Producing internal threads

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by Patrick Nehls

How to select the best thread producing tool


Producing internal threads is one of the most common metalworking processes. We are familiar with cut taps and how to use them, but you have options. Thread forming and thread milling are also great solutions and can be economical under the proper conditions.

Cut taps:There are two main operations for cut taps: through-hole and blind hole threads. Through-hole taps, or spiral point taps, present little challenge with respect to chips. The design of the tap winds up the chips and pushes them forward and out of the component. Blind-hole taps, or spiral flute taps must direct the chips out of the hole. This presents challenges in machining operations. Tapping often produces long tangled chips. You need to properly manage the chips to prevent them from damaging the work-piece or the tap.

Along with the spiral point and spiral flute taps, there is a third style–the straight flute tap, which can be used in blind and through holes. This unique tap is used only in short chipping materials. In the case of blind holes, we rely on the coolant to flush the chips out of the hole.

Thread formers:When chips are a problem, thread forming may be a great option because it produces no chips. The design of the former displaces the material to produce the threads. Formers do not have traditional flutes, so the overall cross section is greater than cut taps. This robust design also leads to less broken taps.

There is a trade off when switching from cut taps to thread formers. With a cut tap, the tolerance of the drilled hole is less critical than with a former. Formers require a tighter tolerance because of the material displacement. If the hole is too small, there will be too much material for the former to displace. This can lead to tap breakage. If the drilled hole is too large, the threads will not be completely formed.

You must also consider where the threaded component will be used. For aerospace, medical and food service components, thread forming is generally not allowed. As the threads are formed, there will be a gap at the crest of the thread. There will be no guarantee this gap will be free from contamination.

Thread milling:Thread milling is another option for producing threads. One tool can be used to produce threads of different diameters or tolerances where they have the same pitch. Thread milling offers the manufacturing process a high level of control over pitch diameter and tolerance holding. The process produces very small, well formed chips that are easily evacuated. Thread milling can be used for blind as well as through holes.

When it comes to the cost of the threaded hole, thread milling can be more expensive, mainly with processing time. For this reason, it is often used on high value components where broken taps must be avoided.

The equipment required for thread milling must be capable of handling complex programming and machine movements. But the programming is not too much of a problem with various software programs available. The operator simply enters the coordinates of the threaded hole along with the tool data and the software will generate the required machine code.

In sum, there are three styles of tools we can choose from to produce threaded holes–cut taps, formers, or thread mills. The tool you choose depends on the results you are after. Cut taps are the norm for high production where the chips can be managed. In high production where the component allows thread forming and the processing of the drilled hole can be controlled, thread forming can be an economical alternative. For high value components, or components where the quality of the thread must be very tightly controlled, thread milling may be the best solution. SMT

Patrick Nehls is product manager for Walter USA LLC, Waukesha, WI


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