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No Turn of the Screw PUBLIC ACCESS

Taking Them Out Isn’t Always Easy.

[+] Author Notes

Associate Editor

Mechanical Engineering 124(12), 42-45 (Dec 01, 2002) (4 pages) doi:10.1115/1.2002-DEC-5

This article reviews about Torx fasteners that are not that easy to remove and needs better access for the removal. There had been no way to fit any other tool but a screwdriver through the hole by which he had originally tried to unscrew the fastener. Better access means that to remove the tilt wheel assembly, a job involving pressed pins and more special tools. Torx fasteners were originated in the late 1960s,when the Camcar division of Textron, developed their design to overcome some of the limitations of other screw styles. The Torx design resisted cam-out better than did Phillips screws. It also reduced the drive angle to 15° from the high 60° angle of hexagonal socket head screws or the still higher 90° angle of the square socket, or Robertson, screw. The new Textron design also relies on a vertical sidewall as a way of increasing a drive bit’s engagement.

To get at the Chevy's failed ignition lock cylinder, the Saturday-morning mechanic hoped a shortcut would trim the three-hour job, as detailed in the shop manual, down to an hour or so. After pulling the steering wheel and relocating the turn signal/high beam/cruise control cluster, he was able to find a single Torx screw buried beneath a few wires and the tilt-wheel spring. That screw was all that stood between him and his sliding the cylinder straight out of the column. Piece of cake, he thought.

But the cake flopped. The little Torx screw refused to budge. Instead, the drive bit rounded off the fastener’s flutes and rendered the screw head useless. The weekend warrior wouldn’t be buttoning up this job so soon after all.

To remove the screw now he’d have to arrange better access. There was no way to fit any other tool but a screwdriver through the hole by which he had originally tried to unscrew the fastener. Better access meant he’d have to remove the tilt wheel assembly, a job involving pressed pins and more special tools.

The setback quickly swallowed our mechanic’s enthusiasm, leading to panic and more failure. This day’s neat little project had suddenly expanded into an undertaking that would take him days to resolve.

Nearly 30 years ago, writer Robert Pirsig compiled a list of “gumption traps” common to motorcycle maintenance. To this mechanic, who’d just fallen into one of those traps, Pirsig probably would have prescribed a spell of fishing until his enthusiasm for the job returned. Anything would have been a better choice than the large drills and chisels and hammers to which he eventually turned.

Torx fasteners weren’t all that well-known at the time Pirsig wrote his book. On the Web, anyway, they’ve since garnered a reputation for stubbornness.

One fellow says he’d like to stuff his broken prop shaft down the throat of the man responsible for putting Torx on that particular assembly. Another says he could deal with the many sharp edges and fussy little parts he’d encounter on the way to taking his steering column apart, but the Torx fasteners tell him he had better leave the job to the pros.

Perhaps a call to Alden Inc. of Wolcott, Conn., to pick up one of the company’s X-Out tools could have salvaged some of the mechanic’s Saturday. X-Out works on the stripped heads of almost any fastener, according to company vice president Peter Bergamo. Chucked into the end of a reversible power drill, the tool bites down into the head while turning counterclockwise.

The snapped hexagonal bolt, kin of the stripped screw socket, calls for one of the company’s small or large extractors that combine left-handed drills with matched diameter easy-outs. After center punching the broken bolt, the mechanic drills down into it about a half-inch, depending on the bolt size. Then, the mechanic twists the accompanying extractor into the hole in the same direction as the drill bit rotates, backing out the bolt fragment.

With luck, the extractor does the trick and the mechanic returns to his original quest. But what if the same careless hand that rounded out the original fastener breaks off the extractor inside the hole? Now, the mechanic’s facing not only a broken screw but a hardened tool steel extractor that’s stuck fast.

Alden necks down a portion of the extractor shank to avoid just this instance. But, among extractor makers, this seems to be an exception rather than a rule.

The next step—and it’s a big one—is to remove the most transportable portion of the assembly from the car and haul it down to the tool and die shop. At the shop, employing an electrical discharge machining, or EDM, process called spark eroding, a machinist could burn out the bolt and broken extractor.

At least one company builds a portable EDM machine that can be carried to the work. A broken tap, die, bearing, stud, or extractor can be burned out on the spot.

“It’s not something you’d use every day,” said Tapbuster Ltd. president Mick Moran.

“What we’re really selling is time,” he said from his office in Warwickshire, England.

Moran has successfully extracted studs of up to 48 mm diameter with the 50-lb. machine. The process takes time—two or three hours to erode a 5- to 6-mm hexagonal hole down the middle of a broken half-inch stud, for example. But factoring in the teardown time needed to get an engine out of a bus, say, and the time needed to install a replacement, the portable EDM machine eventually passes a breakeven point as it erodes away not only any conductive material but also the high cost of downtime itself.

Material hardness makes no difference to speed, although the constituents of stainless steels—nickel, chromium, and cobalt—can slow the process some.

