Blacksmiths have forged strap hinges for many uses and in varying sizes. Large wooden barn and entry doors have been seen with bold strap hinges in most countries all throughout history. One can drive through the countryside and see them still today. The varying usage of strap hinges can be from very small ones on trunks and cabinets to extremely large ones on heavy wooden farm gates and antique prison doors. Forging strap hinges takes a knowledgeable blacksmith of intermediate level and a hot fire.
- Skill level:
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Things you need
- Coal or gas forge
- Cross peen hammer
- Flat chisel
- Hacksaw or bandsaw
- Round metal file or die grinder
- Flat metal file
- Belt sander (optional)
- Bench grinder
- Drill press or vice and hand drill
- 2 pieces of 1/8-inch-by-2-inch or wider mild steel flat bar of desired length
- 4 inches mild steel round bar of desired hinge pin size
Light the coal forge. If using a gas forge, light the forge when you're ready to start, to conserve on fuel.
Calculate and cut the total length of stock needed for forging the type of strap hinge desired. Smaller strap hinges do not require an overlapping end, whereas larger ones need overlap for stability and strength. The overlap is a short length of metal left on the end of the knuckle or rolled part of the hinge where the hinge pin is located. This extra metal is positioned over the strap and fastened closed when the hinge is mounted. The length of overlap needed will depend upon the size of the overall hinge and the amount of weight the hinge will bear.
Heat one end of one 1/8-inch flat bar until yellow in colour.
Form the strap end to desired shape using the cross peen hammer, chisels, and hacksaw. After the desired shape is achieved, reheat strap to the yellow forging colour. Use a punch and hammer or drill press to make three or four holes in the strap for mounting. Take care not to punch or drill holes in the area where the overlap will occur.
Smooth the edges and holes of the strap with a metal file or grinder.
Turn the piece of metal around to work on the other end; this is the knuckle and overlap end of the hinge. Heat the knuckle end of the strap in the forge until yellow in colour. At this time, the part of the stock metal being heated is used to forge the knuckle roll where the pin is located to hold both sides of the hinge together.
Holding the strap using tongs on the anvil with the face (the side of the hinge that will be seen) when it is mounted up and off the edge of the anvil, bend the end of the strap down with the face or flat side of the cross peen hammer to start the roll of the metal. As the hot metal is hit with the face of the hammer, it will bend down starting the rounded part of the hinge called the knuckle. The metal bends at an angle that looks like a curve rather than a 90 degree square corner.
Reheat the metal and continue to bend the knuckle end using back blows (turn the stock over and hit with hammer using blows that are aimed at the blacksmith) until the roll is approximately half of the knuckle or half of a circle . Insert round bar and finish the roll or knuckle, reheating as necessary. By doing this process, the flat stock of the hinge is encasing the round bar (pin) by rolling the flat metal all the way around the round bar or pin; this is called the knuckle. Finish the knuckle roll using the peen side of hammer or use a flat chisel and hammer. Make sure the knuckle is completely surrounding the pin and the overlapping metal is flat against the strap.
Drill one or two holes in the hinge where the overlap occurs to stabilise the knuckle and pin when mounted. If forging the holes, reheat the hinge at the overlap area until yellow, then punch the holes through both the overlap and strap using a punch and hammer.
Make the corresponding strap using the same technique with the length of the material calculated to the measurement of the door casing (what the door is being attached to) where the strongest interior materials are located. The size of this strap varies greatly depending on the size of the over all strap and what it will be attached to. If the strap is going to be used where a door casing or other enclosure, the distance to be measured is on the door casing itself. When the hinge is attached on the exterior of the casing there will be a need to add extra supporting braces in the area where the hinge is to be attached. For a hinge that is to be attached in the interior side of a casing, the width of the casing is to be measured and the strap should be shorter. the casing for this type of mounting should have the no voids between the casing and the studs due to the weight of the door being mounted.
Cut out the centre 1/3 section of the knuckle of one side of the hinge and the corresponding outer sections of the other with hacksaw or bandsaw, file, sand, or grind to ensure a smooth fit.
Upset one end of round bar and smooth with file then cut to length matching width of hinge.
Assemble the hinge and insert pin. If the pin (rod) is too tight, reheat the entire hinge to yellow and work the hinge to loosen.
Tips and warnings
- To calculate the length of metal needed to make the knuckle of a strap hinge, one can use pi times the pin diameter plus half the pin diameter again to obtain the length of added material needed.
- A rule of thumb on how big a hinge pin should be and how much weight one can hold for large doors is that the pin needs no more stress than 9,000 psi in a shear; or the stress equals the load divided by the cross sectional area of the pin. Most forged hinges are larger fabricated ones, and usually can handle the bearing load or stress.
- Any large or heavy wooden door/gate over 6 feet high but under 9 feet high must always be fitted with three hinges.
- While in the smithy, always wear eye protection to prevent shards of hot metal from entering the eyes.
- Synthetic clothing will melt to skin when hot metal touches it; always wear cotton, wool, or other natural fibre clothing.
- Black metal can still be very hot. Use precautions and gloves when you're unsure if metal is hot.
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