Confused about screws

[TheTiddles]

The original patents for things often don’t list all the advantages, the patent for the flushing toilet doesn’t call out the u-bend that is ultimately what allowed it to be adopted, despite it being on the drawings. Sildenafil was a pulmonary hypertension drug till they realised it was rather effective on droopy dongles, the list is extensive, that’s why patents work the way they do, you can amend them and the benefits they provide.

If you’ve not seen Phillips heads being used on a production line to limit installation torque, you might not be 100+ years old that you’d need to be to have seen it. Compact torque limited drivers became common before the last war both in pneumatic and manual versions though manual installation has been pushed for many critical applications such as watch making, aircraft and yacht fitting where the damage from a slip can be so expensive.

 

[Jonm]

Interesting thread this. I have a couple of comments re the above Jonm:

1. After over 50 years in aviation (military and large civil - 20 pax and up, mainly but not exclusively maintenance), I have "almost never" seen PH heads used (and definitely never re the square drive Robertson - also a claim sometimes made for those - IMO - horrible things)!

2. MAYBE in "light/sports/general aviation" aeroplanes but not on big-uns IME;

3. If you read the EUMRO text above, it "smacks" of America and not "English English". For example I've NEVER heard of "cam out" being called "throw-out" in English, and in line 5 of the blurb, one reads "most" and not almost, which is what yer average English speaker would write. Just saying that EUMRO is clearly a supplier and NOT a manufacturer (of tools or fasteners it seems) so I think they're pushing some US stuff. Not necessarily a bad thing at all, but not "standard English practice" either in my experience - even though most big commercial aircraft (apart from Airbus) come from N. America these days;

4. Still re EUMRO, IME, maintainers aren't so interested in cam out damaging the screwdriver/bit, but definitely about bloody great scratches on aircraft skins (skins are, generally speaking, depending on exactly where they are, are an integral part of the whole load-bearing structure. Scratches in load-bearing skins are generally speaking NOT to be tolerated at all.

Not saying any of the above is all definitely wrong Jonm, but definitely doesn't jibe with my own experience.

P.S. There's a member here "Inspector" who if I remember is a Canadian working/has worked in aviation. I wonder what his take is on the above - especially "cam-out" v "throw-out"?

But separately, isn't it "interesting" that so many of us have our own opinions "prejudices" (?) re screw heads? (Personally - Robertson? I hate the bloody things and only use them because of my Kreg Pocket Hole jig)!

That is very interesting, this issue is well outside my area of expertise. My original comment was based on what I had been told and understood the situation to be. This post has made me look at it more closely..

I think we are talking about two different things, you are talking about current practice in the aeroplane industry and I was looking at what the original design criteria was for the phillips screw head. My inclusion of the EUMRO advert relating to the aeroplane industry has confused the two issues.

Clearly the original patents related to the tools available in the 1930’s and 40’s. The 1942 patent, number 2,474,994 which I linked to above is very clear that the revised Phillips design is intended “to provide driving torque up to that capable of destroying the thread on the screw before the throw-out force becomes great enough to force the drive out of the recess.”


I repeat the link and photos of pages 1 and 7 with the relevant text highlighted.
Here is the link
https://pdfpiw.uspto.gov/.piw?PageN...1=2474994.PN.%26OS=PN/2474994%26RS=PN/2474994
Page seven goes in to details of the then available screw driving tools and the undesirability of vertical faced slots which would not throw-out.

It seems to me that the Phillips head as modified by the 1942 patent was designed to be driven with torque limiting screwdrivers and not to cam out in normal circumstances. Page 7 states “thus there is sufficient throw-out effort to force the driver clear of the screw recess on the application of excessive driving torque and thereby prevent reaming or marring of the recess and any damage to the driver blade”

Cam out is generally undesirable and I can see that in a modern aircraft setting it is completely undesirable. This 1942 patent though is very clear that the Phillips head defined in the patent was designed to cam out in an over tightening situation as being the least worse outcome.

As I said at the beginning, I think my inclusion of the EUMRO advert has muddied the waters. I agree that it is great to hear different experiences. On this issue mine is very limited.

 

[glenfield2]

The boat I’m rebuilding was put together with slotted brass screws. Nearly 30 years later I was able to unscrew almost all of them with relative ease leaving the wood undamaged beyond the screw holes.
Says a lot for a) the quality of old brass and b) the merits of hand screwdrivering.

Pozi and the rest are great - I use them all the time - but it’s all too easy to drive them under the surface of wood and cause hell n’all damage if you have to get one out.

 

[Jonm]

Slightly off topic though with strong connotations. Using screws which are self drilling causes some users grief because they don't clamp the two parts being joined up tight. The screw cuts it's own thread into both parts so they cannot be pulled together. So either clamp tightly before screwing or drill a clearance hole in the first part so that the screw is able to pull the joint together tightly.

