That may sound counter-intuitive at first, but it’s the result of experience gained through extensive testing over time. I’ll explain those in just a moment, but the important thing to remember when drilling a ¼” tap is that changing your thread size does not always change your drill bit size. There are four different drill bits suitable for ¼” taps, and which one you’re using depends on your threads per inch and your tap drill size. You do need different drill bits for various ¼” taps. Do Different ¼” Taps Need Different Drill Bits? I can’t list the relative hardness and softness of every material here, so if you need to know, talk to the manufacturer or test your drill on some extra pieces of the material before you begin your project. Wood is generally a soft material because even so-called “hardwoods” are far softer than treated metal. In this context, hard materials include things like hardened steel, cast iron, or alloy steel. Taps can be either 75% ( better for softer materials) or 50% (better for harder materials), so what you’re drilling into can impact your ¼” drill bit size. The second component is tap drill size, which is not the same thing as general drill bit sizing. Fine threads strip easier than coarse ones, though, so they’re not always the next choice. However, higher thread pitches (sometimes called “fine” threads) can be useful if you’re in situations where you need to make precise adjustments or drill into sturdy materials. This is what most instruction manuals assume you’re using unless they specifically state otherwise. In most cases, the lowest (or “coarsest”) thread pitch in a given sizing range is the best choice. For ¼” drill bits, the threading options are 20, 28, or 32 threads per inch. The first component is the threads per inch, which is also known as the thread pitch. * Values are calculated for Length of engagement of 5xPitch.Drill bit sizing is significantly more complicated than it looks to people who are just entering the field, but it’s easy to keep track of once you learn how people size things. For calculations of different engagement length use the Thread Calculator * Values are calculated for Length of engagement of 5xPitch. * Permisable values for thread Class 3A # Description Value (Inch) Value (mm) es Allowance 0 (Ref) 0 (Ref) d Major Diameter 1.2336 - 1.25 31.333 - 31.75 d 2 Pitch Diameter 1.152 - 1.1572 29.261 - 29.393 d 3 Minor Diameter For calculations of different engagement length use the Thread Calculator'
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