One of the bird box entrance reducers I installed nigh onto a decade ago is still on duty, although downy woodpeckers definitely want a larger hole:
Another reducer had gone missing over the years, so I made one from a length of PVC pipe:
It started as 1-½ PVC pipe, 1-⅞ inch actual OD and should fit into a 1-½ hole, so I measured 1.5 × 3.15 around the circumference, bandsawed out the excess, draped it over a 1-½ Forstner bit, toasted it with a heat gun, and squashed it so it’s just a little bit bigger than the (enlarged!) hole in the box.
Now the entrance is 1-¼ (-ish), just like it should be:
The bird box in the front yard has been attracting starlings, in addition to serving as a hawk perch:
The oblong hole required advanced manufacturing techniques:
The front face should be too slick for larger birds and the little ones will zip right into the hole:
The two starlings who’d been evaluating the box seem to have moved on; we doubt they’re now homeless.
The OpenSCAD source code as a GitHub Gist:
| // Bird Box - oval entrance reducer | |
| // Ed Nisley KE4ZNU 2020-02-12 | |
| //- Extrusion parameters must match reality! | |
| // Print with 3 shells and 3 solid layers | |
| ThreadThick = 0.25; | |
| ThreadWidth = 0.40; | |
| HoleWindage = 0.2; | |
| function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit); | |
| Protrusion = 0.1; // make holes end cleanly | |
| inch = 25.4; | |
| //---------------------- | |
| // Dimensions | |
| EntranceID = 1.25 * inch; | |
| BoxHole = [1.5,2.25] * inch; | |
| BoxWall = 0.75 * inch; | |
| HoleOC = BoxHole.y - BoxHole.x; | |
| FlangeWidth = 5.0; | |
| FlangeThick = 5*ThreadThick; | |
| $fn = 12*4; | |
| //------------------- | |
| // Build it | |
| difference() { | |
| union() { | |
| linear_extrude(height=BoxWall + FlangeThick) | |
| hull() | |
| for (j=[-1,1]) | |
| translate([0,j*HoleOC/2]) | |
| circle(d=BoxHole.x); | |
| linear_extrude(height=FlangeThick) | |
| hull() | |
| for (j=[-1,1]) | |
| translate([0,j*HoleOC/2]) | |
| circle(d=BoxHole.x + 2*FlangeWidth); | |
| } | |
| translate([0,0,-Protrusion]) | |
| cylinder(d=EntranceID,h=2*BoxWall); | |
| } |
The Smell of Molten Projects in the Morning 
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