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6th May 2024, 01:04 PM #1
handplaning a matched joint a real world example
Some time ago, meaning probably a decade + I did several blogs on here of the maths of hand planes. Unfortunately they were lost with an upgrade. Suffice to say I did the calculations for how much hollow would remain in a matched joint of boards 2.4m long with an absolutely flat plane and a particular blade projection if starting from a hollow edge. As the blade projection increased, thicker shaving, the hollower the plane would leave the board. Also to say that any plane convexity increased the hollowing action in the same way as increasing blade projection so even if you only had a small blade projection at the mouth, the plane would perform more badly by the amount of the convexity added to the blade projection. Similarly if comparing a shorter plane to a longer plane, the shorter plane would leave the board hollower than the long plane. Even if you have a long plane that is flat in the middle but relieved at the toe and heel, it would perform only as good as the shorter plane that matched the length of the flat area.
In terms of actual numbers a flat #7 with a 0.1mm blade projection (a 0.1mm shaving) would leave a hollow of 2mm on a 2.4m board. Taking a 0.05mm shaving would leave a 1mm hollow. Halving the blade projection (or the convexity) halves the hollow.
Look at Jim Kingshott's video at 57min to visualise this https://www.youtube.com/watch?v=RKMd2-zRLH0&t=3973s
Last week I jointed the concave edges of a couple of River Sheoak boards 3m x ~320mmx28mm. The first board I planed throughout with the #7, full passes until nice full shavings. The hollow in the edge was ~2mm checked with a laser (see photo 1). In theory the hollow would have been 2.5mm for the number 7 taking a 0.1mm shaving. At the point where the hollow is this much, the plane no longer recognised a curve and took full even shavings.
The second board I roughed off the hollow quickly with a wooden jack plane until visually it looked flat. Then I used the #7 with a finely set blade until there was full and even shavings. The board was flat by laser (see photo 2) and resting the first board on top showed a gap of 2mm in the middle.
Summary if jointing hollow edges for a matched joint. Even with a flat plane you will still have to do some partial passes on ends to straighten the boards and reduce the gap between boards. Without the part passes the gap in 2.4m boards is likely to be 2-4mm using a flat #7, this amount of gap would have to be assessed for glue up. Certainly my sheoak boards required a lot of clamping pressure to remove the 2mm gap so I will have to do part passes on the first board.
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