1. Mark the square
2. Mark the center
3. Drill a round hole
4. Take a triangle file to open up the corners
This works well, and I use it in metal whenever I want to use a carriage
bolt. This would obviously be too slow for production, though.
Rotary broaching is a great way to make small square holes in precision machined
products. Take a look at this video which shows internal and external forms:
(the video shows hexagons but square holes are made the same way)
Also, see www.buybroaches.com for more information about the tools.
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