Screws and Glues
Tuesday 04 November 2008
Basic fixings with Michael Huntley
In the last few articles we have looked at tools, timber and some basic joints. This month I want to discuss some basic fixings. DIY stores contain a huge array of metal fixings and adhesives and I have selected the most frequently used from the boxes in our workshop and will try to navigate a path through their complexities.The Screwfix catalogue is a mine of useful information and it is very rare that what I order isn't with me by post the next day.
1 GLUE BASICS: What a range of glues there are! From the left: Standard PVA (polyvinyl acetate); Titebond II - a molecularly stronger (crosslinking) PVA adhesive; Titebond III - a stronger proprietary brand, and approved for indirect food contact and exterior work; Extramite - resin glue that is mixed with water and is totally waterproof once set; PU (polyurethane) glue - this waterproof glue foams up and is a 'gap filler' (but remember that joints with gaps in them are not strong so try to avoid needing a 'gap filling' glue); Gorilla glue is another very strong PU glue; 'Super glue'; Contact adhesive - useful for sheets of Formica
2 Don't use too much glue, and spread it around with a thin sliver of timber or an old kitchen knife
3 This is as much glue as you need...
4 ...because you will have to clean off the squeeze out afterwards
5 SCREW SIZES: Screws come in different head sizes and lengths. This may sound like an advertisement for Screwfix but their catalogue is the only document I am aware of that tabulates old-fashioned head sizes like Nos. 6, 8 or 10 gauge against a modern metric size. No. 6 gauge is 3.5mm, No. 8 is 4mm and No. 10 is 5mm. For small brass hinges you will need a No. 4 gauge or smaller; gauges go down to No. 1 but you will need a specialist supplier to find these. For a big project in the garden like the arch I am just building I use a No. 14 screw
6 SCREW TYPES: Some of the different types of screw are shown here. From the top: Self-tapping with pan head - this is for non-ferrous metals; Waxed countersunk screw - no need to use the old woodworker's trick of rubbing the screw across a candle or bar of soap, this screw is lubricated at the factory (countersunk means that the head recesses into the surface of the timber); Winzer Wurth 'Ecofast' woodscrew with 'improved' Pozi recess; Long black decking screw; 'Ultimate' woodscrew with special driver recess larger than Pozi; Small brass screw - brass is used on higher-quality work but brass is softer than steel so on hardwood you will need to use a sacrificial steel screw first to cut the thread; Domed head brass screw - for use when a countersink is not possible, e.g. on a strap hinge; Small steel countersunk screw
7 HEAD TYPES: Here is a variety of heads. From the left: Three slotted heads; Square head; Hexagonal head; Variety of Prodrives
8 AVOIDING ACTION: Here two pieces of timber are actually being driven apart by the screw instead of being drawn together. This is happening because the screw thread is pulling the top piece of timber up the shank without driving into the lower timber. It can be avoided by clamping the timbers together before driving the screw, allowing the screw to drive evenly into both pieces. Just forcing the screw until the timbers close the gap will strip the thread and probably drive the head too far below the surface. Proper pilot and clearance holes will also help
9 DOWELS AND BISCUITS: From left: Compressed beech biscuits for use in biscuit jointers or with routers; Standard dowels; Stepped Miller dowel - this needs a special drill bit, but is worth using
10 PLUG IT: The plastic plugs on the left are for
plasterboard. Always buy the best plugs you can afford and make sure they are fully inserted in the hole before starting to drive the screw. Plugs are designed for specific hole sizes so use the correct masonry bit