How to bend or curve supawood
Having come across this post on instructables for cutting kerfs in supawood or mdf to bend the board to make a curved or round panel, I just had to share this technqiue. When making your own furniture, not being able to bend supawood or mdf limits the scope of what you can and cannot do, so having an understanding of how to cut kerfs for bending the board is one that I definitely had to share...
The circular bathroom cabinet shown above is made using supawood / mdf. The main unit curves underneath the circular vanity top, and two curved doors allow access to the storage unit underneath the basin.
In order to avoid bumping into sharp corners in this small bathroom, a round bathroom cabinet was made. Shop around and you will find it impossible to find a circular bathroom cabinet, so knowing how to kerf board and bend into curved shapes is a definitely a skill to add to your knowledge base.
For a curved panel you need 2 pieces of 9mm board. Both pieces are kerfed on one side and placed together on a mould with the kerfed sides facing together - see below. In this way you will end up with curved doors that have a smooth surface on both the outside and inside.
Whatever curved project you are making, the sections to be curved should be a bit wider and longer than you need. Start by finding the centre of the pieces; draw a dashed line at this point and make sure it extends onto the sides and back. You will use these lines to align the two sections when gluing together. Working from the centre, draw two solid lines at 5mm from the center line: one to the left and one to the right. The next solid lines are marked at a 10mm interval on both sides. Repeat this until you have covered the length of the half circumference of the outer circle. For this project, 32 lines on the left and right add up to a total of 650mm.
Practice makes perfect
On a piece of 9mm board draw a few lines that are 10mm apart. You will use these lines to find the right depth of the cuts. Adjust the cutting depth of your circular saw or table saw so that the blade will make a cut approximately mm deep. If the cut is not deep enough, it will be difficult to bend and might even break. If it is too deep, the material will be too fragile. Once the blade is set to the right depth make a few test kerfs. Keep any sawdust as you will need it later on to make the glue thicker.
GOOD TO KNOW
When calculating the length you need, keep in mind that the piece that will be placed on the outside will have a bending radius that is 2* 8 = 16mm larger than the piece on the inside. Not being good at calculating curves, my suggestion is to practice this method on some offcut boards. This will give you an idea of how much extra board you need to complete the curve and still have two pieces that are the same size once curved.
The kerfs in the board are filled with a paste made up of sawdust and wood glue. The paste needs to be mixed to a consistency that is easy to spread and fills up the kerfs. Use a putty knife to work the putty into the kerfs, working from the centre along the cuts towards the outsides to prevent air pockets. After filling, dilute some glue with a little water and use a brush to smooth out the grains of the glue / sawdust paste. Repeat for both sheets of 9mm supawood / mdf.
You will need to cut a mould to hold the shape of your cut board.
ABOVE: Here you can see how a box-frame has been assembled.
The edges of the box are curved so that the board can be placed over this while the glue dries.
To make these curved doors, two 9mm sections of supawood / mdf were cut with kerfs, filled with a mixture of wood glue and sawdust and then nailed onto the mould with panel pins. The entire assembly is clamped around the outside to ensure a firm fit onto the mould.
The panel pins aren't fully nailed through and allow for easy removal once the glue has dried. The small holes can later be filled in with wood filler.
ABOVE: Shown here is one curved door panel that was left of the mould until the glue had dried. You can clearly see how the kerfed sections are on the inside, facing each other, so that the outside of the panels are smooth.