Thread Heads

How many holes do you tap a year? A few thousand? 10,000? 500,000? Whatever the number, you know that tapping can be a frustrating, time-consuming and unpredictable process.

Despite the challenges involved, there are manufacturing companies that tap incredibly large numbers of holes-millions each year-and they perform these operations at high speeds. (See accompanying article.) CUTTING TOOL ENGINEERING asked three of these manufacturers to share some of what they’ve learned about producing threads.

The first company taps machinescrew holes as small as 2-56 in aluminum-about 3 million annually. The second, a supplier of aluminum die castings, tapped 8 million holes in 1999 and expects that number to triple in 2000. The third is an automotive supplier that spent ’99 cranking out 10 million 12mm- and 13mm-dia. tapped holes in steel wheel-bearing hubs.

If you’re looking for tips on high-volume, high-speed tapping, there’s no better resource than this group.

When you make a cellular telephone call, there is a good chance that the phone’s signal will be routed by equipment that incorporates heat sinks from Wieland (Figure 1). Heat sinks are used to dissipate the heat generated by electronics components.

Darrell Reid, Wieland’s program manager, said the company prides itself on its efficient processes and its ability to produce high-quality threads quickly and accurately. What distinguishes this seven-year-old company in the marketplace is turnaround"one week is typical. In fact, when Wieland recently accepted an order for 10 prototypes in five part families, it delivered all 50 parts in just three days.

Figure 1: Wieland uses bottoming roll-form taps to thread mounting holes in heat sinks. The heat sinks cool electronic components that are used in wireless telecommunications hardware.

Machine Selection. Wieland runs 14 Haas machining centers and seven Brother CNC drilling/tapping machines 24 hours per day, six-and-a-half days per week. Reid said that the machining centers have rigid-tapping capabilities, but that the process, which requires the spindle to decelerate, stop then accelerate again, is simply too slow for his shop.

Instead of rigid tapping, Reid uses self-reversing tapping heads from Tapmatic Corp. The self-reversing heads allow the machining centers to tap at 2,000 rpm without reversing their spindles.

This method is not only faster than rigid tapping, but it also extends the life of the machine spindle by eliminating the effects of reversal wear. Each head holds a single tap and the entire unit remains resident in the machine’s toolchanger. No manual head changes are needed.

However, continuously running the heads requires them to be regularly maintained and repaired. According to Reid, a head must be rebuilt every 15,000 to 20,000 holes. He keeps 28 of them on hand for Wieland’s 14 machines. Repairs are done by an in-house toolmaker, who has an extensive inventory of parts at his disposal. If necessary, a head can be rebuilt and back in production in an hour.

An outside vendor can’t duplicate the convenience and efficiency of making repairs in-house, said Reid.

Pushing Speed. Tapping is such a large portion of the cycle time when producing heat sinks that Reid is always on the lookout for proven ways to tap holes faster. When a Brother distributor claimed its specialized CNC drilling/tapping machine could do the job, Reid said, "Prove it and I’ll buy it.”

In the past 18 months, Reid has purchased seven of these machines. The Brother Model TC-22A taps at 8,000 rpm and the Model TC-31 taps at 6,000 rpm. The TC-31 includes a pallet changer to cut spindle downtime to just 2.3 seconds"chip to chip. Reid said it is so fast that "you barely see the tap going into the hole.”

While a drilling/tapping machine costs about $30,000 more than a machining center equipped with tapping heads, it pays for itself in productivity. A typical 20-part fixture with a total of 240 holes is tapped in 23 minutes on the machining center. The drilling/tapping machine executes the same program sequence in just 11 minutes.

But the drill/tap machines aren’t a cure-all. They’re not economical to use for smaller lot sizes or when tapping parts with just a few holes. But since some heat sinks have as many as 250 tapped holes, the transition to CNC drilling/tapping machines will continue at Wieland.

Tapping Challenges. Reid is always wary of aluminum chips, which can wreak havoc on tap life and thread quality. While he cannot prevent the formation of chips during drilling and milling operations, Reid does avoid producing them during tapping by using bottoming roll-form taps in the heat sink’s blind holes. Roll-forming taps, which are only usable on soft materials, eliminate chips and form a cleaner, stronger thread than taps that cut. Reid said that roll forming actually workhardens the thread as the form is generated.

Reid stocks several brands of roll-forming taps coated with TiN to extend tool life. The average tap costs about $12. Reid recently began experimenting with a chrome-clad tap, which saves the company about $2 per tap. However, he said he needs to conduct more tests before deciding whether chrome-clad taps are a good long-term investment.

