- Is the pilot hole diameter for the forming tap appropriate?

During forming tap processing, if the amount of material displacement (burr formation) is too large, the crest height of the thread may exceed the target range, causing the minor diameter to become too small.

 

Note: The pilot hole diameter table for forming taps are for reference only.

 

Since the degree of material displacement varies depending on the workpiece material, please consider increasing the pilot hole diameter. 
Additionally, if the drill is not sharp enough, the hole diameter may shrink immediately after drilling. 
This occurs because the dull edge pushes the material aside instead of cutting it, and the material springs back to its original shape due to elasticity once the drill is removed.
This phenomenon frequently occurs when processing stainless steel or mild steel using carbide drills designed for general steel.

 

Please check for cutting edge wear and ensure the drill is compatible with the workpiece material.