1.1. Non-Copper Area:

• Load the Gerber file first, mirror it if necessary but don't create immediately the regular isolation.

Fig. 1.2 : Very first operation

• Instead, under "Non-Copper regions" (figure below), provide a boundary margin that is the distance between the selective isolation area and the edge of the board. Its value is positive for a boundary external to the PCB, negative for an inner one (after milling, a copper strip will remain around the selective milled area Fig. 1.11).

• Here, it is not absolutely necessary but it has been done for illustrating the process.

Fig. 1.3 : In red: the Non-Copper area boundary, stuck to the copper tracks and 1mm far away the PCB edges

• A new object has been created:

Fig. 1.4 : the new geometry object containing the non-copper area

• Save the project.

1.2 Mask:

• On this non-copper geometry, draw the shape of the area to be selectively milled. This is the transparent part of the mask that will cover the rest of the board where the copper will nominally be milled (refer to the isolation milling in the manual). Any operation involving two separate geometry objects i.e two layers and any operation between a geometry object and a Gerber (that's not a geometry object) can't be made.

• A variant consists to merge two geometries: The first one is the "non-copper geometry", the second one is the mask to be applied to the board. In fact this is a workaround to the limitation we were facing to in the above paragraph.
  • Create first a "new geometry" object ("New blank Geometry" button on the figure below) and draw the mask outlines ("Edit" button). Then select the shape and update the geometry, the shape's color turns from blue to red.
  • Go to the project left pane, merge the "new geometry" object with the "xxx.gbl_noncopper" object (Menu "edit", "join geometry").
  • A new geometry object: "Combo" appears in the project left pane. It contains the non-copper area plus the freshly drawn shape.

Fig. 1.5 : Creating a separate geometry for the mask and merging it with the non-copper geometry

• Open the "non-copper" object, turn the plot "off", do the same for "new geometry" object. Keep the "combo" object visible.

NOTE: I don't know where I'm going to go yet with this variant and what its advantages and its drawbacks are but I hope to be able to directly process a mask issued from the CAD software. I also see a possibility to adjust or modify in a more easier way a shape drawn on another layer (geometry object).

• At this point, both the mask and the non-copper area are on the same geometry object.

• Save the project.

1.3 Selective milling preparation

• Under "Paint Area" (figure above), in order to reduce the milling time, select a bit with a rather large tip.
• It should not be very critical if the tool can't go within narrow paths in between some very close tracks. The isolation milling with a thinner bit will do the job after this procedure is completed.
• In this example, we'll take a 0.5mm flat tip 30° V-shaped bit. Refer to this page [2] to deternine the "Tool dia." value for V-shaped engraving bits.
• The tables give a width of 0.527mm for a cutting depth of 50 µm.

• Still under "Paint Area", select "Standard Method", click on "Generate", a message asks to "click inside the desired polygon". Click inside the area where the copper is going to be removed, taking care not to click on a track. The surface is painted except where copper tracks are. In paragraph 1.5, we'll see a comparison between the results obtained with the Standard Method and the seed-based Method.

• A new object "combo_paint" is created. It corresponds to the left view on the below picture. Note that the edges, imported after the painting operation, have been added. From now on, the edges will be represented.

• Save the project.

Fig. 1.8 : The "painting area" result, with the board edges, gerber disabled (left) and with it (right)

• From the "Combo_Paint" object, under "Create CNC Job", generate the CNC job with the same tool as for the painting operation.

• It creates the new object "Combo_paint_cnc":

Fig. 1.9 : "Combo_Paint" object. Last operation before writing the g-code

As Selective Milling and Isolation Milling do not share the same tool, the g-code files shall be generated separately. Then, they may be used separately or merged together but the result shall be edited to affect the right tool to each operation. By default, Flatcam doesn't mention in the g-code files the tool used for milling. It's up to the designer to mention it in the upper box ("Prepend to g_code") of the "CNC Job Object" before writing the g_code file.

1.4 Board Completion:

• Writing the g-code:
  • Nothing spectacular, simple job if it doesn't need to enter into the g-code details.
  • g-code may be attached at the top and the end of the file in order to provide (not exhaustive) handling directives, tool characteristics, machining instructions or commands (see above) or, for instance, instructions for raising the spindle at the beginning and at the end or the machining phase.
• Now, it's time to complete the board:
  • Normal isolation milling. It will run over the whole board, including the already machined area.
  • PTH & NPTH drilling.
  • Edge cutting.
  • Eventually engraving the markings (part values, references, labels, logos, ...).

Fig. 1.10 : Left: Nominal isolation milling. Right: nominal isolation with selective isolation milling added

Fig. 1.11 : Completed board with nominal & enhanced isolation, edges and holes

1.5 Comparison of the "Paint Area" methods:

Fig. 1.12 Method Comparison Table

Fig. 1.13 : Milling paths obtained with the "Standard" method

Fig. 1.15 : Milling paths obtained with the "Seed-Based" method

Fig. 1.15 : Visual comparison. Left: "Standard" method, Right: "Seed-Based" method

It's easy to see that the "Seed-Based" method is resulting in a better aspect than the other method is ending with. However the milling operation is heavily impacted by the number of times the tool has to jump over the tracks. One may logically suppose that on slow CNCs, this will drastically increase the overall machining duration. On to the other hand, the simulation isn't considering the stiffness of the CNC frame nor its inertia that may result in denting effects when the bit stops against a track before jumping over it. These are just concerns that need to be verified on a real board. This document will be updated when the results are known.

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If you think that this tutorial may be improved, feel free to drop me a line. Your message will be welcome. Thanks in advance. youri.margarin@yandex.ru