Hole Saw Centering / Enlarging Tool

This is a tool intended to mount on the arbor of a hole saw, inside a larger hole saw, to allow you to drill a larger hole centered on a smaller one. (Normally this is more difficult because most hole saws are designed with a pilot drill bit, expecting you're drilling the first hole, but if you want to enlarge a hole, you may have to freehand it. This tool lets you center the hole saw on the old hole.)

There are threads and holes on the bottom to accommodate the US de facto standard “quick change” hole saw arbors with pins, which keeps the tool firmly in place. (This has been tested to be compatible with Milwaukee and Kobalt arbors, and should be compatible with DeWalt, Harbor Freight Hercules, and Craftsman, among others.) On the other end, there is a pair of holes intended to let you unscrew the tool.

I've uploaded two sizes here: 3" and 4.5", as well as the FreeCAD file so you can create your own: just open up MainSketch and change the circle's diameter to your desired size. I would recommend using the expression “3 in - 2 mm” (or similar, as appropriate to your desired size), to make sure there is a bit of clearance between the tool and the hole you're drilling, but the tolerances are up to you.

If this doesn't meet your needs, you may also wish to look at Shorco's “Hole Saw Hole Enlargement Guides”. His upload came significantly before this one, although after I initially designed these: looks like a classic case of simultaneous ideas, possibly brought on by Thanksgiving week projects. This model has additional locking positions for arbors and finger holes for unscrewing the tool without getting your fingers near the edges of the hole saw, though they take about 25% more filament and come in far fewer pre-made sizes than Shorco's.

Printing and Recommended Settings

• Material: doesn't matter much, PLA is fine.
• Supports: none
• Layer height: 0.25mm
  • It doesn't really matter, but this is a convenient layer height that will generally print fine.
• Infill: 5%, any pattern
  • You don't really need much infill here: there shouldn't be much pressure on it, so save your filament.
• Orientation: with the two finger holes up
  • The part is designed to not need any supports in either direction, but you'll have fewer overhangs in this orientation.

License and Included Files

This model is licensed under the CC BY-SA 4.0 license, but please let me know if you intend to sell copies of it. (You're welcome to do so, I'm just curious.) The FreeCAD file is included, in case you wish to make any modifications or produce different-sized tools.