Week 7: CNC machining

This week we followed the Computer-Controlled Machining class (see video here)

This page is about the individual assignment.

Test part (group assignment)

Design and milling settings

This week I missed the initial group activity. “Fear not!”, with the precious help from Nico I repeated the process building a test part.

I designed my test part in Solidworks and built different sections to test:

  • slots to check tolerances
  • thin elements to check the part resistance
  • curves with different radiuses
  • manually designed dog-bones

Screenshot-00-Test-part-solidworks

We are using RhinoCAM for generating the toolpath, a ShopBot machine for our production process and a plywood material.

For RhinoCAM some very good video tutorials are available. The process is usually made by few steps.

Setting up the stock dimensions and coordinates origin.

Screenshot-02-Stock-settings

Screenshot-01-Make-flat

We place our design inside of the stock. Often a 2D simplified view is good enough for the toolpath of flat objects. We create fixing points on our stock to use were we will put screws (or other fixing tools).

We create and configure milling processes. In this example we have “Engraving” for the fixing screws and dog-bones holes, “Pocketing” for pockets of different depths, “Profiling” for the external outline cut.

For all processes there are common settings:

  • “Control Geometry”: the path that the tool should follow as reference,
  • “Tool”: in our case a 6mm mill),
  • “Feeds & Speeds”: we load from the defaults from the tool settings,
  • “Sorting”: usually “Minimum distance” sort is good enough and does the work fast,
  • “Entry/Exit”: the “None” strategy is good enough for plywood”, for harder materials a Line/Arc setting could be better.
  • “Clearance Plane”: Usually automatic is good enough
  • “Cut/Parameters”: “Global Tolerance” should be a low value eg. 0.0001, there are then various settings to check base on the process (see. below). Following the diagram and the help panels in the interface helps understanding each parameter.

Screenshot-05-Test-Engraving

In the example above to “Engrave” the dog-bones through the wood board we set “Total Cut Depth”=15.3mm, the board thickness was measured 15.2mm. With “Rough Depth/Cut” to 5.1mm, which is the depth of each tool pass.

Screenshot-06-Pockets-params1 Screenshot-07-Pockets-params2

In the example above we use the “2 1/2 Axis Pocketing” process to cut a 10mm pocket. Some good parameters include increasing the “Stepover Distance” to 60% “Tool Diameter”, “Stock” to 0. Usually “Conventional (Up cut)” is better for a faster process, instead use “Climb (Down Cut)” for a better surface finishing at the top layer (that is good for pockets). Finally is important that in “Cut Levels” we use a “Total Cut Depth” of 10mm because that is the depth of our pocket.

Screenshot-08-Profile

In the example above for the last proces we make the parts outline using “2 1/2 Axis Profiling”. Here is useful to choose manually the “Cut start side” (especially if you transformed your part into a 2D flat object).
Specific settings for this process is the “Bridges/Tab” section. process to cut a 10mm pocket. Bridges are small material leftover connections that don’t get cut so that our part stays connected to the stock. Not creating bridges might cause our piece to move around and detach during the mill operations. There is also a manual setting available that uses “Pre-Defined” regions where we can manually place exactly the position of our bridegs.

Finally it is useful to check the proces to use the “Simulate” functionality to see an animated view of the tool path and the resulting part emergin from the stock.

Screenshot-09-Simulation

When we are ready we can finally use the “Post” functionality to generate the ShopBot G-Code files. In this case I’ve been generating two files: one for the placing the screws to fix my plywood board on the sacrificial layer and one to actually cut my test part.

Download Solidworks 3D design (zipped)

Download Rhino file with milling settings (zipped)

Download Shopbot G-Code file for screws (.spb file) Download Shopbot G-Code file for test part (.spb file)

Production

We are ready to produce our piece!

IMPORTANT: It is important to follow safety istructions when using the mill and wear protective glasses.

