54 lines
2.4 KiB
ReStructuredText
54 lines
2.4 KiB
ReStructuredText
Burn correction
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===============
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The burn correction -- aka kerf -- is integrated into the low level
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commands of Boxes.py. So for the most part developer do not need to
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care about it. Nevertheless they need to understand how it works to
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catch the places the do need to care.
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Burn correction is done by increasing the radius of all outer
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corners. This moves all the straight lines outward by the same
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amount. This has the added benefit of not needing to change the length
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of the straight lines -- making them independent of the adjacent
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angles. An issue arises when it comes to inner corners. If they do
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have a radius reducing it by the burn value does the right thing. But
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for small radii and sharp corners (radius zero) this results in a
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negative values. It turns out flipping over the arc for negative radii
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allows keeping the lengths of the straight lines unchanged. So this is
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what Boxes.py does:
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.. image:: burn.svg
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This results in the straight lines touching the piece. This leads to
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overcuts. They are not as nice as proper dog bones as might be used by
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a dedicated CAM software. But as Boxes.py is meant to be used for laser
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cutting this is deemed acceptable:
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.. image:: overcuts.svg
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Programmer's perspective
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------------------------
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For this to work it is important that outside is drawn in a counter
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clock wise direction while holes are drawn in a clock wise direction.
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:py:meth:`boxes.Boxes.corner` adjusts the radius automatically
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according to **.burn**. This propagates to higher level
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functions. Parts shipped with Boxes.py do take the
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burn out-set into account and execute callbacks at the correct position.
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In case developers move to a feature inside of a part or executing
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callbacks while implementing a part they need to be aware of the burn
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correction. :py:meth:`boxes.Boxes.cc` does correct for the out-set if
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called without an **y** parameter. But if a value is given one has to
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add **self.burn** to compensate. Note that the **x** value typically
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does not have to be corrected as the callbacks are executed from right
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underneath the part.
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A similar approach is necessary when moving to a feature drawn inside
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the part without the use of callbacks. Here you typically have to
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correct for the out-set at the outside of the part and again for in-set
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of the hole one is about to cut. This can be done in **x** or **y**
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direction depending on whether the cut ist started vertical or
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horizontally.
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