Wed 12 Nov, 2008
Tags: Design, Drawer, SketchUp, Tutorial
Cutting dovetails in the shop doesn’t require dimensioned shop drawings. Heck, Frank Klausz doesn’t even measure or mark them before cutting. But for many people a drawing is helpful, and for completeness I add them to my shop drawings. That way they pop out at you when viewing a textured rendering in SketchUp, as shown in the picture at right. Since SketchUp models for shop drawings need to be drawn “exact”, we need to know the detailed pin and tail dimensions. Calculating these dimensions can be a little tricky, but with the aid of a simple Excel spreadsheet it’s child’s play. Drawing them in SketchUp can be equally easy if you follow a methodology. In this post I provide you the Excel tool and a basic tutorial. To download them click on Dovetail Calculator and Drawing Dovetails. This tutorial assumes you have a basic understanding of SketchUp at least beyond the novice or beginner level. If you do not, first refer to the Beginner SketchUp Tutorial on my web page.
Open the Dovetail Calculator and review the drawing provided which explains the terminology used (or click on the picture at left for an enlargement). I am sure you are aware that the pins are the smaller portion of the dovetail joint and the tails are the larger portion. Start with the board width you are using, perhaps for a drawer side or a carcass side. You must decide three things. First the size of a half pin; the partial pins at the ends of the dovetail joint. Second the number of tails. And third the size of the pin gap. The pin gap can be as narrow as your dovetail saw kerf if you cut dovetails by hand. You also want to choose a combination that winds up with dimensions that can be measured by you measurement tools, for example increments of 1/64″. A dimension of 1 23/40″ isn’t useful; though you can as easily draw it in SketchUp it is not useful in the shop.
In the upper left hand corner the Dovetail Calculator has a table. All cells except input cells are locked to prevent erasure of equations. Only the input cells for Board Width, Number Of Tails, Half Pin Dimension and Pin Gap are changeable. Start by inputing the Board Width. In the tutorial I use a board width of 10″, so input that. Next put in the number of tails desired. The tutorial uses 8, and we will see why shortly, so input that. I usually use a half pin of 1/4″. This is small enough to be visually pleasing and large enough to withstand normal side pressure created by a tight fit without splitting in hardwoods. This is largely a matter of taste and personal preference. If you are using a softwood, one that easily splits, you might want to make it larger. Input 1/4″ for now. Lastly input the pin gap. Some woodworkers like a very narrow gap, even as small as the kerf created by their dovetail saw. It’s a show off thing; it says “I am a really talented woodworker. See my small pins?”. But if you hand cut dovetails, tails first, this creates marking problems and almost requires you use a marking knife versus a pencil. Again it is a matter of taste. For now use 1/4″.
The results are a tail width of 31/32″ and a tail width plus pin gap of 1 7/32″ (1/4″ + 31/32″). These are usable dimensions; they can be used by your measurement devices to set a pair of dividers for example.
Now play with the Dovetail Calculator by substituting integers 1 to 10 for the number of tails (fractions of tails aren’t useful). Notice that inters 1, 2, 3, 4, 6 and 8 give results that are increments of 1/64″ which means any of these integers can be used as the number of desired tails. Integers 5, 7, 9 & 10 should be avoided, at least form the point of view of marking them with typical measurement devices used in the United States. The Dovetail Calculator can easily be modified for metric system measurement devices.
Experiment with other choices for Board Width, Half Pin Dimension and Pin Gap to get a feel for the results. OK, set the inputs as originally suggested (10″, 8, 1/4″ & 1/4″). The important results to remember (write them down) are 8 tails, 31/32″ tail width and 1 7/32″ tail plus gap width. Close the Dovetail Calculator and open the Drawing Dovetails tutorial. Notice there are scenes along the top labeled Step 1 through Step 14. Follow along with me, but don’t try to change the tutorial drawings themselves. Instead, open another blank drawing and draw your own from scratch using the explanation here and the tutorial as a reference.
Look at Step 1. We start out by drawing a rectangle 10″ wide by 26″ long. This is the arbitrary side dimensions of a drawer we are going to model. The sides, back and front will be 1/2″, but don’t worry about that for now.
Step 2. Using the Move tool bring the end lines of the box in 1/2″ on each end. Notice the construction line and green axis mark the original length of the rectangle. We are moving these ends in to account for the 1/2″ thickness of the front and back, which will become obvious shortly.
