Threading shouldn’t be an afterthought, though it doesn’t seem to rank high on everyone’s top 10 problems that require immediate attention. That is, until you consider the potential loss that scrapped parts can result in after a significant amount of machining has been invested in high-value parts.

Where the material being threaded is aluminum there are tried and true ways to ensure the reliability and consistency of the threading process. Selecting the right tool for the right application begins with consideration of the key properties of aluminum, the pros and cons associated with the main choices of tools, cut taps or thread mills, and advances in tooling technology including coatings and coolant-through capability.

The objective is a quality result, including reliability of process, consistency of surface quality and thread making that is easier, faster or both. A good start is revisiting the advice in the 10 Questions for Success asked by Mark Ford, director of global product management-threading for YG-1, Vernon Hills, Ill. (see Manufacturing Engineering, November 2021, “Threading Tech Provides Answers to the Right Questions,” Page 54). In the following comments, one of the questions that is raised concerns the material being threaded—in this case, aluminum. A host of industry suppliers suggest cost-effective ways to thread aluminum with their current advanced products.

Threading isn’t something that shops throw a lot of money at unless, as in the case of threading aluminum, it becomes a problem. According to Luke Pollock, product manager for Walter USA LLC of Waukesha, Wis., everybody struggles with it because it can be difficult to control chips. “Threading is one of the last machining operations at a point where the manufacturer has most of his machining cost already invested in the part. When you scrap the part you aren’t just scrapping the thread, you are scrapping all the process in the part,” said Pollock.

Walter’s threading solutions include cut taps, thread mills and roll form threads, but the most common way to thread in aluminum is to use a cut tap. “It’s always a trade-off choosing the right process. We associate longer tool life using a carbide thread mill than using a HSS cut tap, but with a thread mill you can tweak the tool path so that as the thread mill wears you are still getting a thread that gauges properly even though the tool is worn. With a cut tap you don’t have the option of doing that. Once the cut tap wears there’s nothing you can do except replace it,” Pollock explained.

“The trade-off that we normally associate with switching from thread mills to taps is cycle time,” said Pollock. “Typically taps are thought of having faster cycle times than thread mills, but I would make the argument that sometimes faster and even cheaper shouldn’t be the main concern. Sometimes, the main concern is what I call process reliability,” said Pollock. “With thread milling, we’re talking about repeatability, security and less risk of breaking the tap or scrapping the part. This is an underrated aspect of threading.”

Pollock recommends a sharp polished tool to avoid birdnesting and damage to taps that gummy and stringy aluminum chips can produce in both blind-hole and through-hole applications. “[In] through-hole the chips are a little easier to control because you are pushing the chips through the part and out of the bottom. For blind-hole applications chips have to be evacuated out of the top of the part in the direction that the tap is coming from. It’s very common to have chips wrap around the tap or cling to the tap or birdnesting, which can damage tool life by recutting chips. The result is that the operator has to stop the process and manually clear the chips or risk not being around to catch the problem the next time.”

Pollock points out that thread mills are much smaller tools that create smaller chips than produced by cut taps and a good way to address bird nesting caused by cut taps and difficulties of evacuating chips from both blind-holes and through-holes. Walter’s TC620 thread mill is a skip tooth or multiple row thread mill that is used to lower cutting forces so that feed rate and speed can be increased. “We are machining the thread in a shorter cycle time because of the low cutting pressure, feeding the tool and actually machining the thread faster because of the low cutting pressure. It also makes the tool really universal for application so that it can be used for steel and aluminum with the same geometry and the same coating.”

Another facet of aluminum, a material that isn’t as strong as steel or titanium, is the use of a Helicoil thread insert to salvage an aluminum part or add strength to an original design. Pollock noted that it is more difficult to pull out of the coiled thread than the part material once the Helicoil is threaded into a hole that is drilled oversize. Either a standard coarse thread (STI-UNC) or a fine pitch thread (STI-UNF) is possible.

Sandvik Coromant of Mebane, N.C., offers a comprehensive approach to internal and external threading solutions, said Jim Nielsen, round tool specialist. Silicon content is a major factor to consider when tapping internal threads in aluminum materials, he noted.

“The higher the Si content, the shorter chipping the material generally becomes; however it’s more abrasive. The lower the Si content, the more likely the gummier the material becomes,” said Nielsen.

“Cut taps are the most common choice for threading aluminum as they are readily available with good productivity,” he continued, “especially when the geometries are chosen for the type of aluminum material you are threading. Form taps are also a good choice in material for less than 10-12 percent silicon content because you do not need to worry about chips jamming or chip issues. Thread forming is a chipless solution as you form the threads versus cutting the threads. The reason the Si content is important relates to aluminum castings, as the higher Si content tends to be more brittle and the component can fracture.”

Nielsen points to continuing advancements in geometries, coatings and substrates, allowing for higher speeds with increased tool life and better thread quality. “Remember, when you are tapping, the feed rate is locked in based on the pitch of the thread, so you only can play with speed [measured in RPM] and/or tap geometry.”

Coolant also plays a key role with lubricity minimizing material welding or sticking to the tap. Coolant, along with proper speeds based on the specific design/coating, as well as the recommendations of the manufacturer, must be carefully considered. “I have seen where a full synthetic can actually break down the material and cause welding within two to three holes. It’s important to pay close attention to the coolant as well,” Nielsen cautioned. “There must be additives chosen specifically for the aluminum in these cases.”

When you consider the size of the market for threading aluminum, automotive and aerospace industries come immediately to mind, as do applications for electronics for office equipment, according to Ford from YG-1. “Carbide roll form taps really shine in automotive applications where the switch from cast iron to cast aluminum for engines and components has been made. We’re talking about tool life that is required measured in hundreds of thousands of holes,” said Ford.

