What a grid nest does and does not do
This calculator lays parts out in a plain grid: rows and columns of identical rectangles, all the same way round, with a kerf gap between them and a margin at the sheet edge. It tries the part both ways round and keeps whichever fits more, unless you have told it the grain direction is fixed. That is the simplest possible nest and it is deliberately conservative.
Real nesting software does considerably better on anything that is not a rectangle. It rotates parts individually, interlocks concave outlines so one part sits inside the notch of another, fills the strip left over at the end of a sheet with smaller parts, and mixes different parts from the same job onto one sheet. On a job of L shaped brackets or curved signage the difference between a grid figure and a proper nest can be twenty percent or more of the material. If you have the software, use its number. If you are estimating before the file exists, this figure is a floor you will not do worse than.
Kerf, margin and the two numbers people forget
Kerf is the width of material the process removes. A router bit takes its own diameter, a saw blade takes its plate plus set, a plasma torch takes a visible slot, a laser takes very little but not nothing, and a waterjet takes a taper as well as a width. Parts drawn edge to edge with no kerf allowance either come out undersized or cannot be separated. The field here is kerf plus whatever extra gap the process wants, and on sheet goods being held down by vacuum it is worth leaving more than the bare cutter width so the offcut skeleton has something to hold onto.
| What eats the sheet | Typical size | Why it is there |
|---|---|---|
| Edge margin | A fraction of an inch to a few inches | Clamps, vacuum zone boundary, damaged or out of square factory edge |
| Kerf between parts | The cutter width | Material the process removes and cannot give back |
| Skeleton | Kerf plus a little | What holds the offcut together so it does not lift into the cutter |
| End strip | Whatever is left over | The remainder after a whole number of parts — the real cost of a grid nest |
The end strip is where most of the loss lives. A part that fits seven and a half times across a sheet fits seven times, and the half is gone. This is why changing the part size by a small amount, or changing the sheet size, can move the yield more than any amount of clever arrangement.
Grain, pattern and finish direction
Setting the direction to fixed removes the rotated option, and on some materials you have no choice. Plywood and veneered panels have a face grain that runs one way and rotating a part turns the grain ninety degrees where it will be visible. Brushed and directional finishes on metal do the same. Printed or patterned sheet stock has an obvious orientation. On any of those, the better nest is not available and the yield drops, which is a real cost of the material choice and worth knowing before you quote it.
The reverse trap is assuming direction matters when it does not. If the part is going to be painted, edge banded, or hidden inside an assembly, letting parts rotate is free yield.
Scrap allowance
The allowance in this calculator adds parts to the requirement before the sheet count is worked out, which is the right place for it. What it covers is setup pieces, the part you cut before you noticed the offset was wrong, parts damaged in handling, and parts that fail inspection. Five percent is a modest default for a process you have run before. On a first run of a new part, on a material that chips or delaminates, or on anything with a finished face that shows every scratch, it should be higher.
Watch the spares figure the calculator reports. Because sheets come whole, you often end up with a good deal more spare capacity than the allowance asked for. When the spare count is close to a full sheet, look hard at whether one fewer sheet still covers the order, and if it does not, consider making the extras deliberately rather than throwing the capacity away.
Related
For a full one dimensional or mixed cutting plan rather than a repeated rectangle, use the cut list optimizer. Sheet goods by the panel: plywood sheets, drywall sheets and wall panels. For what happens after the nest, see laser cutting time and cost, plasma cutting cost and metal weight.
Questions people ask
Why does real nesting software beat this?
Because it is not restricted to a grid. It can rotate individual parts to any angle, slot concave shapes into each other, place small parts in the gaps left by large ones, mix several different parts from the same order onto one sheet, and fill the end strip that a grid throws away. For plain rectangles all it can add is the end strip trick and the gain is small. For irregular outlines, brackets, letters, curved panels or anything with a notch in it, the gain can be very large. Use this figure to estimate and to sanity check, and the software output to buy material.
What kerf should I enter?
The width your process actually removes, plus any extra gap you want between parts. Measure it rather than assuming: cut two lines a known distance apart on scrap and measure the remaining strip. Router bits are close to their nominal diameter, saw blades remove more than the plate thickness because of the tooth set, plasma removes a wide and slightly tapered slot that varies with amperage and standoff, and laser and waterjet each have their own characteristic. If you are also relying on the offcut skeleton to hold parts down, add to the figure rather than running the parts tight.
Is a lower offcut percentage always better?
Not always, and it is worth being suspicious of the number. Offcut percentage counts every square inch not inside a finished part as waste, including large usable remnants. A sheet with one clean twenty by forty inch piece left over is not the same as a sheet reduced to strips, even if the percentages match. Shops that keep a remnant rack and actually use it have a real yield much better than their calculated one. Look at the shape of what is left, not just the number.
Does this handle different parts on the same sheet?
No. It nests one repeated rectangular part. For a mixed job, run it once per part to get a feel for the sheet count, then treat the sum as an upper bound, because a mixed nest will almost always do better than the parts nested separately. If the job is genuinely mixed and the material is expensive, that is the point at which nesting software or the cut list optimizer earns its keep.
Should the edge margin be bigger than half an inch?
It depends on how the sheet is held and what state its edges are in. Vacuum tables need the part inside the sealed zone and the boundary is rarely at the true sheet edge. Mechanical clamps need physical room and cutting into one ends badly. Factory edges on sheet goods are often damaged, out of square or slightly out of size, and on a job with tight tolerances you may want to trim a reference edge before you nest anything. Half an inch is a common working default. Measure your table and your stock and use what is true for them.