Lets not beat about the bush, the No.8 Jointer is a big handplane. Twenty four inches long and just a pinch over 3 inches wide with a broad 2-5/8" cutting iron. It is also one of the most widely misunderstood. If you try to use a No.8 like a 'go-to' everyday plane for stock removal you will end up exhausted and frustrated in a matter of minutes. Like the smoother, it a dedicated tool for a specific task, in this case, accuracy (and therefore speed).
With its wide blade, the No.8 is wonderful for getting large surfaces truly flat, but to understand why, it is easier to consider it's main function - edge jointing.
|Irons from No.4 (top) No.7 (middle) and No.8 (bottom)|
You always approach flat from concave, so we need to hollow the surface by taking stopped shavings, beginning with the cutting edge on the surface and lifting off before you reach the end. When the hollow becomes greater than the depth of cut over the length of the sole, the plane will cease to cut.
Switch to full length cuts and the plane will take a shaving only at the beginning and end of the board, successive pairs of shavings will get progressively longer until they join up. With the first continuous shaving the edge is straight.
The advantage of a really long plane is that it produces a shallower hollow, so you can straighten an edge or level a surface more accurately, and in fewer passes, than you would be able to with a shorter one.
The image above is a complete hoax, I planed the edge with a No.3 smoothing plane and then a low angle block plane to make the hollow big enough to see (about 1/8" or 3mm), but it illustrates the point. With a finely set No.8, the light gap under the sole would be two or three thousandths of an inch. Because the reference points at the toe and heel are further apart (forming the chord of a bigger circle), the hollow created is shallower and therefore takes fewer passes to establish and fewer to remove. The longer the board, the more time you save - exponentially.
In a collective workshop you could easily get away with one jointer plane as a workshop asset between half a dozen makers, and even then it wouldn't see daily use. But when used appropriately and with due respect for its size, the 'big gun' can make extremely short work of getting large surfaces very flat, and pairs of long edges up to 5/4" thick, very straight indeed.
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