Shop-made welding clamps are mostly two pieces of steel and a bolt. A C-clamp is a flat bar body with a nut welded to one jaw and a bolt running through it. A corner clamp is two pieces of angle iron squared to each other with a screw in each leg. A table dog is a round bar that drops into a hole and a cam or bolt that pushes against the work. None of them need a lathe, and all of them are better than the alternative, which is holding hot steel with your free hand.

What to make them from

Plain hot-rolled mild steel. In the United States that means ASTM A36, the Standard Specification for Carbon Structural Steel, which covers the shapes, plates and bars sold as ordinary structural stock. It welds without fuss, it costs almost nothing, and the offcut bin at any fabrication shop is full of it. Flat bar between 1/4 and 3/8 inch thick makes a clamp body stiff enough to matter; anything thinner springs open under load and your work moves at exactly the wrong moment.

  • Flat bar - 1/4 x 1-1/2 inch and 3/8 x 2 inch cover almost everything here
  • Angle iron - 2 x 2 x 1/4 inch for corner clamps and table stops
  • Square tube - 1 inch, plain black, for bar clamp extensions. Not galvanised, for reasons below
  • Round bar - 5/8 or 3/4 inch to match the holes in your table, if you have a fixture table
  • Nuts and bolts - coarse thread, 3/8-16 or 1/2-13. Plain steel, hex head, at least 3 inches long
  • Cut-off wheel, drill, tap set, and a welder - stick, MIG or TIG will all do this; nothing here is thin or delicate

One thing to skip: do not build a clamp around a hardened fastener and then weld near it. Heat from an arc will soften a heat-treated bolt in the zone it reaches, and a screw that has been annealed halfway along its length will bend under clamping load instead of holding. Weld the nut to the clamp body, leave the bolt free to turn, and if you need a pad on the end of it, weld the pad to a separate washer that spins on the bolt tip rather than to the bolt itself.

Four clamps worth building

1. The welded C-clamp

The workhorse, and the one that teaches the pattern for all the others. Cut three pieces of 3/8 inch flat bar - a back about 6 inches long and two jaws about 3 inches each - and weld them into a squared-off C. Drill the top jaw to clear your bolt, sit a nut over the hole, and weld the nut down on the outside face so the threads stay clean. Run the bolt through, and grind a shallow dome on its tip or fit a swivel pad.

  1. Cut the three pieces, then dress the cut ends square on a grinder. A C-clamp that is not square applies its load at an angle and walks off the work.
  2. Tack the C together and check it before you weld it out. TWI make the point that it is necessary to clamp the plates or use wedges to maintain the joint gap during tacking - which is a small joke here, because you are using clamps to build clamps, but it is also the correct method.
  3. Drill the top jaw to the bolt's clearance size, not the tap size. The bolt must spin freely through the jaw and be held only by the nut.
  4. Weld the nut on all six flats if you can reach them, and keep spatter out of the threads - run the bolt through the nut before welding and back it out afterwards while everything is still warm.
  5. Weld the fillets on the inside corners of the C last. Those are the joints that carry the load, and you want them laid into a part that is already aligned.
  6. Clean it up, chip and brush the welds, then load it hard against a scrap before you trust it on anything that matters.

2. The 90 degree corner clamp

For frames, boxes and anything with a mitre. Two lengths of 2 x 2 x 1/4 inch angle, welded back to back at a right angle to form an L in plan, with a nut-and-bolt screw pressing into each leg. The work sits in the inside corner of the angle and the screws push it against the two faces, which sets the angle for you. This is the clamp that makes the frame projects on our welding project ideas list go quickly instead of slowly.

Two details decide whether it is any good. First, check the corner with a machinist's square before the final weld, not after - a corner clamp that is out by a degree makes every frame you build out by two. Second, relieve the inside corner. Grind a small chamfer or drill a hole where the two angles meet, so weld spatter and the parent metal's own corner radius have somewhere to sit instead of holding your work away from the face.

3. The table dog and stop

If your bench has a grid of holes, this is the cheapest useful thing you can make and you will make a dozen of them. A short length of round bar sized to slip into the hole, with a piece of flat bar or angle welded across the top. Dropped into a hole, it becomes a hard stop to push work against; two of them set a straight line across the table in seconds, and a third turns that line into a right angle.

Make the spigot a slip fit, not a press fit - about half a millimetre under the hole. A dog you have to hammer out is a dog that stays in the drawer. If your bench is a plain steel top with no holes, the same idea works as a stop welded to a scrap plate that you clamp down, or as a magnet-backed stop for light work.

4. The long-reach bar clamp

The one thing worth buying rather than making is the screw mechanism, and an inexpensive F-clamp gives you one for less than the cost of the steel. Cut the clamp's own bar off, weld a sleeve of 1 inch square tube to the fixed jaw, and slide a longer piece of bar through the sleeve. You keep the threaded head and the quick-release action, and you get whatever reach you need - two feet, four feet, across a table.

Keep the heat off the jaw's screw while you do it. Clamp a wet rag or a copper block between your weld and the thread, weld short, and let it cool between passes. The jaw casting on a cheap clamp is often ductile iron rather than steel, so weld to the steel sleeve and the steel bar and let the sleeve grip the jaw mechanically wherever you can, rather than trying to lay a structural weld onto the casting.

Clamping fixes distortion and creates residual stress

This is the part almost no DIY clamp article mentions, and it is the reason clamps are a welding subject rather than a woodworking one. Holding a joint rigid while it cools does stop the parts pulling out of shape. It does not stop the metal wanting to move. That want has to go somewhere, and it goes into the weld as residual stress. TWI put it in one sentence: welding with restraint will generate additional residual stresses in the weld which may cause cracking.

