MYP Design Joining and Fastening Notes

Permanent or Temporary: Decide Before You Reach for the Glue

1. A permanent joint cannot be undone without wrecking the parts, while a temporary joint is built to come apart again.
2. Glued dovetails, pop rivets and welded seams are permanent, while screws, nuts and bolts and cam lock fittings are temporary.
3. Temporary joints let a product be flat packed, repaired and taken apart for recycling, which is why a self assembly bookcase uses cam locks and dowels instead of glue.
4. Permanent joints are usually stronger for their size and cheaper per joint, since one pop rivet costs a few pence against a bolt, nut and two washers.
5. Sustainability leans on this choice: a kettle held together with screws can have its element replaced, while one welded shut by ultrasound is thrown away whole.

Wood Joints: More Work Buys More Strength

1. A butt joint simply pushes two square cut ends together, and it is the weakest joint there is because end grain soaks up glue like a straw and holds almost none of it.
2. A lap joint removes half the thickness from each piece so they overlap, which multiplies the glue area and stops the two parts twisting against each other.
3. A housing joint sinks the end of a shelf into a trench cut across the side panel, so the timber under the shelf carries the weight and the glue only stops it sliding out.
4. A mortise and tenon puts a tongue cut on the rail into a rectangular hole chopped in the leg, and the shoulders around the tenon are what stop a frame racking out of square.
5. A dovetail cuts angled tails and pins that physically interlock in one direction, so a drawer front stays attached to the sides even after the glue has given up.
6. That strength costs time: a butt joint takes two minutes with a saw, and a set of hand cut dovetails takes most of a lesson.

Adhesives Have to Suit Both Materials

1. PVA is the standard wood glue, clamped for about 30 minutes and at full strength after 24 hours, and only the exterior grades survive damp.
2. Acrylic cement is a solvent, not a glue, and it works by dissolving the surface of both pieces so they fuse into one piece of acrylic as it evaporates.
3. Contact adhesive goes on both faces and is left until touch dry, then it grabs the instant the surfaces meet, which is why laminate is slid into place on dowels.
4. Epoxy resin comes as resin and hardener mixed in equal amounts, bonds materials that match nothing else, metal to polymer to ceramic, and fills small gaps as it cures.
5. Hot melt glue sets in seconds and is weak, so it belongs on card models and mock ups rather than anything that carries a load.
6. All of them need surfaces that are clean, dry and keyed, and a quick rub with 120 grit abrasive on smooth polymer makes a noticeable difference to the bond.

Screws, Bolts and Rivets: Fixings You Can Plan Around

1. A woodscrew pulls two pieces together as its thread bites into the lower one, so the top piece needs a clearance hole that the thread passes straight through.
   1. Miss the clearance hole and the screw jacks the pieces apart, leaving a gap you cannot close however hard you drive it.
2. The lower piece gets a pilot hole to stop the timber splitting, and a countersink lets the head sit flush with the surface.
3. A nut and bolt clamps parts together without cutting any thread into the material itself, a washer spreads the load, and a nylon insert nut stays tight under vibration.
4. A machine screw runs into a hole tapped with a thread, which is the answer when there is no room to get a spanner behind the part.
5. A pop rivet joins thin sheet from one side only: push it through both sheets, and the gun pulls the mandrel until the body swells and the mandrel snaps off.
6. Rivets are permanent, and the only way off is to drill the head away with a bit the same size as the rivet body.

Soldering, Brazing and Welding: Joining Metal With Heat

1. Soft soldering melts a lead free solder at about 220 degrees Celsius to join copper and brass, and the parent metal never melts at all.
2. Flux strips the oxide film off the surface so the molten solder can wet the metal and run into the gap by capillary action.
3. Brazing and silver soldering work at roughly 600 to 900 degrees Celsius with a brass or silver alloy filler, giving a much stronger joint on steel bicycle frames and copper pipework.
4. Welding melts the parent metals themselves, usually by MIG or arc, so the finished joint is the same metal as the parts and roughly as strong.
5. All that heat distorts thin material, so a 1 mm steel panel is tacked at intervals and allowed to cool rather than welded in one long run.
6. Each process has its own protection: extraction for solder fumes, and a shaded welding mask because the arc puts out enough ultraviolet to burn your eyes in seconds.

Choose the Joint for the Load It Carries

1. Start by naming the load: tension pulls the joint apart, shear slides one part across the other, and racking twists a frame out of square.
2. A loaded shelf is mostly shear, which is why a housing joint outlasts a glued butt joint under the same pile of books.
3. A chair frame gets racked every time someone tips back, so chair rails get mortise and tenon joints rather than two screws through the leg.
4. A drawer front is pulled in tension several times a day, which is exactly the load a dovetail is shaped to refuse.
5. Then ask whether the product has to come apart for transport, repair or recycling, because that answer can overrule the strongest option.
6. Write the reason next to the joint in your design folder, naming the load and the material, since for criterion C the thinking is worth as much as the photograph.