{"id":686,"date":"2016-12-13T16:33:18","date_gmt":"2016-12-13T21:33:18","guid":{"rendered":"http:\/\/jsevy.com\/wordpress\/?page_id=686"},"modified":"2019-08-06T15:21:41","modified_gmt":"2019-08-06T19:21:41","slug":"the-long-compass","status":"publish","type":"page","link":"https:\/\/jsevy.com\/wordpress\/index.php\/lutherie\/the-long-compass\/","title":{"rendered":"The Long Compass"},"content":{"rendered":"<p>The long compass is a really simple and really useful device for accurately drawing a portion of a large arc (a short segment of a circle of large radius, to be precise). From what I&#8217;ve heard, it&#8217;s an old boatbuilder&#8217;s technique. It&#8217;s used to draw a portion of a circular arc given 3 points on the arc &#8211; generally only useful for small portions of arcs of large radius, like a 15-foot radius arc for which you need a 2-foot-long segment of the circle (e.g., for the back braces on a guitar).<\/p>\n<p>The procedure for making and using a long compass is as follows:<\/p>\n<ul>\n<li>Mark the 3 points of the arc on the piece of stock to be cut into an arc; you&#8217;ll have 2 outer points and one inner point between and slightly above the others:<\/li>\n<\/ul>\n<blockquote><p>\u00a0 \u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 inner * point<\/p>\n<p>*\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 *<br \/>\nouter\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 outer<br \/>\npoint \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 point<\/p><\/blockquote>\n<p>(I usually put the inner point equidistant from the two outer points, but that isn&#8217;t necessary for the method to work.)<\/p>\n<ul>\n<li>Drive a small brad into the positions of the two outer points; the long compass assembly will &#8220;ride&#8221; along these as you draw the arc<\/li>\n<\/ul>\n<ul>\n<li>Take two straight sticks, and place them along the two lines connecting an outer point with the inner point, forming a very shallow upside-down &#8220;V&#8221;. (Put the sticks above the brads, so that the &#8220;crotch&#8221; of the &#8220;V&#8221; is right at the inner point &#8211; see the picture below.) The sticks should overlap a bit in the region of the inner point, so they can be fastened together, and each should be a little longer than the distance between the two outer points. Fasten the two sticks together so they will maintain the angle they make, i.e., so the angle of the &#8220;V&#8221; becomes fixed.<\/li>\n<\/ul>\n<ul>\n<li>The stick assembly is the &#8220;long compass&#8221;; the arc is drawn by putting a pencil into the vertex (&#8220;crotch&#8221;) of the &#8220;V&#8221;, and then sliding the &#8220;V&#8221; along the brads, always keeping it in contact with them. As the pencil travels in the crotch as the sticks are slid, it will trace out a perfect circular arc through the 3 points! Truly magic!<\/li>\n<\/ul>\n<p>The following diagram illustrates the long compass in action:<\/p>\n<div align=\"center\">\n<p><img loading=\"lazy\" decoding=\"async\" src=\"http:\/\/jsevy.com\/luthierie\/compass\/Long_compass.jpg\" alt=\"Long compass diagram\" width=\"609\" height=\"368\" \/><\/p>\n<\/div>\n<p>The compass thus made is only good for the one radius. You can of course make a general-purpose compass that&#8217;s just two sticks with a hinged joint that permits the angle to be adjusted, so you can use it for multiple-radius circles.<\/p>\n<p>Hope this helps to &#8220;round out&#8221; your lutherie! \u00a0 \ud83d\ude09<\/p>\n<p>&nbsp;<\/p>\n<h3><a href=\"http:\/\/jsevy.com\/wordpress\/index.php\/lutherie\/\">Additional Lutherie Resources<\/a><\/h3>\n","protected":false},"excerpt":{"rendered":"<p>The long compass is a really simple and really useful device for accurately drawing a portion of a large arc (a short segment of a circle of large radius, to be precise). From what I&#8217;ve heard, it&#8217;s an old boatbuilder&#8217;s technique. It&#8217;s used to draw a portion of a circular arc given 3 points on &hellip; <a href=\"https:\/\/jsevy.com\/wordpress\/index.php\/lutherie\/the-long-compass\/\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">The Long Compass<\/span> <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"parent":10,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-686","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/pages\/686","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/comments?post=686"}],"version-history":[{"count":13,"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/pages\/686\/revisions"}],"predecessor-version":[{"id":1295,"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/pages\/686\/revisions\/1295"}],"up":[{"embeddable":true,"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/pages\/10"}],"wp:attachment":[{"href":"https:\/\/jsevy.com\/wordpress\/index.php\/wp-json\/wp\/v2\/media?parent=686"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}