High-speed tapping makes it nearly impossible to see a broken tap while the spindle is rotating, so unless the operator hears it, a broken tap is not discovered until a tool change. Reid’s solution to the problem, surprisingly, is not an automatic tool-breakage-detection system, an option that’s available with many machine tools. Instead, an M-code in the program automatically stops the machine at a tool change, and a program note tells the operator to check for tap damage. If there is a problem, the operator simply determines the location, blocks the line of code for the bad hole and reruns the cycle from that point with a new tap.

Fortunately, taps do not break very often. A recent test on a drilling/tapping machine running at 8,000 rpm yielded an average of 12,000 holes per tap. Despite the fact that some of his jobs exceed three hours, Reid’s worst experience was a 20-minute loss in run time.

Coolant Tips. Reid does not use through-coolant tools because of the small tap sizes and the relatively shallow hole depths. Therefore, it is especially important that the shop keep the coolant at the right concentration to maintain lubricity. Extruded aluminum tends to gall"or adhere to"the tap. To circumvent this problem, Reid uses Castrol Clear Cut 6519, a full-synthetic coolant that has a 10 to 15 percent concentration to maintain lubricity.

All coolant systems on Wieland’s machines are equipped with dual-stage, in-line filters that capture small chips. The coolant sump must be monitored carefully, but the vigilance pays off. Typical sump life for each machine is about eight months.

ADC taps 25,000 holes per day, or about 8 million a year. The company expects that number to triple in 2000. About half of ADC’s customers are in the automotive and agricultural markets, while the other half are industrial and commercial customers, including ITW Miller Electric, Westinghouse Air Brake, Amana and White-Rodgers.

The 50-year-old ADC dedicates about one-third of its 180,000 sq. ft. of manufacturing space in its two plants to machining die cast products for these customers.

Machine Selection. ADC taps on a variety of machines, which are assigned jobs according to part volume and configuration. Vertical and horizontal machining centers and multiaxis lathes lead the company’s tapping arsenal. Occasionally, a manually operated lead-screw tapper is used if the part volume is low or if the tapped holes are located in the same plane and relatively close to one another.

While the holes that ADC taps may not have a deep thread, they are often hard to reach or are located on multiple planes. As a result, many of the company’s machines are fitted with an M-code-fired indexer or an A/B-style fixture to position parts quickly and accurately.

Figure 2: ADC found it most economical to tap a washing machine hub (below) on a dedicated lead-screw tapping machine that was part of a four-machine cell. The tapper machines eight holes at once.

The type of machine used to tap any given part can also change over time, depending on the volume of parts to be tapped. If a customer places a large order of parts, a CNC drilling/tapping machine is used. If for some reason the order size is reduced, the work is moved to a machining center or a manual machine.

If the holes are located in the same plane and tightly clustered, ADC has found that a dedicated multihead lead-screw machine is often the most economical option. A washing machine hub that the shop machines is a good example (Figure 2).

The company has a four-year contract to produce the part. The hub has eight tapped holes of the same size, all located in the same plane and relatively close to one another. A manually operated lead-screw tapper was acquired to tap the holes. The tapper and related tooling for it cost half as much as a machining center.

Pushing Speed. Mike Lumpp, ADC’s senior process engineer, began high-speed tapping with self-reversing tapping heads, but he now uses rigid tapping exclusively. He said that maintaining self-reversing tapping heads was too time-consuming and expensive, given the high volume of holes ADC taps.

Like Wieland, ADC has also found that multipurpose machining centers with high-mass spindles rigid-tap at slower rates than the smaller CNC drilling/tapping machines. When there is a long run of parts with high hole counts, Lumpp shifts the work to these machines.

Drill/tap machines’ smaller-mass spindles let them accelerate to full speed in less than 100 milliseconds, then decelerate just as quickly. In a side-by-side comparison at ADC, a CNC drilling/tapping machine running at 2,000 rpm had shorter tapping cycles than vertical and horizontal machining centers with 10,000- to 15,000-rpm spindles.

A lower-mass spindle also means faster rapid-traverse rates, because the machine can accelerate to full rapid and stay there longer before it must decelerate to move to the next hole position. This approach has proven very successful for Lumpp. The main reason he still rigid-taps on the machining centers at all is because of the limited work envelope of the drill/tap machines.

Tapping Challenges. ADC is making the front cover of a new 6-cylinder engine due out this spring from General Motors Corp. Each of the aluminum covers has 15 holes, which will be tapped on ADC’s 16 new Brother Model 32A machines. These machines will tap each hole at 4,000 rpm and will exit the finished hole at 6,000 rpm. ADC expects to tap 3,700 covers per day in 2000, or 17 million holes annually.

Tap choices at ADC are job-specific, but the general rule is to use bottoming roll-form taps for blind holes and cut taps for through holes. Lumpp buys taps from a number of brand-name manufacturers. He works with ADC’s sister company, National Coatings, to experiment with the latest coatings. He believes that TiN-coated taps are still the best value for tapping die cast aluminum.