“Aluminum in general is a material with a high machinability rating, but cast aluminum with a greater silicon content is more abrasive. The neat thing about the material is that it creates mostly a short chip. It’s almost as if you are machining cast iron,” he said.

For cast aluminum, YG-1 has a line of taps, called YG Tap Cast, that are good for both cast iron and cast aluminum. The line, which is mostly sold for cast aluminum, gives users the options for both axial and radial coolant fed tools, as well as a non-coolant option, he said.

YG Tap ALU Tools, developed largely for wrought aluminum applications, use fewer but larger flutes to provide enough flute space to get chips up out of the hole for spiral flute taps for blind-holes and spiral point taps for through-holes.

Chip evacuation is always a major consideration when threading aluminum. “For through-hole applications, you can use a spiral point tap which is designed to drive the chip forward of the tap ejecting the chip on the opposite side of the hole,” Ford explains. “The tap never really has to fight the chip coming out of the hole. That’s always a major issue in blind-hole tapping. For aluminum, it’s really about having a big flute with a really smooth coating or having a really polished surface to allow the chips to move freely out of the hole and not wrap around the tap,” said Ford. “YG-1’s Hardslick anti-galling coating allows tools to move over the coating without sticking,” he said.

Another option with cast or wrought aluminum applications is using roll formed threads. “That’s where our Syncro tap really shines. We’ve had a lot of success with Syncro tap in aluminum. The third option is thread milling which provides excellent control over consistently holding size, especially for threading high value parts,” Ford said.

“Productivity is best with machines that use synchronous tapping cycles. We want the speed and feed to be synchronized during the entire cycle. The machine must be able to hit much higher spindle speeds during the tapping cycle. Automotive needs high output with higher spindle speeds. The synchronous toolholder is the best way to achieve very consistent gauging, very good thread finish and much better tool life.”

For commercial aircraft applications where there are many through-hole applications in aluminum and parts are joined by nuts and bolts for maintenance and service reasons, Ford says that parts are tapped with STI taps or screw thread inserts that are harder and stronger than the aluminum. “It allows for maintenance taking the insert out and putting a new one in without having to scrap the part,” he says.

As reported in Manufacturing Engineering (November 2021, Page 58), Emuge-Franken has collaborated with Audi A.G. to develop a new threading tool technology to produce high-quality internal threads in aluminum alloys. Called Taptor, the threading tool combines a twist drill with a tap and is said to eliminate one entire machining process, the pre-drilling cycle.

Taptor features a unique drill section with a few threading teeth that are arranged axially at a short distance from the drill tip to produce threads in blind- and through-holes. The core and thread are produced simultaneously in one operation. Audi has applied for a patent for the process, which is divided into two steps.

In the first step, the movement of the tool corresponds to synchronous tapping. The drill feed per revolution matches the thread pitch. In the second step, the tool cuts free at the drilling depth and the thread teeth create a circumferential undercut. The Taptor is then retracted through the generated thread.

Used in conjunction with the Emuge Speedsynchro Taptor collet toolholder, which has an integrated transmission gear, high cutting speeds can be achieved, especially in automotive applications. Emuge-Franken reported time savings of 41 percent, or two seconds per thread when using the Taptor/Speedsynchro Taptor collet holder combination when machining a cylinder head side with 26 threads M6 2xD in cast aluminum at 0.47” (12 mm) deep. An undercut is created at the bottom of the thread, the length which corresponds to the lead-in chamfer of a conventional threading tool.

Emuge-Franken says that this process can be used on any machine that supports synchronous tapping, has a spindle with torque support and a tool magazine that can accommodate the Taptor Speedsynchro holder.

Allied Machine & Engineering of Dover, Ohio, is well known as a drilling company that supplies tools for complete holemaking, including boring, roughing, reaming, threading and finishing holes. Allied’s thread mill product lines include a solid carbide thread mill line and an indexable thread mill line. According to Justin Ripple, product manager, the solid carbide thread mills feature Allied’s proprietary AM210 coating for faster threading operations in aluminum with superior penetration rates and improved cycle times.

“Indexable tooling is a better choice for job shops because it gives them the versatility to easily and quickly change thread forms with one thread holder body which is in the spindle or tooling block,” Ripple said. “For high-production applications, solid carbide thread mills run faster, produce very precise threads and deliver high penetration rates. This is why solid carbide is the preferred recommendation for mass production applications.”

Thread milling also helps avoid the biggest concern of having gummy aluminum adhere to the tool when tapping. Allied’s AccuThread solid carbide line benefits from coolant-through capability and the higher lubricity of the AM210 coating, which reduces chip adhesion to the tool, Ripple said. “Formulated specifically for Allied’s thread mills, AM210 coating allows higher speed and reduces built up edge. Tool life of AccuThread solid carbide thread mills, which don’t take a deep depth of cut is able to handle the abrasiveness of silicon aluminum.”

What is required for thread milling is three-axis machine interpolation. Ripple recommends three-axis machine technology with spindle speeds typically 10,000 rpm and higher. “The great thing about thread milling is that you aren’t taking too deep of a cut, so lighter machines, CAT40 or even smaller can effectively machine as long as they have simultaneous movement.”

As an aid to shops, Allied Machine offers Insta-Code programming that is downloadable from its website. Programs are generated simply by entering machining conditions and the desired thread profile. Insta-Code is used by Allied and its customers and has proven to be a valuable engineering resource, according to Ripple. Also available from Allied Machine’s website is a link to its Thread Mill Pocket Guide which provides comprehensive information about threads and their cutting conditions.