I totally agree if you are using screws where the thread goes from tip to head.

I recently completed a new house build, spent about £2000 on screws, vast majority driven without pilot holes and they pulled up tight without splitting the wood. The reason being they were reisser cutter screws, they are designed to be used like this, they have a smooth shank at the top end so they do not grip the top piece of wood. I attach a copy of their advertising blurb.

Only downside is price. My screw boxes have every length from 9mm to 150mm all cheap Toolstation screws and generally I pilot through the top piece of wood. I then buy expensive screws when necessary.

 

[Adam W.]

The boat I’m rebuilding was put together with slotted brass screws. Nearly 30 years later I was able to unscrew almost all of them with relative ease leaving the wood I damaged beyond the screw holes.
Says a lot for a) the quality of old brass and b) the merits of hand screwdrivering.

Pozi and the rest are great - I use them all the time - but it’s all too easy to drive them under the surface of wood and cause hell n’all damage if you have to get one out.

I like slotted brass screws too.

 

[AES]

I like slotted brass screws too.

They certainly do look great in applications where the head can be seen - especially if they're "clocked" (all the slots lined up).

That is very interesting, this issue is well outside my area of expertise. My original comment was based on what I had been told and understood the situation to be. This post has made me look at it more closely..

I think we are talking about two different things, you are talking about current practice in the aeroplane industry and I was looking at what the original design criteria was for the phillips screw head. My inclusion of the EUMRO advert relating to the aeroplane industry has confused the two issues.

Clearly the original patents related to the tools available in the 1930’s and 40’s. The 1942 patent, number 2,474,994 which I linked to above is very clear that the revised Phillips design is intended “to provide driving torque up to that capable of destroying the thread on the screw before the throw-out force becomes great enough to force the drive out of the recess.”


I repeat the link and photos of pages 1 and 7 with the relevant text highlighted.
Here is the link
https://pdfpiw.uspto.gov/.piw?PageN...1=2474994.PN.%26OS=PN/2474994%26RS=PN/2474994
Page seven goes in to details of the then available screw driving tools and the undesirability of vertical faced slots which would not throw-out.

It seems to me that the Phillips head as modified by the 1942 patent was designed to be driven with torque limiting screwdrivers and not to cam out in normal circumstances. Page 7 states “thus there is sufficient throw-out effort to force the driver clear of the screw recess on the application of excessive driving torque and thereby prevent reaming or marring of the recess and any damage to the driver blade”

Cam out is generally undesirable and I can see that in a modern aircraft setting it is completely undesirable. This 1942 patent though is very clear that the Phillips head defined in the patent was designed to cam out in an over tightening situation as being the least worse outcome.

As I said at the beginning, I think my inclusion of the EUMRO advert has muddied the waters. I agree that it is great to hear different experiences. On this issue mine is very limited.

View attachment 113538View attachment 113539

You have a good point Jonm. Remembering that metal structures for aircraft only started to really come in in the 1930s, and only really took off (sorry!) with WWII, then the patent blurb (and the EUMRO blurb) make a lot of sense. And you're quite correct, I WAS indeed thinking about aircraft built in the 1960's and on.

 

[spanner48]

The boat I’m rebuilding was put together with slotted brass screws. Nearly 30 years later I was able to unscrew almost all of them with relative ease leaving the wood I damaged beyond the screw holes.
Says a lot for a) the quality of old brass and b) the merits of hand screwdrivering.

Pozi and the rest are great - I use them all the time - but it’s all too easy to drive them under the surface of wood and cause hell n’all damage if you have to get one out.

30 years - and they still come out whole? You're lucky; or you have a boat that's not been in saltwater. If it had, they would have dezincified, and crumbled out as reddish dust.

Bronze is better. But I was on a 1907 Bristol Channel Pilot Cutter where the 1" bronze keel bolts were destannifying, and crumbling to dust after 70 years. Luckily, we put in stainless replacements, before the keel fell off . . . .

 

[glenfield2]

30 years - and they still come out whole? You're lucky; or you have a boat that's not been in saltwater. If it had, they would have dezincified, and crumbled out as reddish dust.

Bronze is better. But I was on a 1907 Bristol Channel Pilot Cutter where the 1" bronze keel bolts were destannifying, and crumbling to dust after 70 years. Luckily, we put in stainless replacements, before the keel fell off . . . .

Inland waterways boat so no salty stuff except on the chips.

 

[Jonm]

Most of the tools I have seen on production lines are set to the right torque so no cam out, not good if the screws are a nice matt black.

I am sure you are correct, and others have made similar comments. I think we are all mostly correct. Modern torque wrenches and training should ensure correct torque is applied.

However the point is that the patents for these screws go back to 1932/42. The idea, as defined in the patents, is that in normal use they do not cam out, it is only if excessive torque is applied that they cam out rather than “break the screw or ream the recess”. My post above gives extracts from the 1942 patent which is very clear on this point.