Oversized Holes and Bell Mouth: Causes and Solutions
Understanding Oversized Holes and Bell Mouth Oversized holes and bell mouth are two related but distinct quality problems. An oversized hole is uniformly larger than the drill diameter along its entire length. Bell mouth is a local enlargement at the hole entry that tapers to the correct diameter deeper in the bore. I have seen both problems on the same part. The entry is 0.1mm oversized from bell mouth, and the rest of the hole is 0.03mm oversized from drill runout. Each problem has a different root cause and a different fix. ...