Edge Planing Experiment
Start with a perfectly straight edge on a shortish board, say 15 to 20 inches long.
Take 10 full length, reasonably fine, through shavings, with a bench plane such as a 5, 6 or 7.
Test the edge carefully with a precision straight edge such as a Starrett (No.386-24 available from Lie-Nielsen) and I predict a small bump, or falling away of the ends, will have ocurred! The errors will be small, just a few thousandths of an inch, but they will be there. The longer you plane the worse the error will be.
This effect is not necessarily caused by faulty planing technique, though this is one possibility. I believe that the geometry of a bench plane, i.e. a flat sole with a blade protruding by a few thousandth’s of an inch, is not correct for maintaining or producing a straight surface without some technique.
The classic technique used to counteract this fact, is the use of ‘stop’ shavings, where the beginning and end of the surface are not planed, to induce a slight hollow in the length.
On work of the size specified, one simply takes ‘stop’ shavings till the plane no longer cuts, and follows with one or two ‘through’ shavings. The edge/surface will now be virtually perfectly straight. I find that with care my surfaces remain minutely hollow. Perhaps one thou” over 15 “. Since we can never achieve perfection, this seems like a good result to me. I would much rather have a one thou” hollow than a one thou” bump!
Longer surfaces are a different kettle of fish and one really needs a longer straight edge, though the flexibility of timber becomes more of an issue, and less precision is needed.
Edge joints can be tested by offering up and test clamping.
I would like to thank all those who contributed to this discussion recently on www.woodcentral.com forum, particularly those who took the time and trouble to have a go.
To sum up; Even the best planes won’t do it for you automatically, ‘Technique’ is an absolute necessity.
My planing techniques for precision component preparation are described fully on my second DVD. (Face/Datum side, face edge, thicknessing and square ends).