Humidity Effects on wood products
30 MARCH, 2013 BY JIM HILLThere are three key steps in manufacturing 5-piece Cabinet Doors that will determine the sizing accuracy of the finished door. If each of these steps is held within a tight tolerance, the finished product will be accurate to within the desired 1/64 inch, or about 15-thousandths.
The first of the three critical operations is moulder accuracy (sometimes americanized as “molder”). The second critical operation is stile & rail length cutting accuracy and the third is consistency in stock removal during the operation of machining the outside edge. Our standard is to hold each of the three critical operations to a tolerance of 5-thousandths of an inch. This produces a finished cabinet door with a worst-case sizing tolerance of 15-thousandths, or 1/64-inch.
The stile stock moulding operation starts the process of insuring sizing accuracy and, without close attention to this step, consistency in sizing the finished door is almost impossible. When cabinet door stile stock is moulded, the width of this stock is the most critical factor in accurately sizing a 5-piece Kitchen Cabinet Doors. If the moulded width of the stile stock is held within a tolerance of 5-thousandths of an inch, the subsequent operations that affect sizing can be standardized.
At The Door Stop length cutting of stiles and rails is performed on CNC machinery so achieving our 5-thousandths tolerance for this operation is actually easy. Cutting accuracies of better than 5-thousandths are normal. The outside edging operation is more difficult to hold because the edging cutter’s diameter changes from sharpening, so slight variations do occur. Our average edging tolerance does achieve the 5-thousandths inch goal, but the standard deviation shows a range from 2-thousandths to 9-thousandths inches.
Because the moulder accuracy is key to Cabinet Doors and Drawer Fronts sizing, let’s look at some examples of inaccurate moulder adjustment and the effects these errors will have on the finished size of a 5-piece cabinet door. First we will assume the moulder is set up correctly and the stile stock, which is targeted at 2 1/4–inch width, is actually oversized by 5-thousandths (2.255 inches). Assuming the rail length cutting and the edging operations are both perfect, the finished door will be 10-thousandths oversized. Most cabinet makers will be measuring the cabinet doors with a tape-measure, so the 10-thousandths error on this door will be considered perfect and within the acceptable range of even the most demanding cabinet professional.
Now let’s assume the moulding operation is not within a width tolerance of 5-thousandths and is actually over by 15-thousandths of an inch. That will make the stile width 2.265 inches, with the additional 15-thousandths doubled to 30-thousandths by the same error in each of the two stiles. This door will be oversized by 1/32 inch even if the other critical sizing operations are perfect.
This 1/32-inch error, while acceptable by the standards of the industry, is detectable with a tape measure.
The challenge to the manufacturer of Kitchen Cabinet Doors is to size the door as accurately as possible by setting tight measurement quality tolerances on the operations that affect door sizing. Sizing is even more critical when building Replacement Cabinet Doors that will be replacing doors in frame-less cabinets, where tolerance is much tighter. These tight-tolerance applications require very accurate sizing that many door manufacturers find challenging. At The Door Stop, we build every door to the tightest tolerances, not just the doors going into frame-less cabinets.
The variable beyond the control of the manufacturer is the normal swing in relative humidity and the effects humidity changes will have on the moisture content of the wood in the doors. All cabinet doors will react to changes in relative humidity by either absorbing, or giving-off moisture. Unfinished doors will react sooner to humidity changes, but even properly finished doors will eventually reach a balance between the relative humidity and the door’s internal moisture content. These moisture content variations will result in dimensional changes to the stiles and rails of the cabinet door.
These dimensional changes can be greater than the total manufacturing sizing tolerances. The definitive study on humidity and its effects on wood moisture content, and the changes it causes to wood dimensions was conducted by the US Forest Service, a department within the US Department of Agriculture. The Forest Service, published the Wood Handbook: Wood as an Engineering Material in 1999 (and 2010), which outlined the expected moisture-content caused dimensional changes in various woods. The 1999 study focused on wood flooring but applies to all wood products kept inside the house and subjected to normal fluctuations in relative humidity. The 2010 study is extensive and covers almost all commercially used woods.
The 500+ page 2010 study can be viewed or downloaded here (Chapters 4 & 13 deal with Moisture and expansion properties) http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p
I have tried to simplify the findings by focusing on plainsawn (Flat Sawn) Red Oak for this example. But, woods other than Red Oak and differing sawing-grain directions, like quarter-sawn, have different Coefficients for Dimensional Change. Because plainsawn woods are more popular and tend to have a greater dimensional reactions to moisture variations, I have used plainsawn in my example. The bottom-line finding of the USDA research is that a 4% change in the moisture content of the 2 ¼-inch stile of a 5-piece Red Oak cabinet door will be expected to cause a dimensional change of 0.0332 inches, or about 1/32-inch, per stile. That means that a 4% change in moisture content is expected to expand, or shrink the width of a 5-piece Red Oak cabinet door by 1/16-inch, which dwarfs the 1/64-inch worst-case tolerance of the cabinet door as it finishes the manufacturing process.
This 1/16-inch moisture-caused change is 4-times the combined total tolerance of the critical steps in the making of a cabinet door, and certainly needs to be considered when designing cabinets. This is especially true when considering the spacing between butt-doors on cabinets. Without proper spacing, butt-doors may not close properly during high humidity periods.
These humidity-caused sizing swings may seem extreme, but keep in mind that these calculations reflect the humidity of the worst-case the country has to offer; the desert regions of the southwest. In the southwestern deserts relative humidity routinely changes from 5% in the dry month of June, to over 90% when the monsoon storms come in July. This wide humidity swing accounts for the 4%+ change in the moisture content of wood products in the southwest. Cabinet doors installed in other parts of the country will typically be expected to experience about half this dimensional change, or about 1/32-inch.
Design provisions to accommodate dimensional changes in the raised panels of cabinet doors are in place allowing the panels to float within the stiles and rails. The panels used in recessed panel doors are either Medium Density Fiberboard (MDF) or MDF core with wood veneers, which are dimensionally stable and react very little to humidity changes.
Looking at the once popular, and hopefully never to return to popularity, Slab Cabinet Door as an example , you can see the makings of a dimensional train-wreck. With the Slab Door we are not just dealing with the moisture-related dimensional properties of a 2 1/4-inch stile. We are now looking at the dimensional properties of a glued-up slab between 12 and 24 inches wide.
Using the same moisture-content calculations for plainsawn Red Oak from the Forest Service Study, we would expect to see a 12-inch wide slab door react to a 4% moisture content change with a width change of over 3/16-inch and a 24-inch slab changing width by over 5/16-inches.
Clearly width changes of these magnitudes are unacceptable and homeowners requesting slab doors need to be made fully aware of the dimensional instability of this cabinet door design.