A compass ruler, why when you can use a standard compass. The standard compass still requires the ruler to measure the appropriate radius for the circle to be drawn and has a limited useful radius. While a beam compass is more specialised offering a greater range than the standard compass, possibly including graduations and extension pieces but is a relatively expensive option. A ruler is a must have in the workshop and is relatively inexpensive in comparison to the former. But the size of the circle is only limited by the length of the ruler and is infinitely adjustable along its length without recourse to extension pieces. The elements to be designed to accomplish the function are inexpensive, few in number and can be scaled to the appropriate ruler size.