Measuring and Marking
Monday 13 October 2008
Michael Huntley has a look at tools for measuring and marking
The old adage of 'measure twice, cut once' is very sensible, and almost all my students have learnt that their marking technique can be improved. Once you've made sure your timber is level and straight it's simply a case of using the correct sharp tool, marking accurately and cutting square. Then woodwork should be a doddle!
GET IT TAPEDFirst, you'll need a flexible tape - if you are left-handed, buy a tape for left-handers, so you won't have to read it upside down.
Try to use the smallest tape that you can for each job, and never trust the tape for any measuring for which you need to be accurate to the nearest 0.5mm. There are degrees of accuracy, for drawer apertures 0.5mm is fine, for a shelving unit 1mm is fine, for a garden feature you could work to the nearest 5mm, although as a general rule always try to be as accurate as you can.
Note that the tape's hook end can be pulled out for external measurements and pushed in for internal ones. On the tape body the length of the casing should be printed. It might say '+52mm' which means add 52mm to the reading on the tape if you are trying to measure an internal dimension.
CHECK IT OUTHook the end of the tape over a piece of timber and mark the 600mm point with a pencil. Then use the 600mm metal rule to check your mark and make a scribe line with the utility knife exactly at the 600mm point. The scribe line should be in the middle of the pencil line.
SET SQUARESquares come in various sizes, from a little 50mm engineer's square to a 500mm roofing square. Keep the stock (the thick part) clean and make sure the blade doesn't get bent or bruised. To check a square is square put it against a known straight edge such as a new sheet of ply or MDF and scribe a line against the blade. Invert the stock through 180 degrees and check your line. If the blade is anything other than 90 degrees the scribed line will not
agree with the blade in the inverted position.
The large roofing square is good for big projects, but use a decent rule rather than relying on the printed scales. Combination squares are very useful, and can be used for marking parallel lines at any distance from 1mm to 300mm from an edge; they also act as a depth gauge as well as a mitre gauge.
PENCILS AND CHALKHard pencils like a 2H will produce a very fine line and I always keep one sharpened to a fine point by my bench alongside a thick 'carpenter's' pencil which is used for lettering like 'TOP' or 'REAR LEFT' and for writing on unplaned timber. Chalk is very useful on dark timbers, or use a white pencil, but don't use felt tips for marking timber because the ink drives into the wood and is very hard to remove. I know some woodworkers who use fine clutch pencils very successfully as well.
ON THE LEVELThese come as small boat-shaped levels about 225mm long or as builder's levels 1m or 2m long. Only afford one? Then get the smaller one first. The longer the level, the more accurate the reading.
CHECKING FOR WINDHold the timber up to the eye and sight down it to see if it is straight. Beware of 'wind', pronounced as in 'wind up the clock' not 'the wind is blowing'. Wind is evident when a piece of timber is parallel sided but twisted in its length.
Check for wind by using winding sticks. To get the idea just place two pencils, preferably with light-coloured bodies, across the timber at each end, then look down the length of the timber. Do the pencils line up with each other or does one appear to be at a different angle to the other? If they line up the timber is 'true'; if they don't the timber is in wind.
While wind can sometimes be planed out, generally it is best to use that timber somewhere out of sight or not at all.
STRAIGHTEDGE TALKINGThe other possible defect you may come across is 'banana wood'. This refers to bowed timber. It is not in wind, the pencils will look level, but the timber will have a bow in its length. Use a known straight edge to test for a bow.
You can either buy a straightedge or a long level with a straight edge or you can keep an original edge from a sheet of MDF or ply and use that as a reference edge until you find a nice straight, true and square timber that you trust.
Finding trusted reference surfaces can be a problem. Your bench should provide a long flat surface, but if you haven't got a bench then try to get a sheet of MDF and cut it to about 1200 x 750mm and fit 75 x 50mm bearers underneath to keep it flat. You could then mount the MDF surface onto two Workmate-type benches for a flat and true working surface.