This pattern doesn’t hold true with all drill bit sizes, though, even among those with similar patterns. However, going from 28 to 32 is only a 1/7th increase in thread count, so these are close enough that you can use the same drill bit size. Going from 20 to 28 threads is about a 40% increase, so that’s why you see a difference in size. The finest thread pitch in ¼” taps sees more use than some of the less-common screw options, and it’s why the 7/32″ and #1 are among the most useful drill bit sizes to keep in your toolbox. What Size Drill Bit Do You Use for a ¼” 32 Tap?įor a 32-pitch situation, use a 7/32″ bit for 75% thread or the #1 for 50% thread. This means that while the 28 tap is technically the middle of the default range, it’s not as popular in projects as the 20 tap is. However, as I mentioned above, it’s actually the coarsest threads that people use most. It feels like a good baseline to start measuring things from, so it is. ¼” taps are an industry standard for many reasons, but part of it is that it’s simply easier to remember than something like 7/16″. In many ways, a ¼” 28 tap at 50% thread is the default for the entire industry, which makes it stand out a little from almost all other thread sizes. The best size of drill bit for a 28-thread project is the #3 for 75% thread, or the #1 for 50% thread. What Size Drill Bit Do You Use for a ¼” 28 Tap? Getting a little more precision when working with hard materials isn’t a bad thing. I’ve seen some people suggest a 13/64″ bit for the 50% thread instead, not the 1-64 tap with drill bit size #53 and that also works. The best size drill bit for a 1/4″ 20 tap is a #7 if you’re using 75% thread or a 7/32″ if you’re using 50% thread. What Size Drill Bit Do You Use for a ¼” 20 Tap? The best sizes of drill bits to use for a ¼” tap are the #7 (#8 drill bit in a pinch), the #3, a 7/32″, or a #1. Instead, always refer to charts until you’ve memorized the drill bit sizes for each tap size. The fact that changes don’t have a perfect curve means there’s no formula for guessing which size you should use. Instead, they follow an uneven curve, with noticeable bumps around the #55, #30, #3, and N sizes. Factors like overall width play into how much drill bit sizes change when you adjust the thread count or move between hard and soft materials, and it’s not quite a linear progression.Īs you can see on this chart of US number and letter drill sizes, changes aren’t linear. 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. 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.
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