Wed 24 Feb, 2010
Tags: Bed, Design, LayOut, SketchUp, Sleigh, Swan-neck, Trundle
The Trundle Bed design is complete and with this post so will the Trundle Bed Design series be. The next Trundle Bed post will be Part 1 in the Trundle Bed Crafting series where we will chronicle the build of this bed. But let’s not get ahead of ourselves and complete this post first before strapping on our shop belt. As always I will make plans for this project available to my readers on my Free Plans page.
Since the last post the only design decisions were that of choosing joinery and allowing for material expansion/contraction through seasonal changes. All panels in the bed will ride in a grove 1/2” deep by 1/4” wide. The panels must be sized to allow for seasonal expansion/contraction, and we would like the panels to be centered in the groves. To accomplish this I will use a relatively new product on the market called Space Balls. Space Balls are flexible polymer balls 0.260” in diameter to fit snuggly in a 1/4” grove. By placing a number of these all around a panel which is appropriately sized they assure the panel will always be centered, eliminate panel rattle and allow for smooth expansion/contraction. Space Balls can be purchased from a number of places, including on the internet from McFeely’s.
The trick to using Space Balls and to sizing panels is to know precisely how much a panel will expand/contract over the course of a season. You have seen me make these calculations before using an application called Wood Movement Master from Kite Hill Software Inc. Unfortunately this application is no longer available and supported. So, while I will use it here, you can find similar free calculators via a search of the internet. If you want to do the calculations by hand see Understanding Wood by R. Bruce Hoadley, an excellent reference for almost anything wood.
The first picture left above shows the results of a calculation for the bed’s Headboard Panel. This panel is quite wide, 22 29/32”. The bed will reside on Cape Cod, Massachusetts, so I choose Massachusetts, Coastal as the “Ultimate furniture location” which specifies the seasonal extremes of Equilibrium Moisture Content (EMC). The species is Cherry which defines the shrinkage percentage. The other critical parameter is the type of lumber, flat sawn in this case. Flat sawn lumber expands/contracts about twice as fast as quarter sawn lumber in most hardwoods. In the lower right you can see the results. This panel will undergo a total change in width of 3/8” over the course of a season. If I cut the panel now it will be at its narrowest because this is just past the peak of the dry season in Massachusetts. Space Balls recommends that panels be undersized 3/8” overall. I suspect this is a typical number for the typical cabinet door panel width. I decided to undersize this panel by 1/2” overall, which is how I arrived at the 22 29/32” panel width. This should work quite nicely.
One other subtlety about this panel, it is not the same width across its length; therefore the expansion/contraction will not be the same absolute amount across its length. See the second picture on the right above. Most of the expansion will take place near the center of the top edge where it will place the most force on the Swan Necks. This is a place where the Swan Necks are not physically attached to anything and hence will act as a lever prying the joint at the Headboard Posts apart. This is another reason why it is important to get this calculation right, that is, to minimize that maximum force generated by expansion. It is also why I will use lag bolts and glue on this joint to make it as strong as possible.
While we are on subtleties there is one more to consider. The width of the panel given is good if I cut it right now. If, on the other hand, I get lazy and don’t get around to cutting it until late summer I need to adjust the cutting width for the planned expansion. If I cut it just past the peak summer humidity I may want to cut it 3/8” wider, lest it be too narrow and create an unsightly gap next winter. It is important to keep expansion/contraction in mind throughout the course of a design and crafting of a piece. Wood is still alive even though it is cut and dried.
Finally we can generate a cut list. Thanks to Cut List 4.0.7 Ruby Script (see Cut List 4.0.7 Ruby Script Connects SketchUp & CutList Plus ) we can pass information directly from SketchUp to CutList Plus and generate a Parts List and Material Requirements as shown above left and right. The rough lumber calculations are based 20% waste, which may be optimistic for hardwood calculations. It should be adjusted for the individual work habits. In addition to the rough lumber requirements shown above, two sheets of 3/4” plywood are needed for the platforms.
Now that we have completed the design I will make the SketchUp model file, CutList Plus file and a complete shop drawings package in PDF format available on my Free Plans page. The shop drawings in PDF format are thanks to LayOut, a presentation package that comes with the professional version of SketchUp. Now it is time to go in the shop and build this puppy. I will see you next for Trundle Bed Crafting – Part 1.