Welding workholding and fixturing: magnetic welding squares/angle holders, switchable magnetic clamps, welding C-clamps and locking pliers, welding tables with slotted tops, and ground clamps

magnetic welding square  · 

Magnetic welding squares and angle holders: what the pull rating means

A magnetic welding square, also sold as an angle holder or arrow magnet, is a block of ferrite (ceramic) magnet sandwiched between two steel pole plates. The plates are cut to a fixed angle, usually 45, 90 and 135 degrees on the common arrow shape, so two pieces of steel meet at a known angle while you tack them. There is no switch. You pull the part off with leverage or a twist, and the magnet is always on.

The pull rating printed on the box, in pounds, is a direct-pull figure measured on thick, clean, flat mild steel. Real work is less generous. Mill scale, paint, rust, or a gap from a bowed plate all act as an air gap and cut holding force. Thin sheet saturates before the magnet delivers its full pull. A part that tries to slide sideways sees only a fraction of the rated number, so shear resistance is the figure that matters when a heavy plate is trying to drop out of the corner. Buy a higher rating than the part weight suggests, and expect to add a clamp for anything heavy.

Switchable magnetic clamps: when on/off is worth paying for

A switchable magnetic clamp has a rotating magnet inside a steel body. Turning the lever aligns the magnet with the pole faces to hold, and turning it back shunts the flux inside the body so the part releases cleanly. That control is the main advantage over a fixed square. You can place a part, adjust it, then lock it down, and you are not prying a magnet off hot steel.

More money typically buys stronger magnets, a V-groove base that grips round tube and bar, flatter ground faces and a more positive detent in the on position. Look for a lever that does not creep toward off, and faces that stay free of burrs, because a gouged face loses contact area. Keep switchable magnets away from pacemakers and other implanted devices, and store them with a steel keeper so they do not pull in chips from the bench.

Arc blow and heat: the limits of magnets on a weld

Magnets bring two welding-specific problems. The first is arc blow. A magnetic field near the joint, or one built up in the workpiece by the current path, deflects the arc, causing wander, spatter and incomplete fusion. DC welding is more prone to it. If the arc starts steering itself, move the magnet, move the ground, or switch to a clamp.

The second is heat. Magnets lose strength when hot, and permanent magnets can lose it permanently if they get hot enough. Neodymium types are generally more heat-sensitive than ferrite ones, which is one reason squares for welding are commonly ferrite. Use a magnet to hold the fit-up for tack welds, then take it off before you run a full bead beside it. Magnets also do nothing on aluminum or 300-series stainless, which are not attracted, so those materials need clamps.

Welding C-clamps and locking pliers: throat depth, pads and spatter

Clamps hold parts mechanically, so they work on any metal and apply real clamping force. Welding C-clamps usually have a forged or cast frame, a swivel pad on the screw to avoid marring or twisting the work, and a screw that can be protected from spatter. Throat depth, the reach from the screw to the frame, decides whether you can get to the middle of a plate. Jaw opening decides what thickness stack fits.

Locking pliers with C-clamp or sheet metal jaws are faster to place and release, which suits tack-and-check fabrication. Their weakness is that clamping force is set by how far you adjust the screw, and a slipped adjustment leaves a loose part. Spatter on threads is the common failure. It cooks the screw and makes it bind, so spray the threads with anti-spatter or cover them. Do not let welding current pass through the clamp contact or screw threads, since arcing there pits the threads and ruins the clamp.

Slotted-top welding tables versus a table you build

A slotted or perforated welding table gives you a flat reference surface with a pattern of slots or holes for bolt-in stops, pins and clamps, so you can square parts without chasing magnets across a bench. Plate thickness drives rigidity and heat sinking. A thicker top stays flat under clamping and tack welding. Thinner tops are lighter and cheaper but can warp under heat or from unevenly applied clamp pressure. Check the stated flatness tolerance and whether the hole or slot grid is supported by compatible clamps and stops.

Large tables are heavy and often ship by freight, so confirm delivery requirements before you order. Smaller tables, folding tops and kits ship as parcels, and a modular top on a rolling cart can suit a garage. Building your own from steel plate and tube costs less in cash and a lot more in time. Welding the frame distorts it, so you must leave the plate loose until the legs are square, then add leveling feet. A homemade top rarely matches the flatness of a machined one without grinding or shimming. Either way, bare steel rusts, so wipe it with oil or use anti-spatter.

Ground clamps: ampacity, contact area and cable

The ground clamp completes the circuit, and a poor one shows up as a weak or erratic arc, a hot clamp or a warm cable. Ampacity, the current rating in amps, should meet or exceed your welder’s output at your duty cycle. Spring clamps are quick but can have a small contact area. Screw-style and heavy-duty clamps hold better and carry more current. A copper-bodied clamp conducts better than plated steel, though it costs more.

A magnetic ground clamp is convenient on a table or oddly shaped parts, but a magnet relies on face-to-face contact. Mill scale, paint or a gap raise resistance, which heats the clamp. Also, a strong magnet that sits in the current path can contribute to arc blow. Crimped lugs should be tight, and the cable gauge (AWG) must suit the length and amperage. Clean the contact point to bare metal, and ground as close to the weld as practical. On a table, grounding the work directly beats relying on the top, because paint and rust add resistance.

Matching tools to the job and budget

For a first kit, a pair of 90-degree magnetic squares, two or three locking C-clamps and a solid screw-style ground clamp handle most tack-and-weld fit-up. Add a switchable magnet when you need to hold odd angles or reposition parts, and a slotted table when you repeat the same assembly. Under-buying the ground clamp is the most common mistake, because its failure looks like a welder problem. Over-buying magnets for thin sheet is the second, since a bigger pull rating does not help on a material that saturates.

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FAQ

Are magnetic welding squares strong enough to hold heavy plate?

They hold flat parts well, but the rated pull is measured straight off thick, clean steel. Sideways force, rust, paint and thin material cut holding power considerably. For heavy plate, use a clamp or a switchable magnet alongside the square.

Can I use a magnet on aluminum or stainless steel?

No for aluminum, and generally no for 300-series stainless, because they are not attracted to a magnet. Use C-clamps, locking pliers or a table with bolt-in stops. Some stainless grades are magnetic, but you should not count on it.

Is a magnetic ground clamp as good as a screw-style clamp?

It is convenient, but contact depends on a clean, flat face and sufficient ampacity. A rusty or painted surface raises resistance and heats the clamp. A screw-style clamp gives more consistent contact, and a strong magnet can add to arc blow on DC.