PCB making - the automation part

The last article covered all the chemical aspects of my PCB manufacturing. This one will briefly cover the computer design and cnc drilling.

First of all, here are a few links:

• board schematic
• board layout
• drill rack data
• excellon drill file (very similar to g-code)
  
• excellon drill tool description
• supplemental library
  

I could not find a suitable object for the large current sensing resistor, so I had to explore how to create one in Eagle. That's the only component in the library file. I think I ended up borrowing some people's ideas, and not doing a very good job overall, but in the end my model does the job reasonably well - it does look like a resistor in the schematic, and it is the correct size component with the right size leads in the board layout. All of these files were generated from eagle. If you want to re-generate the drill files, or generate gerber files, follow this link. I am a novice in the area of PCB design and manuacturing, and that page helped me immensely.

It was not long before I realized that just making a schematic and getting an excellon file is not quite enough. If you have an excellon machine, then this might be all you need, but I did not have an excellon machine, and needed to convert the drill file to g-code. The output from eagle looks a lot like g-code, but differences are significant enough that it is not feasible to adjust the file by hand. One needs a utility to do that. Unfortunately, at that time my internet connection was down for several weeks in a row (significant packet loss + abysmal verizon customer service = no internet), so I could not locate a suitable tool. Lack of internet also meant that I could not even find out any information on how to decipher the format. And to add insult to injury, I am not really all that familiar with g-code programming. So, in essense, I needed to take a file in an unknown format, translate it into another unknown format, and feed that to my mill. And do all of the above without being able to use any sources of information that are normally at my fingertips on the internet! All that was before I knew anything about canned cycles that are great for this kind of task. I remembered only the G00 and G01 commands, and I managed to misinterpret even them! Well, after some trial and error and experimentation I finally came up with this PHP script. You are welcome to look at it, laugh and enjoy total lack of understanding of g-code on my part. But... it works! And that's all that mattered! It'd be trivial to make it accept parameters from a web page instead of the command line, so if you want to give it a shot, please feel free.

In the end, I found that benefits of CNC drilling of any reasonably complex board really outweigh any costs of having to learn how to do it. The time invested into figuring that out is well worth it - the consistency of the resulting board is impressive, repeatability is perfect, there are no missed holes, no errors, no holes drilled to the wrong diameter, no broken drill bits, no sore back, eye strain and inhaling of fiberglass dust (because your nose absolutely has to be within a couple of inches away from the drill bit if you have any hope of seeing where you are drilling the next hole). That, and you can drill in subdued light or totall darkness, if you want to. Why is subdued light so important? Because you can spray boards with photoresist, then drill them, and then use the holes to align the artwork to both sides of the board before exposing to light, like I outlined in one of the earlier articles. That is now my preferred way of making PCB boards. It allows to acheive excellent alignment beteen the traces and the holes.