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Week 7: Computer Controlled Machining

Group Members: Sonam Tshering, Sonam Pema Yangchen, Sonam Pelkeath Lhazin

Group Assignment: • Do your lab’s safety training • Test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine

Safety Training

Our local instructor walked us through the machine’s components, emergency procedures, correct operating procedures, and safe handling of tools and materials before we were allowed to operate the machine.

ShopBot PRS Standard CNC Specifications

The CNC machine at our lab is the ShopBot PRS.

  • Workspace: 2400mm x 1220mm
  • Speed: We used the speed of 10,000 RPM to cut our design
  • Software: ShopBot Control Software for operation
  • Fixture: Cutting material is attached using clamps
  • Setting Origins: We use ShopBot3 software to set the X and Y origins. Proximity sensors are used to detect the edges of the material. For the Z axis, you have to manually set the origin through zeroing — you place a metal zeroing plate on the surface of the material, attach the clip to the end mill, and the machine slowly lowers the bit until it touches the plate to find the exact surface height.

Important Components

Here you can find more details on some of the most important components of the CNC machine.

Emergency Stop Button

  • 🔴 Red button: This is the emergency stop button, which will immediately stop the machine when pressed.
  • 🔵 Blue button: This is the reset button, used right after turning on the machine.
  • 🟢 Green button: This is the start button, which makes the end mill begin rotating. Always press this before cutting — if the end mill isn’t spinning when the machine starts moving, it will snap.

Control Box

The control box is where the machine is turned on and off. It also has a key switch to keep the machine engaged or disengaged — this is a safety feature so that no one accidentally starts the spindle while someone else is nearby. The wrench used for switching the end mill is also stored here.

End Mill

An end mill is the cutting tool used in CNC milling. Unlike a regular drill bit, it can cut both downward (plunging) and sideways (milling), which allows it to carve out shapes and profiles in flat material.

Clamps

Clamps are used to hold the sheet of material flat and secure on the machine bed. If the material moves even slightly during cutting, the whole cut will be off — so clamping it down properly before you start is really important.

Testing: Runout, Alignment & Fixturing

1. Runout

Runout refers to how much the tool wobbles as it spins. Too much runout causes inaccurate cuts, poor surface finish, and the tool wears out faster.

During our safety training session, our instructor showed us how to check for runout on the ShopBot. The process involves:

  1. Securing a dial indicator against the shank of the end mill (not the cutting flutes).
  2. Rotating the spindle slowly by hand and observing the dial indicator reading.
  3. Acceptable runout on the ShopBot PRS is typically under 0.05mm. Values above this mean either a worn collet, debris in the collet, or a bent tool that should be replaced.

Our instructor confirmed the machine’s runout was within acceptable range before we started cutting. We visually verified this during our test cut by observing consistent cut width along the full length of our profile cuts with no visible deviation.

2. Alignment

Alignment makes sure the spindle, tool, and workpiece are all correctly positioned relative to each other before cutting begins.

We verified alignment through the following steps:

  • X/Y Alignment: We used the ShopBot3 software with proximity sensors to detect the edges of the material and set accurate X and Y origins. This makes sure the toolpath is positioned correctly relative to the actual material.
  • Z Alignment: We used the zeroing plate method to set the Z origin precisely at the material surface, making sure we get the correct cut depth throughout.
  • Material Squareness: We positioned the plywood sheet flush against the corner of the machine bed before clamping, so the material edges were parallel to the machine axes. This was visually verified before starting.

After cutting, we measured our test piece and confirmed it matched the design dimensions exactly (see Print-to-Design Comparison below), which validates that our alignment was correct.

3. Fixturing

Clamps securing the plywood to the machine bed

Fixturing refers to how the workpiece is held securely in place during cutting.

We secured a 15mm plywood sheet to the ShopBot bed using clamps at multiple points around the edges of the board. Key steps we followed:

  • Placed clamps close to the cutting area without blocking the toolpath.
  • Verified the material was flat against the bed with no bowing or lifting.
  • Checked all clamps were tight before starting the cut.
  • Added tabs in VCarve to keep cut pieces attached to the sheet until the cut was fully complete, preventing parts from shifting and catching the end mill.

Modeling and Cutting

We kept our cutting design simple by sketching it in Fusion 360 and then exporting it as a DXF file.

For the cut we decided to use a plywood board with a thickness of 15mm. We clamped the wooden piece on top of the ShopBot workspace. We will be using the 6mm end mill with 2 flutes.

Generating the Toolpath

We used the software VCarve to create our toolpath. You can follow along below.

  1. Open VCarve and set up the width and height of your material.

  1. Go to File > Import > Import Vectors and select your .dxf file.

  1. To move your design, select it and click the move icon in the toolbar.

  1. Go to Toolpaths > Profile Toolpath.

  1. Keep the start depth at 0 and set the cut depth equal to the thickness of your material.

  1. Go to Add Tabs to Toolpath > Edit Tabs, which opens the Toolpath Tabs window. Increase the number of tabs to suit your design — we used 2 tabs. Click Add Tabs, then Close. If the lines of a shape aren’t recognized as one closed outline, select all the lines that belong to it and go to Edit > Join Vectors.

  1. Press Close to finish the toolpath settings.

  1. Save the toolpath file.

  1. Open ShopBot and import your toolpath file. Set your X, Y, and Z origins. You’re ready to start cutting.

To set the Z-axis origin, we use the zeroing plate method described above.

This is the simple design we made to test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for our machine:

Here is a definition of the key terms involved in this assignment:

  1. Runout — Measures how much the tool wobbles as it spins. Too much wobble causes inaccurate cuts.
  2. Alignment — Making sure the spindle, tool, and workpiece are all lined up correctly before cutting.
  3. Fixturing — How the workpiece is held in place, using clamps, vises, or a vacuum table.
  4. Speeds & Feeds — Speed (RPM) controls how fast the tool spins; feed rate controls how fast it moves across the material. Both affect cut quality.
  5. Materials — Different materials like wood, plastic, and metal each need specific tools and settings.
  6. Toolpaths — The route the CNC follows to cut: contour (outline), pocketing (removing material), drilling (holes), etc.

Cutting Settings

These are the settings we used to cut our group assignment design:

  • RPM: 10,000
  • End Mill: 6mm, 2 flutes
  • Material Thickness: 15mm
  • Tabs: 2

Here are the results!

The dimensions of our original design:

  • Length: 185 mm
  • Width: 130 mm
  • Diameter: 20 mm

The dimensions of the cut piece:

  • Length: 185 mm
  • Width: 130 mm
  • Diameter: 20 mm

Safety Measures

  • “Look, listen, and smell” during operation — watch for smoke, unusual sounds, or burning smells.
  • Wear PPE (Personal Protective Equipment): safety goggles and ear protection.
  • Never leave the machine running unattended.
  • Keep hands, hair, and clothing away from all moving parts.
  • Tie up long hair before operating the machine.
  • Use suitable feed rates and spindle speeds for the material being cut.
  • Check that the collet and end mill are tight before every cut.
  • Always keep the emergency stop button within reach.
  • Clean the work area and store all tools and bits safely after use.

Conclusion

Through this assignment, we were able to gain hands-on experience using the ShopBot PRS CNC machine — from setting up the workspace to generating toolpaths in VCarve. Understanding key concepts like runout, alignment, fixturing, speeds, and feeds was essential for getting accurate results. The cut went smoothly, and we came away with a much better understanding of how computer-controlled machining works. Our final cut piece matched the design dimensions exactly, confirming that all settings were correct. Overall, we had a great time!