It’s said that some of the most impressive projects begin with a simple drawing on a paper napkin. Well, maybe most are not quite that simple — but the initial parameters of this next project seemed almost as sketchy, although pretty straightforward. It ultimately turned into a challenge of creativity, ingenuity and craftsmanship.
The contractor’s request was for 400 linear feet each of custom milled exterior crown molding in three different profiles (5”, 8 1/4” and 9 1/4”) in 16-foot lengths, milled from PVC stock. PVC was requested rather than mahogany because the 9 1/4”gable line had “a subtle curve.” This seemed like a reasonable request except for that “subtle curve” !!! The 9 1/4” crown needed to flex, by the contractor’s description; however, because of the profile’s size, the PVC material would need to be a glued assembly. Once glued, the material loses it flexibility and will not bend like the contractor might think, and this “subtle curve” aspect was too vague a description upon which to proceed. It was time for a field trip.
As it turned out, the “subtle curve” was anything but subtle… more serpentine than anything else. “Continuous curve gambrel roof design” is a more accurate description: five independent gables, each with its own unique geometry, and not a straight line to be found. It was decided then that PVC was not going to flex enough to form the contours of any of these gables. It would need to be solid mahogany and it would have to me made to glove fit all the gable contours. Yikes !!!!
The decision in the end would be an African Mahogany crown. The gables would need to be field templated in order to replicate their true form, then the crowns could be milled to their exact contours. Also, the overall length of any one gable from ridge to soffit ranged from 30 to 40 feet, making installation a challenge. Decisions to break the runs and where, in order to create an assembly of parts for each gable, were done on site at the templating stage. In the end, each gable crown assembly would consist of between 8 and 10 individual parts.
One thing became evident as the complexity of the project grew: the need for tracking and monitoring all the parts. We took photos of all the gables, and each gable was assigned a code. The breaks in the crown assemblies were noted on the photos and an additional sequential code was assigned to each part location. This part sequencing would ultimately be the single most important coordinating feature of the manufacturing and installation processes.
Back in the shop is where the fun began!!! How to create and mill these serpentine creatures? After several brainstorming sessions, we decided the final product could not be milled in one solid piece but instead would be milled in stages. The profile was broken down into 5 separate cuts. The key piece would be a laminated section made to replicate the contours of the field templates. The genius of this is that once milled up, this key laminated core becomes the control piece for milling the adjacent parts of the profile.
Let me ‘splain. You can’t bend 1 1/2” solid African Mahogany, but if you rip it down into 1/8” veneers it will flex. And if you restack these veneers, then glue and clamp them in a contoured form, when the glue dries they will hold the new contoured shape. Sand and square edge this laminate, then glue a second piece of mahogany to it and mill that second piece. Add a third piece to that group and mill the third cut and so on. Ultimately we are sequentially building out the full profile, using a key laminate core, with precision accuracy. BAM !!!
Let the Games Begin
Yes! We can do this !!
First order of business was to break down the profile into manageable parts and order the milling knives. Next, we needed to check our field templates and their sequences and hand them over to our CNC wizard. The CNC wiz was able to take all the field templates and create a program that replicates each contour with pinpoint accuracy. Running the program, the CNC produced glue-up forms that replicated all the field templates for the project. The forms would be 3 3/4” thick using 5 , 3/4” sheets of MDF. Each form would be a top and bottom set and numbered to its corresponding position in the crown assembly. Eighteen form sets were built to produce the 35 crown sections that would ultimately be installed on the 5 gables. (The 5 Gables! Sounds like a B&B).
To create the key laminated core, 2” thick planks of African mahogany were resawn into multiple 1/4” slices, then planed into 1/8” veneer strips. Nine veneers each were then restacked within each form and glued together using West System Epoxy, ultimately taking on the shape of the form contours.
5 days, many BF of Mahogany and 5 gallons of West System Epoxy later, each key veneer core had been made up. All the contours from the field templates had been replicated in mahogany. All that was left now was to build out the crowns to their full 9 1/4” profile, right? Yah, right!
“Let’s git’ er done !!!”
So now all the key laminated cores are finished. In the meantime, Mr. CNC has been busy running the same program that created the glue-up contour forms — only this time cutting 2’ 1/2” thick solid mahogany. The CNC cut all the additional build out for each of the 35 crown sections. The new mahogany was then glued up to each of their respective key lams and milled one build up at a time.
Now for the genius of the key laminated core piece. Remember the crown profile and how it had been cut up into parts to be milled in multiple passes? To the bottom of each key lam is glued a separate piece of rough stock cut from the CNC. At the shaper, the next machining step, a profile knife is mounted that will mill the newly added mahogany. Mounted above the knife is a guide bearing that will ride along the key laminated core. The bearing directs the cut to follow the exact contour of the key lam. After this milling, the next piece of solid mahogany is added to the bottom of this growing profile in the same manner, and the next milling knife and guide bearing are called to duty. Just as in the previous milling, the guide bearing, an 8 1/4” diameter bearing, is positioned to ride along the key laminated core while its knife mills the lower profile. The last two profiles, the end cap features, are milled separately and glued to their respective ends and, “Shazzam, we got crown!!!!!” The epitome of awesomeness!