How much space between parts? It depends on what is doing the cutting.

Every nesting tool asks for one number: the minimum distance between parts. Too small, and cuts merge, thin sheet warps, or the router bit chews into a neighbour. Too big, and you pay for material you never use. There are published starting points for each process, and they are not the same number.

Why there is no single number

The gap protects something different on each machine. On a plasma table it keeps the web between two cuts alive and gives heat somewhere to go. On a laser it keeps the burn from eating small features. On a router it is bluntly mechanical: a spinning bit has to pass between the parts, and the tabs that hold everything down live in that same web. That is why sensible values range from 3 mm on a laser job to nearly 40 mm on a router sheet with tabs.

Plasma: the web between cuts must survive

Kinetic, a plasma CAM vendor, recommends a plate gap of about 10 mm (3/8") as a starting point in its Primecut documentation: more on thick plate, because pierces and lead-ins need clearance, and less on thin sheet metal, where kerf and lead-ins are smaller.

Below that rule of thumb sits a hard floor: never let the web between two cuts fall under one full kerf width. The kerf is removed roughly half on each side of the programmed line, so an outline gap of two kerf widths leaves a surviving web of one kerf. With a 1.2 mm kerf that means 2.4 mm outline to outline, minimum. If two kerfs overlap, the web burns away, the skeleton loses stiffness, and cut parts start to move and tip in front of the torch.

Practice matches the arithmetic. PlasmaSpider users report running 2 to 3 kerf widths between cuts; one reports 3.3 mm (0.13") on 14 gauge with fine-cut consumables, which works out to roughly 2.5 kerf widths. Those are forum reports, not a vendor specification, but they land in the same range as the CAM documentation.

Heat is the other constraint. Trade articles in The Fabricator and Canadian Metalworking make the same point about thin gauge: tightly nested parts concentrate heat, and thin sheet answers with warp. Threads on the Langmuir forum tell that story from the hobby side. On thin material, generous spacing is cheaper than a warped sheet.

Hobby laser: material thickness is the yardstick

Stanford's laser cutting process guide recommends keeping features at least one material thickness apart, for heat. Ponoko's help documentation warns that features less than 0.5 mm apart can burn away entirely. Put together: on 3 mm plywood or acrylic, 3 mm between parts is a sane default, and 0.5 mm is the absolute floor for anything that should survive the cut. The laser kerf itself is small, but charring and heat build-up scale with how much energy lands in one spot.

CNC router: the bit has to fit, and so do the tabs

Router spacing has a mechanical floor: the gap must be wider than the bit that passes through it, and in practice wider still, because tabs usually live in that web. SendCutSend's routing guidelines call for at least 10 mm between parts and 20 mm around small parts, with tabs about 4.8 mm (0.1875") wide and half the material thickness tall. A hobby CNC FAQ at cncprojects.co.uk suggests roughly 38 mm (1.5") between parts when cutting with a 12.7 mm (1/2") bit and holding parts with tabs. The numbers differ because tab strategies differ; the shape of the rule is the same. Start from one bit diameter as the bare minimum, then add whatever your tabs need.

Turning the rule of thumb into the spacing field

Check what your nesting software means by spacing before you type a number in. Some tools measure outline to outline and expect you to include the kerf yourself; others take kerf as a separate value and add it on top. The difference is a full kerf width of web, which on plasma is exactly the difference between a clean skeleton and overlapping cuts.

One thing no placement-only nesting tool knows about is your toolpaths. Pierce points and lead-ins need room inside the gap too, and a tool that arranges outlines cannot see where your CAM will put them. If you cut plasma with, say, 6 mm lead-ins, that length has to fit between parts, so add it to your spacing value yourself.