I have tried to search this in google but I haven't found a complete explanation, so I ask this here. I also have read other answers related with hex bit socket, but it still not satisfied.

How the mechanism in screwdriver hold the hex bit? The specific part that I'm asking is the groove area in the bottom part of this image:

hex bit shank

I have screwdriver like in this image:

T handle screwdriver

I noticed that the bits and the screwdriver bar (that have notches in its side) all have 1/4 inch size. Also, the screwdriver can hold the bit from the first image securely. And when I browse the cordless screwdriver and impact drivers, all have the same 1/4 inch hex chunk.

There is an article in Wikipedia about Hex shank, but it doesn't explain about the groove part of the bits, and what the history behind its design.

I'd like to know the mechanism inside the screwdriver to do this. I also want to know the history of why we stick with 1/4 inch size. While in the wrench, we had 1/4 inch, 3/8 inch, and 1/2 inch male square head:

wrench 1/4 3/8 1/2

  • You can get 3/4 and 1” square drive as well...
    – Solar Mike
    Mar 13, 2019 at 5:50
  • Hello, and welcome to Home Improvement. Details of tool mechanisms may not be a good match for our site; let's see if you get any good answers. Mar 13, 2019 at 11:13
  • 2
    Notch is most likely for a locking mechanism to hold the bit into the device. Ratchets have a push button and a ball bearing to secure the socket. Ratchets also come in various sizes based on needs of material and basic torque. Imagine trying to unholy a 3" bolt and nut with a 1/4" drive or vice versa.
    – Jeff Cates
    Mar 13, 2019 at 11:17
  • @SolarMike I see. Is there are any adaptor to convert between sizes? In the square head, there are adaptors to enable 1/2 wrench to fit 3/8 and 1/4 socket key. Mar 14, 2019 at 0:36
  • @DanielGriscom well, this is the only StackExchange sites that discuss power tools, so I hope there are members here with the knowledge about how those tool work:-) Mar 14, 2019 at 0:38

2 Answers 2


I'm going to throw out some opinions here that I don't have hard evidence to back up in form of citations.

The Wikipedia article you liked kind of mentions this, but the reason hex shank (6 sides) is a thing is most likely because drills normally have chucks with 3 jaws. A shank with 6 sides is a natural result of that.

The extendable bits you show with the small notches are only good in very low force situations. I have a driver with a bit like that and it never gets used because when I really need to press on a screw, the bit slips into the handle. The small notches are designed so that a metal ring can tighten around the shank in the grooves to hold the bit. The grooves are small, so it can only support so much weight.

The larger round groove in the hex shank is made to hold the bit from slipping in or out and the deepness of the groove means it can support much more weight. The circular profile allows locking to be done with a thick ball bearing rather than a thin metal ring. Ball bearing locking mechanisms are found in many well made padlocks because of their simplicity and ability to resist very strong forces.

enter image description here

Ball bearing locking mechanism from LockLab

The 1/4" diameter is based on the typical torque these tools are used at. Cordless drills are the main users of hex shank, and then driving screws is the main end goal. Screws have a maximum torque they can accept before breaking, and the drill obviously has a maximum force it can apply. It's arguable that todays technology can over-torque a 1/4" shank, but in the past a cordless drill just wasn't that strong. Anything thicker would be overkill for the common task at hand.

  • Looks like a reasonable set of opinions and assumptions.
    – FreeMan
    Mar 13, 2019 at 16:49
  • Thanks, it makes sense. So in the square driver, the ball is in the male side, while in this hex driver, the ball will be inside the female side and the male only has notches or groove. Mar 14, 2019 at 0:35
  • 1
    Yes, that is correct. On the socket wrench, some balls are spring loaded and some are actuated with a button. Any hex shank holder I’ve seen is operated with a collar.
    – JPhi1618
    Mar 14, 2019 at 0:47

since your focus is on the 'groove at the bottom' of that driver bit... Those are for a "quick release" bit holder. Here is an example: enter image description here In this picture, the short 'driver bits' are meant to be pushed into the 'quick-release' holder. The driver bits snap into place and are held securely by some ball-bearings inside the blue collar, and to release the bit you must pull the blue collar backwards.

Of course you can use these "quick release bits" in any hex holder, but the purpose of the groove at the back is to hold them securely into a quick-release.

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