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What is the optimal screw drive shape (such as philips, internal hex socket, or slot) that will allow the maximum amount of torque to be applied without having the drive hole stripped? The torque can be applied to the drive hole like in a philips head or to the head itself such has a hex head bolt. The screw in question would be made of heat-treated steel. The screw would be inserted into a threaded hole but it would be nice to know about a screw that is self tapping.

Currently i mostly use philips head or hex head screws for drywall, wood, metal and plastic (abs). Of coarse my drill starts cam out and occasionally starts to strip my screw. Recently i used a square holed screw and the bit was so secure that the drill started to twist violently. This was not something i was used to as iv mostly used philips. So i was wondering based on the material strength what hole size would be optimal for high torque situation? 

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    Do you mean 'what type of screw head'? I had to read your question a couple of times because my first thought was "I think all screw holes are round..." – DA01 Oct 12 '15 at 3:22
  • Hint: use a power-driver with a torque adjustment. Set the torque low enough so that (1) the driver doesn't cam-out, (2) you don't snap the screw, and (3) you don't hurt your wrist by the driver twisting in your hand. – DoxyLover Oct 12 '15 at 3:57
  • My question was just relatively for my interest. In many cases applying to much torque could damage the screw or as Doxy stated in the three examples. But you are right doxy about setting the torque limiter. I was playing around with that which then lead me to fathom this question. – user26409 Oct 12 '15 at 13:50
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I would say the optimal screw hole shape that will allow the maximum amount of torque to be applied, would be no hole.

Removing material from the head of the screw; no matter what shape, will weaken the screw slightly. The only way to maintain the full strength of the screw; and therefore achieve maximum torque, is to not remove any material. So a hex head screw (not to be confused with a hex drive screw), would allow for maximum torque.

Hex head screw

  • This is the correct answer IMHO - but the OP did ask about hex-headed bolts too (if you read carefully - "The torque can be applied to the hole like in a philips head or to the head itself such has a hex head bolt." – RedGrittyBrick Oct 12 '15 at 10:43
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For most home building applications, the 'high torque' option that would be most commonly available would be the (appropriately named) torx screw:

enter image description here

Another option would be the Pozidriv:

enter image description here

Made by Phillips, it looks like a Phillips, but has 4 added notches at a 45 degree angle to the main 4. It's designed to not cam out like a Phillips is.

Other screw heads used for high-torque applications that you wouldn't necessarily see in home building applications:

Polidrive (used in the automotive industry): enter image description here

Spline: enter image description here

Torque-set (aerospace industry): enter image description here

Triple-square: enter image description here

As for square drive heads, while they do offer more torque than flat or phillips head bits, they have a tendency to 'round out' if the bit isn't properly seated. The main benefit to square heads is that the screw 'sticks' to the bit all by itself, so makes it a lot easier to get in the right spot in a lot of situations.

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This is something of a religious debate.

It Is Claimed that Phillips was actually designed to "cam out" above a certain level of torque. I'm not sure I believe that.

It Is Claimed that square-recess, or torx, or any of several other designs are best for transferring torque with least risk of cam-out and resulting damage. I've used square and it does seem to be an improvement, though it isn't perfect. I haven't used enough of the other variants in enough situations to have a valid opinion.

Some of the new designs allow "trim-head screws", which are about as close as a screw can get to finishing nails. I used trim-head square drive screws for some window trim last year, and didn't feel too bad about leaving them visible.

Quick point: You probably do want the driver to cam out at a torque below that which would snap the screw...

  • Indeed, phillips heads are designed to cam out when used with the proper bit. This was particularly useful on the assembly line. A newer variant by the same company, the Pozidriv, is designed not to cam out (even though it looks similar). – DA01 Oct 12 '15 at 3:24
  • Ah. Wasn't sure whether Posidriv was a functional variant, a competing patent, an attempt to renew patent protection, or other. – keshlam Oct 12 '15 at 3:38

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