Why Do Phillips Screwdrivers Cam Out? The Real Reason The Bit Slips

Phillips screwdrivers slip out of the screw, an effect called cam-out, because the four arms of the cross are cut on a slight slope. Twist hard enough and that slope turns part of your effort into an outward shove that lifts the driver up and out. The popular claim that Henry Phillips built the screw to slip on purpose is a myth, because his own patents list a driver with “no tendency to cam out” as a main goal, and the real aim was a screw that centers itself under fast factory power tools.

Picture the last time you built flat-pack furniture. You are on the final panel. You lean into a Phillips screw, the driver bites, and then it happens. The tip rides up out of the cross with a nasty crunch and skates across the wood. The little cross is now a chewed-up mess, and the screw is going nowhere.

You have just been beaten by a piece of hardware that costs less than a cent. And if you go looking for why, you will meet the same tidy answer everywhere. Henry Phillips, the story goes, designed the screw to slip on purpose. It was clever, they say, a built-in safety valve to stop factory workers overtightening.

It is a great story, and it is everywhere. It is also wrong, and the proof is sitting in Phillips' own patents. Let us start with what is actually happening in that stripped little cross.

What Does It Mean When A Screwdriver "Cams Out"?

Engineers have a name for that sickening slip. They call it "cam out."

Cam out is when the screwdriver rides up and pops out of the screw head while you are turning it. It does not happen at once. It happens once you push past a certain amount of twisting force. Below that point the driver stays put. Above it, the tip climbs the walls of the recess and jumps clear.

The word "cam" is the clue. A cam is a shaped surface that turns motion in one direction into another. Your Phillips screw plays that trick on you. It turns your twist into a lift, and the lift throws the driver out.

So the driver is not slipping. It is being pushed. To see why, we need to look at what a screw actually is. Hold on to one idea from here on: a screw is a ramp.

Why Is A Screw Just A Ramp Wrapped Around A Nail?

Picture a staircase next to a wheelchair ramp. Both reach the same door. The stairs are steep and quick. The ramp is long and gentle. It lets you trade a short hard climb for a long easy one.

A screw is that ramp, rolled up. Look at the thread winding up the shaft. Unroll it in your mind and you get a long, gentle slope. Engineers call this shape a simple machine, and the screw is one of the classic six. It is, in plain terms, "an inclined plane wrapped around a cylinder."

That wrapped ramp does something useful. When you turn the head, the thread pulls the screw forward into the wood. Your gentle twist becomes a strong forward push. The screw "transforms rotational force, or torque, into a vertical force." A small turn buys you a big squeeze. That is the whole point of a screw.

Here is the catch. A ramp works both ways. It can also turn your twist into a shove you did not want. In a Phillips head, a second set of little ramps hides in the cross itself. Those are the ones that bite you.

So Why Does A Phillips Bit Climb Out Of The Cross?

Look closely at the four arms of a Phillips cross. The walls are not straight up and down. They lean. Each wall is cut on a slight slope, so the cross is a touch wider at the top than at the bottom. Those sloped walls are called tapered flanks.

Now push your driver in and twist. Your force lands on those sloping walls. And a sloped wall can only do one thing with a sideways push. It splits it. Part of your effort goes where you want, around, turning the screw. The rest goes where you do not, straight up the slope, lifting the bit.

That upward part is the villain. The Phillips cams out because of "angled contact surfaces, which create an axial force." That force pushes the driver up and out as the twist climbs. "Axial" just means along the line of the screw, straight up toward your hand.

At low force, the upward push is small. Friction and your grip hold the bit down, and the screw turns. But the harder the screw fights back, the harder you twist. The harder you twist, the bigger that upward push grows. Pass the tipping point and the lift wins. The bit rides up the slopes and leaps out, and the soft metal of the cross gets shredded on the way. That is a cruel joke to play near the end of the job.

Did Henry Phillips Design The Screw To Slip On Purpose?

Now for the famous story. You have probably heard it, because it is repeated as fact almost everywhere.

The claim goes like this. Phillips wanted the screw to slip. On a fast car assembly line, a worker with a power tool could crank a screw too tight and snap it. So Phillips, the tale says, designed the cross to cam out on purpose. Once the screw was snug, the driver would pop free and warn the worker to stop. A slip, in this telling, is not a flaw. It is a safety feature.

It is a lovely story. It turns a daily frustration into secret genius, and you can see why it spread. It feels like something a clever engineer would do.

There is only one problem. The people who wrote the patents said the exact opposite.

What The Phillips Patents Actually Say

We do not have to guess what Phillips wanted. He wrote it down and filed it, and the patents are public.

Start with U.S. Patent 2,046,839, filed in 1935 by Henry Phillips and Thomas Fitzpatrick. It lists the goals of the design in plain words. One is a screw head shaped for firm grip with a driver. The patent's phrase is blunt: there should be "no tendency of the driver to cam out of the recess." It says it twice, adding that a well-fitted tool "will have little or no tendency to cam out."

Read that again. The central goal was to stop cam-out, not to cause it. The inventors treated slipping as a defect to design away. It is hard to build a feature on purpose while writing "no tendency" toward it into your own patent.

