Tapping new threads into metallic materials is unusual. However, it can still happen in your household. When drilling the pilot hole before tapping, the hole should be smaller than the tap but large enough for the tap to cut the edges.
If you’re using the 1/4-28 tap drill size, our team crafted this article to help you with the sizing and thread percentage.
What Size Drill Bit Do I Need for a 1/4-28 Tap?
The right drill bit size for a 1/4″ 28 taps can help you finish your tapping project conveniently. This table contains the screw size, drill size, fractional equivalent, and decimal equivalent in inches and millimeters.
| Screw Size | Drill Size | Closest Fractional | Decimal (in) | Decimal (mm) |
| 1/4-20 NC | #7 | 13/64” | 0.201 | 5.105 |
| 1/4-24 NS | #4 | – | 0.209 | 5.309 |
| 1/4-28 NF | #3 | 7/32” | 0.213 | 5.41 |
| 1/4-32 NEF | 7/32” | 7/32” | 0.219 | 5.558 |
| 1/4-40 NS | #1 | – | 0.228 | 5.791 |
The idea of tapping new threads is simple, but it can get complicated if you have no idea which size to use. The bolt will not fit well with the threads if the pilot hole is too large.
Likewise, if the pilot hole is too small, it will be difficult to screw the bolt into the drilled hole because it will extend too far into the center.
Screw Size | Major Diameter | Thread Per Inch (TPI) | Minor Diameter | 75% Thread (Brass, Aluminum, & Plastic) | 50% Thread (Iron, Steel, & Stainless) | ||
| Drill Size | Dec. Eq. | Drill Size | Dec. Eq. | ||||
| 1/4 | 0.25 | 20 | 0.189 | 7 | 0.201 | 7/32 | 0.219 |
| 28 | 0.206 | 3 | 0.213 | 1 | 0.228 | ||
| 32 | 0.212 | 7/32 | 0.219 | 1 | 0.228 | ||
The only difference between the 50% and 75% is the materials you use. The 75% thread is the standard tap drill for softer or average materials.
Softer materials include plastic, aluminum, and brass. If you are going to drill on harder materials, use 50% thread for a tighter screw or bolt connection.
If you are unsure which thread percentage to use, check the instructions from the manufacturer’s guide for the correct tap drill size. Look at the recommended size of the drill bit on the table above. Alternatively, you can also use a calculator for this.
Other related articles:
What to Consider When Drilling a 1/4" 28 Tap
Once you have the material type, drill bit size, and thread percentage determined, it’s essential to address the drilling process, including pecking and other considerations, when working with a 1/4″ 28 tap.
Drilling Process Explained
After deciding which drill bit size to use, it’s time for the drilling process. At least two clamps can mount the metal into place to reduce the chances of the metal hurting you or moving out of place.
Wear safety gloves and goggles to protect your face and hands as well. Mark the area where you need to make a hole. If you don’t want to leave any marks, use painter’s or masking tape [1]. If the material is too flexible or thin, use a solid backing so it will not be deformed.
You can make a center punch using a hammer to make a spot for the drill bit to rest. When drilling, keep the angle at 90 degrees. Any other angle can cause problems when screwing or tapping the bolt.
To be sure, you can use a guide or drill press if you’re using a handheld drill. Lastly, sharpen the drill bits and use cutting fluid to prevent overheating.
How to Utilize 1/4" 28 Taps
Tapping allows you to create threaded holes for a bolt or screw to enter. Therefore, you have to prepare a bolt when tapping for visual comparison. After drilling the initial hole, clear the edges and remove any sharp parts.
You may also add a chamfer to easily tap into the hole. You can use normal drill bits, but it is easier to chamfer with specialized drill bits. Clean the dust, debris, and other chips around and inside the hole.
Use a 90 degrees tap guide to set up the tap. Use just enough pressure on the hole to prevent having a crooked tap. Do it slowly but surely.
Pecking
Pecking involves getting into a hole in a short distance and then backing off. It prevents the drill bit from overheating or breaking under too much force and torque.
You can do pecking by revolving a full turn of the tap inside the hole, then half turn out. Repeat the process until the hole is tapped completely. It can take a long time, but this process is worth it.
Conclusion
Now that you know which drill bit to use for a 1/4-28 tap drill size, you may now begin with your project. You may use the chart for tap drill size that we have prepared to use the right bolt and drill bit size for your project.
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