Electronics Production¶
Group assignment: - Characterize the design rules for your in-house PCB production process - Submit a PCB design to a board house
The Monofab SRM-20 Milling Machine¶
The Roland DGSHAPE MonoFab SRM-20 (SRM stands for Subtractive Rapid Manufacturing and 20 refers to its model number in the series) is a compact, desktop 3-axis CNC milling machine designed for rapid prototyping, education, and small-scale manufacturing. It’s the machine we use for all our PCB milling in the lab. It’s one of the most commonly found machines in Fab Labs around the world, since it’s reliable, easy to use, and doesn’t take up much space on a workbench. The machine is capable of milling a wide variety of materials including wood, foam, acrylic, and soft metals, but for our Fab Academy assignments we’ll mainly be using it with modeling wax and copper-clad PCB boards. Despite its small size, it delivers consistent and accurate results, which makes it a great fit for producing circuit boards in-house. You can learn more about it here.
SRM-20 Desktop Milling Machine Technical Specifications¶
Before jumping into how we used the SRM-20, it helps to understand what it’s capable of. Knowing the machine’s specifications lets us plan our work properly, from choosing the right materials and tools to making sure the setup is safe and the results are accurate. Things like cutting area, spindle speed, and resolution are all important in how we approach each job. Understanding these details also helps us avoid mistakes, like trying to cut something outside the machine’s limits or using the wrong settings for a particular material. We got our information from here.
| Specification | Details |
|---|---|
| Model | SRM-20 |
| Cuttable Material | Modelling Wax, Chemical Wood, Foam, Acrylic, Polyacetate, ABS, PCB (Printed Circuit Board) |
| X, Y, and Z Operation Strokes | 203.2 (X) x 152.4 (Y) x 60.5 (Z) mm |
| Workpiece Table Size | 232.2 (X) x 156.6 (Y) mm |
| Distance From Collet Tip to Table | Max. 130.75 mm (5.15 in) |
| Loadable Workpiece Weight | 2 kg (4.4 lb) |
| X-, Y-, and Z-Axis Drive System | Stepping Motor |
| Operating Speed | 6 - 1800 mm/min (0.24 - 70.87 inch/min) |
| Software Resolution | 0.01 mm/step (RML-1), 0.001 mm/step (NC code), 0.000039 inches/step (RML-1 or NC code) |
| Mechanical Resolution | 0.000998594 mm/step (0.0000393 inches/step) |
| Spindle Motor | DC Motor Type 380 |
| Spindle Rotation Speed | Adjustable 3000 - 7000 rpm |
| Cutting Tool Chuck | Collet Method |
| Interface | USB |
| Control Command Sets | RML-1, NC code |
| Power Requirements | Machine: DC 24V, 2.5A / AC Adapter: AC 100-240V ±10%, 50/60 Hz |
| Power Consumption | Approx. 50 W |
| Operating Noise | During operation: ≤ 65 dB (A) / During standby: ≤ 45 dB (A) |
| External Dimensions | 451.0 (W) x 426.6 (D) x 426.2 (H) mm |
| Weight | 19.6 kg (43.2 lb) |
| Installation Environment | Temperature: 5 to 40°C (41 to 104°F) / Humidity: 35 to 80% (no condensation) |
| Included Items | USB cable, AC adapter, Power cable, Cutting tool, Collet, Set screw, Spanners (7, 10 mm), Hexagonal wrenches (2, 3 mm), Positioning pins, Double-sided tape, Start-up guidance card |
Characterizing Design Rules¶
We used the characterized files from Prof. Neil to guide our design rules.
Preparing Files in Inkscape¶
Before importing into MODS, we need to convert our PCB exports into the correct file format.
- Open Inkscape and import your exported file by going to File > Import and selecting your file
- Once imported, the image may appear small on the canvas,to fix this, go to File > Document Properties and click Resize to Content to make the canvas fit the image properly, or press Ctrl + Shift + R. Do not resize the actual design itself, as the dimensions are already accurately set from the PCB export
- Make sure the background is not transparent, in Document Properties, set the background color to white so the image renders correctly in MODS
- Do this separately for your F.Cu (front copper layer) and Edge.Cuts (board outline) files, since each one needs to be exported as its own individual file
- Once everything looks correct, go to File > Export and set the format to PNG
- Before exporting, make sure the DPI is set to 1000 for high resolution output.This ensures the toolpaths generated in MODS are precise and accurate
- Save each file separately and make sure they are clearly named so you don’t mix them up when importing into MODS
Generating RML Files¶
To generate the cut files for our machine, we used MODS, which is a browser-based CAM (Computer Aided Manufacturing) tool that converts board files into the RML format that the SRM-20 understands and can work with.