So where did the "camming" idea come from? A second patent does use the word, but not the way the myth claims. It talks about a "camming action" that pushes grit and dirt out of the recess, so a bit of dust cannot stop the driver seating. In the same patent, the goal for the driver is the reverse of slipping. The driver, it says, is "locked in its proper centralized working relation with the screw." Any danger of it leaving is "thus entirely eliminated."

That is the smoking gun. "Camming" in the patents means clearing debris. The driver is meant to lock in and stay. The whole slip-on-purpose legend rests on a word that meant something else.

Who Really Invented The Phillips Screw?

Here is a twist worth its own aside. Henry Phillips did not even invent the Phillips screw.

The cross-shaped screw was dreamed up by a man from Portland, Oregon named John P. Thompson. Thompson filed for two patents in 1932, one for the screw and one for the driver, and they were granted in 1933. He had spotted a real problem. The old slotted screw, the one with a single straight groove, was a pain on a fast line. It was hard to line the driver up, the tip slid out the open ends, and every screw needed a closely matched bit. A cross fixed all three.

But Thompson could not sell it. Manufacturers were not interested. So the rights passed to Henry Phillips, then a manager at an Oregon mining firm. Phillips had the salesmanship Thompson lacked. He set up the Phillips Screw Company and talked the American Screw Company into making the thing.

His pitch was never "it slips." His pitch was that the screw centers itself. Point a power driver at a cross and it drops into place on its own, even if your aim is a little off. On a moving line, with a spinning tool, that self-centering trick saves a second on every screw, and there are a lot of screws in a car. General Motors bit. They tested Phillips screws on the line and built the 1936 Cadillac with them. (A few sources put the Cadillac rollout at 1937.) By 1940, 85% of American screw makers had licensed the design. The selling point was speed and self-centering, and slipping was never in the sales brochure.

If It Wasn't Designed To Slip, Why Does Everyone Say So?

So the myth is wrong. But myths usually grow from a seed of truth, and this one does too.

Cam-out was real. Phillips screws did slip, and the inventors knew it, even as they tried to cut it down. Then something unplanned happened. The early power tools on those lines had crude clutches. A clutch is the part that stops the tool once the screw is tight. These early ones were bad at it, and kept twisting when they should have quit.

On those bad tools, the slip turned out to be handy. When the clutch failed to stop, the screw would cam out and save itself from being cranked to death. A flaw became a lucky backup. Over the years, people saw the useful result and assumed it was the plan all along. That is how an accident gets promoted to a feature in the retelling.

The true position is a middle one. Cam-out was not the goal. The patents wanted firm grip and self-centering. But cam-out was a tolerated side effect that sometimes helped, and helpfulness got mistaken for intent. It was not designed to slip. It just slipped in a way that, now and then, did you a favor.

Why Does Canada Use Square Robertson Screws?

If sloped walls cause the slip, the fix is obvious. Get rid of the slope. That is what the better drives did, and it is why serious workshops have mostly moved on from Phillips.

Take the Robertson screw, the one with a square hole. It was made by a Canadian, Peter Robertson, and it is everywhere in Canada. Its walls are square and nearly straight, so there is barely any slope to lift the driver. A Robertson bit sits down in the square and grips. It even holds the screw on the tip on its own, so you can drive one-handed. Unsurprisingly, Robertson screws "significantly reduce cam-out" next to Phillips.

Pozidriv And Torx: The Cross And Star That Grip

Pozidriv took the cross and fixed its walls. It looks like a Phillips at a glance. The difference is the flanks. "A Phillips screwdriver has slightly tapered flanks." A "Pozidriv screwdriver has parallel flanks." Parallel walls do not split your twist upward, so the bit stays down. The Japanese JIS cross is another near-lookalike, cut to a slightly different profile of its own.

Then there is Torx, the six-pointed star you see on electronics and cars. Its lobes are close to straight-walled, so almost none of your twist becomes lift. Torx moves far more torque than a cross, and it "resists cam-out" by design. If you have wondered which drive is better for heavy work, this is your answer.

So Why Do We Still Use Phillips Screws Everywhere?

Here is the real puzzle. If Robertson and Torx grip so much better, why is your junk drawer still full of Phillips screws? Why does the screw with the known flaw still win?

The answer is that the flaw and the strength are the same shape. Those sloped walls that spit the driver out are also what guide it in. The self-centering that sold the screw to General Motors is still the best thing about it. For most jobs, a hinge, a light switch cover, a bookshelf, you do not need much force. You need to seat a screw fast and move on. A Phillips does that for almost nothing.

It is cheap to make, easy to find, and forgiving to aim. It is good enough for the low-force work that fills a house, and "good enough and cheap" is a hard combination to beat. The everyday hardware around you is a long list of good-enough winners.

So the next time a Phillips driver climbs out and chews up a screw, you can let go of the myth. Nobody engineered that slip to teach you a lesson. It is just a ramp, doing the one thing ramps do, at the worst possible moment. The screw that centers itself so kindly on the way in is the same screw that throws you off on the way tight. It was never a warning. It was only ever geometry.