Manufacturers and professional installers use metric tap drill charts to tell which size drills and taps to use in combination to create female screw guides. This allows homeowners or other workers to later add screws and complete construction of predrilled items.
Metric tap drill charts rely on the metric system, and users can compare the diameter of the drill bit in millimeters to the chart to determine which size of tap creates female screw guides for that type of hole. Users then drill a hole of the appropriate length and insert the tap to begin creating the guides. Twisting the tap carves out guides of the desired size, ensuring a snug and secure fit for the treads of screws. Drill charts work equally well with both metal and wooden components, and the guides created using taps along with the charts work for both machine screws and wood screws.
Imperial tap drill charts are also available. These allow users to compare drill diameters in inches instead of millimeters and fulfill largely the same purpose. Most guides cover drill bits up to 36.5 millimeters, or 1.44 inches, in diameter. Tread depths may vary by chart, making it important for woodworkers or metalworkers to know what depth they desire to ensure they use the right charts.