Cuttable pattern rules: what gets checked and why

Most stained glass pattern generators, especially AI image tools, produce a picture that looks like a stained glass panel without checking whether any of its shapes can actually be cut in glass. Every pattern on this site is checked against three specific geometric rules, computed from the pattern's real vector coordinates at the size you choose, before it is published.

Minimum piece width: 3/8 in

A piece narrower than about 3/8 in at its narrowest point has very little glass left once the edge is ground down, and tends to snap under grinding pressure rather than survive to become part of the panel. This is computed as the shortest side of each piece's minimum bounding rectangle, not just its widest measurement.

Minimum interior angle: 30 degrees

A sharp point under about 30 degrees is difficult to grind cleanly: the grinder wheel tends to round it off or chip the tip rather than hold the exact angle. Every interior angle of every piece on every pattern is computed and checked against this floor.

Minimum reentrant curve radius: 1/4 in

On patterns with a concave, inward-curving cut, such as Crescent Moon's inner curve or Tiffany Arch Fan's arch, a curve tighter than about 1/4 in radius is hard to grind to shape without over-grinding the surrounding piece. This check applies only to patterns with a real concave curve; straight-line patterns like Diamond Lattice have nothing to check here.

What this does and does not cover

These are geometric checks against known bench constraints, run against the pattern's actual coordinates. They confirm a piece can be cut and ground at the size shown. They do not account for the specific glass you use, since some glass is more brittle than others, and they do not replace your own cutting technique or tool condition. No pattern on this site has been physically cut and tested in glass as part of this check; the verification is geometric, not physical.

Try these checks live on the free pattern with the Pattern Sizer tool.

Bench procedure for checking a resized pattern

Print the pattern at the final working size, not at a preview size. Before cutting glass, measure the paper or card copy with a steel rule and confirm one known overall dimension. A printer set to fit to page can change a 10 in panel by 1/4 in or more, which is enough to change several small pieces from workable to fragile.

Mark the narrowest area of each small piece with a pencil. Do not judge only by the center of the shape. A leaf, feather, flame, or border triangle often becomes weakest near one end. Use a clear ruler with 1/16 in marks, or calipers if the piece is under 1/2 in wide. If the narrowest practical glass left after grinding will be close to 3/8 in, treat the piece as a caution piece even when the drawing passes the rule.

Check sharp points with a small angle gauge, a printed 30 degree reference, or a bevel gauge set from a protractor. Place the reference at the inside corner, not along the outside edge of the lead line or foil allowance. The usable glass corner is what matters. If two cut lines meet at a very fine point, plan the cutting order so the short side is scored last, because a short score next to a weak point is easier to break by accident.

For inward curves, use circle templates, drill gauges, or a paper circle cut to 1/4 in radius. A 1/4 in radius is a 1/2 in diameter circle. If that circle cannot sit in the hollow of the curve without crossing both cut lines, the curve is tighter than the bench limit. For more on changing a design without changing its proportions, use the sizing notes in how to size a pattern.

Tools, tolerances, and working dimensions

Use measuring tools that are more accurate than the decision you are making. A ruler with only 1/8 in marks is too coarse for checking a 3/8 in minimum. A digital caliper is useful, but it should be used lightly on paper because pressure can bend the copy and give a false reading.

Check	Useful tool	Bench tolerance to allow	Practical note
Narrow piece width	Clear ruler or calipers	Leave about 1/16 in extra when possible	Grinding can easily remove several hundredths of an inch from each side.
Sharp inside angle	30 degree paper gauge or bevel gauge	Aim above 30 degrees, not exactly on it	Foil and solder will not rebuild a chipped glass point cleanly.
Concave curve radius	1/2 in diameter circle template	Prefer a larger radius on textured or brittle glass	The grinder face contacts more glass on a wide curve than on a tight hollow.
Printed scale	Steel rule	Keep printer error under 1 percent	A 12 in panel printed 2 percent small loses almost 1/4 in overall.

Keep separate copies for measuring and cutting. A copy used for repeated measuring gets dented and softened at corners. If you cut pattern pieces from that same sheet, the paper edge may no longer show the intended line. For beginners, a second clean copy often saves more time than it costs, especially on panels with many small pieces. The basic workflow is covered in the beginner guide.

Frequent mistakes and recovery methods

The most common mistake is shrinking a pattern after it has already been checked. A design that passes at 12 in wide may fail at 9 in wide because every narrow bridge, radius, and point shrinks in the same proportion. If you must reduce the panel, recheck the smallest pieces before cutting any glass.

If a piece is too narrow, combine it with a neighbor and remove the dividing line. This usually changes the look less than trying to cut a fragile sliver.
If a point is too sharp, blunt the point on the pattern before cutting. A flat end of 1/16 in to 1/8 in is often enough to make the piece survive grinding.
If an inward curve is too tight, open the curve by moving the deepest part outward. Keep the new line smooth, because a lumpy correction is harder to grind than the original curve.
If a printed copy is off scale, do not compensate by cutting outside the lines. Reprint at the correct scale and confirm the measurement again.
If a cut begins to run toward a point, stop trying to force the break. Make a relief score from the waste side, then remove the waste in smaller sections.

Another mistake is ignoring glass choice. A smooth cathedral glass may tolerate a small piece better than a heavily textured sheet of the same outline. Rippled, hammered, plated, or very streaky glass can break along internal stress lines that do not match the score. When a difficult piece is important to the design, cut it from a larger scrap first and save the main sheet until the method is proven.

Special cases that need extra judgment

Border pieces look simple, but they can be weak when they form long strips. A strip 3/8 in wide and 8 in long is not the same risk as a compact 3/8 in detail. Long strips twist during handling, and the score can run off near the end. When possible, divide a long border with a joint that aligns with nearby pattern lines.

Foil and came panels also behave differently at the same drawn size. Foil follows small curves closely, while came needs space for the channel and has less tolerance for tight direction changes. If you are choosing between those construction methods, compare the practical limits in foil versus came before finalizing very small details.

Mirrored patterns need the same checks after flipping. The geometry does not change, but cutting habits do. A right handed cutter may find one direction of curve easier than its mirror. For difficult concave curves, place the pattern on scrap glass and rehearse the score direction before committing to the final color.

Nested shapes, such as rings, crescents, eyes, and flower centers, deserve a final dry review. Look for the narrowest bridge between the inner and outer edge, then check the deepest hollow of the inner curve. These pieces often pass one rule while being close to another, so they should be cut early while your hands are fresh and the grinder bit is clean.