Why the recess matters
The recess in a screw head is the part your bit actually touches. A good match transfers turning force efficiently, keeps the bit centered and lets you push firmly without the bit slipping out. A poor match wears away the recess, damages the bit and leaves you with a stripped screw. Since you have to buy bits to match the screws, it pays to know the common types.
Cam-out
Cam-out is when the bit rides up and out of the recess under torque. It happens because the sides of the recess are angled, so the turning force is partly converted into an upward push. That push can chew the recess and the bit, and it can mar the workpiece surface.
Some designs are made to cam out on purpose. The Phillips recess is often said to have been designed so that a driver would slip out at a certain torque, which protected assembly line machines and parts from overtightening. The details of that history vary in the telling, so treat it as a common explanation instead of settled fact. Modern power tools have adjustable clutches for this purpose, which is why designs with less cam-out are now favored.
The common drive types
Slotted. A single straight slot. Simple and found on older hardware and some decorative work. The bit slips sideways easily, so it is hard to keep centered with a power driver. It is still common on electrical device screws and fine hardware.
Phillips. A cross-shaped recess with tapered sides. It centers well, which made it popular for assembly, but it cams out readily under high torque. Phillips bits come in numbered sizes such as #1, #2 and #3, and using the wrong size is a major cause of stripping. #2 is the most common size for general construction screws, though the correct size depends on the screw.
Pozidriv. It looks like Phillips with extra small marks between the arms of the cross. It has straighter walls and less cam-out, but a Phillips bit in a Pozidriv screw, or the reverse, fits poorly and tends to strip. Pozidriv screws are common in furniture and cabinet hardware, especially of European origin.
Torx (star). A six-lobed star shape. The lobes fit tightly, allowing high torque transfer with very little cam-out, and a bit seats in the recess securely enough that you can sometimes hold a screw on the bit sideways. The sizes are labeled with a T and a number, such as T20 or T25. Torx is widely used in deck screws, structural screws, automotive parts and electronics. A variant with a tamper-proof pin in the middle requires a bit with a matching hole.
Square (Robertson). A square socket with slightly tapered walls. It has long been popular in Canada and is common in construction and cabinet work elsewhere too. It grips well, resists cam-out and allows one-handed starting because the screw can stay on the bit. Square bits are sized by number, and common colors identify them in some regions, but marking is not standardized, so check the size on the bit.
Hex (Allen). A six-sided socket tightened with an L-shaped hex key or bit. It is common in machinery, furniture assembly and set screws. Hex keys come in metric and inch sizes, which are close in size but not interchangeable, and a poor fit strips the socket.
Combination or hybrid drives. Some screws accept more than one bit. A combination of square and Phillips, for example, lets the same screw work with either bit. These are convenient, though the best grip comes from using the bit the recess was designed for.
Comparison at a glance
| Drive | Cam-out | Notes |
|---|---|---|
| Slotted | High | Bit slips sideways; fine for hand tools |
| Phillips | Moderate to high | Very common; sized by number |
| Pozidriv | Lower than Phillips | Do not mix with Phillips bits |
| Torx | Very low | Good for high torque |
| Square | Low | Screw stays on the bit well |
| Hex | Low | Needs a close metric or inch match |
Choosing bits and using them well
- Match the type and size exactly. Test the bit in the recess. It should fit snugly with almost no wobble. If it rattles, try the next size.
- Press firmly while turning. Push in as you turn. This is the single biggest habit that avoids cam-out.
- Replace worn bits. A rounded tip lets go early. Bits are inexpensive compared to a ruined project.
- Use impact-rated bits with an impact driver. Ordinary bits can shatter under the repeated blows. The accessories for an impact driver are covered in another article.
- Keep the bit straight and in line with the screw.
- Control the torque. The clutch on a drill is intended to stop before you strip the head. A drill and a driver differ in how they deliver torque, so use what suits the screw.
- Choose screws with better drives for tough jobs. If you drive many screws into hardwood or work overhead, Torx or square recess screws can save time and frustration.
When a recess strips anyway
If a screw head gives out, the article on removing stripped screws covers how to get it out. A better grip on a stripped recess can come from a rubber band, a different bit or an impact driver.
Safety
Wear eye protection because bits can break, especially when overloaded. Unplug the tool or remove the battery before swapping bits, and secure the workpiece so a slipped bit does not hit your hand. This is general information, so follow the instructions provided with your tools.