Which tap geometry for which hole
The choice is driven by whether the hole is blind or through, and whether the material produces a controllable chip.
| Geometry | Chip direction | Best for | Watch out for |
|---|---|---|---|
| Spiral flute | Back out of the hole | Blind holes in steel and stainless | Chips can jam in the flute on deep holes |
| Spiral point | Forward through the hole | Through holes, high speed | Not usable in blind holes |
| Straight flute | Accumulates in flute | Brittle materials, cast iron, short holes | Needs frequent reversal to clear chips |
| Forming (fluteless) | None; material is displaced | Ductile materials, blind holes, no-chip requirement | Needs a larger tapping drill; can split thin walls |
| Thread mill | Washed out by coolant | Large threads, difficult materials, oversized or repaired holes | Slower than tapping per hole |
Tapping drill size and thread percentage
A common cause of tap breakage is a tapping drill that is too small. The nominal thread percentage is a range, not a fixed value, and dropping from 75 percent to 65 percent thread engagement reduces the torque substantially while keeping full strength in most applications.
Forming taps need a different hole size again. Because the material is displaced rather than cut, the hole is larger than for a cutting tap of the same thread, and the hole must be calculated from the material rather than copied from a cutting-tap chart.
Thread milling instead of tapping
Thread milling cuts the thread with a helical interpolation using a single small cutter. It has three practical advantages: one tool covers a range of diameters at the same pitch, a broken tool does not leave a tap stuck in the part, and the thread can be cut in materials where tapping torque would break the tool.
It is also the standard remedy for a damaged thread, because the milling tool can recut an existing hole or open it to a repair size without a dedicated oversize tap.
Coatings and substrates for taps
Taps have limited chip room and limited coolant access at depth, so coating choice matters more than it does on a mill.
For stainless steel and other work-hardening materials, a sharp PVD-coated edge run at a firm feed is what limits hardening at the thread flank. For cast iron, an uncoated or TiCN-coated tap frequently outperforms a harder coating, because the material is abrasive but produces short chips.
Frequently asked questions
Should I use a spiral flute or spiral point tap?
Spiral flute for blind holes, because the chips are pulled back out of the hole. Spiral point for through holes, because the chips are pushed forward and out. Using a spiral point tap in a blind hole packs the chips and breaks the tap.
When is a forming tap better than a cutting tap?
Forming taps produce no chips, which suits blind holes, horizontal operations and any application where a stray chip is unacceptable. They need a larger tapping drill and a ductile material, and they should be avoided on thin walls where the forming pressure can distort the part.
Why does my tap keep breaking?
The usual causes in order are an undersized tapping drill, misalignment between the hole and the tap, a material that has already work-hardened from a previous operation, and insufficient lubrication at depth. Check the hole size and the alignment before changing the tap.
Can one thread mill cover several thread sizes?
Yes. A single thread mill covers a range of diameters at one pitch, which reduces tool inventory and makes it practical to cut low-volume special threads. It also lets you repair or resize an existing thread without a dedicated oversize tap.
Do you make taps with non-standard shanks or lengths?
Yes. Special lengths, reduced shank diameters, extended reach and non-standard chamfer leads are all made to drawing, as are taps to a customer's own specification for OEM programmes.