An onion skin keeps a thin sheet connected over a broad area until the final separation pass. Its thickness must survive stock variation and cutting force while remaining removable without tearing, excessive recutting, or a dangerous loose part.
Decide Whether an Onion Skin Fits the Material
An onion skin retains a continuous layer beneath the profile, which can distribute holding force more evenly than isolated tabs on thin sheet. It works only when stock thickness and bed flatness are known well enough to leave material everywhere. Measure the actual sheet at several points before choosing the remaining layer.
Measure the Real Sheet and Reference Plane
Variation must be included in the risk calculation. If the sheet is thinner in one region or the spoilboard is not parallel to motion, the first pass may cut through and release a part unexpectedly. Surface or map the support as appropriate, and use a conservative coupon rather than setting skin thickness from nominal stock alone.
Rough Without Weakening the Remaining Web
Plan the final separation pass for a sheet that may already be less stiff. Reduce the chance of small parts tipping, moving into the cutter, or losing vacuum as the skin disappears. Cut order, direction, bridge locations, and support beneath the part all belong to the final-pass plan.
Choose a Final Separation Method
Inspect the edge left by the first and final operations. A heavy skin can require a second pass that adds heat, fuzz, burr, or dimensional change; a very light skin may tear before the cutter returns. Compare removal force and finish quality with a tabbed coupon when both methods are feasible.
Inspect the Skin Before the Last Pass
Do not rely on onion skin where the remaining layer cannot be measured or where released parts can fall into an unsafe area. Thin plastics, warped sheets, and inconsistent laminates may need a different workholding method. Any vacuum or adhesive method must also be suitable for the material and machine environment. The skin must survive the same sequence planned in Choose a CNC Cut Order That Keeps Nested Sheets Stable; a stable first profile does not prove the final skeleton will remain flat.
Record a Material-Specific Skin Range
Store measured thickness range, support condition, programmed skin, actual result, final-pass settings, part size, and accepted edge. Recheck after changing the sheet supplier, spoilboard condition, cutter, or nested layout. The method is released for a material-and-setup combination, not for every thin sheet.
