With a 5/16″ tap, the right drill bit is essential. This tap comes in two sizes, 18 or 24 threads per inch, each needing a specific drill bit. As a seasoned DIYer, I’ve navigated this conundrum and I’m here to guide you. Dive in to discover the perfect drill bit for your 5/16″ tap.
Figuring out what size drill bit you need for a 5/16″ tap starts with understanding a bit about sizing differences. When I talk about the 5/16″ tap, keep in mind that there are two measurement variations; an 18 tap and a 24 tap.
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While the two are 5/16″ taps, they have different measurements relating to their length and thread pitch. These varying measurements lead the taps to require other-sized drill bits.
To figure out what size drill bit you need for your 5/16 tap, look at the tap’s specific length or thread pitch. From there, you can determine the appropriate drill bit you’ll need.
How Tap Sizing Works for 5/16″ Tap
Tap sizing depends on several variables, including the tap’s diameter, desired full thread percentage, the thread pitch, and the drilled hole size. For a 5/16″ tap, either an 18 or 24 inch, the diameter remains the same, but the thread pitch differs.
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The full thread percentage doesn’t matter whether you have a 5/16″ tap or an 8-32 inch tap because it depends on your desired outcome. Typically the whole thread percentage ranges between 70 and 85 percent, but most users and professionals recommend 80%.
When you have the specific measurements for the one you plan to use or own, you can calculate the needed drill bit size.
For instance, the diameter for both the 18 and 24 5/16″ taps is 5/16″ or .3125. Once you have the diameter, you can insert it into the formula DH=DMB -.0130(% of full thread desired/Ni). This formula looks a bit complicated, but don’t worry.
- The DMB is the diameter measurement you figured out above, .3125.
- The full thread percentage depends on what size thread you want, typically between 70 and 85, but I’ll use 75 for this example.
- The other variable, Ni, is the other part of the tap. Let’s use the 5/16″-18 tap, for instance. The Ni for that tap is 18.
Insert the numbers for the 5/16″-18 tap into the equation to look something like this:
DH=.3125 – .0130(75/18)
Then, solve for DH. You should get around .2583, which you then round up to the nearest drill bit size. In this case, the most relative drill bit size corresponding to the 5/16″-18 tap is 17/64, which corresponds to an F drill bit.
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You can find the relative drill bit size with some simple math as well. For example, I found 17/64 as the closest bit in relation to the 5/16″-18 (DH=.2583) tap because, when divided, 17/64 is only a couple of thousands away.
The number of threads per inch influences the length of the tap. The 18 inch tap isn’t capable of producing as many threads as the 24 inch tap and is, therefore, shorter in comparison. Because of this length difference between the two taps, they will need different drill bit sizes even though they have the same size diameter.
Their lengths directly influence the calculations since thread pitch is a component of calculating the needed drill bit size. If you ignore the thread pitch component for the 18 and 24 inch taps, you won’t get a drill bit that fits, which will not only prevent the device from working correctly but won’t work at all.
To save you the trouble of going through the sizing formula, a 5/16″-18 tap, you’ll need an F drill bit because it most closely matches up to that tap’s drilled hole size measured in inches-the DH I solved for in the previous calculations.
Based-off that formula, the 5/16″-24 tap will require an I drill bit. Just like for the 5/16″ 18inch tap, the I drill bit’s measurements most closely relate to the 5/16″-24 tap’s DH.
Get Drilling with a 5/16″ Tap
Now that you know the fundamentals behind the 5/16″ tap measurements and drill bit sizing, you can put your knowledge to work and get to work using it. Just make sure you’re aware of what specific size you have-either the 5/16-18 inch or the 5/16-24 inch-as both have different compatible drill bit sizes. Once you’ve got that squared away, go on and get to work.
Source: https://t-tees.com
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