Quickstart Guide
1. Kerf bend basics
Kerf bending allows us to bend flat materials (most often plywood) into curves, often these curves are arcs (sections of circles) but with some clever math we can get any continuous shape.
This tecnique has its limitations: the curves must be continuous and the bend radius must be sufficiently large (bends cannot be too sharp).
2. A simple arc bend
In this example we will learn how to create
the curved sides of this cabinet
This is a top view of the side of our cabinet
I want to bend a piece of plywood to form a 90 degree angle to connect two sides of this cabinet. The plywood I'm using is 20mm thick and I will be using my table saw to cut it. My table saw makes cuts with a width of 2.7mm. Let's list the parameters I will need to input:
Tool type: Saw
Cut Width: 2.7 (either measure the cut with calipers or look for the width written on the blade)
Cut Depth: 17 (I got this by multiplying 20×0.85, which is a good starting point)
Curve type: Arc
Arc radius: 150 (this measurement is taken on the uncut side of the curve, rendered in red in the drawing)
Arc angle: 90
Now I press "Generate Kerf Pattern" and a the output appears in two formats:
The cut list: the simplest way to get the output. It's a list of all the cuts that must be made, their distance from the left edge of the workpiece and the side on which to cut. By default (with output_offset=0) these measurements will correspond to the center of the cut, so you will align the center of the blade with the measurement.
The output DXF: for use in CAD or CNC routers, cuts are organized by layer and color to make importing into CAM software easier.
Ideal cut depth suggestion feature:
Each cut bends the board by a given angle (let's call it kerf_angle, if you're curious
the formula is 2×atan(cut_width/ 2×cut_depth)), because of this the angle that the workpiece will bend will always be a multiple of kerf_angle, if arc_angle is not divisible by kerf_angle your arc will end up with a slightly larger or smaller angle span (this software always rounds to the larger one).
The "suggest ideal cut depth" feature helps us deal with this issue. With the values of our example the best possible arc approximation is 90.8° instead of 90, this is close enough but the software suggests we increase the depth to 17.15 in order to be even closer. We run the calculation again with that value and the arc approximation is now 90°.
3. Bending more complex shapes
Let's keep going with the same tool and material parameters as before, we'll now focus on different types of curves:
Bezier curves:
These are simple to use, you will need 3 or more points. With 3 control points this curve is mathematically equivalent to a parabola, which can come in useful. These curves may also have both negative and positive curvature (an S shape for example) which will require you to make cuts on both sides of the board (side A and B).
Splines:
This type of curve also allows for the creation of more complex geometries but I find it harder to use, also there are occasionaly some display errors, enable the 'Display Extra Geometries' option to check for inconsistencies.
Example: bending a parabola (or similar shape using splines)
Let's start with inputting our tool and material parameters as in the previous example. This time we will need to select "Bezier" in the Curve Type section. This will show us 3 default points of the graph, we can change them by dragging the control points on the graph or by typing our coordinates on the left. Clicking on the graph adds new points allowing for more complex curves
Most CAD softwares support bezier curves (sometimes under different names) so I find it useful to draw the curve in CAD then manually transfer the coordinates of the control points.
As outlined in the paragraph below whether our curve points upwards or downwards is important, for simple use cases I suggest you draw your curve pointing up (U shape) and draw the uncut side of your final piece. If you choose to draw it facing down (inverted U) you will need to draw the side with the cuts.
Advanced use case:
Understanding the handling of complex shapes
This software takes curves (which mathematically have no thickness) as input but in the real world we work with materials with thickness (eg. 20mm in our example) this means that the two sides of our finished piece will follow different shapes (eg. in our first example the inner side had a radius of 130 and the other of 150).
In the previous example I simply said to input the measurements of the uncut side of the material. This is a great rule of thumb whenever you are only cutting on side A. If your curve is concave you should always point it upwards (making a U shape) so that the output cuts are all on side A, then draw the uncut side.
If this is all you need to do then you can skip the headache, don't read the next part.
But what if your curve has both upwards and downwards concavity (eg. an S shape)? You want to orient your drawing so the left side has upwards concavity. The shape that you draw will correspond to the side of the piece that on the left side is left uncut.
4. Using tapered ball end mills
To avoid getting those triangular gaps in the workpiece you can use a tapered ball end mill instead of a saw blade to make the cuts. Select tool type "tapered end mill". The cut depth parameter is self explanatory, the angle needs to be the total angle of the endmill, basically the angle of the triangular cut it creates. This measurement is shown as "B" in the picture, measured in degrees. Also try to minimize tip diameter (2R in the picture).