Step 3. Using the Tape Measure tool draw lines parallel to the top and bottom 1/4″ IN from the edges. This creates a half pin construction line. If you haven’t already, use the Tape Measure tool to create a parallel line 1/2″ OUT from the right end. This creates a tail width construction line.
Step 4. Using the Protractor tool start at the intersection of the top parallel 1/4″ IN construction line and the parallel 1/2″ OUT construction line and form a construction line 8 degrees from the top parallel 1/4″ IN construction line. I will explain why 8 degrees in a moment. Now, using the Tape Measure tool draw a line starting at the previously described intersection down along the parallel 1/2″ OUT construction line and make it 31/32″ long, the width of a tail. Again using the Protractor tool start at the end of this construction line marked with the construction cross and draw a construction line 8 degrees sloping upward (opposite slope to the previous angled line). You have now formed the outline of a tail.
The angles used for dovetails are somewhat arbitrary, but most dovetail markers use two angles, generally a combination of 6:1, 7:1 and 8:1 run over rise. The larger the first number the shallower the angle. Shallow angles are appropriate for hardwoods while steeper angles are more needed by softwoods. I use the 7:1 side of my marker which corresponds to an angle of 8.130 degrees or simply 8 degrees. Coincidentally this angle is also very useful for beveling the bottoms of drawers or raising door panels.
Step 5. With the Line tool outline the tail. It requires two short angled lines and one line the width of the tail. Refer to the zoomed view of the drawing. At this point you can choose Edit/Delete Guides to remove the construction lines.
Step 6. Using the Select tool with the Ctrl key select the three lines you just created to form the tail. With the Copy/Move tools and the Ctrl key copy these lines 1 7/32″ to the left and parallel to the green axis. Immediately type the characters x7 to replicate this move seven times. If you type anything after this step before typing x7 the multiple copy feature will not work. Seven is the number of desired tails, eight, minus 1, and 1 7/32″ being the tail width plus pin gap width.
Step 7. Use the Eraser tool to delete the opposite end line which will also eliminate the surface. In addition erase the short lines inside the tails. Refer to the drawing. Now select all lines and using the Move/Copy tool make a copy alongside the original. While still selected, use the context menu (right click) choose Flip Along/Group’s Red (assuming your drawing looks exactly like mine).
Step 8. While still selected move the second set of lines into position as shown to complete the dovetail outline. Now create a surface by drawing a diagonal line from one corner to the other as shown.
Step 9. With the Erase tool eliminate the diagonal line. The surface should remain. Now with the Push/Pull tool give the drawing a thickness of 1/2″, the thickness of both sides, back and front.
Step 10. Select the dovetailed board and make it a Group or a Component using the context menu. Using the Rotate tools followed by the Move/Copy tool place it as shown on the origin. Using the Move/Copy tool with Ctrl, make a copy and move it in place as shown, that is, along the green axis and 19 1/2″ from the first so that the outside dimension is 20″. With the Rectangle tool and opposing corners of the sides as extreme diagonals for reference, draw a rectangled surface as shown.
Step 11. Use the Push/Pull tool to give the front a thickness of 1/2″. Select the entire drawing. Choose Edit/Intersect/Intersect with Model.
Step 12. Move each side away from the front and with the Push/Pull tool move the tail surfaces in to create the pins. Notice I have moved some and left the others so you can see what I am doing. Do the same to the other end.
Step 13. The Intersect with Model tool leaves a few lines that should be cleaned up. Use the Orbit tool to view the back side of the front and erase the lines on the inside of the pins. Notice I have erased some lines and left the rest so that you can see what I am doing. When the lines are cleaned up make the front a Group or Component.
Step 14. Using the Move/Copy tool and Ctrl make a copy of the front. Using the context menu Flip Along/Group’s Red flip it. With the front as the reference assemble all pieces and you have your basic drawer or carcass construction.
This methodology may seem long and complex as you followed along with me, but once you get the hang of it and do it on your own you will find drawing dovetails is quick and easy. I keep a pointer to the Dovetail Calculator on my desktop so that I can access it quickly. I draw a lot of dovetails and find it very useful.
In reality drawer construction is slightly more complicated because the front and back dovetails are different in traditional style drawers (see The Design & Construction of a Traditional Drawer and The Crafting of a Traditional Drawer). I will leave it to the student to figure out how to modify this methodology and Excel spreadsheet to take this into account. Right now I am tired and hungry and am signing off.
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