Thread: Testing a straight edge

"straightforward"

Jill, how about using a stringline as a reference. Oh and by the way , you only have, at best, ONE straightedge

Ian

To check a straight edge for straightness take the straight edge in question and place it on a flat surface and carefully draw a line, then turn the straight edge over and lay it so that the edge now lines up with the line that you had previously drawn, if the straight edge is straight the edge should line up with the line without any gap.
Cheers
Malcolm Eaton

For those who may not be aware you can use a similar method for checking a straight edge to be straight for checking if a Square is Square ( 90 deg).
Take the Square place the stock against a straight piece of wood and draw a line at what should be 90deg, turn the Square over and line up the blade with the so call 90deg line, if the blade lines up exactly and there is no gap then it is " Square" 90deg.
QED
Malcolm Eaton.

"Every action has an opposite and equal reaction."
Assuming one is straight and the other is not, turnining one over should mean a gap in the centre. If there is no chage it means the one you are turning is straight. Now try turning the other one and see what you get. My monies on the angle being out.

Jill, Malcolm's method gets my vote, as it shows you not only if the edge is straight, but where the deviations are. If you have endless time, you could correct it by filing, sanding and scraping the edge until it is straight, checking it as you go.

How straight is "straight"? Remember no edge is perfect, and how good it needs to be depends on what you are expecting it to do. If you are setting up jointer tables & the like, it needs to be very close, but for some jobs, a deviation of less than a mm over 1.8 metres may be acceptable. If one of your 'straight' edges is providing the bulk of the error you are seeing, the other may be useable.

Cheers,

If one is contemplating using aluminium extrusions as straight edge if you have ever had the opportunity to see how the extrusions are produced particular how they are checked for straighness and the manufacturing tolerances you would think again. The extrusion spews out of a die in a semimelted state like spaggetie and then mechanically straighten and checked by "eye". The sections are in long lengths and are cut into shorter lengths say about 6 metres before distributing to the suppliers. The problem is those lengths can have built in stresses so when the 6 metre lengths are cut into shorter lengths those stresses can cause bowing twisting etc, it is there fore very important that the piece being sellected to be a straight edge is checked thoroughly particular by the method I had previously recommended. Perhaps the best material for a straight edge in metal would be to have it cut from sheet material with the edge cut by guillotine even then it should then be checked.
Being a woodworker perhaps the way to go is employ the old method that I was bought up with and that is to select a suitable piece of oregon say about 150mm x 25mm wide, shoot the edge straight by hand with the trusted no 7 and then finally check the straightness by my suggested method and if necessary removing any deviations until the edge is straight to your satisfaction then mark the piece with letters " Straight edge".
Cheers
Mac

Cedar for straight edges.

"Cheapish" straight edges can be picked up from time to time - check out the various lengths of levels available from the tool/car places. Have picked up a couple of these - straight, strong, tall enough to provide tool support, wide enough to be clamped - not the sort of thing I would do with my "good" levels.