The Tapbuster uses a copper sleeve as an electrode, which the machinist locates within 0.1 to 0.3 mm of the broken part. A spark jumps the gap anywhere from 2,000 to 250,000 times a second, vaporizing a bit of metal each time. A coolant is needed.

Given all the bother of using an extractor, it might be easier just to phone in one of these EDM house calls as soon as you’re holding a broken bolt head in your hand. But it seems like overkill just to remove the cheap little screw, stud, or bolt that couldn’t have cost any more than pennies upon its initial installation.

On the way to developing that all-important “feel,” mechanics break fewer and fewer bolts, for sure. Heat, penetrating oil, impact, and patience they learn to apply in just the right measures can coax out even the most stubbornly seized fasteners. But what about the rest of us?

Torque, insertion speed, and bit wear are fastening concerns on the assembly line. But manufacturers must consider service issues, too, as this one did for a magnesium steering column build.

Grahic Jump LocationTorque, insertion speed, and bit wear are fastening concerns on the assembly line. But manufacturers must consider service issues, too, as this one did for a magnesium steering column build.

A collection of special threaded fasteners designed for thread rolling in magnesium also depicts variations of the Torx-based screw head.

Grahic Jump LocationA collection of special threaded fasteners designed for thread rolling in magnesium also depicts variations of the Torx-based screw head.

Torx fasteners originated in the late 1960s, when the Camcar division of Textron developed the design to overcome some of the limitations of other screw styles. The Torx design resisted cam-out better than did Phillips screws. It also reduced the drive angle to 15 degrees from the high 60-degree angle of hexagonal socket head screws or the still higher 90-degree angle of the square socket, or Robertson, screw.

Cam-out wasn’t always a bad thing, though. Before torque-limiting screwdrivers debuted, the original Phillips cruciform socket allowed the drive bit to disengage once a fastener had been driven home. This feature led to their use on Cadillacs in the mid-1930s, said Michael Mowins, president of Phillips Screw Co. of Wakefield, Mass.

Before then, the design drew no attention from some of the day’s major screw manufacturers to whom inventor Henry Phillips showed it. But, in 1933, Eugene Green, the American Screw Co.’s new president, pronounced the Phillips screw the wave of the future at about the same time a shop foreman there was developing a method for cold heading the cruciform socket. By the beginning of World War II, there were 40 licensees worldwide, Mowins said.

Neither hex nor square sockets suffer from high cam-out rates. But their steep drive angles can create radial stresses along a fastener’s recessed walls, said Terry Tripp, director of Textron Fastening Systems’ Applications Engineering Group. High stresses lead to failure of the recess or the tool. To eliminate these problems, the company’s latest design—Torx Plus—uses elliptical geometry to create a zero-degree angle between the driving and driven faces of the tool and the screw.

You realize that this one, individual, particular screw is neither cheap nor small nor unimportant. Right now this screw is worth exactly the selling price of the whole motorcycle, because the motorcycle is actually valueless until you get the screw out.—Robert Pirsig on the stuck fastener, Zen and the Art of Motorcycle Maintenance, 1974.

The new Textron design also relies on a vertical sidewall as a way of increasing a drive bit’s engagement. Because straight walls leave it to chance that a spinning bit will catch the drive flutes during assembly, the company offers an optional ramped head design that helps guide a turning drive bit down into the recess.

Torx Plus doesn’t mean that you’ll have to start buying new bits. In fact, unless you’re a mechanic working for a General Motors or Harley-Davidson dealer, you can’t. It’s simply too early in the service cycle, Tripp said. It may be a while before all 170,000 retailers that sell Torx bits stock the new design. But acceptance of the design—which won patent protection in 1992—is growing at a quicker rate than it did for the predecessor, Tripp said.

For now, backward compatibility ensures that an old T-50 bit will take out a new 50IP screw. The newer bits won’t fit the older screws.

When it comes to new shapes, Phillips hasn’t been resting either. In 2002, the company introduced Mortorq to aerospace manufacturers, Mowins said. The recess resembles the traditional Phillips cruciform with the flutes twisted slightly. It’s said to be forgiving of both driver misalignment and paint buildup.

Tolerating misalignment between driver and screw is a particularly strong suit of the Phillips screw and its variants. Straight walls can generate vast torques, Mowins said, but do so at the expense of accommodating misalignment.

Poor alignment between our mechanic’s screwdriver and his Torx screw was one possible explanation for the flute’s failing, Mowins reasoned. Neither he nor Tripp suggested the real cause might have been a heavy-handed mechanic.

Screw heads seem to last 20 to 30 years, Mowins said. In the 1960s, Phillips introduced Pozidriv just as the patents on the original cruciform shape began running out. The design incorporated negative draft in the driving sidewalls.

Since the ’70s, the company’s ACR designs have competed with Torx. ACR relies on ribs, both in the cruciform socket and on the drive bit, to create a stick fit between them. Phillips’ Torq-Set design, with its offset cruciform, can withstand 50 percent more torque coming out than going in. This makes it a popular choice among aerospace manufacturers who need to design for frequent disassembly.

