Complex 3D and 2D Routed Site Model by Danielle, Crissy, and Ellie (Including Drawing Set-Up)
This step by step guide will walk you through how to set up a 3D .STL file and overlay it with a 2D .DXF file to achieve a sloped site base with flat buildings plots.
The CNC router is not a magic machine, it is only as good as the files prepared for it, so I hope this tutorial will help. If you are ever unsure with anything just come find me in the workshop and I will be more than happy to sit down with you and walk you through it.
The first thing to understand is that the 3D file and the 2D file need to line up on top of each other perfectly. Here we are using 3DSMAX/Sketch-up and AutoCAD, but all programs work in similar ways.
The shape of this job is not square and is a strange shape, it is also fairly small. This causes problems when CNCing because the material needs to be secured to the bed, normally the vacuum is strong enough, but when the material is this small ( less than 500mm x 500mm) it can move, to stop this we usually use masking tape around the edge and corners. Therefore the girls decided to build a rectangular box 15mm away from the extents of the site, (15mm so an 8mm drill bit could get round). The corner of this rectangle was at 0,0 on both the 3D and 2D files making it easier to line up both files, it also meant I could put this corner at 0,0 on the router and secure it safely.
Often when cutting 3D files that are not rectangular we build a ‘slither’ into the cut file, this is usually at 2mm as that is enough to maintain the suction of the bed on the material. On this occasion it was better to work with the girls and engineer this into their drawings. The rectangle spoken about above on the 3D drawing now engineered into a ‘slither’. The rectangle is turned into a surface, its Z co-ordinate is not 2mm like I spoke about above but is the lowest point of the slope, (the job was to fit inside another site model, the hole it was to fit in was 4mm deep, therefore the lowest point of the sloped site was 4mm, and so the surface is at 4mm). This can be confusing and is different for every job and as I said above I will be more than happy to see the best approach for your specific file.
The job was cut in 3 stages; the first was to cut the the building plots, the building plots are all to be cut at different heights, so they only go into the sloped site 1mm each at the lowest point. Each depth of area clearance has a different layer, as illustrated in the AutoCAD print screen, the layers are named by how deep they are to be cut from the top of the material. (more help on the process of drawing set up can be found here; https://lsaworkshop.wordpress.com/2014/11/13/introduction-to-cnc-routing-using-2d-dxf-drawings). In this instance the girls over-layed the Cad file on the slope in sketch-up, the z co-ordinate was the thickness of material (15mm), they then moved each building plot down until it vanished below the sloped surface, the distance it had moved from 15mm was the name given to the layer for that area clearance in AutoCad.
After the building plots were cut the second stage is the 3D relief cut. The file is fairly simple as it is simply a slope with a slither at 4mm, but for more help on 3D files there are plenty of examples buried within this blog, or again come see me. The advantage of doing this after the 2D area clearances is the tendency for ply wood to splinter when doing 2D mills, as this is done first the 3D mill then cleans up the surface. If the 3D mill was before the area clearances the surface of the slope would suffer from splintering that would need sanding.
The final stage is to cut out the shape from the rectangle slither, as seen in the AutoCad print screen there is an outside cut, this is then simply cut out with and 8mm bit leaving two taps to keep it secured to the bed.
Then is the small task of making every building from timber in the workshop. This can be a long process but the girls have done a truly fantastic job of it, the slight tonal differences between the two chosen woods illustrates existing and proposed buildings brilliantly.
I’m a big fan.