As the Tap Turns

Tapping isn’t easy and one of the toughest challenges is preventing oversized threads. New tools can help.

Taps are one of the most problematic cutting tools. Because part manufacturers typically perform tapping in the final machining stages, a misapplied tap can scrap a workpiece that already has a significant amount of value added to it.

Unlike taps, thread milling and single-point turning tools often remove material in time-consuming multiple passes to achieve final part dimensions, surface finish, straightness and perpendicularity. In contrast, by the time a tap fully advances and retracts from the hole, the threading process is already completed and multiple passes are not needed. 

However, because a tap contacts all sides of a hole when threadmaking, chip evacuation can be problematic. Thread milling and single-point turning tools, on the other hand, enable relatively easy chip evacuation.

In contrast to drills and reamers, for example, where end users can alter the speeds and feeds using a pecking cycle to obtain the needed results when holemaking and finishing, no option exists for altering the feed rate with a tap. The tap’s feed rate is confined by its pitch diameter and is much higher than most other types of cutting tools. Cutting speed, however, depends on workpiece material, condition and hardness. 

All images courtesy of YG-1 Tool.

Tapping Challenges

There are several major challenges that must be overcome when tapping.

The workpiece closely surrounds the tap, which creates friction and therefore increases torque and machine power consumption.

Workpiece materials tend to shrink when tapped, also increasing torque requirements.

When applying spiral-fluted taps, rubbing between the chips, tool and workpiece creates friction and heat, which might cause tap size problems in materials that tend to behave in a thermoplastic manner, softening when heated and hardening when cooled. That leads to nonconformance when cutting critical threads. 

Chips crawl up along the helix with spiral-fluted taps, which prevents some of the coolant from penetrating the cutting zone, also increasing friction and heat.

Spiral-fluted taps tend to break when they reach the full blind-hole depth and stop to reverse because they don’t penetrate through and relieve pressure. The tool stops while it’s still forming chips and those chips lock the tap. To prevent breakage, the tap must break loose from all impediments.

When tapping a short-clearance blind-hole with a spiral-fluted tap, the chamfer, or lead, length is short and limited. That causes the tap to undergo a heavier torque load compared to a spiral-pointed tap, also called a gun tap, which has longer lead lengths. 

Torque changes during tapping occur from the time the first chamfer tooth touches the workpiece and continue as the tap rotates and feeds into the hole until the tool stops and reverses, returning to the starting position. This causes changes in chip formation and flow, leading to unstable cutting and changes in repeatability and performance.

In one revolution, a tap should remove all of the volume of material according to its standard. For example, a standard-size coarse (UNC) ½-13 tap will remove more stock than a standard-size fine (UNF) ½-20 tap. Unlike other metalcutting operations, end users cannot change the amount of stock removal when tapping because the tap thread standard dictates the amount of stock removal per tap revolution.

When cutting a full thread profile, a taps enters into a prepared hole that’s larger than the tool’s end diameter. The taps then lands on its first chamfer tooth, begins to shave metal and leaves a specific amount of material for the second tooth to remove. As the tap continues to rotate and infeed, a third chamfer tooth enters the cut. It removes the material left by the second chamfer tooth, preparing the pathway for the first full tooth, which removes only a small pyramid peak of material to finish the final thread form. All the other teeth along the length of cut—linearly and peripherally—only lead and do not cut.

The chamfer teeth, which do the main cutting, undergo a heavier torque load and significantly impact the final thread results. Basically, this is why a tap is considered a single-point cutting tool.

Big Problem

In addition to the challenges previously noted, tapping can create oversized threads, which can ruin a part. When a tap advances into a hole, it should advance in one revolution, one pitch. In some cases, the pressure of the feed is greater than its lead. This pressure makes the tap advance slightly faster than it should according to its lead. That puts pressure on the top flank angles and shaves material that shouldn’t be removed (Figure 1).

Figure 1. A nut with a normal thread and a nut oversized because of a thin thread.

When a tap reverses and exits a hole, it sometimes retracts faster than it should because of spindle backlash and poor condition of the tapping attachment. When that happens, the tap is shaving the bottom flank of the threaded hole. As a result, the shaved surfaces make the tooth width space too wide. The periphery drill hole diameter, both in the root and the crest, may not be affected at all, but a thread gage doesn’t measure that. Again, it measures the tooth width space. When the tap shaves either the trailing or leading flanks, a GO gage measuring the tooth width space will indicate the space is wider than it should be, large enough to allow the NO-GO gage to advance. This causes part defects.

An oversized thread and too large a pitch diameter refer to an exceedingly wide tooth space, allowing the normal width of the gages to advance further before it touches the teeth. It is almost always the shaved thread angles that cause the oversized condition.

When an oversized condition occurs, users often incorrectly blame the poor, innocent tap. In most cases, it’s because of the tapping equipment or the reaction, or axial, force as a result of flute geometry. The reaction and cutting forces influence the direction a tap is being pushed or pulled, either into or out of the hole. Those forces push spiral-pointed taps out of the hole and push spiral-fluted taps forward into the hole.

As a result of uncompensated axial forces, thread miscutting, or shaving, problems may occur, creating part defects.

Problem Solvers

Metalworking professionals often consider taps to be a tool of last resort because of these problems. Different workpiece materials and machining conditions lead to an almost unlimited variety of tap geometries, material substrates, surface treatments and coatings. Such an array causes headaches when choosing the right tap for the job.

However, because tapping can be the preferred threadmaking option, it is important to find solutions that prevent a tap from advancing slightly faster than it should according to its own lead, prevent the tooth width space from being shaved too wide and prevent oversized threads. These solutions can help achieve an ideal thread form.

One such offering is the patented Combo tap for blind- and through-holes from YG-1, based in Inchon, South Korea. The tool’s special thread form geometry solves tapping problems by acting like a brake to prevent the tap from overfeeding (axial miscutting) and producing oversized threads. In addition, the geometry compensates cutting forces, reducing tap wear and extending tool life.

The tap also allows for increased thread relief, reducing torque, machine power consumption and friction and enabling smoother tapping with better chip evacuation. Finally, it reduces tap inventory because the multifunctional tap can thread a large variety of materials, including carbon, alloy, stainless and tool steels. CTE

About the Author: Avi Dov is an international application engineer for YG-1 Tool Co., Vernon Hills, Ill., with over 40 years of experience in the cutting tools industry. For more information about the company’s cutting tools, call (800) 765-8665, visit www.yg1usa.com.