|Nailing and Screwing - Course: Making of Wood Joints. Trainees' Handbook of Lessons|
When in the manufacture of supports and girders more than two boards shall be joined by means of nails, then these joints have to be made as two-shear or multi-shear nailed joints. These joints have to be statically calculated. Correspondingly, the kind, size and number of nails to be used are specified. Depending on the existing joint surface and in consideration of the minimum distances, nailing patterns (nailing points) are fixed. Nailing is done by means of patterns.
Single-shear nailed joints are produced when two parts are nailed together. In doing so, the nails are normally tensile-stressed in their longitudinal axis. This ability of the nail to absorb tensile forces is called "extraction resistance". Among other factors, the extraction resistance is determined by the strength of the wood, the nail length and the shank dia. of the nail. When driving in the nail, the wedge-type point punches through and destroys a part of the fibres of the wood. The other fibres are pushed aside and compressed and clamp the nail shank. The strength of a nailed joint increases proportionally to the number of nails if the minimum distances between the nails and the distances to the edge are observed.
The minimum distances between the nails are calculated proceeding from the diameter of the nail shank (d). When nailing in a row with the grain and side by side, the distances are not less than 10 d. The distance from the edge in grain direction must be at least 5 d, the distance from the board ends at least 10 d. The following rule applies: "Nail as many nails as necessary, not as many as possible!"
Frame corners are nailed with 2 to 4 nails which shall have a distance from each other as large as possible.
Nailings in a row (rear walls of furniture, glass strips) are made in dependence on the nail size at a distance of 100 to 200 mm.
Why are minimum distances between the nails and distances from the edge to be adhered to?
As far as no constructional requirements are in contrast with it, the following basic rules are to be followed:
- Nailing is performed from the soft into the hard wood.
- Nailing is performed from the thin into the thick wood.
- When nailing in hard wood, pre-drilling has to be carried out. The diameter of the drill is 0.8 d of the nail.
- When nailing in cross-cut wood, the nails have to be driven in at an angle so that they penetrate several annual rings, if possible. Nails which are vertically driven into cross-cut wood have hardly any extraction resistance!
- When nailing through round wood, nailing has to be done along the tangent line!
- When nailing at the ends of a board, the nail points are upset so as to reduce their effect of splitting.
Multi-shear screwed joints (pressure- or tensile-stressed) are required in the wood construction work (supporting frameworks such as roof frameworks, bridges, scaffoldings). They are executed with through screw bolts. The screws are fastened with washers and nuts.
Wood screws are used as joining elements and fasteners for wood parts or for the fitting of mountings. The screws are tensile-stressed as to their longitudinal axis. In case of a proper mounting, wood screws have a very large extraction resistance. They must not be driven in with a hammer!
Screw holes are to be pre-drilled!
About 60 % of the length of the wood screw is provided with a thread which draws in or loosens the screw at a rotary motion and light pressure.
For small screws can be precut with a steel square having the respective size; for larger screws, the holes must be pre-drilled. Doing so, the bore-hole with the diameter of the screw shank or slightly less has to be executed some 40 % of the screw length deep. As for large screws, it can be necessary to pre-drill additionally some 40 % of the screw length with the diameter of the thread core. Cup square bolts are pre-drilled with their shank diameter + 0.4 mm.
Pre-drilling of the screw holes saves force to be applied when screwing in the screws and prevents the screw driver from slipping off if too much pressure is applied!