Twelve Pierced Bookmarks, One Blade Width
The workshop has accepted an order for twelve sterling silver bookmarks, 0.9 mm thick, each pierced with a vine motif. The tightest internal corner in the customer's artwork has an inside radius of 0.45 mm, and several openings are only 1.2 mm wide. The bench stocks 2/0 saw blades only; the last eight blades have all snapped in the same tight corner, and the students are drilling 0.8 mm starter holes with a hand-held rotary tool run at maximum speed. The sheet was annealed and pickled before sawing. Eight working days remain, silver is bought at a fixed price, and roughly 30 per cent of the sheet has already been scrapped. The supervisor wants a decision today: is this a blade-quality problem, a tooling problem or a technique problem, and should the job be renegotiated with the customer or pushed through as drawn?
1. Assess the blade and drill choice against the geometry and state the working rule that links blade width to the smallest inside radius.
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Blade quality is the least likely explanation. Eight breaks in one place is a geometry-and-technique signature.
The rule: a saw blade can only turn a curve if the blade is comfortably narrower than the radius it must follow - as a working rule, blade width no more than about half the smallest inside radius, and the kerf must still leave the drawn line. For a 0.45 mm radius that means a blade of roughly 0.2 mm or finer, in the 6/0 to 8/0 range. A general-purpose 2/0 blade is simply too wide: the back of the blade binds on the outside of the curve, the saw jams, and the blade snaps at the point of maximum twist.
Starter holes. The drill only has to pass the blade, so a 0.5 mm drill is enough for a 0.2 mm blade. A 0.8 mm hole in a 1.2 mm wide opening leaves 0.2 mm of metal on each side and puts the hole where the design cannot absorb it. Drill from the face side, centre-punch first, back the sheet with a hardwood block, use beeswax or a light oil, run the rotary tool slowly and peck at the cut rather than leaning on it. Fast, dry drilling work-hardens and snatches thin sheet.
Conclusion: this is a tooling plus technique problem, made worse by the wrong drill size.
2. Diagnose the breakage pattern and list the corrections you would enforce at the bench for the remaining nine bookmarks.
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Diagnosis. Breakage always at the tightest corner = the blade is being twisted to follow a curve it cannot physically turn, usually combined with too much forward pressure and no lubrication.
Corrections at the bench.
- Order the correct blades (0.2 mm or finer) today; the cost of a dozen blades is trivial against 30% scrap.
- Saw at the bench pin, sheet flat and supported close to the cut, blade vertical, cutting on the downstroke with a light, steady, fairly fast rhythm. Let the teeth cut; pressure is the enemy.
- Turn the work, not the blade. The frame stays roughly still; the sheet is rotated into the blade.
- Relieve the corners. Cut into each internal corner from the waste side first, or drill a small relief hole at the corner radius, so the blade never has to reverse direction inside a tight turn.
- Lubricate with beeswax or a light oil; re-anneal only if the metal has work-hardened, and pickle properly afterwards.
- Blade seating and tension: blade correctly in the frame, teeth pointing down and towards the handle, frame tensioned so the blade gives a clear note, and the blade checked for a burr after each break.
- For twelve identical pieces, stack two or three sheets with double-sided tape and saw the stack, or make a template. Repeatability comes from the setup, not from heroic sawing.
3. Recommend what to do about the customer's design and the material budget, showing your reasoning rather than only the conclusion.
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Options and how to judge them.
- Push through as drawn: technically possible with 0.2 mm blades and relief drilling, but twelve pieces at 30% scrap is already a loss and every remaining sheet is at risk.
- Renegotiate the artwork: open the tightest corner to 0.8–1.0 mm inside radius and widen the 1.2 mm openings to 1.6 mm. Visually this is almost invisible in a vine motif, and it lets the workshop use a more robust blade and cut faster. This is the cheapest fix by a wide margin.
- Change process: laser or CNC piercing, or acid-etched/photo-etched blanks, subcontracted. Justified only if the artwork must stay exactly as drawn.
Material budget. Work out the sheet already consumed (30%), the sheet needed at the current yield versus the sheet needed at a realistic yield once the blade and drill are corrected. Quoting is done on metal actually delivered, not on metal bought, so any re-cut has to be charged or absorbed - a conversation to have now, not at delivery.
What I would do: take the corrected tooling to the customer together with a marked-up drawing showing the proposed corner radii, quote the unchanged price on the modified drawing, and quote the as-drawn version at a higher price with the delivery date honestly stated. Document the agreement in writing before the next sheet is cut.