Metalworking: Making Square Holes in Metal

Metalworking: Making Square Holes in Metal
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This is one of those posts in which I expect an expert to rip me top shreds over my terrible technique.  I would deserve it as this is a precision machining technique that I butcher with hand tools and a total lack of precision.

I am going to show you a rough approximation of the machinist technique called broaching.  You broach in order to make square holes in metal.  Its not limited to square holes either, you can make all manner of shapes.  This is most commonly done in making key-ways, but also the magazine well of an AR-15 is commonly made by broaching.

According to Wikipedia:

Broaches are shaped similar to a saw, except the height of the teeth increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one for finishing. Broaching is an unusual machining process because it has the feed built into the tool. The profile of the machined surface is always the inverse of the profile of the broach. The rise per tooth (RPT), also known as the step or feed per tooth, determines the amount of material removed and the size of the chip. The broach can be moved relative to the workpiece or vice versa. Because all of the features are built into the broach no complex motion or skilled labor is required to use it.

Making a Broach:

  • Take a square length of tool steel that is the diameter of the desired hole.

    Cutting Square Holes in Metal
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  • Remove material to taper the ends – the length of the taper is longer depending on how large the hole is to be, the depth of the material to be cut, and the hardness of the metal to be cut.
  • Cut teeth in the bar stock – start with shallow teeth,  get deeper teeth the closer you are to a finished cut.  continue the teeth as you get to the finished dimensions.
  • Harden and the tool.  I find case hardening compound, like the type pictured really helps.

Using the Broach:

    • Drill a hole in the metal, close to but smaller than the finished hole size.
    • Insert the broach and ensure it lines up at 90 degrees from the workpeace or the hole will be crooked.
    • Use a hysraulic press to gradually and smoothly increase pressure until the broach is forced through the hole cutting material and forming a square
    • Use a good amount of cutting fluid as you perform this operation and wear safety gear as there is a lot of stress on the tool.

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