I think the initial "breaking loose" of the screw to get it to turn requires more down pressure and torque than does the following turns... I think back to the days of working on motorcycles and using an impact driver that was hit with a hammer to break Phillips head screws free as an extreme example. A less extreme example is when I've hit the handle of the screw driver with a hammer as I put rotational force upon it to break the screw free. More typically I push down hard with the screw driver to break the screw free and them use primarily rotational torque to remove them. When using a powered driver (usually my PC drywall driver) with various bits the same dynamic occurs... I need a lot more down pressure to break the screws loose than to remove them.
Grant Edwards wrote in news:lrirnm$7r9$1 @reader1.panix.com:
There's probably some truth in that... But remember the bit you used to make the round isn't making a good quality round. It's kinda like those Combo drive screws or the square drives that are almost, but not quite, compatible.
Make a good quality round, use a good quality round bit, and pull rather than push and the screw will come out easily.
(Do be careful not to make the round too round. You might wind up cold welding the bit to the screw. :-))
It's not. Friction is proportional to the normal force applied, so force applied to the head of the screw does increase the friction of the screw threads against the material.
We were talking about screw selection. In my world, I choose what screws I use. If I didn't build the deck, chances are it's a total loss because the fasteners aren't coming out.
Yeah, there IS a screw head design that allows you to pull. The screw has a dovetail slot, curved so the driver can be positioned at one edge and rocked into the slot. This scheme is not compatible with magnet-held hex shank bits.
Except, particularly in a woodscrew (and this IS a woodworking group) the pressure applied against the threads of the screw by the compression of the wood fibers is SO much more than the pressure you are applying to hold the screwdriver in, that the pressure you are applying is totally insignificant. And in th case of a machine screw, the pressure applied to the surface by the head due to the torque of the screw, and it's torsional stress, again makes ANY pressure you are going to apply TOTALLY irrelevent. The friction on the upper surface of the thread due to the installed tension will be 10 or more times what the friction on the bottom of the screw thread would be from the pressure you apply.
In FACT, hitting a screw on the head while turning it is a well known, shop worn method of removing a stuck screw or bolt.
Not nearly as "beyond belief" as thinking the pressure you apply to the head of a screw to keep the driver engaged is going to increase the torque required to remove screws that have weathered a decade due to increased friction in the threads!!!!
If that were true, very little torque would be necessary to remove them.
Exactly what is "tortional stress" and how does "stress" relate to the amount of torque it requires to remove a screw? I've never seen that one in a physics book.
Wrong. The friction on the threads is quite relevant.
No, that is NOT how screw removers work.
I say you're throwing bullshit to see what sticks.
Pardon? The friction provided by the wood gripping the screw is what makes a screw difficult to remove. The friction between the wood and the threads, plus the friction between the wood and the shank.. If anything, putting pressure on the screw would DECREASE the friction on the top of the threads while increasing the friction on the bottom - either netting out or reducing the total friction on the screw - so either keepi ng the required torque the same or less. Certainly no great increase in required torque.
OK - I'm not a physics major - just a dumb mechanic. The tension load placed on the screw by virtue of the torque applied to the fastener puts very high pressure on the interface between the head of the fastener and the surface of the material being bolted together. The pressure can be in the hundreds of lbs. The break-away torque required to overcome the stiction between the screw head and the bolted material is often significantly more than the torque required to continue turning the fastener after it is broken loose. The force required to overcome the "static friction" in the threads - breaking the bonds of rust and corrosion, is also quite substantial. So is the "running friction" between a corroded fastener and the internal threads of the material being fastened, or the nut. So substantial as to render the incredibly small amount of extra friction caused by even
40 lbs of pressure applied to the screwdriver to keep the driver in the head of the screw almost totally incosequential.
Again - you are not reading what was said very well for a PHD in Physics (or someone who plays one on TV) Nowhere did I say the friction on the threads is irrelevant. I said your contribution to the friction on the threads is totally irrelevent. Your effect on the universe is greatly overestimated.
Who said anything about "screw removers" by which I assume you are referring to "impact drivers" which you hit with a hammer, causing a cam to rotate the fastener. I'm talking about beating the bejeapers out of the head of a bolt while pulling on a wrench to break free a seized bolt. It is a VERY effective method of breaking loose large threaded fasteners in old equipment.
Nope. I have removed thousands of stubborn fasteners from old equipment over the last 50 years. Some of them litterally seized by "bullshit" which can REALLY make things stick. Ever try removing bolts from the apron chain of an old shit-spreader????
snipped-for-privacy@attt.bizz wrote in news: snipped-for-privacy@4ax.com:
That device is not what is termed a "screw remover". I have to side with clare on this one - I have such a device in my toolbox - albeit for metal and not wood, but the principle is the same: you insert the tool into the screw head, apply torque, then hit the end with a hammer. The tool has a heavy spring inside and a spiral mechanism - when you hit it with a hammer, it applies more torque than you can apply with your hands - plus vibration, etc.
This sounds like an impact wrench. The impact of the hammer is converted to torque. There is some advantage hammering to break metal-metal binding (essentially a "weld") but you're applying torque constantly. The impact only increases friction for the milliseconds of the impact. Sorry, physics doesn't care about consensus.
And stuck bolts don't care about theoretical physics. Have you ever had to remove any REALLY stuck bolts?? Your physics theory won't remove them. And I can guarantee you would not be able to measure the amount of extra torque required to remove a stuck bolt due to the extra friction (if any) caused by pushing on the screwdriver to hold the bit in the head without extremely accurate lab measurement equipment.(and a good dose of imagination)
snipped-for-privacy@attt.bizz wrote in news: snipped-for-privacy@4ax.com:
Well, "impact driver" -
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Given subsequent posts to the one I responded to, I have no idea what either party is really talking about. My post here may or may not have any applicability.
I have a stuck bolt right now holding a lawnmower blade on that belongs to my neighbor lady, removed two of the blades but the third one will not budge. I have used an impact wrench but someone had rounded the bolt before and even tapping a smaller fraction size on it will not grab good enough. Tried vice grips and pipe wrench--what next, maybe heat. I am afraid it will break and then the mower will be toast, it is an old John Deere her husband had before passing away. Any ideas?
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