Phillips vs Pozidrive and portable drills

A Torque setting works OK in an engineering context where a machine screw is being driven as the thread will have been cut and you then just want to tighten the screw to a specified torque.

However most DIYers and for that matter carpenters will be driving screws into wood which is not consisent, you could set your torque setting so that the tool drives a screw in to the desired depth and then when you come to a bit with a knot in it your clutch will start slipping before the screw is anywhere near driven home. So that is why the torque setting tends to be left high and the electronic brake together with the users reaction will govern how deep the screw is driven, even then it will depend on whether the upper timber has been pre drilled and or CKS or if the woodscrew has additional ribs to help it CSK itself which will require less torque than one with a plain CSK.

As for Phillips heads the main use of these on site is for drywall screws and you don't use a torque setting to sink these correctly. You have a dediated driver or at the very least a specific bit that will act as a depth stop that will sink the screw below the surface of the plasterboard but not far enough to tear the paper. So Torque is left high or on the drill setting so you can bash them in to any hard knots, soft timber, steel studs, etc and the depth stop will do the rest so they all end up at the same depth..

The Patent for Pozidriv was filed in 1942 and finally approved in favour of the American Screw Company in 1949. Phillips was an american businessman, not an engineer. His drive was actually invented by John P Thompson and Phillips formed a company to market it, very successfully. Later Philiips worked with the American Screw Company to develop Pozidriv as an improved Phillips. Phillips and Pozidriv are close relatives.

The patent gives lots of interesting background. 'Good old fashioned' slotted screwdrivers get a well-deserved kicking for bad behaviour when used under power, and it is also explained why the blades are tapered.

For years following the advent of the wood screw and its companion the machine screw, the conventional coupling between the head of the screw and the bit of a screw-driver was formed by a simple kerf cut entirely across the screw head and adapted to receive the substantially fiat blade of a screw-driver. The walls of the kerf were parallel to each other in most cases since the cutting was done with a small circular saw, but the engaging walls of the driver blade included an appreciable angle, not only to prevent weakening the blade but for convenience in its construction, for the usual blade was flattened from a circular rod to provide additional width and to reduce the thickness. Sharpening, after wear, should normally follow the original surfaces but the tendency was to increase the included angle to reduce the amount of material necessary to be removed in the sharpening operation.

This type of coupling was subject to numerous faults, not the least of which was the lateral slipping of the driver through the open ends of the kerf resulting in the marring of the material to which the screw was being applied. When considerable resistance to the driving of the screw was encountered, the substantial taper of the driver blade and the relatively small contact of the same with the kerf, which in most cases. was considerably wider than the thickness of the blade, resulted in a high throw-out force tending to move the driver axially away from the screw. This again caused slipping and marring. When screws were driven almost entirely by hand, conditions were not so bad, but with the increased use of power screw-drivers they were materially augmented because of the higher speeds of driving and the greater torque applied. Even though greater torque was available for driving, no greater force could be applied manually to the screwdriver to hold the same in the kerf against the resultant increase in the throw-out force.

Having used plain slots, Phillips and Pozidriv, I'd say the most important factor is having a correctly sized screwdriver tip in good condition. I'd rate Pozidriv above Phillips, and Phillips above slot heads but they're all rubbish once the head is chewed. All three systems are horribly abused by careless operators: I am one!

Dave

Dave (SOD). Wikipedia says 'The Pozidriv was patented by GKN Screws and Fasteners in 1962'. (So you see that I'm just recycling dubious info. found on the web...). I skimmed the patent to which you linked (how does anyone read this stuff?), but wasn't clear that it was put into production.

Michael G. To design a screw/driver combination to 'cam out' at a desired torque seems pretty heroic, given that the applied axial force, let alone the alignment of the driver to the fastener can't be controlled in the field. And then the driver wears and it all goes to hell. The taper of the driving flanks (ignore the point angle) is, to the Mk 1 eyeball, far shallower than a Morse taper, so one might hope that the screw jams onto the driver, rather than it being a self-releasing taper. I just don't believe that the tiny flank 'slope' will produce a big axial force from reasonable torques.

Anyway, for whatever reason, Phillips and Posidriv drivers can 'cam out', although leaning on an unworn driver with sufficient determination seems to be able to frustrate the (Phillips) designer's supposed intention. If the designer really didn't care about the need for subsequent screw removal, then I still would call him an idiot. There's a lot of them about - especially in the auto industry, where dismantling for service and repair is commonly frustrated by lack of thought - or Machiavellianism.

Grinding a bit of the point off drivers is often worthwhile, allowing better driving flank engagement.

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That's not actually what I said ... but never mind

'Camming out' is the unfortunate collateral consequence of the taper on the flanks of the Phillips screw and its driver ... which were put there for assembly-line convenience.

MichaelG.

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Edit: it is interesting to note the claim made by Phillips, at lines 16-24 in the Patent:

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2046839A&KC=A&FT=D&ND=3&date=19360707&DB=EPODOC&locale=en_EP

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Edit: I've just found this:

https://www.geniusstuff.com/videos/phillips-screwdriver-video.htm

Edited By Michael Gilligan on 08/07/2019 09:39:17

Michael: earlier, you asked why I found it difficult to believe that the 'camming out' was designed in. I was answering, not arguing with you. My original scepticism was in response to MK_Chris' post.

Wikipedia suggests that the 'camming out' was not a designed-in feature, and cites references. I think this is another instance of the harder you look, the confuseder you get...

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I also clarified my 'position' [or so I thought] by writing:

Rather than 'camming out' ... just think of it being like a self-releasing taper on a machine tool spindle. [so much more convenient than the self-locking Morse]

I suspect we have done this to death now ... and we are actually in agreement

MichaelG.

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P.S. for what it's worth ... My Dad worked at GKN in the early 1960s, so I had all this Pozidriv vs Phillips stuff almost 'first hand'

Michael Gilligan:

An earlier development in fastening technology was the tapered thread wood screw which I believe was due to GKN or, perhaps, just N.

My experience is that a 'fail' tends to damage the driver when using Phillips and the screw when using Pozi.

Neil