Registration pins: putting a board back exactly#

A double-sided or tiled job takes the board off the machine and puts it back. The second cut has to line up with the first to a fraction of a millimetre, and the board's own edges can't be trusted for that: they are rarely straight or square. So the board is located by dowel pins fixed in the spoilboard, and holes in the board that drop onto them.

Why two pins of different sizes#

One pin lets the board turn round it. Two pins fix it — but two pins the same size fix it two ways. Turn the board round, or flip it the wrong way, and the holes are still the same distance apart, so the board still drops onto the pins, slid along by however far the holes were from the middle.

Every one of those wrong ways swaps the two holes end for end. Make one pin bigger than the other and the big pin meets the small hole: the board won't go on. So there's only one way it fits, and that's the right one.

A third pin, spaced unevenly, would do the same — but it's one more hole in the middle of the board, where the parts are. Two sizes keeps the pins at the ends.

Why they sit on the flip axis#

Turning a board over about a line leaves every point on that line where it was. Put the pins on the flip axis and the holes drilled on side A are still over the pins after the flip. Anywhere else, the holes would land somewhere new.

Why side B is drawn mirrored, but cut as drawn#

After the flip, the board's left is your right. Side B's sheet shows the board that way, so you draw side B as you'll see it, and its toolpaths are made directly from that. The app never makes side B by mirroring side A's toolpaths: a mirror turns a climb cut into a conventional one.

Why each tile drills the next tile's holes#

On a tiled job the pins stay put and the board moves. Each tile drills the holes the next tile will sit on, before the board moves — so those holes are exactly one tile further along, measured by the machine itself. Slide the board until they drop onto the pins and the next tile carries on precisely where the last one stopped.

The first tile drills its own pair right through into the spoilboard. Those spoilboard holes are where the pins go.

Why the holes are bored, not pecked#

A peck drill makes a hole the size of the bit. Peck both holes with one bit and they come out the same size, which undoes the whole point. Boring helically with an end mill makes each hole its own size.