Polish broken stud clean so weld adheres there first. Fill metal pool inside dirty nut. Apply wrench, coated liberally with the patience left out first time around. Repeat, repeat, repeat. Send pix to ME.

Grahic Jump LocationPolish broken stud clean so weld adheres there first. Fill metal pool inside dirty nut. Apply wrench, coated liberally with the patience left out first time around. Repeat, repeat, repeat. Send pix to ME.

With so many resources exploring fastener heads, there should be nothing left for thread design. But innovation reigns there, as well.

Magnesium, increasingly popular among automakers for its casting and melting properties, dimensional stability, and high strength-to-weight ratio, has one drawback if you make screws for it.

According to Larry Pickett, who is a product manager at Textron Fastening, magnesium doesn’t conform very well to thread-forming fasteners. Threads can crack and chip as they are made, and their condition worsens with the repeated removal and reinsertions of screws during repairs.

Until you take out such a screw, Pickett said, it’s analogous to a broken, wrapped peppermint candy. Any bits of magnesium broken during the initial forming are held intact as long as the screw is in place and unshaken by vibration. But unwrap the candy, or remove the screw, and chips can fall everywhere.

A certain degree of cam-out was incorporated into the design from the beginning. However, what worked on the assembly line has bedeviled handymen ever since.—Witold Rybczynski on the Phillips screw, One Good Turn: A Natural History of the Screwdriver and the Screw, 2000.

To circumvent this trouble, Textron’s Mag-Form screws use threads with a broad flank angle (105 degrees compared with 60 degrees for traditional screw threads) to increase compressive force and decrease shearing force during thread forming.

The new screws for magnesium assemblies were developed very much with the needs of servicing in mind, Pickett said.

In one instance, a magnesium die caster had to find a way to attach a cast magnesium shift tower to a sport utility vehicle’s steel floor.

The casting and floor were eventually mated with Mag-Form screws, and the company is currently producing 7,600 of the castings every month with no assembly or service problems reported, Pickett said.

In another example, a maker of steering columns chose the Mag-Form screws for attaching the magnesium columns to their lower mounting brackets after first trying ordinary tapped holes. Roll forming fasteners that worked for aluminum and steel generated too many chips when working in magnesium, causing the fasteners to reach their torque points prematurely. Since switching to Mag-Form, the manufacturer has installed 1.5 million of the fasteners in steering columns, Pickett said.

Their advancements over the last half-century notwithstanding, threaded fasteners still manage to end up broken and stuck in some very bad places. For collectors, old cars make happy hunting grounds.

When he’s not working as a network engineer for Philips Medical Systems Inc. of Andover, Mass., antique Volkswagen buff John Henry has been known to come upon a broken fastener crying out for extraction. He has seen so many on his way to restoring a 1950s Beetle that he’s developed an odd sort of affinity for them. Of course, every one of them gives him a good excuse to fire up his MIG welder.

Henry doesn’t take credit for discovering the technique, but he has used it to extract some particularly vexing screws. He found it as he tried welding an easy-out into a broken stud with the hopes of bettering the extractors bite. With a reductionist’s aplomb, he eliminated the easy-out and just built up a metal blob with the welder to which he clamped on a pair of Vise-Grips. Nowadays, he welds a dirty nut onto the shined-up end of the stud. The method takes patience. Expect to go through a lot of nuts, he said.

For Henry, coming upon a new fastener always lights a red lamp. He approaches it with the caution of a street dog sniffing a stranger’s hand. Now, he always purchases the correct tool, too.

That last piece of advice our Saturday mechanic already knew. Like most good lessons, it’s one that’s constantly reinforced.

Before starting his repair, the mechanic had bought a replacement ignition cylinder and a couple of tools his Internet search told him he’d need. One was a steering wheel puller. The other was a spring depressor. A third tool, which he left on the shelf, was the pin puller for removing the tilt column pins. That one he wouldn’t need. He had a shortcut.

He should have gone and bought the pin puller as soon as he’d stripped the head of the Torx screw. That would have saved him from stripping the threads in one of the tilt wheel pins by trying to pull it out with an ordinary screw.

Once he finally did buy the puller, it worked flawlessly on the clean pin. But the stripped pin was too far gone. No drill, of course, could match the hardened steel pin.

As a last resort, he drilled out the old ignition cylinder. He added a couple of switches, and he learned a little about automotive wiring.

It’s not pretty, but the Chevy rides again past dealer signs touting zero percent financing.

In the end, the job cost about $30. He saved a hundred or so over what a shop would have charged. Yet, even if engineering is doing for a nickel what others can only do for a dime, the next time he’ll gladly pay the 10 cents to let someone else screw up the fix.

Square-Oriv marries ACR Phillips 11 to a square recess.

Grahic Jump LocationSquare-Oriv marries ACR Phillips 11 to a square recess.

Copyright © 2002 by ASME
Topics: Screws , Fasteners , Design
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