body of each car for qualification to race. This new integrated template combines all the single templates that were used in years past for a more precise fit.

Find your sweet spot or step up the juice

If you’re welding a car, know your material thicknesses and match the machine to the job at hand. Machines rated at different amps will have different “sweet spots” for the wire diameter they were designed to perform the best with.

For this reason, a 180-amp machine will run great on .023 and .030 wire, but if you’re looking to run .035 and up, you’ll need a machine with more versatility and power. Just because a 180-amp machine will accept .035 wire doesn’t mean it’s the best option. Remember, good “wet-out” and proper penetration is what you are looking for.

The main frame-rails that run underneath the car are the thickest frame piece on the car at 120-wall. Most of the primary structure of the car is well over 16-gauge, which is typically welded with .030 wire. Team fabricators will often run .030 diameter wire in their machine because it gives them the flexibility to switch between tack welding and fabricating structural components on the car like 1/8 in. support brackets and cross members.

Spring buckets and connecting suspension components are 1/4 in. to 3/8 in. thick. In this situation, fabricators will TIG them or use .035 wire to avoid making multiple passes. Here, a small MIG machine (200 amps and below) simply won’t cut it. A larger machine such as the Millermatic 212 is more ideal.

The Millermatic 212 is the most versatile MIG machine for the home user because a variety of thick and thin material because it has a large sweet spot for running the .023, .030, .035 wires. If you’re building 1/4 in. workbenches or body panels on a ’69 Camaro, it’ll have you covered. Plus it’s not power hungry and will run just fine off a generator.

A perfect tack weld will blend seamlessly with the base metal after being hit with the grinder.