- Open your browser and go to the MODS website
- Click on Programs
- A list of available machine programs will appear look for and select Roland SRM-20 mill 2D PCB
- This loads the correct workflow specifically set up for milling PCBs with the SRM-20
- Once the program is loaded, you’ll see a set of connected modules appear on the screen
- Click Select PNG (or Select SVG if your file is an SVG) to upload your board file
- Make sure your file is a properly exported PNG or SVG from your PCB design software before this step, as the quality and resolution of the image directly affects how accurate your toolpaths will be
- After importing, you will need to invert the image, this is an important step because MODS interprets black areas as regions to preserve and white areas as regions to mill away. Without inverting, the machine would cut away your traces instead of the surrounding copper
- Next, select Mill traces (1/64) since we are using a 1/64 inch endmill for milling the copper traces
- Set all axes (X, Y, and Z to 0),this defines the origin point for the milling job in MODS. This step is easy to overlook but very important,even if you have already zeroed the axes in VPanel, the machine will still not reference the correct starting point if the origin isn’t also set to 0 in MODS. The coordinates in MODS and the machine need to match.So always make sure the axes in MODS are zeroed correctly before running the job as well
- You will notice two unnecessary modules on the canvas (these are on/off toggle modules that are not needed for our workflow). Remove them by clicking on them and selecting Delete from the top panel
- Once removed, click on Modules from the top right side of the toolbar and select Add Module, then navigate to Files and click Save. Connect this new module to the Roland SRM-20 output node so that once Calculate is clicked, the RML file is properly exported and saved to your computer
- Finally, click Calculate : MODS will process the toolpaths and generate your RML file, which is now ready to be sent to the SRM-20
For Edge Cuts, the process is exactly the same, except in the tool selection step, choose Mill outline (1/32) instead. We use a larger 1/32 inch endmill for edge cuts compared to the 1/64 inch used for traces, as cutting through the full board outline requires a bigger and more robust tool.
Milling the Board¶
Preparing the Milling Bed¶
- This was our inventory:
- Allen Key (Hex Wrench) : A small L shaped tool used to tighten and loosen the set screw that holds the endmill in place inside the collet of the SRM-20.
- 1/64 inch Endmill :A very fine cutting bit used for milling copper traces on the PCB. It’s small and precise, making it ideal for cutting thin traces and fine details on the board.
- 1/32 inch Endmill : A slightly larger cutting bit used for cutting the board outline (Edge Cuts). It’s bigger and more capable of cutting through the full thickness of the board without breaking.
- Tape a sacrificial board onto the milling bed to keep it secure
- Use double sided tape to firmly attach the material you want to mill onto the bed
- Make sure the material is properly secured and sticks well before starting the milling process. We used double-sided tape to secure the copper board onto the sacrificial board to prevent it from moving during milling and to ensure cleaner and more accurate cuts.
Setting Up the Sacrificial Board¶
- Setting up a flat and properly aligned sacrificial board is important for the milling process
- It helps ensure that the cuts are even and accurate
- This prevents problems such as uneven surfaces or inaccuracies in the final product
Selecting and Installing the Bit¶
- Choose the appropriate bit size for the milling process
- Secure the bit in the collet using an Allen wrench
- For tracing, we used the 1/64 inch bit
Setting Up the Machine in VPanel¶
- Open VPanel on the computer connected to the SRM-20
- Make sure the machine is powered on and connected via USB
- Place your copper clad board on the sacrificial board and secure it firmly using double sided tape to prevent any movement during milling
- Using the VPanel controls, manually jog the spindle to your desired X and Y starting position on the board, this is where your milling will begin
- Once you are happy with the X and Y position, click Set X Y in VPanel to zero those axes
- Next, carefully lower the spindle along the Z axis until the tip of the endmill is just barely touching the surface of the copper board — this is commonly known as the Gravity Method, where you loosen the collet slightly and let the endmill slide down gently under its own weight until it rests on the board surface, then tighten the collet back. This ensures the Z axis is set exactly at the board surface without applying too much pressure and damaging the endmill or the board
- Once the endmill is touching the surface, click Set Z in VPanel to zero the Z axis
- Make sure all three axes are zeroed before proceeding,this is your origin point and everything will be milled relative to this position
Loading and Running the File¶
- Go back to MODS and make sure your RML file has been calculated and saved to your computer
- In VPanel, click Cut to open the file dialog
- Locate and select your RML file for the traces.
- Click Output to send the file to the machine
- The SRM-20 will start milling automatically.Do not touch the machine or move anything while it is running
- Once the traces are done, repeat the same process for the Edge Cuts RML file to cut out the board outline
Milling¶
- Once both jobs are complete, carefully remove the board from the sacrificial board and clean off any remaining copper dust.
Results¶
Conclusion¶
Our board came out well on the first try and we concluded that the 0.020 inch range is the minimum required for proper visibility and clean milling of both trace width and spacing. Anything below this range, particularly under 0.015 inch (1/64”), risks the traces being too narrow or too close together, which can lead to incomplete cuts or shorts on the board. We will therefore make sure to stay within this range when designing our circuit boards going forward.