Form Taps vs. Cut Taps: The Full Guide

When you need to thread a hole, it may seem easiest to reach for the nearest right-size tap. After all, the threads will be identical in gage and will accommodate the type and size of external fastener threads. But even before drilling the hole, professional machinists consider the material being tapped, whether the hole is through or blind, and other variables before deciding whether to use a cut tap or a form tap.

While cut taps and form taps are both designed to create internal threads, the way they go about their tasks is completely different. As the name suggests, cutting taps produce threads by progressively cutting away the space between threads into which the fastener’s external threads will fit. Forming taps, by contrast, remove no material. Instead, they move or displace material inside the hole to build up the threads and dig into the grooves.

These different characteristics determine each tap class’s design and appropriate applications. While it is agreed that form taps create stronger and more precise threads, there are plenty of instances where cut taps deliver better results. Regal Cutting Tools has prepared this guide to assist you through the tap selection process.

Cut Taps

Cut taps, or cutting taps, are identifiable by the grooves called flutes that are cut into the longitudinal axis of the tap blank. Flutes are necessary when shaving away pieces of metal (or plastic or other work material) from inside the hole. The flutes perform two functions, evacuating chips out the hole’s opening and allowing for coolant or lubricant to access the cutting edge.

Long the only choice for machinists, cutting taps still find favor among many manufacturing professionals. But as a rule of thumb, you should only use a cut tap when there is a legitimate reason to forgo a form tap, such as:

Workpiece is too hard or unsuitable for form taps – Cut taps can be used on virtually any material, including plastic. Forming taps, however, work best on softer metals like aluminum, copper, brass, lead, and certain stainless steels.
Threading the hole – Form taps tend to distort or deform the both the entrance and the exit hole (in the case of a through hole), which will require a separate tool to repair. This is caused by the force of the displaced material being formed moving past the edge of the part while attempting to create the thread. Adding a chamfer to the hole prior to tapping can eliminate this issue.
Threading a big hole – Thread-cutting taps require less horsepower than thread-forming taps.
Machinery issues – The machinery being used for tapping does not provide the required horsepower needed to utilize a form tap. Manual tapping heads, air spindles, and low horsepower machines are examples of equipment unsuited for form tapping.

Form Taps

Form taps, or forming taps, are also called roll taps because they roll material into the thread form rather than cutting excess material away. They are also sometime referred to as fluteless taps because they create no chips and therefore need no flutes to evacuate them. However, form taps do need to have lubricant grooves to allow lubricant along the longitudinal axis of the tap. These “lube” groves also prevent causing a hydraulic seal and “lock” which can deform or destroy the part as the pressure builds up inside the part creating a pressure vessel.

When tapping soft, non-ferrous metals, and moderate steels, form taps enjoy long lifespans because their fluteless design gives them the rigidity to resist breakage and because of the absence of cutting edge, there is nothing to wear and get dull. If drilling a material creates chips that do not break, but rather form ribbons, the workpiece probably is conducive to thread forming.

One of the most important operational difference with forming taps is the need to drill a larger hole. Since there is no excess material being removed in the form of the cut chips, this difference is compensated by removing the excess material by first using a larger pre-tap hole size. Because material is being forced into the thread form, it may require experimenting with a specific hole size to reach the proper amount of material that will form into the required thread limit size for gaging purposes.

Second, form taps are typically run at speeds 1-1/2 to 2 times the speeds of a cut thread tap. Form tapping needs to be accomplished in a timely manner before the material can work harden from the forming action and create premature tool failure. The forming action coupled with the natural tendency for most materials to work harden will create a much stronger thread when compared to a cut thread tap. However, with the greater wear life and faster speeds, greater productivity can be attained using a form tap versus a cut thread tap.

Moving material into the thread form require more horsepower than cutting it progressively into the required thread form. The form tap will exert greater pressure on the hole walls than a cut thread taps and is typically unsuited for thin wall applications such as tubing or tapping near the edge of a part. Use of a good-quality cutting oil or the appropriate concentration of a water-based lubricant to limit friction and keep things at a reasonable temperature.

In addition to stronger and more durable thread structure, form taps carry several advantages:

No chips that must be fished out of blind holes
High-speed capability for greater efficiency

The professionals at Regal Cutting Tools can help you decide whether cutting or forming taps are best for the internal-threading operation you will be performing.

Contact us for a quote on any tool we have in stock. We also can custom-build taps to your exact requirements. To learn more, send us your specs.