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Category Archives: 6502

Progress on the 6502 computer – Milling PCB’s

While work and other commitments have been sapping a large amount of my time I have been making slow progress with the milling of circuit boards for the 6502 computer project. I have added some pictures of progress below.

I am slowly getting the hang of repeatable and reliable results on the mill. Most trouble stems from the height at which the program starts: Cut too deep and the V cutter wears quickly, the tracks wander a bit (following the weave of the fibreglass mesh in the board) and the over all finish is poor. Cut too shallow and you might run into issues with track isolation.

I seem to have found a sweet spot for starting position and depth of cut specified in the GCode, there are methods for using a probe to set height but I have been using some thin thermal docket paper to test height. I carefully jog the Z axis down until the V cutter just cuts the paper without marking the surface of the copper, I hit the “touch off axis” button to set the zero position and then run the program.

I am still tweaking the design of the boards but I am very close to the final revision. Once this is done I can start milling the final version of the boards and begin construction.

First milled PCB

After much tweaking, fiddling and procrastinating I have finally been able to successfully mill a PCB!

The only minor issue I had was with the drill file created by the PCB-GCode script which would not load correctly into LinuxCNC. Luckily this is a step that I can complete manually using my PCB drill.

The results are excellent, the finish is amazing and I am so very happy I won’t have to mess with PCB etchants and toner transfer ever again!

Mistakes on the mill…

While waiting patiently for the mill to finish running a PCB isolation route job I stepped away for a few seconds to skip a song on my music playlist. As I was fiddling with the music I heard the spindle motor starting to bog down and turned around to see that the cutting tool had started cutting deeper and deeper into the PCB material, the MDF board it was resting on and the PVC mill table…

I quickly hit the emergency stop button, killed power to the spindle motor and raised the Z-axis. At first I thought that the program had decided to plunge into the table but it quickly became apparent that was not the case. The heat of the spindle motor had made it slip down in the motor mount, the Z axis stepper had stayed perfectly still the whole time. I tightened the screws that clamp down on the motor and all seems fine now.

No real damage done to the mill, it’s just a little bit annoying… Ahh well!!

Milled PCB Test

A few pictures of a test run trying out isolation PCB routing. GCode was generated using the PCB-Gcode script for Eagle, using the backplane board from my 6502 project as input.

Part way through the run I noticed that the mill was slightly out of alignment on the X-axis, this was causing shallow track cuts which got progressively worse the further along the X-axis the program went. A quick adjustment of the gantry seems to have fixed this but some further testing is required. I also need to tweak feed rates and cut depths, but the test went very well!

Etching boards – Round 1

My first attempts at etching these boards didn’t go so well. Using the toner transfer method has worked very well for me in the past, but it looks like my current printer doesn’t work as well as the one I used previously.

Below are a few pictures of the backplane boards I tried etching, the first photo based off a slightly older design with narrower traces. While the traces were etched fairly accurately, the surface of the traces has some fairly deep pitting where the toner coverage wasn’t thick enough. The worst of the pitting has eaten through to the fiberglass.

In some places the toner didn’t stick down very well, possibly because the board wasn’t clean enough or the toner hadn’t set properly. Ahh well, looks like more testing is needed.

While it is annoying to have wasted two boards worth of PCB material it is good to iron out these problems now. I have gone through all of the boards and increased trace width and also modified some packages to increase the size of pads and decrease drill size which should help with etching. Now I just need to improve the transfer process!