The ongoing challenge at ADC is tool wear, since the aluminum that it machines is abrasive (about 11 percent silicon). ADC routinely threads 5,000 holes per tap. Sometimes, though, a process can be tweaked to increase tap life considerably. For example, ADC had an order that required it to tap washing machine hubs for Amana. After making minor adjustments to the speed, feed rate and percentage of the tap that engaged the hole, the company was able to tap 120,000 holes with eight taps.

The Brother drilling/tapping machines are equipped with tool-life-monitoring systems that alert operators when taps are approaching a predetermined hole limit.

Like Wieland, ADC does not use tool-breakage-detection systems, since breakage has not been a critical problem. At worst, the company has had to remelt a part when unacceptable threads were produced.

When it comes to regrinding, Lumpp sends some special carbide-tipped taps out to be sharpened. However, he echoes the sentiments of the other experts we interviewed: Reground taps do not offer a good return on investment. Therefore, he replaces most worn taps with new ones.

Coolant Tips. ADC taps with flood coolant or high-pressure through-coolant systems, depending on the requirements of the specific job. In one case, a deep blind hole that needed to be tapped was located at the bottom of a long bore. The solution was to use through-coolant at 1,000 psi. This allowed the coolant to reach the cutting edge while flushing the chips completely out of the bore.

Coolant lubricity is a key factor for tapping highly abrasive aluminum. Lumpp, like most experienced manufacturing professionals, has spent years searching for a coolant that can do the job effectively while not adversely affecting employees or the environment.

"I think [over the years] that we have literally tried every ‘witches brew’ you can think of," said Lumpp.

He finally settled on Trimsol from Master Chemical Corp. He first used it when he was trying to tap a set of six 6-32 holes in each part of a 1 million-part order. The roll-form tap he used kept breaking; the tap manufacturer discovered that the aluminum workpiece was adhering to the tool.

Lumpp used Trimsol to charge the sump and the problem immediately disappeared. He has since upgraded to Master Chemical’s E206, an all-purpose coolant.

According to Lumpp, the key to successful tapping is to make sure that you have coolant that can provide consistent lubricity. The recommended concentration must be maintained through a strict daily monitoring and maintenance program. ADC has a full-time person in each of its facilities who handles the coolant system, which, Lumpp said, is absolutely necessary for maintaining high tapping speeds.

Lumpp also purchased a complete recycling and automatic mixing system for his coolant. The results have been dramatic. The shop was using 55 gals. of coolant concentrate per week for 10 machines. It now uses only 23 gals. per week for 30 machines"less than 1 gal. per machine per week. The only coolant lost is what is recycled with the chips and what is mopped up from the floor.

Coolant disposal in the Chicago area is expensive"about 28 cents per gallon"so the recycling system was a real cost-saver. The total payback period was just three months.

And while Monday-morning stinky sumps and employee dermatitis complaints are every shop’s nightmare, Lumpp has avoided these problems by using deionized water. On a monthly basis, a local contractor replaces two deionizing tanks that filter the city water supply. At $120 per month, this is a very inexpensive and effective solution.

Deionized water also reduces downtime and coolant costs. Lumpp said that no ADC machine sump has been drained and cleaned since the switch to deionized water more than a year ago.

Those seeking advice about the high-speed cut tapping of steel don’t have to look much farther than FAG Automotive. The company is a tier-one supplier of wheel-bearing hubs for the auto industry (Figure 3).

Machine Selection. FAG has dramatically increased the speed at which it taps during the past few years. In 1996, according to the company’s technical leader, Rene Hutchings, FAG was tapping at spindle speeds ranging from 120 to 130 rpm. Today, its seven Chiron and Kira machining centers run rigid-tapping cycles at 900 to 1,200 rpm.

Hutchings credits the rigidity of the machine tools, tooling and fixtures. The whole system must be rock-solid in order to hold tight tolerances at high speeds, he said. Premium-quality taps are harder and sharper and can cut more holes than their less expensive cousins. What they cannot do, said Hutchings, is tolerate any misalignment. At FAG, hole-location tolerances of ±50µm are maintained.

The sources of misalignment"spindle and toolholder runout and lack of fixture rigidity"must be addressed up front and monitored continuously. Failure to do so will lead to tap breakage or the production of bad parts.

To minimize problems associated with misalignment, FAG regularly checks the accuracy of its fixturing and machine tools. It pulls a part from each fixture"72 parts in all"and sends them to the company’s gage laboratory. There, every dimension of every part is measured on a CMM. The measurements are compared to historical data.