The process follows the following steps:

  1. place the board
  2. set the “zero” coordinates for X/Y
  3. set the “zero” coordinates for Z
  4. use the first engraving operation to make small screw holes
  5. fix the board with screws
  6. reset the “zero” coordinates for Z (the board might slightly move when we fix the screws)
  7. run our milling process

Below some images to show some additional important informations during this process (not the entire process).

We first place our board on the surface, we can use a meter to put it well perpendicular to the machine edges.

IMG_10_Placing_Board.jpg

To set the X/Y “zero” we use “Cuts -> C3 - Home X Y Axes using Prox switches” from the shopbot interface. The machine will move to the bottom home edge to fine the machine “zero”.

IMG_11_XY_home.jpg.jpg

From there using the keypad arrows we can manually move the “zero” corner of our board and set our operations origin with “Zero Axes”.

IMG_14b_ZeroAxes.jpg.jpg IMG_12_XY_zero_board.jpg.jpg

After this we use “Cuts -> C4 - Zero Z Axes w/Zzero plate”. For this process we need to place the clamp and the conductive plate below the tip of the tool so that the machine can find the bottom limit.

IMG_13_Z-Zero.jpg.jpg IMG_14_Z-Zero-plate.jpg.jpg

Finally we can open the air valve that is used to blow air on the tool and limit dust/heating.

IMG_15_Air_valve.jpg.jpg

After all the settings are in place we are ready to load the ShopBot G-Code file to start the operations. When doing this is important to follow the steps.

  • Load the file
  • Manually chek the spindle speed setting (12000 rpm)
  • Start the spindle with the green button
  • Press OK to start milling!

IMG_16_File_load.jpg.jpg IMG_17_Spindle_Speed.jpg.jpg IMG_18_Start_button.jpg.jpg IMG_19_Start_milling.jpg.jpg

When the process is finished we can remove the part cutting the extra bridges and clean everything behind ourselves.

Here is the learnings from my test! :)

Lessons learned

  1. Manually designed dog-bones in the 3D model are tricky! They should be correctly calibrated for the toolpath to follow them. It is easier to place engraving points and use an engraving process to make holes in those places.
  2. Small wood parts ~5mm are easy to fly away.
  3. Default bridges size created by RhinoCam are too small for wood: at least double the size.
  4. Curved paths are quite slow for the ShopBot to follow, while with the test settings some straght edges where a bit too rough.

Build something big: a simple chair design

For the personal assignment I decided to start with a simple chair that I designed taking inspiration from one of the opendesk designs.

Screenshot-41-Chair

Worth mentioning that in Solidworks I took advantage of simmetry layers and built my objects faster with mirroring.
I also decided to use rounded corners for my chair for better finishing.

Screenshot-43-Chair-Supporto Screenshot-44-Chair-Seat Screenshot-45-Chair-leg

Download Solidworks 3D design (zipped)

The milling settings for this process are very similar to the test part.
The only differences are:

  • we have dog-bones at two different depths (for pockets and cut through)
  • we have an inside profiling operation to cut holes in our shapes

I put extra care in adding dogbones only in areas where the holes could not be visible in the final surface. I also increased the bridges sizes based on the previous test learning.

Screenshot-46-Rhino-Chair.png.jpg

Download Rhino file with milling settings (zipped)

Download Shopbot G-Code file (.spb file)

IMG_50_Sgabello_milling.jpg

Finally, I was a bit worried about the slots tolerances in the side parts and the top pockets. The result after cleaning the part was indeed good.
Only in one place I used a bit of extra sand paper to smooth a surface to fit into the hole!

IMG_51_Sgabello_final.jpg

Lessons learned

  1. A good tolerance for the wood slots fitting is between +0.2mm and +0.4mm above the board thickness. It is also easier to make the slots slightly smaller and eventually enlarging them with a bit of sand paper finishing on the first try.
  2. To have a better surface finishing use “Down Cut” to do a small engraving, then cut through with “Up Cut”

Next steps

I have some extra design ready to build in the future (stay tuned!)

  • Shoe rack design: with more complex and thin surfaces
  • Digital tree design: potentially for a final project

Happy machining! :wave: