The Jack Plane That Tears Mango and the Chisel That Crumbles
A training workshop owns a 48 mm bevel-down jack plane (randa) used by students on mango and shisham. The finished surfaces show torn grain and a set of raised ridges about 1.5 mm apart; on the push stroke the plane also chatters. The instructor strips the iron and finds four faults at once: a nick about 1.5 mm deep in the cutting edge, a bevel measuring 35°, a back that is hollow along its length so that only the corners touch the stone, and a straw-blue patch about 8 mm behind the edge left by a dry high-speed grinding wheel. The cap iron (chip breaker) sits about 4 mm behind the edge.
In the same rack a 25 mm firmer chisel (chheni) has been honed at 20° and is being used to chop mortises in seasoned shisham; its edge crumbles after a few mallet blows. The bench oilstone is hollowed in the middle by years of use, and the oil can has been empty for weeks.
1. Link each symptom - torn grain, 1.5 mm ridges, chatter - to the specific faults found on the iron and cap iron, and explain the mechanism in each case.
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Ridges 1.5 mm apart. The cutting edge carries a nick 1.5 mm deep, so the middle of the iron no longer cuts. The two horns either side score two grooves and leave a ridge of uncut wood between them. No amount of extra pressure will remove it - the edge has to be ground back past the nick.
Torn grain. Several faults act together. A dull or nicked edge rubs and lifts fibres instead of severing them, and mango is the worst case because its earlywood is soft and porous and its latewood is hard. The cap iron at 4 mm is far too far back: on a smoothing cut it should sit 0.5–1.5 mm behind the edge so that the shaving is broken immediately and the fibres are not levered up ahead of the cut. A 35° bevel is not wrong for hardwood in itself, but combined with a hollow back the edge is unsupported at the cutting line and dulls in a few strokes. Planing against the grain would also tear, but here the evidence points at the iron.
Chatter. The hollow back means the iron contacts the cap iron and the lever only at its corners, so it is not bedded flat and vibrates under load. A deep cut and a slack cap-iron screw make it worse.
Summary of the diagnosis: nick → ridges; nick + distant cap iron + dull edge → torn grain; hollow back and poor bedding → chatter.
2. Give the full sequence to restore the iron - grinding, back work and honing - including how to remove the nick without destroying the temper, and state the bevel you would finally hone for mango and for shisham, with reasons.
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Order: back first, then grind, then hone. Honing a bevel on an unflattened back produces an edge that is a series of arcs and will never be sharp.
- Back. On the coarsest stone, work only the first 15–20 mm behind the edge until a continuous wire edge is raised across the full width, then repeat on progressively finer stones over the same area. For a badly pitted back, use the 'ruler trick' - a thin steel rule under one edge of the stone to put a fraction of a degree of back bevel on the last few millimetres.
- Grind out the nick. Hold the iron square to a 46 or 60 grit wheel and take light passes, dipping in water every few seconds. The straw-blue patch shows the steel reached roughly 250–300 °C and has lost hardness; that metal must be ground away, not honed. Use a slow wet grinder or a freshly dressed wheel, and never let the edge change colour.
- Primary bevel. Grind at 25°, stopping just short of the edge so that a fine 0.5 mm band is left for the stone.
- Hone. Mango: 25° primary with a 30° micro-bevel - the soft earlywood needs a keen, thin edge, and a micro-bevel keeps the honing effort small. Shisham: 30° primary with a 35° micro-bevel, because shisham is dense and abrasive and a thinner edge crumbles or folds. Remove the wire edge on the back, finish on the finest stone, strop on leather.
- Set the cap iron 0.5–1.5 mm behind the edge for smoothing work, further back only for coarse stock removal.
3. Explain why the 20° chisel fails on shisham, recommend a bevel for that work, and describe how to flatten a dished oilstone and why the oil matters.
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Why 20° fails. Bevel angle is the wedge angle of the cutting edge. At 20° there is very little metal behind the edge, so when the mallet drives the chisel into dense, abrasive shisham the edge cannot support the load - it crumbles, folds and dulls within a few blows. The same chisel would survive light paring in softwood at 20°, which is why the angle was chosen, but it is wrong for mortising hardwood.
Recommendation. Hone at 25° for general bench work and 30° for chopping mortises in seasoned shisham. Grind about 5° below the honing angle (20–25° at the wheel) so that each sharpening removes only a narrow band of metal; that keeps the chisel long-lived.
Truing a dished oilstone. A hollow stone cannot produce a flat bevel, and the blade will rock so the edge is rounded over. Rub the stone face down on a flat plate - plate glass or a steel surface plate - with 80–120 grit silicon carbide powder and water, using a figure-eight motion, or use a coarse diamond plate. Check with a steel rule laid on edge across the stone: when no light shows under the middle, the stone is flat. Round the arrises afterwards so the blade does not catch.
Why oil matters. Oil floats the swarf, stops the pores glazing over and reduces friction so the blade does not judder and round the edge. A dry stone cuts slowly, clogs and overheats the edge. Use plenty of the correct oil, wipe the stone clean after use, and keep the stone flat by re-truing it whenever it starts to dish.