Why a fixed 1.5 mm cut allowance is the wrong default

Ask how far outside the placement line to cut an applique piece and you get a number. Hooping Station recommends 1.5 mm. EmbroideryHooping recommends 1.6 mm. Brother PE-Design 11 offers three choices in its ScanNCut export: 0 mm "exact", 1.0 mm for standard cotton and felt, 2.0 mm for thick fabric. All are single numbers applied to every shape, satin width and fabric. Our tool searches per shape and usually lands on 0.5 to 0.8 mm for a 4 mm satin on woven cotton. This post is the argument for why a fixed number is wrong, and why ours is smaller.

What the allowance has to do

The allowance is the distance from the placement line (the outline the machine stitches first, and the centre of the satin band) to the cut line. Its one job: put the fabric edge where the satin will hide it after everything has gone wrong a little.

The satin band on a 4 mm border reaches 2 mm inside the outline and 2 mm outside it. The fabric edge is safe anywhere the band covers it with at least 0.8 mm of bite on the inside and nothing showing on the outside, so between about 1.2 mm inside the outline and 2 mm outside. That is a window about 3.2 mm wide. The allowance should aim the edge at the middle of that window, because the errors push it both ways.

The balanced allowance, worked out in the cut allowance guide, is:

allowance ≈ (half satin width + 0.8 mm bite − d1) / 2

where d1 is how deep the satin's inner end sits inside the fabric outline. On a straight edge of a 4 mm band d1 is 2 mm, and the formula gives (2 + 0.8 − 2) / 2 = 0.4 mm. Add the pull-in compensation on the outer rail (0.25 mm on woven) and the window's centre moves out a little, which is why the search lands at 0.5 to 0.8 mm rather than 0.4.

Now look at what the formula depends on: satin width, bite, and d1, which falls at tips. Then look at what moves the edge away from where it was cut: the fabric's pull-in and the care of placement. None of those is in a fixed 1.5 mm.

What 1.5 mm does to a 4 mm satin

Put the edge 1.5 mm outside the outline on purpose, and the outer rail is 2.0 mm out (2.25 with compensation on woven). The margin for the edge to drift outward before it shows is 0.5 to 0.75 mm. The errors in a normal stitch-out are 0.25 mm from the cutter, 0.5 mm from hand placement, and 0.15 mm of shift during the tack-down, root-sum-square about 0.6 mm. Half the time the drift is outward, and with a 1.5 mm allowance most stitch-outs on a 4 mm satin show fabric past the border somewhere along the outline. Our engine, run on a 100 mm heart at 4 mm satin on woven with normal placement, gives a 47 percent pass rate at a fixed 1.5 mm, 37 percent at 1.6 mm, 85 percent at PE-Design's 1.0 mm, and the low 90s at the searched 0.6 mm.

Meanwhile the inner margin, 2.7 mm, is never used: the fixed number spends the whole window on one side.

What the allowance should depend on

Satin width. Half the width is the first term. A 3 mm satin has a 1.5 mm half and a window of about 2 mm; a 5 mm satin has 2.5 and a window of 4. The same shape needs a 0.3 mm allowance at 3 mm and can take 1.0 mm at 5 mm. A fixed number is right for one width.

Fabric pull-in. A knit pulls the outer rail in by 0.40 mm against 0.25 for a woven. The file compensates, but the pull compensation guide explains why we assume 30 percent of it still shows. That shrinks the outer margin on knits; the allowance must come down or the satin must go up (we do the second, to 5 mm).

Placement care. Hand placement is about 0.30 mm with spray adhesive and two alignment marks, 0.50 eyeballing it, 0.90 for a quick drop. Care moves the pass rate more than the allowance, but a 1.5 mm allowance leaves no outer margin at all for a 0.9 mm error.

Tips. At a sharp point the satin's inner end sits shallower (d1 drops from 2 mm to maybe 1 mm), and the formula asks for a larger allowance there: (2 + 0.8 − 1) / 2 = 0.9 mm. One allowance serves the whole outline, so the tool picks the value that keeps the tip and the sides both inside the window, usually by leaning toward the tip's number and then reporting a lower pass rate if the sides cannot take it. That is where the "typically 0.5 to 0.8" comes from: it is the range where a shape's tips and sides agree. A star at 3 mm satin has no such value, and the tool says so.

What the search does

For each shape the tool builds the satin band, measures d1 along the outline, then tries allowances from 0.2 to 1.0 mm in 0.1 mm steps, running 120 simulated stitch-outs for each and keeping the best; the winner is re-run 300 times for the number you see.

Where the fixed numbers are right

Two places. PE-Design's 1.0 mm for cotton and felt is close to a 5 mm satin's balanced allowance, and its 2.0 mm for thick fabric is a real correction for an edge the satin has to climb. And 1.5 mm is the right margin to leave when trimming in the hoop, where scissors, not the cutter, decide the edge; Hooping Station's and EmbroideryHooping's numbers come from that workflow.

For a pre-cut piece on a 3 to 4 mm satin, the allowance belongs between 0.4 and 0.8 mm, and it belongs to the shape. If you have an existing design and want its allowance computed rather than guessed, the cut file from an embroidery file tool reads the placement line out of the file and runs the same search on it. The Cricut guide covers importing the result at exactly 100 percent, which is the other half of getting the edge where the number says.