What Is Roll Form Tapping
Roll form tapping (also called thread forming or cold forming) creates threads by displacing material rather than cutting it. The tap has no cutting edges or flutes. Instead, it has lobes that push the material into the thread form as the tap rotates and advances into the hole.
This process is particularly useful after deep hole drilling because there are no chips to evacuate from the bore. In deep holes, a conventional cut tap produces long stringy chips that pack in the flutes and can seize the tap. Roll form tapping eliminates this problem entirely.
I have used roll form tapping for oil passage threads in hydraulic components after deep hole drilling. The thread quality was excellent and the tap lasted for thousands of holes.
Advantages and Disadvantages
| Aspect | Roll Form Tapping | Cut Tapping |
|---|---|---|
| Chip generation | None (chip-free) | Chips produced |
| Thread strength | 20-30% stronger (work-hardened) | Base material strength |
| Surface finish | 0.4-0.8 um Ra | 0.8-2.0 um Ra |
| Tap life | 3-5x longer | Standard |
| Torque requirement | 50% higher | Standard |
| Material requirement | Ductile only | Any machinable material |
| Tap drill size | 5% larger than cut tap | Standard |
The rolled thread has a work-hardened surface that resists galling and stripping. I have pull-tested both thread types and the roll-formed thread consistently fails at 20-30% higher load than a cut thread in the same material.
Material Requirements
Roll form tapping requires ductile materials. The material must flow under pressure rather than crack. I use roll form tapping successfully in:
- Aluminum alloys (2000, 5000, 6000 series)
- Low-carbon steel (1018, 1020, A36)
- Stainless steel (304, 316 – annealed condition)
- Brass and bronze
- Copper
Materials that do not work for roll form tapping include:
- Cast iron (brittle, cracks instead of flowing)
- Hardened steel above 35 HRC
- Titanium (work-hardens too rapidly)
- High-carbon steel above 0.5% carbon (tends to crack)
If the material is marginal, I do a test tap in a sample part before running production. The test tap tells me immediately whether the material will form clean threads.
Tap Drill Sizing
The tap drill diameter for roll form tapping is larger than for cut tapping because the formed thread displaces material into the bore. The threads need room to form without excessive force.
I use the following formula for tap drill diameter:
Tap drill size = Major diameter - (0.0065 x Thread pitch in mm)
For a M10 x 1.5 thread:
- Roll form tap drill = 10 - (0.0065 x 1.5) = 10 - 0.00975 = 9.42 mm
- Cut tap drill (standard) = 8.50 mm
| Thread Size | Roll Form Tap Drill (mm) | Cut Tap Drill (mm) |
|---|---|---|
| M6 x 1.0 | 5.55 | 5.00 |
| M8 x 1.25 | 7.40 | 6.80 |
| M10 x 1.5 | 9.42 | 8.50 |
| M12 x 1.75 | 11.40 | 10.20 |
| M16 x 2.0 | 15.20 | 14.00 |
The tap drill size is critical for roll form tapping. Too small a hole creates excessive torque and can break the tap. Too large a hole produces incomplete thread form. I hold the tap drill tolerance to +/- 0.03 mm for consistent thread quality.
Process Parameters
Roll form tapping requires higher torque than cut tapping. The machine spindle must deliver about 50% more torque at the tapping speed. I check the spindle torque rating before running roll form taps.
Recommended speeds for roll form tapping:
| Material | Speed (m/min) | Lubrication |
|---|---|---|
| Aluminum | 20-40 | Oil or emulsion |
| Low-carbon steel | 10-20 | Heavy oil |
| Stainless steel | 5-10 | Sulfur-based oil |
| Brass | 30-50 | Oil or dry |
I use a dwell at the bottom of the tap cycle. The dwell allows the material to flow completely into the thread form before the tap reverses. I set the dwell to 0.5-1.0 seconds for most materials.
The coolant used for the deep hole drilling operation may not be adequate for roll form tapping. Roll form tapping generates more friction heat than cut tapping. I use a dedicated tapping lubricant applied through the spindle or as an external mist.
Thread Depth Considerations
In deep holes, the thread depth is limited by the tap length. Standard roll form taps are available up to about 3x diameter in thread depth. For deeper threads, I use special long taps or a thread milling operation.
I ensure that the deep hole drilled bore has a clean, deburred entry for the tap. A burr at the entry can cause the tap to start crooked and damage the thread. I use a countersink or chamfer tool to create a 45-degree lead-in at the thread entry.
Key Takeaways
- Roll form tapping eliminates chip evacuation problems in deep hole tapping.
- Thread strength is 20-30% higher than cut threads due to work hardening.
- Use a tap drill diameter about 5% larger than for cut tapping based on the formula.
- The material must be ductile – cast iron and hardened steel do not work.
- Apply dedicated tapping lubricant for best results and tap life.