Quick answer: Choose a spiral flute tap for most blind holes, a spiral point tap for through holes where chips can be pushed forward, and a straight flute tap for short holes or materials that make powdery chips, such as cast iron. The tap style should follow chip evacuation first.
Cutting taps all make threads by removing material, but they do not move chips in the same direction. That one difference explains many broken taps, rough threads, and unstable hole results. Before choosing coating or brand, confirm whether the hole is blind or through, how deep it is, and what kind of chips the material makes.
Spiral flute tap: best starting point for blind holes
A spiral flute tap pulls chips back out of the hole. That makes it useful when the bottom of the hole is closed and there is no space for chips to leave forward. It is commonly used for blind holes in steels, stainless steels, and ductile materials where chip control is the main risk.
The tradeoff is strength. Spiral flutes remove more material from the tap body, so the tool can be less rigid than a straight flute or spiral point design. In deep holes, small diameters, or hard materials, machine synchronization, tapping holder stability, and correct tap-drill size become critical.
Spiral point tap: through-hole efficiency
A spiral point tap has straight flutes with a point geometry that pushes chips forward. If the hole goes all the way through the part, chips can exit from the far side instead of packing around the cutting edges. This is why spiral point taps are often selected for through-hole production.
Using a spiral point tap in a blind hole is risky because chips are pushed toward the bottom. Once chips have nowhere to go, torque rises quickly and the tap can chip or break.
Straight flute tap: simple, rigid, and material-specific
A straight flute tap is simple and relatively rigid. It can work well in short holes and in materials that produce small powdery chips rather than long continuous chips. Cast iron is a common example. Straight flute taps are also used in some hand tapping and special-purpose conditions.
That said, a straight flute tap is not a universal choice for blind holes in ductile materials. If chips curl and stay in the flute, packing can damage the thread or overload the tap.
Selection checklist for cutting taps
- Blind hole: Start with spiral flute unless the material or hole geometry points elsewhere.
- Through hole: Spiral point is often safer because chips can be pushed forward and out.
- Powdery chips: Straight flute may be appropriate in cast iron and similar materials.
- Small diameter: Check tap-drill size, runout, synchronization, and holder float before blaming the tap.
- Deep thread: Confirm coolant, chip evacuation, chamfer length, and whether thread milling is safer.
When a standard tap is not enough
If the hole is very deep, the thread must stop near the bottom, the material springs back, or the machine is unstable, a standard catalog tap may not be the safest route. HEYI can review custom tooling, tap chamfer length, tap-drill size, or whether thread milling is better for the part.
For a useful review, send the thread size, pitch, blind or through-hole condition, thread depth, material, hardness, machine type, coolant, current tap style, and the failure mode. Use the RFQ form when drawings or photos are available, or contact HEYI for a quick first check.