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A drill square is a specialized drilling aid used primarily by sheet metal technicians and aircraft mechanics to ensure a handheld drill bit enters a surface at a perfect 90-degree angle.

It is an incredibly simple but highly precise tool designed to eliminate “egged” or slanted holes, which can compromise structural integrity in aviation and precision manufacturing.

https://www.sas-canada.com/store-2/drill-square-medium-drill-guide-90-degrees-2/

How a Drill Square Works

• The Design: It is a small block shaped like an “L” or a cube, traditionally machined from hardened tool steel. Precision 90-degree “V-grooves” are cut along the outside edges or faces.
• The Technique: You place the flat base of the square flush against the metal surface you intend to drill. After placing your drill bit point on your intended hole center or center-punch mark, you slide the drill square over until the flutes of the drill bit rest flat inside the V-groove.
• The Result: By keeping the drill bit pressed firmly against the V-groove as you drill, the tool physically forces the drill to remain perfectly vertical, eliminating human alignment errors even in tight or awkward spaces.

Where It Is Used

You will most commonly hear about this tool in aviation maintenance, specifically during structural sheet metal repair and riveting. In aircraft skin repairs, fastener holes must be exactly perpendicular to the skin; a tilted hole can cause a rivet to sit crookedly, resulting in a structural defect.

Specialty tool suppliers like SAS Canada manufacture these for aircraft technicians. Other trades, like woodworkers or DYI’s, may find this tool an invaluable addition to their tool box.