Sunday, 19 June 2011

Calibrating the XAML to Drill application

We've added a calibration PDF to the xaml2drill files posted earlier. We used this to work out what our "scaling" value should be, when converting xaml into g-code.

It turns out it's pretty simple (and obvious) but here's what we did anyway:
In ExpressPCB we placed a number of pads in a small square



Print top copper layer to a PDF and open in Inkscape, then save as .xaml
Load the .xaml into our VB app and set the scaling to one
(so we can see the exact output from the .xaml before it is modified)

The resulting g-code:


G0 Z0
G0 X0 Y0
G0 Z2
G0 Z0
G0 X0 Y60
G0 Z2
G0 Z0
G0 X0 Y120
G0 Z2
G0 Z0
G0 X0 Y180
G0 Z2
G0 Z0
G0 X180 Y180
G0 Z2
G0 Z0
G0 X120 Y120
G0 Z2
G0 Z0
G0 X60 Y60
G0 Z2
G0 Z0
G0 X60 Y0
G0 Z2
G0 Z0


Just by looking at these values, we can see that in our conversion, a value of 60 is the same as 2.54mm. Or, more simply, 0.1". This means that to convert our .xaml into g-code that uses inches as units, we need to set the scaling to 600 (60 divided by 600 = 0.1)

This suddenly seems quite obvious. If our images are drawn at 600dpi, it makes sense that we should set the scaling to 600 to get from screen pixels to inches!

From this simple test we concluded:
To convert the .xaml to g-code in inches, scaling = 600
Since 1 inch = 2.54mm, to convert inches to mm we should multiply by 2.54
So to convert .xaml to g-code in mm, scaling = (600/2.54) = 236.2204724409449

How accurate you want to be when scaling is a matter of how accurate your CNC machine cuts. As far as we're concerned, deviation of up to 0.3mm per hole is still quite tolerable, so we use scaling 236.22 for millimetres and 600 if we want the g-code in inches.