A Tiny House Round Top Window
Difficulty Level 9
- Table saw
- Ear Plugs or ear muffs
- Eye Protection
- Belt Sander
- Jig, Scroll saw, or Band saw
- Tape Measure
- Small and large pipe/bar clamps
- Cordless drill
- Hand Plane
- Power Planer
- Paint Tray
Materials Needed – Approx $ 350
- 1 -1/8” x 4 x 8 Bendable Plywood
- 6 pcs – ¾” x 2 x 4 MDO Board
- 1 – 5/4 x 8’ x 4’ hardwood
- Insulated Glass – To be ordered after frame is completed
Savings approximately $1,500.00 *
*Labor costs not included
When it comes to making curved window and furniture parts, woodworkers have several options; They can cut or shape the curve from a single, thick piece of wood, or they can steam the part to make it pliable enough for bending, or they can use bendable plywood. Some woodworkers cut a series of thin saw kerfs into the back of a piece of wood to make it bendable. And finally, there is lamination bending. For many applications where the side layers of veneer show, I find that lamination bending—in which thin plies of wood are glued up on a curved form—is often the best method. It uses material efficiently and produces tight curves with little spring back. For curved window jamb parts where only the 1st 1/8” of jamb appears, my preference is using bendable plywood.
Bendable plywood – Description: Plywood with a single face veneer and core plies with all grain running perpendicular to face to allow cross-grain bending. Uses: Used mostly as a substrate for building cabinets, etc., with rounded corners. Sheets with clear face veneers are suitable for furniture. Available Sizes: 1/8″ and 3/8″ are common, though thicker sheets are produced. Sold in 4×8′ sheets. Common Grades: Able to conform to tight radii without splitting or cracking with no need for kerf-bending or steaming. Pros: Flexibility allows radiuses corners, decorative shapes. Cons: Not designed for structural use, quality of face veneer varies greatly. Where to find it: Building-supply stores and hardwood retailers. We pick up our bendable plywood and hardwood from Youngblood Lumber in Minneapolis, MN.
Lamination bending is very economical
The procedure is simple. Glue is applied to all of the plies, and then they are stacked and clamped around a curved form. The piece ends up with the same curve as the form. This technique is a good way to bend woods that feature interesting grain patterns, like curly or bird’s-eye figure. You pay a premium for such wood, so cutting the curved piece from a single thick board can get rather expensive. And that fancy grain doesn’t hold up well to the stresses of steam bending. But by gluing up a stack of plies to make a curved piece, you need only use the figured wood for the show side of the work piece. The inner plies can be any straight-grain wood. You end up with a strong part that looks like a solid piece of expensive wood.
One of the features of tiny houses is special sized round/arched windows; Gothic, arched, triangular, or round tops. To purchase a round top window that fits the size and geometry of a tiny house is expensive at best. Manufacturers do not like making special order windows with round tops that are narrower than 30” and that size is does not work well in gables of tiny houses. The best way around this is to make your own Gothic, arched, or round top windows yourself. That is what we do at Tiny Green Cabins. We make the jambs and then order the glass from a local glass shop. But 1st you need the form for making the jambs. We will be making a round top window that has 24” between the jambs.
A form supports the bend
To control the shape of a bend until the glue dries, you’ll need a bending form. A typical form consists of a solid core with several outer segments. The core, often called the male form, matches the inside shape of the bend, while the outer segments, the female forms, match the outside shape of the bend. Particleboard, plywood and medium-density fiberboard (MDF) all are acceptable materials for making a form.
When clamped to the plies placed on the core, the outer segments serve as clamp pads, distributing the pressure evenly along the full length of the work piece. With this method, all ofthe laminations end up squeezed tightly together, ensuring a good glue bond. The height of the form should equal the width of the laminations, plus a bit extra. Generally, that means you’ll need to face-glue several pieces of MDF.
But before making the form, two curves must be drawn full size on paper: One line represents the inside curve of the lamination; a second line parallel to the first represents the outside curve.
