The most popular measuring tool is a retractable steel rule. Most are cheap, versatile, easy to use and widely available. They are generally accurate to within 1mm which is good enough for cutting wood to size. But they cannot be used as a straight-edge for marking out as well as for measuring. For this you need a rigid rule.
Rigid rules
Dewalt 18V Cordless
A metre stick is probably the cheapest accurate rigid rule. These are pieces of wood, plastic or aluminium alloy a metre long marked with measuring scales. Their size makes them cumbersome to carry around and sometimes awkward to use. Smaller rigid rules - ordinary 300mm wood or plastic rulers - can be used for rough measurements up to their length: never use a 300mm rule to measure lengths over 300mm accurately. Aluminium alloy and plastic rules can be more durable than wood ones and are less prone to problems caused by changing humidity. Plastic and wood rules are likely to be damaged by using them as a guide for a trimming knife.
Rigid steel rules are for making accurate measurements. They come in a number of lengths, normally 150mm, 300mm, 600mm and 1 metre. They can be used as a straight-edge for assessing flatness and as a guide for a trimming knife. The cheaper ones often rust and their scales become difficult to read un¬less they are cleaned with a fine grade waterproof silicon carbide paper. Expensive ones are often either chromium plated or made of stainless steel.
Folding rules
The traditional woodworking rule is the one metre folding rule made of box¬wood. This rule combines stiffness with portability. Before buying, check the hinges to make sure they are not loose and that the rule remains flat when it is unfolded. Those with a bevelled edge to reduce parallax error are best. Synthetic and metal folding rules are also available. These are more robust than wood ones but neither type should be used as a guide for a trimming knife.
Retractable rules
Retractable steel pocket rules have flexible steel tapes which coil up inside a small storage compartment. The tapes are usually marked in metres, centimetres and millimetres and come in various lengths - generally between two metres and three and a half metres. The tapes are usually cither 6 or 12mm in width though wider ones - 16 to 20mm - are also available.
The zero end of the tape has a right-angled steel clip attached to it. This gives some protection to the end of the tape, stops the tape vanishing inside the case and acts as a retaining clip when measuring. The clip should be titted so that it can slide about 1mm along the tape (the thickness of the steel clip) to expand the tape slightly for external measurements and to contract it for internal ones.
The storage compartments are generally either plastic or steel and contain a coiled spring to help the tape retract after use. With some rules, the tape has to be pushed back into the case - the spring acts as a sort of guide. With others, the spring pulls the tape back into the case many of these have a tape lock so that any length of tape can be held out of the case. Tape locks are particularly useful when making measurements in poor light conditions or in awkward positions when it is difficult to read the tape.
The shape of the case is important when making internal measurements. Some retractable rules have square cases marked for length and are much more accurate than those in round cases. Some rules have a clear plastic window on top of the case through which you can see the scale and so read off the measurement.
There are a number of things you should avoid doing with any retractable steel rule:
o do not pull hard on the tape around sharp corners - it may break or split
o do not let the tape hook smack into the mouth of the case when it retracts - slow it down by hand
o do not use a steel rule next to bare electrical wires or connections
o do not pull on the tape when it reaches the end of its length
o do not use a steel tape in wet conditions - water may get into the case and cause the tape to rust and the mechanism to seize. If you have to use a steel tape in damp conditions, make sure that you dry it afterwards and wipe it over lightly with an oily rag.
If you suspect the accuracy of a measuring tool, you could ask your local Trading Standards Office to check it.
Home-made rules
For some jobs it is possible to make your own rules from pieces of wood or card. For example, when you want to measure a particular length many limes a straight piece of wood can be used as a rule though it should not be stored for future use as a change in its moisture content will change its dimensions. As another example, a set of marks or measurements can be accurately transferred from one place to another by copying them on to a piece of card or paper. As a simple extension of this idea, complex curves or joints can be drawn on card, cut out and then used as a jig or template to ensure that every time the shape is marked out it will be identical.
Black Friday DEWALT Bare-Tool DC390B 6-1/2-Inch 18-Volt Cordless Circular Saw 2011 Deals
Nov 14, 2011 09:54:31
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Black Friday DEWALT Bare-Tool DC390B 6-1/2-Inch 18-Volt Cordless Circular Saw Feature
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Black Friday DEWALT Bare-Tool DC390B 6-1/2-Inch 18-Volt Cordless Circular Saw Overview
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