At Fab Lab Amsterdam we have a Shopbot CNC milling machine.
To test our machine, we created nine 4x4 cm squares, and milled them using different settings for speed and feed.
We decided to set the tool settings to 10 000 rpm and 10 mm/sec, and then changed the settings manually for each square as we were milling.
Width 270
Height 270
End mill: 5 mm.
Path depth: 3.0
Stepover: 2.5
Cut depth: 15.5 mm (outside square)
Cut depth 7.5 (cut through half the material)
We tested:
Speeds: 10 000, 12 000 and 14 000 rpm/min
Feeds: 10, 20 and 30 mm/s
We made a design in Fusion 360. Exported it as a DXF file.
We picked plywood 15mm. We attached it to the plywood to the bed using 5 screws.
Speed: 10000
Feed: 10
Plunge: 20
Machine vector:
Outside right
Direction: climb
Tabs: constant: 4
Length: 6
Thickness: 6
All settings the same as the outside square except:
Machine vector: inside
No tabs
Speed and feed changed for each square. See the image for the values.
No big differences between any of the squares. Bas thought that we could even have milled faster. If we had to pick a favorite, it would be 20/14000, since the edges have less crud on them. 30/12000 looked the worst.
Cuts in the grain of the veneer are cleaner than perpendicular to the grain. So orient your design’s longer edge along the grain.
Runout
10/10000, orientation perpendicular to the grain: 0.0135 (4-3.973=0.027/2)
10/10 000, orientation with grain: 0.0435 (4-3.913=0.087/2)
30/14 000, orientation perpendicular to the grain: 0.014 (4-3.972=0.028/2)
30/14000, orientation with grain: 0.036 (4-3.928=0.072/2)
Alignment: 90 degrees
While Bas was teaching us to use the CNC, we decided to use the direction "Conventional" instead of "Climb" on one of the cutout shapes. What ended up happening is that because the drill bit was spinning in the opposite direction, it actually ended up becoming unscrewed during the milling. The machine made some seriously wrong sounds, then lots of sparks flew out.
The machine was immediately turned off, and the drill bit was stuck in the wood, deep inside it. After we took the wood and bit out, what we ended up seeing was the following image:
The drill bit, being unscrewed, ended up going deeper and deeper into the wood and into the sacrificial layer. The image actually shows a gradual deepening of the trace, from the top left, going and getting deeper and deeper until it completely fell off, which is the big hole in the bottom right.
This was not during the group assignment and not really related to it, but is a very interesting thing that happened, and also the reason why we only used the "Climb" direction in the group assignment.