Honing Guide Projection Settings: Exact Millimetres for Every Chisel and Plane Iron Angle
Projection equals the guide's effective pivot height divided by the tangent of the target angle. For a typical side-clamping guide that puts a 25 degree chisel bevel at about 31mm of projection and a 30 degree plane iron at about 30mm. Measure your own guide's pivot height once, build a stop block, and the number never changes again.
For a typical side-clamping guide, a chisel held in the narrow lower slot needs about 31mm of projection for 25 degrees and 25mm for 30 degrees. A plane iron in the wide upper jaw needs about 37mm for 25 degrees and 30mm for 30.
Those numbers are not magic and they are not universal. They fall straight out of one measurement — the effective pivot height of your particular guide, meaning how far the blade's clamped underside sits above the stone. Projection equals that height divided by the tangent of the angle you want. Measure the height once, and every setting for the rest of the guide's life is arithmetic.
This is a setup measurement page, not a comparison of which bevel angle to choose. The problem it solves is the slow one: sharpen freehand-ish in a guide with a slightly different projection each time and you re-establish a marginally different angle every sharpening. Over a year that rounds the bevel into a convex curve, the edge stops meeting the stone properly, and each session takes longer than the last.
The projection table
Three pivot heights cover almost every roller guide in circulation. H = 14.5mm is representative of the narrow chisel slot on Eclipse-pattern side-clamping guides and their many clones. H = 17.3mm is the wide upper jaw on the same guides, where plane irons go. H = 22mm covers taller-roller guides and several shop-made designs.
Projection is measured from the front face of the guide's jaws to the cutting edge, with the blade seated flat and square.
Target angle tan of the angle Projection, H = 14.5mm (narrow chisel jaw) Projection, H = 17.3mm (wide plane-iron jaw) Projection, H = 22mm (tall-roller guides) 20 degrees 0.364 39.8 mm 47.5 mm 60.4 mm 22.5 degrees 0.414 35.0 mm 41.8 mm 53.1 mm 25 degrees 0.466 31.1 mm 37.1 mm 47.2 mm 27.5 degrees 0.521 27.9 mm 33.2 mm 42.3 mm 30 degrees 0.577 25.1 mm 30.0 mm 38.1 mm 32.5 degrees 0.637 22.8 mm 27.2 mm 34.5 mm 35 degrees 0.700 20.7 mm 24.7 mm 31.4 mm 40 degrees 0.839 17.3 mm 20.6 mm 26.2 mm Pick the column matching your guide and jaw, read the projection, and cut a stop block to that length. Verify your own H using the method in the next section before trusting a column outright. Two things to notice. The projections get shorter as the angle gets steeper, which is counter-intuitive until you picture the triangle — a short blade sticking out of a fixed-height guide meets the stone at a steep angle. And the spacing between angles compresses at the steep end: the difference between 35 and 40 degrees is only 3.4mm on the chisel jaw, so a 1mm error there is worth about 1.5 degrees, while the same error at 20 degrees is worth about half a degree.
Find your guide's pivot height in five minutes
Do not assume a column. Guides vary between manufacturers, and clone guides vary between production runs of the same model.
Clamp any chisel at exactly 30mm projection, measured with a rule against the jaw face. Hone a fresh bevel on a coarse stone until the scratch pattern is continuous across the whole bevel — that is the only way to know the angle you actually cut rather than the one you intended. Then measure the resulting bevel angle. A digital angle gauge on the back of the blade with the bevel flat on a reference surface works; so does measuring the bevel's length along its face and using the blade thickness, since sin of the angle equals thickness divided by bevel length.
Now solve for H: pivot height equals projection times the tangent of the angle you measured. Clamp at 30mm, measure 26 degrees, and H is 30 times 0.4877, or 14.6mm. Write that number on the guide in permanent marker and never work it out again.
Do it separately for the narrow slot and the wide jaw. They are different heights on the same tool, which is exactly why so many people get a consistent result on chisels and an inconsistent one on plane irons.
Primary and secondary bevels without re-setting anything
The efficient way to sharpen is a primary bevel ground once and a tiny secondary bevel refreshed each time. Only the secondary touches the fine stones, so a full honing takes a couple of minutes instead of twenty.
Re-clamping the blade to a shorter projection to raise the angle works but throws away the whole point of a repeatable setting. Shim the roller instead — leave the projection alone, lift the back of the guide slightly, and the angle at the edge rises.
