To check your straight edge is
straight follow these steps and refer to the adjacent photos.
I have done this in small scale on a piece of
white paper so I can take clear photos of it, but if you have long
straight edges you will have to do this test on a suitable length
bench or table which is reasonably flat and the white paper isn’t
necessary. As in the photo, lay your straight edge down on a flat
surface and with a very sharp pencil or marking knife, mark 3 short
(10mm) lines on the surface: one at either end of the straight edge
and one in the middle of the straight edge. Then end for end the
straight edge and align exactly the two ends on the same side
of the straight edge with the pencil lines you marked prior. This
needs to be done very accurately. Then draw another short line in the
middle of the straight edge. Remove the straight edge and if the two
lines marked in the centre of the straight edge DO NOT lie exactly on
top of one another your straight edge is not straight.
The 2nd photo (exaggerated for
clarity) shows where the two lines don’t match up indicating this
straight edge is not straight and it either has a hump in the middle
or a hollow. You can work this out by aligning your straight edge back
on the two lines marked at the end of the straight edge and if you can
see the 2 lines marked in the centre then the straight edge has a
hollow, if you can only see one line then the straight edge has a
hump.
To correct this problem you will need to have
a know flat surface with some 150 grit sand paper glued on to it. The
flat surface will need to be at least nearly as long as the straight
edge. A piece of 10mm thick glass layed on a cast iron surface like a
saw table will do the trick. As you can imagine the longer the
straight edge is the harder it is to set up to fix the straight edge
and do the job. Once you have set up to do it, it is a matter of just
taking the high spots off the straight edge by sanding them on
the flat surface a bit at a time. Continually re-checking if the
straight edge is straight using the method described above.
Lastly, without this sounding like a sales pitch to get you to buy a
good quality Paul William’s straight edge, I can assure you it is very
difficult and time consuming getting anything over 400mm long back to
straight.
If you haven’t checked you straight edge for
straight I highly recommend you do and as a general rule the pressed
metal rulers we tend to use as straight edges are usually not very
straight.
