Sat 17 May, 2008
Tags: Drawer, SketchUp
A SketchUp drawing of this drawer can be downloaded by clicking here.
There are many ways to design and construct a drawer. For example you may use plywood for the material and dadoes or nailed butt joints as the primary joinery. On the other extreme is the hand cut dovetailed joinery, floating beveled bottom, and made of hardwood design-and-construction. The latter is the subject of this post. This traditional design has been perfected through the ages and is bullet proof when considering strength and environmental conditions. Provided you follow a few – sometimes not so obvious – design guidelines, this drawer will last for centuries.
I use this design in all my furniture and have no desire to vary from it regardless of the architecture or period of a piece. It can easily accommodate a flush, inset, beaded, lipped or any other style front. It can be used with hardware sliders, wooden inset sliders or simply slide along guides. I can’t think of a design criteria this drawer cannot meet. It is always nice to design an original piece or sub assembly; but when the ages have provided you a design that can not be improved, I believe it is time to surrender and use it.
Let’s take a quick look at the overall design. Click on the picture at left above to enlarge it. The drawer is a five-sided box. The front and back are joined by the sides using dovetailed joinery. The tails are on the sides and the pins on the front and back. The tails and pins are beveled in such a direction that they resist being pulled apart thus providing a very tight mechanical joint; a snuggly fit dovetailed drawer will make one believe no glue is needed. The bottom slides into the sides in groves and comes to rest at the front where a tongue on the front edge of the bottom seats in a grove in the front. A side benefit of this is that the bottom tends to square up the drawer at the time of glue up. Now let’s look at the design in more detail.
Still looking at the picture at left above you can see the sides with their tails. Sides and back are usually 1/2″ thick and the front 3/4″ to 7/8″ thick. The sides dovetail into the pins in the back to form a through dovetail. A through dovetail is a joint where the pins and tails are visible on both corners. The sides dovetail into the pins in the front to form a half blind dovetail. A half blind dovetail is a joint where the pins and tails are visible only from the side and not the front. The thicker front provides for the side’s tails to protrude into the front 1/2″, locking with its pins, but the pins are faced and disguised by the remaining 1/4″ to 3/8″ material. Not shown in this picture is a grove on the inside face of each side which supports the bottom. Since the thickness of the bottom is 1/2″, like the sides and back, this grove begins 1/2″ up from the bottom edge, is 1/4″ wide and 1/4″ deep. It will be clear later that this will provide a 1/4″ clearance between the bottom side of the bottom and the bottom edge of the sides.
The sides will expand/contract in width. This needs to be compensated for. I have calculated that it will change a total of 7/64″ over the course of a year. Since it is going into a 5″ tall opening, and since May is about the driest time of the year and my material has been in my shop for a very long time, I will cut it to 4 7/8″ wide.
The back is usually 3/4″ narrower than the side width, allowing for the bottom to extend under it to its back side. Click on picture above right to enlarge it. Note that a side has three tails in the front and three tails in the back. However these sets of tails are not the same width. The front tails are evenly and symmetrically spaced. The back tails are all the same width but biased toward the top of the side. In addition to the three tails there is a 3/4″ square tail on the bottom edge. The reason for this will become obvious later.
The length of the back and front are determined by the opening width. I like to leave 1/32″ to 1/16″ on both sides for clearance. If there were no Cock beading on this draw I would leave 1/32″ on each side or 1/16″ overall making the drawer front length the opening width minus 1/16″. However, because of the extra step of a Cock Bead, and the fact that I can’t use my block plane to shave the bead without destroying it’s curves, I will leave a total 1/8″ making the drawer front length the opening width minus 1/8″.
This particular draw design has what is traditionally called Cock Beading around the perimeter of the front. The beading is approximately 3/32″ in radius and hence stands proud of the front surface by 3/32″. The top bead extends to the back face of the drawer front, while the bottom and side beads end at the beginning of the pins. This is done both to maintain the strength of the dovetail joints and to show them off; if you had cut dovetails you want someone to see them!
