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Fixturing and Fit-Up for Cobot Welding: Get the Part Right and the Weld Follows
ArmMate
When a cobot weld comes out wrong, the first instinct is to blame the program. Nine times out of ten, the problem started before the arc: the part sat somewhere slightly different from where it sat when the path was taught.
A welder compensates without thinking. A cobot does exactly what it was shown. So the fixture and the fit-up carry more weight in a cobot cell than they do at a manual bench. This is how we approach both.
Why a small shift matters
The AWS Welding Digest makes the point with one number: a 1/4 inch variance in contact tip to work distance can drop the amperage and significantly affect the joint's integrity (AWS Welding Digest, weld setting optimization for collaborative robots). The same article lists torch angle, travel speed, amperage and voltage, and shielding gas as variables a cobot holds steady. It can only hold them steady relative to where the joint actually is.
If the part moves, every one of those settings moves with it.
Locate the part, do not eyeball it
A good cobot weld fixture answers one question the same way every load: where exactly is this part?
- Three point location. Pins or locating blocks set the base part in X and Y. Three points define a plane. More than that and you start fighting the part.
- Hard stops for the second part. The upright, tab or gusset lands against a stop, not against a line scribed by hand.
- Clamp near the joint. Toggle clamps or pneumatic clamps close to the weld keep the part from walking as it heats.
- Modular tables. A hole grid table with standard pins and clamps lets you build a fixture for a new part in an hour and tear it down for the next job.
- Clearance for the torch. Clamps and stops cannot sit where the nozzle needs to travel. Check every weld with the torch at its real angle.
We would rather see a simple, repeatable fixture on a basic cobot than a sophisticated cobot on a loose table.
Fit-up starts at the saw and the laser
The fixture can locate a part. It cannot fix a part that was cut wrong.
- Laser or plasma parts with dross or burrs on the mating edge change the gap.
- Bent tabs or brake parts with springback land in different places.
- Saw cut tube with a heavy burr will not sit flush against a plate.
Before you teach the first program, pull ten parts from the bin and fixture them. If the gap varies noticeably from part to part, fix the cutting process or add a deburr step. That fix also helps your manual welders.
Tack sequence and heat
Distortion moves the joint during the weld. Plan for it:
- Tack in a balanced order so the part does not pull to one side.
- Weld from the stiff end toward the free end.
- Use stitch welds where the drawing allows, to spread heat.
- On multipass joints, give each pass its own taught path and settings.
Touch sensing and seam tracking
Modern welding cobots have two tools that handle small variation. They solve different problems, so it helps to keep them straight.
Touch sensing works before the arc. Miller's description of its cobot system is a clear example: the welding wire acts as a probe, searches for the part in a set direction and applies a shift value to the original location, which helps with poor part alignment, bad joint fit-up and inconsistent joint geometry (Miller Electric, Copilot cobot systems).
Seam tracking works during the arc. The same source describes the robot weaving across the joint, averaging the amperage and adjusting its position from live feedback, which reduces the effects of part distortion and part to part variation.
Both add seconds. Both are worth it on real shop parts. Neither one replaces a fixture. Think of them as the last few thousandths, not the first quarter inch.
Changeover for high mix work
In a job shop, the fixture is also your changeover. Build it so the next job is quick:
- Dowel located sub plates that drop onto the table in one position.
- Labeled clamps and pins stored with the program name.
- A photo of the loaded fixture saved next to the cobot program.
- One taught reference point per fixture, so a touch up takes minutes.
The aim is a changeover that a second person can do as well as the first.
A short pre weld checklist
- Fixture clean, spatter knocked off pins and stops.
- Part seated against every locator.
- Clamps closed and clear of the torch path.
- Nozzle and contact tip clean.
- Program name matches the fixture on the table.
How we help
We screen weld jobs, review your parts and fixtures, and integrate cobot welding stations as part of our industrial cobot work. See how it works for the steps we follow from first visit to a running cell, or meet the team on our about page.
FAQ
Do I need a custom fixture for every part?
Not always. A modular hole grid table with standard pins and clamps covers many part families.
Can touch sensing make up for poor fit-up?
It helps with small shifts and gaps. Large or random variation needs a fix at the cutting or forming step.
What should I fix first?
The parts that feed the fixture. Consistent parts make every later step easier.
Book a free site assessment
Bring us your worst fit-up job and your best one. We will look at both on your floor. Book a free site assessment at armmate.com/contact, email hello@armmate.com, or call (949) 441-4828. ArmMate, 15642 Sand Canyon Ave #51176, Irvine, CA 92619. https://armmate.com