Enlarging (or Decreasing) Circular Blocks

Several people have asked how I drafted the larger blocks in my Steam Punk quilt. It's an application of basic high school geometry. No kidding. The geometric principle is this: similar figures have proportional lengths and congruent angles. That means you always multiply lengths by the same amount while angles stay the same.

Steam Punk quilt with larger and small blocks

How do you do it?

1. Measure your original block and decide how large or small you want the new block.

I won't give the exact measurements of Steam Punk; you'll have to buy that pattern. But let's pretend the original finished block was 4" and the blades intersect the side 1.25" from the corners (Fig. 1 below.)

Fig. 1 - example of original block at 4-inches

2. Calculate the scale factor (new finished block size/original block size) to determine how long the new segments will be. My new block finishes 8" so my scale factor (8/4) is 2.

Draw a new block 8" on each side (4" x 2) and mark the center of the block. Using the scale factor already!

Mark points on all four sides 2.5" from the corners (1.25" x 2). Scale factor again!

Fig. 2 - example of drafting an enlarged circle block

3. Draw lines from the center to these points on the side. (Fig. 3)

Using a compass with the sharp point in the center of the block and the pencil at the side point 2.5" from the corner, draw new curves for the blades. If your compass won't expand that far, use your ruler. Place the end on the center of the block and align the ruler through that 2.5" point.

Notice the measurement on the ruler. On my example it's about 4.25".

Fig. 3 - enlarged block with main lines drawn

4. Now rotate the ruler slowly in an arc, keeping the center at the ruler endpoint. Use your pencil to mark many small dots at the same length until you reach the 2.5" mark on the next side (Figs. 4 and 5.) Connect all those points to mark the blade curve.

Note: If you're making a really large block, use your rotary ruler. This tiny ruler just made the photo easier to shoot and see.

Fig. 4 - drawing a circle or arc on an enlarged block

Fig. 5 - enlarged arc is drawn

Once your block is drafted, check the central angles by placing your original blade and triangle templates on the new block pattern at the center. The sides should line up perfectly. The templates are just shorter than your new length.

The radius of the center circle will also be multiplied by the scale factor. So if it was one-half inch originally, it will be one inch in this example (1/2" x 2).

Trace each template, add seam allowances and you're ready to go!

Hope this helps. Let me know if you have questions.

Enjoy the day, the holidays and the New Year!
Ann