Tabs, onion skin or vacuum for a production run: which one do shops actually use, and how do I set up an onion skin?

What each method costs you

• Tabs hold well but must be cut and sanded off every part. Production users also point out that the machine has to slow down and speed up at every tab, so the cut itself takes longer. The official documentation says 3D tabs (the tool ramps up and down instead of stopping to lift) often let the machine run quicker and smoother than 2D tabs.
• Onion skin leaves a thin layer under the whole cut, so nothing is released until a second step. In a published shop discussion on holding pieces, the contributors who stated a preference chose onion skin over tabs.
• Vacuum alone depends on area. Holding force is pressure times sealed area, so a small part has almost none. One shop reports holding parts as small as 3 x 7 inches with an onion skin on a 25 hp blower.

Many shops combine them: vacuum for the sheet, onion skin for the small parts.

Skin thickness shops publish

There is no single number. Published examples: 0.020 inch (a machine maker's tutorial), 0.03 inch with the final cut at half feed, 0.05 inch (cutting 0.70 deep in 0.75 material, then a second pass to full depth), and 3/32 inch finished later with a trim router. Thinner skins release more easily but need flat sheets and an accurate thickness measurement. Test on your material.

Setting it up in the software

1. Measure the real thickness with calipers and enter it in Material Setup.
2. In the Profile toolpath, type the Cut Depth as a calculation. Z (or T) stands for material thickness, so typing Z-0.02 followed by the = key gives a 0.020 inch skin.
3. Or open Edit Passes... and use Set Last Pass Thickness, which lets you state how much material the final pass removes.
4. To release parts on the machine, add a second Profile toolpath on the same vectors with its Start Depth at the first cut's depth and the remaining thickness as Cut Depth. Put it last in the toolpath list.
5. To release parts off the machine, tape the face and sand the back (the tutorial starts at 80 grit), or use a drum sander. The same tutorial warns against a planer because small parts can come loose.

Pro tips

• Tab Thickness is measured from the bottom of the Cut Depth, not from the bottom of the material. If the cut goes into the spoilboard, the tab is thinner than the number you typed.
• One shop that uses tabs on small parts sizes them about 1/8 inch thick by 1.25 inch long so a knife removes them.
• See the entry on cut order for sheet work before running small parts.