Hertel K008371AS Spiral Flute Tap 1/4-28 Unf Bottoming Hss

Product Description

The HERTEL K008371AS is a spiral flute tap designed for threading 1/4-28 UNF holes in a bottoming configuration. Built from high-speed steel (HSS), it delivers consistent thread cutting in ferrous and non-ferrous metals where a full-depth thread is required close to the bottom of a blind hole. The tap features three right-hand flutes with a fast flute style and a 52-degree helix angle, which promotes efficient chip evacuation in short-chipping materials. The H3 thread limit and 3B class of fit make it suited for standard tolerance threaded assemblies. Overall length is 2.5 inches.

This tap is used in machine shops, manufacturing, and maintenance applications where 1/4-28 UNF fine-thread fasteners are assembled into blind holes requiring maximum thread engagement. The bottoming chamfer — typically 1 to 2 threads of taper — allows threading nearly to the bottom of the hole after a plug or taper tap has started the thread. It is installed by machinists, tool-and-die technicians, and maintenance mechanics using a tapping head, CNC machine, or hand tap wrench. It does not support coolant-through delivery.

Key Features

• 1/4-28 UNF thread form for fine-pitch applications requiring high tensile strength
• Bottoming chamfer for threading to the bottom of blind holes
• 52-degree helix angle with fast flute style for effective chip removal
• H3 thread limit with 3B class of fit for standard tolerance assemblies
• High-speed steel construction for durability across a range of workpiece materials
• Right-hand thread and right-hand flute direction for conventional tapping operations

Compatibility

Designed as a direct-use bottoming tap for 1/4-28 UNF threaded blind holes in standard 3B tolerance applications. Verify pilot hole diameter per UNF tap drill charts before use.

Installation Notes

Secure the workpiece and ensure the pilot hole is drilled to the correct UNF tap drill diameter before beginning the threading operation. Use cutting fluid appropriate to the workpiece material to reduce heat and extend tap life. When using by hand, apply steady, even pressure and back off a partial turn periodically to break chips. Avoid forcing the tap against the bottom of the hole to prevent tap breakage.