PCB Design rules

When using the Roland SRM-20 for PCB milling, it is important to adhere to specific design rules and parameters to ensure precision and quality. Here are detailed guidelines for feeds, speeds, plunge rate, depth of cut, and tooling:

1. Feeds and Speeds

• Spindle Speed: The SRM-20 typically operates at a spindle speed range of 3,000 to 7,000 RPM. For PCB milling, a common setting is around 6,000 RPM.
• Feed Rate: The recommended feed rate for milling PCBs is typically between 10 to 20 mm/s (600 to 1200 mm/min). Adjust based on material and bit size.
• Plunge Rate:For PCB milling, a plunge rate of 5 to 10 mm/s is suitable to prevent damage to the material and the milling bit.

2. Depth of Cut

• Trace Cutting Depth: typically set to around 0.1 to 0.2 mm. This shallow depth ensures precision without excessive wear on the milling bits.
• Outline Cutting Depth: for cutting through the PCB material, a depth of around 1.6 mm (the standard thickness for most PCB substrates) is used, usually done in multiple passes to avoid stressing the bit.

3. Tooling

• End Mill Size: for general PCB milling, a 0.8 mm or 1.0 mm end mill is commonly used for traces and detailed cuts.
• Engraving Bits: for cutting through the PCB material, a depth of around 1.6 mm (the standard thickness for most PCB substrates) is used, usually done in multiple passes to avoid stressing the bit.
• Tool Change: ensure you have the appropriate collet size for your bits, typically a 3.175 mm (1/8 inch) collet for the SRM-20.

4. General Design Rules

• Trace Width: ensure trace widths are no smaller than the tool diameter. Common practice is to have a minimum trace width of 0.2 mm.
• Isolation Distance: maintain an isolation distance that matches or exceeds the tool diameter to ensure proper separation between traces.
• Pad Sizes & Via Sizes: design pad sizes considering the drill bit diameter. Ensure pads are at least 0.5 mm larger than the hole diameter and ensure vias are large enough to accommodate the drill bit, with a typical minimum diameter of 0.6 mm for vias.

Practical Tips:

• Multiple Passes: for deeper cuts or thick materials, use multiple passes to achieve the desired depth incrementally. This reduces stress on the tool and material.
• Surface Preparation & Cooling: ensure the PCB material is flat and securely fixed to the milling bed to prevent movement and inaccuracies and although not always necessary for PCB milling, using an air blower to clear away debris can improve the milling process and tool life.

Workflow for sending a PCB to a board house

for sending the pcb to the board house, for the most part first we start with design files from CAD software whether it be KICAD or EAGLE in SVG or PNG format with that we used the mods website to generate files readable by the PCB software controller so you can do this by opening the web and right clicking the dots in the top left corner then program- open program - selected the machine which is the SRM 20 Roland - mill 2D PCB program in mods. you opened where i had to load the picture for traces (SVG & PNG), and the mill raster cell was set to:

• tool diameter: 0.3962 mm


• cut depth and max depth: 0.1016 mm


• Offset number: 2.5

Set-up Milling machine and Milling PCB

In FabLab Rwanda we have SRM-20 monofab milling machine

So for setting up the machine, i had the copper as the material to produce my pcb, a double sided tape to hold it on the bed of the machine and my v bits and Endmill tools to cutout the pcb. I proceeded with putting double sided tape on the copper material and laying it on the bed and then putting the tool(v bit) in the collet of the machine.

Setting up the controller of the machine

the machine has a controller, the Vpanel roland SRM-20 so we opened it to load the files generated by the modsprojects.org so we started by the testing files in order to identify and pick the the suitable traces size to use once we get to pcb design but also identify how efficient the vbit tool to be used can do traces. And so at first when you open it (first picture) the interface is simply contains coordinate system, tool movement buttons, curse step for moving fast or slow the axis, move to origin, set origin point, adjusting speed and other four buttons for set up (for machine configuration), cut(loading files), pause and cancel for stoping/cancelling the work. So when the software is opened the machine moves to the machine origin position, but its still has the origin of the last work to check this, click on XY button in the move section under TO ORIGIN and it goes to the origin of the last work done on the machine, you eliminate this by moving the axis to the point we want to be our work's origin and when we get there, go to Set Origin Point click on XY button to set xy origin point of our work, For the Z axis, after putting the tool in the collet i used the method explained in class to set the Z axis which is done by lowering the tool with the machine so that it gets nearer the pcb surface and then loosen it up by opening the collet and the tool fall on the surfaces, retighten the collet to hold the tool to that point click the Z button under Set Origin Point and then raise it up a little bit before it starts milling. To load files you click on cut it opens a window and click on deleteall to remove previous work, then click on add should directly take to the generated files, open them and then click on outpout and the machine starts right away.

As you can see we came up with some good result but due to how small the traces are and the path of the tool the pcb got chipped on its right side as we were trying to remove the pcb from the board.