D. Ugolini Project #2: Bridge

My goal in project #2 was to make a 28mm scale bridge like this one from Warlord Games:

The CAD drawing took only about thirty minutes (link here).  That's where the fun ended.

The idea was to cut out the bottom arches (see below), flip it over, and then cut down to the bridge surface and rails.

The original goal was to cut around the bridge with an extra 1/2" on each side, plus tabs sticking out, so that I could flip it over and replace it within the foam frame, with the tabs forcing it to the right spot.  The problem was that the bit is not long enough to cut all the way through 2" foam.  Instead I cut the perimeter halfway down, cut away the excess with the bandsaw, cut the perimeter again into a piece of plywood, and then inserted the foam into the new plywood frame.

Some other fun issues:

• I had to delete practically every line from my drawing, including construction lines, to get the toolpath right for the perimeter cut.  It wanted to follow just about any path other than a simple rectangle.
• The code wouldn't post unless I created a separate setup for the bridge railing/surface cuts.  But then that second setup defaulted to both a different origin point and a different stock size.  It took two failed cuts to find both problems.
• The X-Carve belt slipped at least three times; twice along the y-axis while making perimeter cuts, and once along the x-axis due to the pressure of my hand while changing the router bit.

But the biggest issue of all was that the z-axis was improperly zeroing.  I could barely raise the bit above the 2" foam plus the brass disk for probe zeroing, so when I started the zeroing process, the bit would barely drop, then retract to the rail and bounce against it a couple of times.  What I eventually figured out was that while the z-axis was railing, the Easel software thought it was still retracting.  Then when it got ready to cut, it would lower the router the same amount, which was now nearly a quarter-inch into your material.  Among other things, that's what caused all the gouges as the router moved from one cut to another, as you can see in the third photo above.

I figured this out halfway through my fifth attempt.  The arches were thus too high, but I had just enough material between the arches and the top surface of the bridge that I could still finish.  I decided to manually zero the z-axis to eliminate the problem.  Unfortunately, manually zeroing resets every axis.  So I had to:

• Measure from the corner of the bridge to find the XY zero, and mark it on the wood frame
• Find the zero on the z-axis.
• Move the router away from the bridge.
• Lower it to just above the frame, counting how many steps it took in the process.
• Move the bit until it was just above the marked origin point.
• Move the bit up the same number of steps I just lowered it before.
• Now it's at the origin on all three axes, so manually zero.

And it just worked.  There are a number of extremely thin spots, and one rail split at the end (held together by tape and wood glue).  But I'm already 15 hours into this project, so that'll have to be good enough.

Note that the arches are not centered under the rail.  That happened every time, and does not exist in the CAD file.  I have no idea why that is.