6 Passes with a Diamond Card File Restore a Silky Zubat 330 Cutting Edge

Impulse-hardened teeth on the Silky Zubat 330 sit at roughly 60 to 63 HRC on the Rockwell scale, and that hardness controls the whole repair. It lets the saw cut green wood cleanly for two or three seasons without attention. It also makes a conventional triangular saw file skate across the teeth instead of biting. The induction-hardened zone is only at the tips, perhaps 1 to 2 mm deep, while the blade body stays softer and flexible. A fine DMT diamond card file, around 600 grit, is the abrasive that removes material from this steel. Six light passes along the gullet face of a worn section expose fresh metal. Go past ten passes on the same stretch and tooth height starts to change, which can pull the cut off line.

Why a normal file slides over the Zubat

A standard Bahco or Nicholson slim taper file is made for steel in the 45 to 55 HRC range. That covers an unhardened saw plate or a softer carbon blade. On the Zubat, the file glides over the tooth face, leaves no shaving, and starts rounding its own teeth within a few strokes.

Diamond abrasive solves the hardness problem. The grit is bonded in nickel over a steel or plastic core, and diamond is harder than the blade metallurgy it is asked to abrade. The file still needs a steady angle, because the abrasive can cut the tooth as well as refresh it.

The diamond card dresses the tooth tips and the leading edge, bringing them back to a burr-free point. It does not re-cut the full tooth profile. The Zubat’s four-sided geometry, alternating bevels, and raker pattern are factory-ground and effectively unrecoverable by hand. The usual dulling is smaller: slightly rounded tips from seasoned oak, or a scrape against a fence staple. That narrow damage is why a short count of passes works.

Counting the passes on a worn section

Take a 330 mm blade where the last 100 mm of teeth have dulled after dead apple wood. At 7 teeth per inch, that length holds about 27 teeth. Draw the fine diamond card once along the outer bevel of that section at the factory angle, and the file removes only a few microns of steel. One stroke reaches every tooth in that stretch.

Six total passes, usually three per side on alternating teeth, bring the points back without measurable shortening. A coarse 325-grit diamond plate used for a full re-profile would need thirty or more strokes, and the tooth line would be visibly lower afterwards.

The tolerance at the tip is tiny. A fraction of a millimetre separates a saw that clears the kerf from one that starts to bind. The original tooth shape is already there; the job is to uncover a clean biting edge at the very end of it.

Pressure causes most of the damage. The diamond card should cut on the draw stroke under its own weight plus a light guiding hand, perhaps 200 to 300 grams of downforce. Hard pressure makes the abrasive dig into the brittle hardened tips and can chip the edge being saved.

Work a section at a time. If the card is riding the old bevel, the sound stays even and the tooth tips brighten together. If one tooth catches harder than the rest, the file is probably leaving the factory angle or hanging in a gullet.

Stopping early is safer than correcting an over-filed run. Once a few teeth in the stretch are lower than their neighbours, the saw can wander in the cut, and restoring an even line is far harder than removing a little more rounding from the tips.

Rust belongs on a different job list

The softer body steel below the hardened teeth rusts quickly after the factory coating wears through. 0000 steel wool with light machine oil clears surface oxidation in a minute or two and leaves the tooth geometry alone. Deep pitting near the teeth moves the blade into replacement territory, because the cutting edge no longer has sound steel behind it.

A Bahco Laplander wants a different file

The same diamond-card method goes wrong on a Bahco Laplander 396-LAP and on most Bahco pruning saws. Those saws use harder-set, conventionally hardenable teeth that a proper Bahco pruning saw file can re-cut. The matching tool is a slim triangular file chosen for the tooth pitch.

On that pattern, sharpening means restoring the whole tooth profile. Each gullet is filed to a consistent depth, and the set can be re-established with a saw set tool. The geometry is repeatable by hand in a way the Zubat’s impulse-hardened pattern is not.

Put a triangular file on the impulse-hardened Zubat and the file wears away while the saw barely changes. Drag a diamond card across the softer Laplander teeth and it can remove steel too aggressively, wiping out the alternating set that gives the blade clearance in the kerf.

The Zubat is sold as a premium consumable blade. After perhaps four seasons of hard use, replacement becomes the expected path. Silky sells the separate 330 mm replacement blade for that reason. When six diamond passes no longer make the saw cut cleanly through a 40 mm green branch, the blade has reached that threshold.

Testing the restored edge

Use wood to judge the edge. Draw the restored Zubat through a 30 mm piece of green willow or fresh hazel and watch what comes out of the kerf. With healthy impulse-hardened tips, the saw throws long, curled shavings and usually clears the cut in six to eight strokes.

Rounded tips behave differently at the start of the stroke. They drag before they bite, and the waste comes out as fine powder. The same 30 mm cut can take twelve strokes or more. Powder is the useful sign: that section usually wants another two or three light passes.

The thumbnail drag is a faster check when several parts of the blade need attention. Touch the nail carefully across the flat outer face of the teeth, never along the cutting edge. Fresh tips grab with a distinct catch. Rounded ones slide with much less resistance.

The thumbnail check is crude beside an actual cut. Relying on feel too much can send the file back over teeth that were already sharp, lowering them compared with their neighbours. Uneven tooth height is what makes a hand-dressed pruning saw drift away from the intended line.

Repeated dressing has a limit because the induction-hardened layer is thin and every pass removes a little of it. After enough restore cycles, the file reaches softer steel below the hardened band, and the saw may hold an edge for only a single afternoon of cutting. The exact boundary on a given blade shows itself only when the cut test is repeated after each restore. Long shavings and powder can appear only a few teeth apart on a blade that looks uniform in the shed light. Which teeth still have hardened tips under the edge, and which have already crossed into the softer steel below?