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WEEK 7: Computer-Controlled Machining

This week's assignment was about Computer-Controlled Machining or CNC.
As last weeks, we had a group assignment and an individual assignment. For the group assignment, we had to do the lab safety training, and then we had to test some parameters of our CNC like runout, alignment, speeds, feeds, materials and toolpaths.
For the individual assignment we had to design, mill and assemble something big (human scale)

Group assignment:

For the group assignment we worked together, first to design a file to test the machine, then to set the machine up to cut, and finally to do the cutting.
We also attended to an explanation about the operation with the machine and safety training by one of our local instructors.
We also got some training about Rhino CAM, wich is the software used in our lab to make the files for the machine.
The machine we have in FabLab Barcelona:

The file we designed to do the tests had different shapes to test different characteristics of the machine. We designed a couple of combs to test tolerances and adjustments. We also tested different speeds and rpm within the chip load range. The file had also some cuts to test the flexibility in both directions of the board. Pocketings were also tested with a pentagonal piece to test different depths.

Once we had the design, we built all the operations with Rhino CAM, where we setted up parameters such as speeds and cutting depths.
To set up the speeds, we needed to calculate the Feed Rate. We used the following table to see the chipload of the material (plywood)

And then we calculated the Fed Rate wich is: Feed Rate = RPM x number of flutes x chip load.
We tested the following RPM and Feed Rates:

Once we had our gcode file, we went to the machine in order to prepare everything to cut.
The machine has its own specific software, which allows us to move it, set the origin, launch the files and some other controls such as speeds.
The steps we followed to set up the machine and cut were:

  1. Turn on the machine and go to absolute x0,y0
  2. Put the material in the machine
  3. Put the endmill
  4. Set the relative x0,y0 origin of the board
  5. Go to a middle point of the material an set the z automatically with the tool that the machine has
  6. Fix the material to the sacrificial board with screws. We might have a gcode to mill little dots or marks where the screws sholud go
  7. Once we have the screws screwed, we have to set the z again to adjust it better with the board fixed
  8. Then we can launch the files to cut
  9. Once cutted, we can take the screws out and clean everything

The test file after cutting:

Speed tests:

Clearance tests:

Pocket tests:

Flexible tests:

Conclusions:

For the clearance tests, we saw that the tolerances are the opposite as the laser cutter, so here we should to leave a bit of clearance in the joints. Anyways, the wood we were using was quite soft so in the end we were able to fit the joints with no clearance. I would say that for other harder woods 0.2 to 0.3 mm would be better.
Same with the pocketings, they should be a bit deeper than the piece we are inserting.
About the flexible pieces, they worked better perpendicular to the fiber of the wood but in the end they broke.

Individual assignment

For the individual assignment I wanted to design some piece of furniture. I decided to design a table wich can placed in different positions, so that we can change the height in order to use it in different ways.
In the final design, it can be used as a bench, coffee table, regular table or high (counter height) table.

To design it I used Rhinoceros, where I first designed the model in 3D. I started with basic shapes, and then by trial and error I was improving it and adding more detail until the final result. Then I took all the edges and put them into the XY plane in order to make the routine for the CNC machine with RhinoCAM.
For the joints, I made pockets where the other pieces fit. I made them with no tolerance to not allow the pieces to get loose.


First, I decided to test the design in the laser cutter in order to see if it can be built, and also to check the stability and the scale between the different pieces.

The result was satisfiying

To make all the files for the CNC, I used Rhino with RhinoCAM.
We had a 122x244cm plywood board for each one, so I had to fit all my design within it. I also added a couple of passive speakers for my phone in some areas that were not used.

Then I built all the operations in RhinoCAM. There was a tricky part in the design, wich was a pocketing in both sides in two pieces, so I needed to flip the pieces once cutted in one side in order to do the pocketing in the other side. To do that, I had to make a couple of holes that went 15mm into the sacrificial board in order to fix a metal bar wich allowed me to flip the piece in the same point.
The endmill I used has the following parameters:

And these are all the operations created:

Engraving:

I used engraving to mark the holes for the screws.
The feeds and speeds were: 4500mm/min and 18.000RPM, and the cut direction Natural
I set the clearance plane to 20mm to avoid any colission, the cut depth to 3mm (half of the endmill). The entry and exit dind't need any aproach because it was just to mark holes.

Pocketing:

I used pocketing to make the insertions for the joints, and also to make the insertion for the cap of the bench.
The feeds and speeds were: 4500mm/min and 18.000RPM, and the cut direction Natural
I set the clearance plane to 20mm to avoid any colission, the cut depth to 3mm (half of the endmill).
I chose "Offset spiral" for the cut pattern, starting from inside, with a stepover of 40%.
For the aproach I chose along path with a angle of 10 and linear rectracting motion with an angle of 45

Profiling:

I used profiling for all the regular cuts.
The feeds and speeds were: 4500mm/min and 18.000RPM, and the cut direction Natural
I set the clearance plane to 20mm to avoid any colission, the cut depth to 3mm (half of the endmill), and the total depth to 15.1mm.
I chose "Climb" for the cut direction.
For the aproach I chose along path with a angle of 10 and none retract motion. For the Tabs, I set 4 tabs for each piece with 4mm length and 4mm height.

For the machine itself, I followed this procedure:

  1. Turn on the machine and go to absolute x0,y0
  2. Put the material in the machine
  3. Put the endmill
  4. Set the relative x0,y0 origin of the board
  5. Go to a middle point of the material an set the z automatically with the tool that the machine has
  6. Launch the engraving routine to mark the screws
  7. Fix the material to the sacrificial board with the screws
  8. Once we have the screws screwed, we have to set the z again to adjust it better with the board fixed
  9. Launch the pocketing file
  10. Launch the profiling file
  11. Without taking the board, I flipped the pieces that needed to be milled in both sides, and screwed them again
  12. Then I launched the last routine to mill the other side that was the insertion for the cap of the bench
  13. Once cutted, I took the screws and retired the material from the machine

Details of the different pocketings and insertions after sanded:

Then I mounted it with a hammer. The clearance I setted up for the joints was cero, because the wood is quite soft and in this way I obtained a really tight joint imposible to take out.

The final result was a very stable piece of furniture that can be used as a bench, a regular table or a high table. We also tested the operation and it worked really well. Special thanks to Nil Peguero who helped me as a model

Files:

Rhino test file
Rhino file of my design