Fasteners and Joining

Two pieces of material can be held together in a surprising number of ways, and the exam's questions are all versions of one question: what is actually doing the holding?

Four ways to join two pieces

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A nail resists shear well and pull-out poorly. Friction is all that holds it, so a load along the nail's length can withdraw it, while a load across it is carried by the steel.

A screw's threads bite into the material, which is why the same size screw holds far more than the same size nail. The unthreaded shank under the head matters: it passes freely through the upper piece so the threads can pull the two together. A screw threaded all the way to the head would jack the pieces apart instead of clamping them.

A bolt does not thread into the work at all. It passes through a plain hole in both pieces, and a nut on the far side does the holding. That is the distinction the subtest asks for, and it is why a bolt makes the strongest removable joint: the clamping force is set by how tight the nut is, not by how well the material accepted a thread.

A rivet is a permanent fastener. It goes through both pieces and its far end is upset into a second head. Removing one destroys it, and that is the property that identifies it.

Bolts, nuts and washers

The two washers do two different jobs and this is asked. A flat washer spreads pressure so a nut does not sink into softer material, and it gives a smooth surface to turn against. A lock washer keeps the fastener from backing off under vibration, by springing against the nut.

Thread pitch is how many threads there are per inch. Coarse threads assemble faster and tolerate dirt and damage. Fine threads hold slightly more tension and resist loosening better, which is why fine threads turn up on precision work.

Bolt grade marks the steel's strength. The radial lines on an inch-series bolt head and the number on a metric one both mean the same thing, and replacing a high-grade bolt with a low-grade one of the same size is a failure waiting to happen.

Nails and screws

Nail size is given in pennies, written with a lowercase d, and a larger number means a longer nail. A 16d nail is longer than an 8d.

A pilot hole prevents splitting when a screw goes near the end of a board, and it is also what lets the screw pull the pieces together rather than spinning in place.

Soldering, brazing and welding

All three join metal with heat, and the exam's question is always which one melts the base metal.

Only welding melts the base metal. Soldering and brazing both work by a filler flowing between the parts and sticking to them, which means the joint's strength is the filler's, not the parent metal's.

Soldering is the electrical one. Electronics and copper plumbing are soldered, and the low temperature is the reason: components and thin copper would be damaged by more heat.

Brazing is stronger than soldering because its filler is a brass or bronze alloy melting far hotter, and it joins dissimilar metals well.

Flux is used in all three. It cleans the surface and stops it from oxidizing while it is hot, and without it the filler will not wet the metal at all.

Adhesives are worth a line. Epoxy, anaerobic thread locker and structural adhesives all join without heat, and a thread locker is the answer for a fastener that keeps vibrating loose when a lock washer is not enough.

Know it by sight

What you can skip

Across the 59 questions on this topic in our bank:

• Thread designations. UNC and UNF never appear, and thread pitch comes up once. Know that coarse threads grip soft materials and fine threads hold better against vibration.
• Masonry anchors, toggle bolts, dowels and biscuits never appear.
• Welding processes by name. MIG and flux-core never appear; the joining questions ask what welding, brazing and soldering are and how they differ.

Where people lose points

Saying a bolt threads into the material. A screw does; a bolt takes a nut.

Swapping flat and lock washers. Spread against resist loosening.

Thinking a rivet can be unfastened. It is destroyed to be removed.

Saying brazing melts the base metal. Only welding does.

Thinking a smaller penny number is a longer nail. Larger is longer.

Ignoring bolt grade when replacing a fastener. Grade is strength.

Work one in under a minute

Two steel plates must be joined so that the joint is as strong as the plates themselves and will never need to come apart. Two more must be joined so they can be taken apart for service. What is used for each?

Weld the first pair. Bolt the second.

Welding melts the base metal of both plates so they fuse into one piece, which is what makes the joint as strong as the material. Soldering and brazing would put a filler between them, and the joint would only be as strong as the filler.

The second pair rules welding out entirely, because a welded joint has to be cut apart. So do rivets, for the same reason.

A bolt and nut is the strongest removable joint, because the clamping force is set by how tight the nut is drawn, and neither piece has to accept a thread.

Note what the question was really asking: permanence. Permanent means weld or rivet; serviceable means bolt or screw, and that single split answers most items in this topic.

Where this leads

The screw thread is an inclined plane, the bolt is held by torque you now know how to measure, and the safety lesson closes the subtest.