Shim under the roller Chisel at 25 degrees primary, 31mm projection Plane iron at 30 degrees primary, 30mm projection What it is for 0.5 mm (two playing cards) 25.8 degrees, +0.8 30.7 degrees, +0.7 A whisper of relief; barely worth the trouble 1.0 mm (a stack of four cards) 26.5 degrees, +1.5 31.4 degrees, +1.4 Light touch-up between full honings 1.5 mm (a US penny, or 1.65mm for a UK penny) 27.2 degrees, +2.2 32.1 degrees, +2.1 The standard microbevel. This is the one to use 2.0 mm 27.9 degrees, +2.9 32.8 degrees, +2.8 Slightly more durable edge for site work or knotty stock 3.0 mm 29.4 degrees, +4.4 34.1 degrees, +4.1 Approaching a full second bevel; more steel to remove each time 4.0 mm 30.8 degrees, +5.8 35.3 degrees, +5.3 Too much. Re-grind the primary instead Keep one shim of a known thickness taped to the sharpening bench and use the same one every time. The absolute angle matters far less than using the identical shim at every session. The arithmetic, if you want to shim by a different amount: the new angle is the arctangent of (H plus shim thickness) divided by projection. A thicker shim raises H, which raises the angle. That is the entire mechanism — you are not tilting the blade, you are lifting the pivot.
What each guide will actually hold
Jaw capacity is where guides fail people, usually at the extremes. Narrow chisels slip sideways out of side-clamping jaws; wide irons foul the roller.
Guide family Blade widths held How the angle is set Pivot height band Where it lets you down Eclipse-pattern side-clamp, narrow lower slot About 3mm to 38mm By projection, measured Low — near 14-15mm Anything under 6mm can rock in the jaws; check the edge is square after clamping Eclipse-pattern side-clamp, wide upper jaw About 38mm to 70mm By projection, measured Mid — near 17-18mm Tapered irons and thick modern blades can sit unevenly; clamp on the parallel section Cam-action guides with their own angle-setting jig Roughly 12mm to 73mm depending on jaw set By a registration stop rather than a measured projection Varies by jaw The jig is the whole system — lose it and you are back to measuring Narrow-blade accessory jaws About 2mm to 15mm By projection Same as the host guide Worth owning if you have mortise chisels; useless otherwise Single-wide-roller shop-made guides Whatever the jaw is built for By projection High — often 20-24mm The wide roller resists tipping but demands a wider stone than you may own Skew and cambered-iron holders Blade specific By projection plus a lateral offset As host A guide will not camber an iron for you; that is finger pressure at the edges Identify your guide here before reading a projection column. If yours sets angles by a registration jig rather than a measured projection, use the jig and treat the millimetre tables as a cross-check. Build the stop block
Measuring projection with a rule every time is where the error creeps back in. A stop block removes it entirely.
Take a scrap of hardwood or 6mm ply about 40mm wide and 12mm thick. Cut one end dead square. Cut it to the exact projection length for your most-used setting — 31mm for a 25 degree chisel bevel on a typical narrow jaw. Then cut a second block at 25mm for 30 degrees, and label both on the face in marker with the guide, jaw, angle and length.
In use: sit the guide on the bench, drop the block against the front of the jaws, slide the blade forward until the tip touches the far end of the block, and clamp. Two seconds, no rule, no reading error, identical every session.
A refinement worth the extra ten minutes is a single block with a rebate cut in each end — 31mm on one side, 25mm on the other. Fewer loose pieces to lose in the shavings, and the rebate registers the blade's edge rather than just butting against it, which keeps the edge square to the jaw as well as at the right distance.
Why the same setting still drifts
You have the number and the block, and the angle still moves a little. Four causes, in the order they usually apply.
Roller wear. A guide with a steel roller running on coarse stones and their slurry develops a flat or a groove. That drops the effective pivot height, which lowers the angle. If your angles have crept shallower over a couple of years, look at the roller before you doubt your measuring.
Clamping pressure. Side-clamp jaws pull a blade slightly downward as they tighten, and how hard you tighten changes it. Consistent is more important than tight.
Stone thickness. A stone that has been flattened repeatedly is thinner, and if the guide's roller runs on the bench rather than the stone, a thinner stone means a steeper angle. Guides that run the roller on the stone itself are immune to this; guides that run on the bench are not, and it is the reason two people with the same guide and the same projection get different angles.
The back of the blade. If the back is not flat where it sits in the jaw, the blade is not at the angle you calculated. This is the one nobody checks, and it is the reason a brand new budget chisel can refuse to give a consistent result no matter what the guide does.
The one-year test
The whole argument for measured projection is what happens over time, so here is how to check that you are winning.
Colour the bevel of a chisel with marker before honing. Take three or four strokes on the stone, then stop and look. If the marker is gone evenly across the entire bevel width and right to the edge, your angle matches the one already ground on the tool. If marker survives at the edge, you are honing shallower than the existing bevel. If it survives at the heel, you are honing steeper.
Do that check on the first stroke of every session for a month. A shop working from measured projections sees clean full-width contact almost every time. A shop working by eye sees the contact patch wander, which is the convex rounding happening in slow motion.
It is also the cheapest diagnostic in the shop. One marker, four strokes, and it tells you more about your sharpening consistency than any amount of examining the edge under a light.