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What to do if the blind hole thread becomes too large?
When it comes to threading, even the most experienced professionals can run into problems. One of the most common and frustrating issues is when a blind hole thread becomes too large. This can lead to misalignment, poor fit, or even complete failure of the threaded component. But why does this happen? And more importantly, how can you fix it?
Blind holes are different from through holes because they don’t go all the way through the material. Instead, they end at a certain depth, which means there’s a bottom surface inside the hole. This makes them more challenging to work with, especially when it comes to threading. The biggest issue is that chips can get trapped in the hole, making it harder for the tap to cut properly. In some cases, the tap might even break if it hits the bottom of the hole.
One of the main reasons blind hole threads become oversized is due to the design of the screw tap. Many taps used for blind holes have spiral flutes, which help push chips out of the hole as the tap moves downward. While this is useful for chip removal, it can also cause the tap to be pulled further into the hole than intended — a phenomenon known as the “corkscrew effect.†This happens because of the helix angle on the flute, which creates an axial force pulling the tool deeper into the material. If the cutting pressure is too high, the tap can move faster than the pitch, leading to misalignment and oversized threads.
To prevent this, it's important to reduce the cutting pressure when working with spiral-fluted taps. This allows the tap to stay aligned and maintain the correct thread size. Another key factor is ensuring that your tap is properly clamped. If the tap has too much play in the chuck, it can shift during retraction, causing misalignment. A high-quality drill chuck with good concentricity and clamping force is essential for maintaining accuracy.
When selecting a tap, always make sure it’s suitable for the material you're working with. Check the coating, geometry, and overall condition of the tool. A worn or damaged tap can easily produce incorrect thread sizes. Also, ensure that the pilot hole is drilled to the correct diameter before tapping. An improperly sized pilot hole can lead to thread failure or oversized results.
If you're still struggling with oversized blind hole threads, consider switching to a straight-fluted tap, especially when working with short-chipping materials. However, for medium or long-chipping materials, spiral flutes are still the best choice — just be mindful of the corkscrew effect and adjust your technique accordingly.
For those looking for reliable tools and expert advice, check out our BAER online shop. We offer a wide range of drill chucks and screw taps designed specifically for blind hole threading. If you need more tips or assistance, feel free to reach out to our customer service team or explore our blog for more in-depth guides. With the right tools and techniques, you can achieve perfect threads every time.
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