Once the curves have been drawn, tape the paper to a single piece of MDF that’s a few inches wider and longer than the curve. Transfer the inside curve to the MDF using an awl. Punch a hole through the line on the paper and into the MDF every inch or so along the full length of both the inside and outside curves. Remove the paper and connect all of the holes to create a smooth curve. A piece of thin plastic or wood batten comes in handy here as a flexible ruler. To make the process easier, use a few small nails to position the ruler exactly on the curve. Once everything looks okay, use a pencil to scribe a line along the full length of the curve. Then repeat the process for the outside curve.
Next, use the band saw or jig saw to cut just slightly on the waste side of the two lines. You end up with three pieces: one for the core of the form, one for the segments and a waste piece from the middle.
Now sand the curved edge of the core exactly to the line (I use a belt sander on edge). The inside curves on the segment piece are shaped to the line with a half-round file and then smoothed with sandpaper. Use the first layer as a template to scribe the two curves on all of the remaining layers, then cut them out a bit on the waste side, just as you did on the first one. Now you’re ready to trim the edge flush using a router and a flush-trimming bit. For applications like this, I like a bit with a shank-mounted bearing because it’s easier to see that the bearing is staying in contact with the template. Also, it vibrates less than a typical end-mounted flush-trimming bit.
Fasten the template to the first layer with a couple of screws. Then trim it flush using a router. At this point, we need to determine how thick the form or how many plys to make it. We need the form to be 1-1/2” wider than the jamb size we will need. To hold the glass in place, we need window stops and the 1-1/2” excess will become the stops. For our purpose, we need 3-7/8” jambs plus the 1-1/2” or 5-3/8” rips Therefore, the form needs to be 5-/1/2” or 8 layers. Repeat the process for each layer, using the original template to guide the router. Once all of the layers of the form have been cut out, glue and screw the pieces together, taking care to make sure the edges remain perfectly flush. When completed, you’ll have two parts: the core and the segment piece. Now use the band saw to cut the segment piece into several parts. On each segmented part, the edge opposite the curve is cut square to the centerline for better clamp alignment. Cut out about 1⁄4 in. between each of the segments to provide adequate clearance. Next, drill holes for the clamps in the core. Then add a base piece to the core, which will make it easier to clamp the form to a workbench. To complete work on the core, use wide cellophane tape to cover all of the surfaces that might see glue squeeze-out. Without the cellophane, the lamination would likely end up glued to the form.
For tight bends, use thinner plies
With the form completed, you need to determine the number of plys and the width of each ply; we earlier determined that the ply width would be 5-1/2”. A general rule of thumb applies here: As the desired bend gets tighter, the plies must get thinner. When the plies are the correct thickness, spring back becomes inconsequential, and failure due to splits or cracks is rare. Since we are working with 1/8” bendable plywood, and we want a ¾” jamb thickness, we want 6 plys to glue together for the jambs.
Milling the plies — Once the ply thickness is known, you can go ahead and cut all of the plies. First, rip stock to a little wider than the work piece to allow for trimming after the part has been bent. Next, crosscut the stock to length, keeping in mind that it’s generally best to cut the plies several inches longer than the finished part. The lamination will be trimmed to final length after bending.
With the stock cut to rough length and width, it’s ready to be glued and clamped to the form.
You need to work quickly during glue-up
It’s best to glue all of the plies in one operation. Apply glue to both sides of each one except, of course, for the two outside surfaces. The goal is to coat all of the surfaces completely. A pair of lightweight rubber gloves will help keep glue off your hands. You’ll want to work quickly because the glue has a very limited open time. To help speed up the process, I lay all of the plies on plastic wrap before spreading glue on one side of all of the layers at once. Then, except for the two outside pieces, I turn them over, glued side down on the plastic, and coat the other side. It’s tempting to glue the layers just a few at a time, but I always glue the entire oozing bundle at one time no matter how many pieces.
Once the surfaces have been coated with glue, the plies are placed one on top of the other to form a single stack. Make sure the two uncoated surfaces are facing the outside of the stack. Also, you want all of the edges of the stack to be reasonably flush.
