Burrs, thread distortion, or ovality can compromise tolerances, cause downstream fitment issues, and lead to part rejections.
Machinists solve this problem by forming taps.
Forming taps offer chipless threading, higher strength threads, and better tool life in ductile materials. But they’re also sensitive to process errors. Without the right approach, they can easily cause burr formation and deformation around the thread zone.
In this blog, you will learn to reduce these issues at the source.
Choose the tap based on the material flow.
Different materials react differently during plastic deformation. Aluminium alloys, for example, flow readily under forming pressure, while stainless steel or titanium alloys require much higher forming forces.
If you’re using a standard forming tap across all materials, that’s likely contributing to burr formation or localized distortion. Tap geometry, including thread profile, lobe count, and back taper, must be selected to match how the material flows during forming.
Manufacturers often provide material-specific tap geometries. Use them as it improves thread quality and reduces unnecessary strain on the workpiece.
Adjust the pilot hole size precisely.
Unlike cut taps, forming taps displace material into thread form. That means the pre-drill hole size must allow just enough material for forming.
If the pilot hole is too small, it can form pressure spikes. This leads to excessive wall expansion, thread bulging, or burrs around the entry and exit points. Oversized pilot holes, on the other hand, result in shallow threads with reduced load-bearing strength.
Use precise drill sizes as per the tap manufacturer’s data. If burrs persist even with recommended dimensions, fine-tune in small increments to match the specific behavior of your material and tapping setup.
Select the correct lobe geometry.
The lobe geometry of a forming tap directly influences material flow, friction, and pressure zones during threading. Taps with more lobes can improve surface finish but may increase torque. Fewer lobes reduce friction but may produce less consistent threads in certain alloys.
If you notice deformation or inconsistent threads, review the lobe design. Some materials perform better with asymmetric or radiused lobe profiles that reduce entry shock and smooth the forming action.
Do not assume a “general-purpose” forming tap will work across all applications. Match the geometry to the job.
Control tapping speed
Tapping speed is often overlooked. However, in forming operations, excessive RPM leads to heat buildup, material softening, and increased friction, especially in blind holes or sticky alloys.
This thermal expansion can deform threads and cause inconsistent pitch or wall thickness near the mouth of the hole. On the other hand, too slow a speed can result in interrupted forming and increased surface drag.
Follow the speed recommendations from the tap supplier, but also observe actual tool behavior. For example, on low-carbon steel, optimal forming speeds may be higher than for stainless. Adjust accordingly.
Use stable high-pressure lubricants.
Forming taps require a consistent lubricating film to control friction and prevent galling. Standard cutting oils may not be adequate under forming pressure.
Use lubricants formulated specifically for forming operations, typically those with high film strength and thermal stability. If burrs or deformation occur sporadically, check the delivery method. Mist or drip systems may not provide enough volume for deep holes or long threads.
Flood or high-pressure lubrication often yields better results. Also, monitor for contamination. Old or dirty lubricant quickly loses effectiveness.
Maintain perfect tap alignment.
Misalignment between the tap and the pilot hole axis causes uneven pressure distribution. This leads to thread distortion on one side and excessive material flow on the other; both of which can produce burrs or deformation, especially in thin-wall sections.
Use alignment fixtures, tapping heads with radial float, or guide bushings to maintain concentricity. Manual setups should be verified using indicators or optical alignment tools.
Even a slight angular offset during tap entry can negatively affect thread form and tool life.
Limit torque fluctuations
Torque spikes during forming usually indicate a process inconsistency. Either from tool wear, misalignment, or material variation. These spikes cause over-compression of threads, localized deformation, or even micro-cracks at the thread roots.
Use torque-controlled tapping systems where possible. Servo-tapping heads or tapping units with torque limiters help maintain stable forming pressure and reduce the risk of damage.
For manual or older machines, monitor torque manually using gauges or record torque curves to identify trends before they result in part quality issues.
Conclusion
Forming taps can outperform cut taps in both strength and consistency, but only when applied with precision.
Thread deformation and burr formation aren’t flaws in the tool. They’re almost always the result of small process gaps: hole size, tap alignment, material mismatch, or lubrication failure.
Focus on those inputs. Tuning them systematically reduces part rejection, improves surface finish, and extends tool life without slowing production.