As mentioned previously the front is usually 3/4″ to 7/8″ thick. It has half blind pins on either end and a grove 1/4″ wide and 1/4″ deep on the inside face. The grove is positioned up from the bottom to align exactly with the groves in the sides. This distance is different on lipped fronts versus inset fronts.
The front will also expand/contract across the width. Like the sides I need to compensate for it. There is also the chance, that because the sides and the front are different material (maple versus birch) that a mis-match in expansion/contraction could cause problems. A quick calculation reveals that they will change the same amount. Therefore I can cut the front to 4 7/8″ as well (before trimming with the Cock Bead).
The length of the front is the same as the length of the back.
The bottom has a taper along each edge. The taper is designed such that the top surface of the bottom just touches the top side of the grove in the sides, and the bottom of the taper touches the bottom inside corner of the grove at just one point. See the picture at left. This single point of contact (actually it is a line if you consider the depth of the bottom) allows the bottom to expand and contract with seasonal humidity and temperature changes without bind, which would likely happen if this were a tongue in a grove. The forces generated by seasonal changes can be quite large, sufficient to break joints or put cracks in a board. Eliminating this potential binding with a taper assures those forces will not be exerted on boards or joints. Making this tapered cut in the shop is a trial-and-error cut. I set my table saw for an 8 degree off normal or 82 degrees. I then creep up on the thickness I want, using scrap wood and testing for good fit after each cut, hoping to avoid the error in trial-and-error. When cut right the taper will seat all the way in the grove, touch the top of the grove and the bottom inside corner, but still slide rather freely.
The bottom has a tongue along its front edge. This tongue seats in a grove on the inside face of the front. This joint is glued so that the bottom is anchored to the front and all expansion/contraction appears at the back as the bottom will alternately extend beyond the back or fall slightly short of the back surface of the back. It is important to know how much this total seasonal change will be and how deep the bottom should be cut at the time of assembly. Suppose for instance that you are building the drawer in May in the Northeast United States. This is just about the driest time of the year. If the stock has been in your shop for some time, it may have reached equilibrium with the surroundings and its moisture content may be low, around 7%. If it has not reached equilibrium it may be high, say 14 – 28%. You need to know the specific moisture content for your situation, and then you need to calculate how much the bottom will expand/contract around its nominal dimension.
I should mention at this point, in case you do not know, wood expands almost entirely in a direction across the grain. It expands negligibly in either the direction of the grain or radially. This is why plainsawn lumber always expands more than quartersawn lumber of the same species.
Let’s take the specific example of this draw bottom. The depth of the drawer bottom is 10 1/2″. I measured its moisture content (MC) to be 7% with my Delmhorst J-2000 meter. The drawer is crafted in mid February in Western Massachusetts (USA). The wood species is yellow birch.
Click on the picture at right to enlarge it. This is the output of a program called Wood Movement Master by Kite Hill Software (no longer in business). You can find many such programs on the internet, many for free and many for thirty-ish dollars. It requires a little interpretation. I am calculating a board centric figure, that is, I just want to know how much the board is going to expand or contract. The board is Flatsawn (Plainsawn) and the bottom is made of a two board glue up. (All other things being equal, a glued up board will have less seasonal change than a single board and the more boards the better to a point. I inputted this bottom was constructed from two 5 1/4″ board to create a 10 1/2″ bottom. Note that the results indicate that each board will move a total of 7/64″ over the entire year for a total movement of 7/32″ or nearly 1/4″. Note also that the bottom is currently at its smallest or most shrunk. Therefore I choose to cut the bottom to 10 3/8″ (10 1/2″ – 1/8″) at the time of glue up. It will be 1/8″ short of the back surface of the back, but six months from now it will stick out 1/8″ further than the back. Since the back is 1/2″ thick there is no chance of anything falling out of the drawer when it is shrunk.
There should be plenty of room behind the finished drawer to allow for the bottom to expand out the back. In this case it needs at least 1/8″. Also, the drawer needs stops to keep it from sliding into the opening too far. Do not place the stop behind the drawer bottom or the position of the front when closed will change with the seasons.
In a future post I will discuss how I hand cut half blind and through dovetails.