Clamp the plies in the form—Now comes the part that’s most exciting—bending the plies. First, though, place the stacked plies, on edge, on the core of the form. Then start adding the clamps and the segments. You have a couple of options when determining the clamping sequence. You can apply the first clamp at the lengthwise midpoint of the lamination and then add clamps as you work toward each end. Or you can start at one end and work toward the opposite end (as shown above). The point is to avoid adding a clamp between two clamps already tightened. Doing so might encourage a bump in the curve. Either way, get the first clamp secured. And as you do, keep the edges of the plies reasonably well aligned. Then add the remaining clamps in sequence. As I work my way along, adding and tightening clamps, I often keep an extra clamp on the plies. This loosely applied clamp, which is about a foot or so from the clamp being tightened, helps encourage the lamination to conform to the bend. Once all of the clamps have been added, take a minute to make sure all of them are tight. When you’re satisfied everything is snugged up properly, it’s just a matter of letting the glue cure. I prefer to let them cure over night before removing the clamps and jambs from the form.
One last point. Big glue-ups like this always come with a certain amount of anxiety. But you can minimize the anxiety level by doing a dry run of the entire clamping process. Smooth the edges—After the glue has dried, remove the work piece from the form. Expect a fair amount of glue squeeze-out along the edges. Because the plies tend to shift a little during glue-up, the edges are going to be less than smooth. So the next step is to flatten, smooth and trim both edges to final width. A hand plane or belt sander makes short work of cleaning the glue, but they aren’t the best tools for getting the two long edges of the work piece straight and parallel. I get one surface as straight as possible and then send the work piece on edge through my my table saw. Depending on how much the work piece is curved, it might have to be steered along the fence and through the blade. Ripping the 2 pieces of½” stops 1st, and setting them aside, I set the fence to rip the jamb. Then using the belt sander and planer, I smooth the edges. The next step is to cut the jambs to length making sure that the overall height of jambs are ½” short of the rough opening.. I like to cut the jambs square so that the jambs cab be shimmed off of the rough sill. Then I mark a line on the inside of the jamb to align the sill to.
The next step is cutting the sill piece of the window. Since we are building a window with inside dimensions of 24”, we want the total length of sill to be 32-1/2” in length. 24” + thickness of jambs ¾ +3/4 = 1-1/2” + dog ear on each side of 3-1/2” which is for the exterior trim to sit on. The dog ear is determined by the width of exterior trim used. The next step is to use the width of the window jamb we are using as the sill width plus 1-1/2 “dog eared depth. We want the sill to slope at 10 degrees so that the water will drain off and not puddle on the sill. Rip one side of the 5/4 board at 10 degrees and then set the fence to 5” and rip the other side at 10 degrees also. Now on the outside underside, you need to run the sill trough the table saw adding a saw kerf about ¼” deep to the sill. This kerf is needed to stop water from wicking back up to the wall and possibly into the walls. Next notch out for the dog ears and we are ready to assemble the window components.
Assembling the window –
Now that we have all the pieces cut, we are ready to assemble the jambs and sill together for the window. Since the sills and jambs have been cut, all we need to do is add some glue to the sill and slide the sill between the jambs making sure we align it with our pencil marks. Screw the jambs to the sill with 3 screws per side. Then cut and install one stop to the jamb. I like large sills on the inside for holding things, so I set the 1st sill in 3” and use brads to hold it in place. Then I use rosin paper and cut to fit it for a template for the glass company. This is the exact size needed, so I inform the glass company to make the glass 1/8” smaller on all sides. It generally takes about 3 weeks for the glass to be ready for pickup. Once the glass is here, I run a bead of caulk around the stop and use a 1/8” rubber spacer at the bottom and slide the glass into place. We like using the aquarium caulk for this. Then another bead of caulk around the edge, then install the final stop. All that is left to do is install the exterior and interior trim, paint and stain as needed and clean the glass after the caulks have cured.