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EQ Mini: Understanding Rotary Cutting

Rotary cutting dimensions are nearly always approximations. For example, the 7/8 inch that quilters conventionally add for half-square triangles is really .85355 inches, rounded off to the nearest 1/8 inch so that we can read it on the ruler. Experienced quilters in fact cut a little “shy” in this case, since .85355 is a little less than 7/8.

This applies, of course, to the dimensions given in EQ rotary cutting printouts. Because of this, the rotary cutting instructions may not always be the best guide to cutting your block. Here are some things that you should know when deciding on the best type of pattern to use for a given block.

EQ Gives the Conventional Results

In simple cases, rectangles and triangles in even sizes, for example, EQ gives the same results as conventional quilter’s wisdom. It adds 1 1/4 inch to the finished size of quarter-square triangles, for example, just like the books.

But, EQ gives rotary cutting dimensions even in cases where the conventional rules of thumb do not apply. Here you need to be aware that rounding off to the nearest 1/8 or 1/16 inch may produce inaccuracies. The following example illustrates the point very well.

06rot4

The inaccuracies produced by rounding off can become significant if the finished sizes of patches are not easily measured on common rulers. The block to the right has 7 patches across by five patches down. If you wanted a 10 inch block, this would mean that each rectangle has a finished size 1.43″ by 2″. 1.43 is not to be found on your ruler.

EQ would add the 1/4 inch seam allowance, round off (say, to the nearest 1/8 inch), and tell you to cut rectangles 1 7/8″ by 2 1/2″. If you then pieced together these pieces (assuming you use accurate quarter inch seams), the finished size of the actual block would come out to be 9 5/8″ by 10, a full 3/8 inch too narrow!

Templates May Be Used as a Check

EQ templates are always accurate. If you are uncertain about rotary cutting dimensions in some cases, print the templates as a check. The actual (unrounded) dimensions for rotary cutting may be measured from the dotted (seam allowance) lines on the templates. First extend the dotted lines until they cross. Then measure the resulting total length and height.

06rot5

Round to 1/16 Rather Than 1/8

If you are able to use the finer measurement, always round to 1/16 inch because it is more accurate.

EQ Does Metric

If you select centimeters on the Measurement section of the Preferences dialog, EQ gives rotary cutting dimensions in centimeters. In this case, EQ rounds to the nearest 1/10 centimeter. This is even more accurate than 1/16 inch. Of course, all your measurements then need to be in centimeters.