PCB-Gcode

[This text in German]

With the schematic and layout editor EAGLE (Easy to Use Graphical Layout Editor), two-sided boards up to the size of half an euroboard - corresponding to 8 x 10 cm - can be created in the freeware version.

The finished board can be ordered from a professional PCB service, but you can also produce the board yourself.

The earlier approach of etching the board in a chemical process never really convinced me, the disposal of the resulting residues in the form of a toxic broth via the pollutant mobile acts as a fun brake.

Since I own a CNC-milling machine and the appropriate software in the form of OpenCNCPilot, I have abandoned the photochemical approach and mill my boards.

The ULP (User Language Program) PCB-Gcode, created by John T. Johnson in 2003, provides initial support in converting the board created in EAGLE into a form of data that can be interpreted by my milling machine. PCB-Gcode converts the polygons of EAGLE into enclosing paths, which are then run by the milling machine - this is called isolation milling. Here is the link to the current version of the ULP. The ULP also works with more recent versions of EAGLE, I work with EAGLE V7.7.

As tool for the creation of the milling paths for the PCB production I usually use so-called V-mills or engraving gravers, I use those with 0.2 mm tip width and a cutting edge angle of 45 or 30 degrees. The simplest and very well suited gravers have a D-shaped cross section and thus one cutting edge. In the ULP you can choose whether engraving should be done with the feed direction ("Always climb") or against the feed direction. In my experience the edges that are cut against the feed are the cleanest, edges that are cut with the feed do not look very nice, usually look a bit frayed.

Unfortunately, the ULP in its original form can create all traces either with or without the option "Always climb", so you have to decide, and it is best to choose "Normal", i.e. not "Always climb", then at least the contours of the traces are clean.

Since I was dissatisfied with this behaviour of the ULP, I analysed the code and with really manageable effort I created the possibility to mill the first contour around the resulting signal trace in the preferred direction, i.e. against the feed, but all further milling paths in the opposite direction, so that now the contours of the remaining copper areas are also cut against the feed. Thus the resulting edges of the copper areas look as good as the signal traces.

The ULP change must be implemented in the "PCB-Gcode.ulp" file starting at line 1288, directly after

// set direction of cut around contour
if (((g_side == TOP) && !CLIMB_MILLING) ||
   ((g_side == BOTTOM)  &&  CLIMB_MILLING)) m_fwd = true; else m_fwd = false;

Here I have inserted the following code:

////////// Harald added 13 Feb 2020
// Provided our tool is rotating clockwise when viewed from the top.
// If we are in the first turn around a trace, we mill it normal (not climbing) since we mill the _inner_ side of the trench,
//  so we move the tool on the right side of the trace counter clock wise around the track.
// If we are in the second or beyond turn, switch to the opposite mode (climbing) since now we mill the _outer_ side of the trench,
//  which results again in "normal" (not climbing) milling for the given side of the trench.
// If the user chose "climbing" in the settings, both cases result in "climbing" milling

if (m_pass_num == 0) m_fwd ; else m_fwd = !m_fwd;
//////////

First a lot of comments, real code is only the last line. Here the first pass (m_pass_num == 0) will keep the direction as set in the PCB-Gcode setup, in all other cases, i.e., if and when additional passes are made to increase the trench between signal trace and remaining copper area, the milling direction will be reversed.

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A further improvement over the original version was suggested by Alexander Arkusha in the end of 2017.

This improvement addresses the problem that in the original version the marks for the holes, called Spot Drills, were drilled with the same depth as the correct holes when the drill file generation is activated, too. Correctly, the spot drills should only mark the surface to prevent the drills from running off when they are placed.

The correction to the code proposed by Alexander is a bit more extensive, the following changes have to be made (I am inserting here the original text by Alexander, which is also quoted exactly the same by John Johnson):

    step 1. ADD FOLLOWING TEXT AT THE END OF "source\pcb-file-utils.h"

    void output_drill_spot_hole(real drill_x, real drill_y, real depth)
    {
        if (SIMPLE_DRILL_CODE) {
            rxy(drill_x, drill_y);
            fzr(depth, FEED_RATE_DRILL_Z);
            rz(DEFAULT_Z_UP);
        }
        else {
            out(frrrr(DRILL_SPOT_HOLE, drill_x, drill_y, depth, FEED_RATE_DRILL_Z));
            update_cur_xy(drill_x, drill_y);
            }
    }
 

   step 2. ADD FOLLOWING TEXT FROM LINE #102 IN "settings\gcode-defaults.h"

    string DRILL_SPOT_HOLE  = RAPID + MOVE_XY + EOL
                            + FEED_MOVE_Z_WITH_RATE + EOL
                            + RAPID + "Z" + real_to_string(DEFAULT_Z_UP) + EOL;
 

    step 3. REPLACE FOLLOWING TEXT BETWEEN LINES ##467-473 IN "PCB-Gcode.ulp"

    if (m_first_spot_drill) {
        output_drill_first_hole(drill_x, drill_y, SPOT_DRILL_DEPTH);
        m_first_spot_drill = NO;
    }
    else {
        output_drill_hole(drill_x, drill_y, SPOT_DRILL_DEPTH);
        }

    ...WITH FOLLOWING TEXT

        output_drill_spot_hole(drill_x, drill_y, SPOT_DRILL_DEPTH);

    step 4. (optional): REMOVE LINE #455 IN "PCB-Gcode.ulp"
        SINCE VARIABLE  m_first_spot_drill BECAME USELESS.

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There are further improvements. For example, there is a patch by Timo Birnschein which splits the milling of the finished PCB into several passes so that the milling cutter does not have to mill the complete thickness of the PCB in one pass. The patch is described on Hackaday.io, the complete code can be found here.

Unfortunately the possibility to generate so called bindings is still missing. They would hold the finished PCB in the remaining PCB material to prevent the milling cutter from tilting. If the raw board is fixed with double-sided adhesive tape, however, it is not necessary to generate those bindings.

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Here the link to the official homepage of PCB-Gcode.

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Translated with www.DeepL.com/Translator (free version - online)