Archive

Archive for June, 2012

New CAN Main Board

I made a few adjustments to the board layout and also to the pcb-gcode setting and milled another circuit board. This one turned out nicely, even with a couple hiccups. First, the board wasn’t taped to the fixture exactly level, so a corner with nothing important didn’t mill through the copper (it was only the board outline). Then a phantom E-stop signal stopped the machine before it was done and had to restart from the beginning (I upped the debounce value to correct this). I thought it was another beverage coaster while watching it mill, but let it finish and it turned out fine. I ran the drill routine with a .035″ bit and everything but the screw terminal block and power jack fit. I was able to make the terminal block fit by sanding down the edges of the pins, but the power jack I’ll just use some wires and plug later.

Freshly milled CAN Main board, version 0.2.

Since the majority of components fit, I soldered this one up to give me an idea of what the next round of changes need to be. The main one is to increase the size of the pads to make it easier to solder and to increase the isolation path around them to prevent bridges to the ground plane. I’ll also have to change the through holes to the right size in the Eagle component library I made. At least the spacings were all right so everything still fit. Another change will be to the text ratio so the thin areas of copper don’t peel off when milled. They probably would have been fine if the second pass wasn’t done, but whatever.

Just needs a power connection and it’s ready to go.

Anyways, the CAN Main board only contains the basic circuit to run a 18F4580 micrcocontroller with the MCP2551 CAN transceiver; power from a 7805, 20MHz crystal oscillator, ICD plug, reset button, CAN status LED and then the rest of the pins are brought out to female headers to plug in a shield. I also included an area for the CAN bus termination resistor and three capacitors with jumpers, and a place to connect the cable shielding to the board’s ground. There is a jumper to bypass the 7805 regulator to use a 5V wall wart directly which are more common these days.

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CNC Milling Circuit Boards

June 1, 2012 18 comments

I finally was able to mill a circuit board using my CNC machine. No more acid etching, but I never had much luck with it anyways. After milling the table and fixture, I was held up by the lack of 1/8″ collet, which Bosch doesn’t make. I bought a nice collet set from Think & Tinker/PreciseBits for a modest price and it seems it was worth it.  I also needed to buy a license for Mach3 to go beyond the 500 line code limit, since the code to etch the board was near 20,000 lines. To produce the G-code from an EagleCAD project, a nice script is available for free, http://www.pcbgcode.org/.  It installs like any other ULP script for EagleCAD and comes with docs to get you going, plus forums and plenty of other websites with tutorials.

First attempt and first scrapped board.

It was neat watching the CNC machine cut the traces and no broken bits or other catastrophes. Unfortunately, I didn’t take the time to properly zero the end of the 45 degree v-bit and cut too deep. This washed out the traces and pads, so it’s scrap. Before I realized this though, I tried fiddling with the settings for the pcb-gcode script and kept milling on the sheet of copper clad board and got a lot more bad traces.

This is only a test.

The correct way to zero the end of the tool was to use a piece of paper between the board and the tool and to jog down at a very low speed (about 3% of max). While the tool is going down, move the paper back and forth till the tip catches it. Now zero the Z-axis in the Offsets tab of Mach3 in one of special G-code offset codes of your choice. Then measure the thickness of the paper with calipers (my piece was .0035″) and subtract it from the Z offset (press the “Save Work Offset” button to bring up a dialog box that allows you to edit the coordinates). This puts the Z-axis 0 coordinate on top of the circuit board without any pressure. The X & Y coordinates are set to where you want to start milling the board, and can be done by eyeballing it, just don’t re-zero the Z-axis. Now the machine is set to mill.

Successfully milled board, for the smart CAN house project.

My settings for pcb-gcode script (not all are included, just what I changed):

Generation Options

Isolation

Default = .0001″

Maximum = .0001″

Step Size = .005″

Etching Tool Size = .007″

Machine

Z Axis

Z High = .5″

Z Up = .1″

Z Down = .002″ (someone posted an excel file in the pcb-gcode forums to calc this, I set mine .001″ higher and it worked)

Drill Depth = .065″ (go a little deeper than the board thickness)

Tool Change

X = 0

Y = 0

Z = .5″ (don’t leave as 0 or the machine will drag the end of the drill bit across the board between tool changes)

Feed Rates

XY = 10″/min

Z = 5″/min

Change any other options you need, then hit “Accept and make my board”. All the files generated will be placed in the project folder with the board. Check out the readme file in the docs folder for explanations, it was helpful. I’ll probably try setting the Maximum Isolation setting to something higher, so the slivers between close traces are removed. I’m sure I’ll update with more progress, or at least to show of stuff I made.