Thread-Forming (Roll) Taps

What it is

A thread-forming tap (also called a roll tap, form tap or fluteless tap) has a lobed cross section instead of cutting flutes. As it turns, the lobes press the wall of the hole outward and upward into the thread form. No metal is removed, so there are no chips to clear. The formed thread is cold worked and often stronger than a cut thread.

How to identify

A forming tap has no cutting edges and no chip flutes. The threaded section looks smooth and slightly polygon-shaped when viewed from the end. Many have a small oil groove along the side so lubricant can reach the bottom of a blind hole. The shank is marked with size, pitch and the pitch diameter limit, the same as a cutting tap.

Forming vs cutting taps

Where it works

Forming taps work in materials that stretch before they crack. Typical good choices are aluminum, low carbon steel, copper, ductile brass, many stainless steels and zinc die castings. They are a poor choice for cast iron, hard or brittle alloys, and most heat-treated steels. When in doubt, check the material elongation and the tap maker's guidance.

Key points

• The drill must be larger than the cutting tap drill, because metal moves inward to form the minor diameter. Use the thread-forming tap drill chart, not the standard tap drill chart.
• Small changes in drill size change thread height a lot. Hold the drill size closely.
• Forming torque is higher than cutting torque. Use rigid holding and the correct speed.
• Use a good forming lubricant. Dry forming quickly damages the tap and the thread.
• A small seam or split at the minor diameter is normal and is how a formed thread looks.

Common mistakes

• Drilling to the standard tap drill size, which overloads and breaks the tap.
• Trying to form threads in cast iron or brittle material.
• Running without lubricant.
• Rejecting good parts because of the normal crest seam.

Governing standards: ASME B94.9 (taps); thread form and class per ASME B1.1 and ASME B1.13M. Tap maker's drill and speed data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.