On thin mild steel in a home shop this rarely bites. On thick section, on hardenable steels, and on anything where a crack matters, it does, and the answer TWI give is a suitable welding sequence and the use of preheating rather than less clamping. The practical version for the rest of us: do not treat a clamp as a way to force a bad fit-up into looking like a good one. If the gap is wrong, fix the gap. Clamping a forced joint stores the force in the weld, and it comes back out days later as a crack you did not see happen.

The other half of the same page is useful and cheerful: TWI's distortion control notes describe tack welding and back-to-back assembly, where two identical parts are clamped together so the welding is balanced around the neutral axis of the pair. It costs you nothing but a clamp and it is the single most effective trick for keeping long thin weldments straight.

Six rules for clamping a weldment

  • Clamp close to the joint. Restraint falls off fast with distance. A clamp six inches from the weld is holding the part; a clamp three feet away is holding a lever.
  • Keep the clamp off the weld path. Obvious until you are halfway down a run and have to stop, reposition and restart - and a restart is where slag inclusions and lack of fusion come from.
  • Do not clamp over your earth return. Current takes the path it likes, and a clamp that bridges from work to bench can carry welding current through its own threads and arc-strike the part.
  • Protect the screw. Spatter welds itself to exposed threads and then the clamp is scrap. A smear of anti-spatter, a scrap of leather, or backing the bolt out of the heat zone all work.
  • Let it cool before you release. Releasing a clamp on a weld that is still hot and plastic gives away most of what the clamp was for.
  • Two light clamps beat one heavy one. Spread the restraint and you spread the stress, which is exactly what you want given what TWI say about cracking.

Where shop-built clamps go wrong

  • Body too thin. The clamp springs, the jaws open under load, and the work creeps. This is the most common failure and the easiest to avoid - go thicker than looks necessary.
  • Welding the bolt instead of the nut. Now nothing turns, and you have made a spacer.
  • Threads full of spatter. Fix it at build time, not afterwards.
  • A jaw that is not square to the body. It applies load off-axis, walks off the work and marks it.
  • Sharp corners everywhere. You will pick this tool up a hundred times a week with gloved hands. Break the edges with a grinder. While you are there, wear the right clothing for the grinding, not just for the welding.
  • No pad on the screw tip. A bare bolt end digs a dimple into everything it touches, and on thin sheet it punches through.

Build or buy?

Shop-built against bought

Build itBuy it
CostNear zero if you have offcutsLow for basic F and C clamps, high for quick-action and toggle clamps
Fit to your tableExact - that is the whole argumentGeneric
Clamping forceWhatever the bolt and your arm giveSpecified, and usually higher for a given size
Speed to useSlow. A threaded bolt takes turnsQuick-action and toggle clamps are far faster
Heat and spatter toleranceExcellent, because it is all plain steelPlastic handles melt, and they are on most of the cheap ones
Worth buildingC-clamps, corner clamps, table dogs, stops, bar clamp extensionsQuick-grip clamps, toggle clamps, magnetic squares, anything with a spring mechanism

The honest split: build the things that are just steel, buy the things that are mechanisms. A shop with a dozen home-made dogs and stops and four bought quick clamps is better equipped than a shop with twenty of either.

Frequently asked questions

What steel should I use to make welding clamps?

Plain hot-rolled mild steel - ASTM A36 or its local equivalent - in 1/4 to 3/8 inch flat bar for clamp bodies and 2 x 2 x 1/4 inch angle for corner clamps. It welds without preheat, it is cheap, and it is stiff enough at those thicknesses. Avoid galvanised stock entirely, because the zinc coating produces a fume hazard when you weld it.

Can you weld a bolt to make a clamp screw?

Weld the nut, not the bolt. The bolt has to turn, so welding it defeats the mechanism, and arc heat will soften a heat-treated fastener in the area it reaches, which leaves you with a screw that bends under load. If you want a pad on the tip, weld it to a washer that spins freely on the bolt end.

Do welding clamps stop distortion?

They reduce movement, but they do not remove the cause. The metal still contracts as it cools, and restraint converts that movement into residual stress inside the weld. TWI warn that welding with restraint generates additional residual stresses which may cause cracking, and recommend a suitable welding sequence and preheating where the material is crack-sensitive. On thin mild steel it rarely matters; on thick or hardenable steel it does.

How much clamping force do I need?

Enough to close the fit-up and hold it, and no more. A clamp is not a way to force a badly cut joint into position - the force you put in gets stored in the weld and released later, sometimes as a crack. If you are leaning on the clamp to close a gap, stop and re-cut the part.

What is the difference between a welding clamp and a woodworking clamp?

Heat and spatter tolerance, mostly. Welding clamps are bare steel with no plastic handles or pads to melt, and their jaws are shaped to sit clear of a joint rather than flat across it. A woodworking clamp will physically hold steel, but the first spatter shower ruins the pads and the threads, and plastic grips smoke.

Can beginners make their own welding clamps?

Yes, and it is a good first project. A table dog is four cuts and one weld. A C-clamp is three cuts, one hole and five welds, and none of the joints are thin, positional or structural in the sense of carrying a person. Build a dog first, then a C-clamp, then a corner clamp, in that order.

Sources

In short

Thick flat bar, coarse bolts, weld the nut and leave the bolt turning. Start with a table dog, then a C-clamp, then a corner clamp, and buy the quick-action ones rather than trying to make a spring. Clamp close to the joint, off the weld path, and remember what the clamp is doing to the weld as well as to the work - restraint buys you a straight part and charges you residual stress for it.