Even if all of the parts fall within tolerance, Hutchings looks for trends in the data that could indicate any drifting in the alignment of the machine spindle, toolholders or fixtures. If there’s evidence of a misalignment, an investigation is launched to pinpoint the source and correct the problem before any nonconforming parts are produced.

Tapping Into Thread Mills

The next step in high-speed threading for FAG Automotive might not be tapping. FAG’s technical leader, Rene Hutchings, is currently conducting tests on a combination tool that drills, thread-mills and chamfers in one pass. The tool, called the ThreadBlaster, is available from BBT Manufacturing Solutions Inc. In operation, the tool is placed in the Z-axis position and begins FAG Automotive is testing a combination drilling/thread milling/chamfering tool that could save the company tens of thousands of machining hours annually. a helical interpolation movement above the workpiece material. It then spirals directly into the solid material, cutting the hole and the thread form in the same pass. At the bottom of the hole, the tool moves to the centerline and rapids out of the part in the Z-axis. The resulting chips are very small, resembling grains of pepper. Because of this, this tool can be used on a variety of materials: aluminum, cast iron and a variety of standard and exotic steels, such as stainless, Hastelloy and Inconel. In a recent demonstration performed for FAG, the ThreadBlaster was used to thread-mill several 3/8-16 blind holes that were 1" deep. The cycle time was just four seconds from hole to hole. The automotive supplier tapped approximately 10 million holes in 1999. It figured that if it had used the ThreadBlaster instead of taps, it could have saved more than 20,000 machining hours. -D. Esford

Hutchings said that because of the emphasis on alignment and rigidity, FAG seldom breaks a tap. Its machining centers run 24 hours a day, six days a week and tap 10 million holes per year. Yet a tap breaks, on average, only once every two weeks per machine.

Pushing Speed. As mentioned, FAG taps at 900 to 1,200 rpm. The speed is determined by the hardness of a particular lot of wheel-bearing hubs, which are forged and annealed before they arrive for machining. Hutchings explained that the annealing process causes variations in hardness from lot to lot.

A harder-than-expected lot can quickly snap taps if the machine spindle is running too fast. On the other hand, a softer-than-normal lot will result in improper chip breakage or tap damage.

The most efficient way to address hardness variations in steel, according to Hutchings, is to use a CNC machine’s feed override. This lets the operator increase or decrease the spindle speed by a specified percentage.

Tapping is monitored closely during the early stages of a part run, with the machinist watching for tap breakage or stringy chips. If taps are breaking, the feed override is dropped below 100 percent. If the chips are stringy, the feed override is bumped above 100 percent until they begin breaking efficiently.

Hutchings said that these problems usually present themselves almost immediately, which makes it possible to stabilize the process before serious damage is done.

Unlike Wieland Precision and ADC, FAG utilizes a tool-breakage-detection system while tapping. FAG’s system consists of a simple electrical switch in the middle of the part fixture. The machine is programmed so that the tap touches the switch after it taps a fixture filled with parts. This sends a signal to the control. If a signal is not received, it means the tap has broken, which triggers an alarm.

Tapping Challenges. FAG uses premium-HSS taps coated with TiN or TiC. While there is nothing distinctive about the design of the taps the company uses, Hutchings said that the coatings make them far more resistant to wear.

Hutchings added that a premium tap costs $18, but it will tap four times as many holes as a tap costing half as much. He also emphasized that the wear benefits of a premium-quality tap will be negated if rigidity and alignment issues are not addressed.

Hutchings has experienced problems getting taps resharpened. He finds that many vendors refuse to recoat taps after resharpening. A resharpened"and therefore uncoated"tap will last less than 30 percent as long as a new coated one. So, for now anyway, Hutchings finds that it’s cheaper to replace taps than to regrind them.

Coolant Tips. FAG uses standard flood coolant because it taps fairly shallow through-holes. Coolant lubricity is the key to thread consistency and tap life, according to Hutchings.

Expert Advice
The three manufacturers profiled have developed their own methods for tapping threads, with each basing its approach on the type of hole it taps, the material being tapped, available equipment and how it has historically tapped parts. Clearly, given the many variables involved, there’s no "best" way to tap high volumes of holes at high speeds.

Still, all of the pros interviewed recommended the following:

• Rigid tapping is slower than tapping on dedicated drilling/tapping machines, but it may be the best solution for a given volume, material, part size or hole location. Consider self-reversing tapping heads to extend spindle life.
• Use a rigid, accurate machine tool. One tapping professional we interviewed was able to retap a hole 10 times in a row on a vertical machining center without exceeding the thread gage specification.
• Use coated taps. Apply the coating the manufacturer recommends for the material you machine. Our three experts favored replacement over regrinding.
• Pay close attention to the coolant’s concentration levels in order to maintain proper lubricity. Lubricity is the key to good tap performance and long tool life.