Difference between revisions of "Screws"
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Using '''screws''' is simple enough, but there are many points that can make for a more satisfying screwing experience.
Using '''screws''' is simple enough, but there are many points that can make for a more satisfying screwing experience.
Revision as of 00:10, 12 February 2008
Using screws is simple enough, but there are many points that can make for a more satisfying screwing experience.
Slotted screw heads are the oldest of all modern head types, and have the poorest performance of all major screwhead types. There is no misalignment tolerance, and heads are easily and often damaged in use. The bit gives no support to the screw, which must be supported by fingers in nearly all cases. And conveniently the driving bit has sharp corners, just what's wanted when the bit is liable to slip off the head onto your always required fingers.
The advantage with slotted head screws is they look nice, and fit aesthetically with old fashioned goods.
Slotted screwdriver tips are easily reground if broken.
Slot heads have an advantage where access is especially difficult, which is that a slotted tip can easily be ground at any desired angle on almost any piece of metal. However screws are not a satisfactory choice when access is this difficult.
Philips & Pozidrive
These give some amount of support to the screw while screwing, provide more grip, and more tolerance of minor misalignment. The bits don't have sharp corners and don't easily slip out of the head, which reduces finger injuries.
Pozi has the slightly better characteristics of the two for most tasks. Philips is preferred for plasterboard screws as the bit disengages from the head more readily when driving torque limit is exceeded (ie it cams out).
Bits are available that can drive these screws at upto 15 degrees out of line.
Pozi size 2 is by far the most common screwhead in diy use, with 1 used for smaller screws. Sizes 0 & 3 are also used occasionally.
Pozislot heads will accept both pozi and slot screwdrivers.
Allen & Square
These have greater interlocking of bit and head, mostly eliminate bit slippage, and hold the screw firmly inline with the bit. Square is more wear tolerant than hexagonal, and is also known as Robertson.
A drawback with allen key driven heads is that attempting to drive them using the wrong size bit can cause rapid bit damage.
Hexagonal ball ended bars are available that will drive these heads at an angle. These are less robust.
Torx are hexagonal, but with a star shape. They have better wear resistance, and are intended to prevent cam out, unlike philips. Torx sizes T5 to T25 are in common use.
Multitiered hexagonal recess
Heads such as ... have the advantage that one bit fits all sizes of screw. The disadvantage is increased wear, less misalignment tolerance and less of a hold on the screw compared to plain allen key heads.
Heads are hexagonal, and a socket or spanner is used.
Spanner heads can not be driven by spanner. They have 2 holes into which the bit is inserted to drive them. The heads and bits do not have as much strength of many other head types.
Security type heads
If caught without the right tool it is often possible to insert a screwdriver and turn to snap off a central pin.
Hex tamper resistant
Has a central pin. Requires a TR (tamper resistant) hollow hex bit.
Has a central pin, and requires a hollow torx tr (tamper resistant) bit.
There are also other variations on the Torx theme, such as external torx, torx plus, and tamper resistant torx plus with 5 lobes instead of 6.
A 3 lobed version of pozi.
A triangle shaped head. These can generally be undone without the right bit, just not as easily as heads intended to be undone easily.
Look like philips but with offset slots. These are designed to cam out when sufficient torque is reached.
Shear head screws
The driving heads shear off when tightened to torque, leaving only a smooth cone shaped head behind. This makes them especially difficult to remove, and they are used as anti-vandal screws. Removal requires drilling or cutting them out.
If screwing gets too tight, lubrication and/or a pilot hole is needed.
Recommended Lubricants include:
- candle wax
- soap, eg bar soap or washing up liquid
- lubricating oils
- vegetable oil
Handy non-ideal lubricants good enough for many purposes include:
- very greasy food
- waxy furniture polishes
- silicone furniture polishes
- handwash, shampoo, hair conditioner
Pilot hole sizes
When tables are not to hand, a good rule of thumb for softwood is to pick a pilot hole drill the size of the inner diameter of the screw shank, and a clearance hole that just clears the outer diameter.
No pilot hole
Small screws can usually be inserted with no pilot hole. When neat results and maximum strength are not important, and occurrence of very small localised splits is acceptable, medium screws may be inserted into softwoods without a pilot hole. Medium screws into bigger timbers such as joists are generally fine without pilot holes.
When going pilot-hole-less, the thinner the screw, the better it will behave, and the easier it will be to insert.
Wood is prone to splitting when screwed with no pilot hole. Blunting the point of the screw reduces the risk of this. Wood is made of hard fibres with a softer infill between them. A blunted screw will not penetrate the hard fibres, but will slide off into the softer material, which will give sideways as its screwed in. A sharp pointed screw will stay on the hard fibres and penetrate there, levering a distance of hard fibre apart, thus cracking the wood.
Wood is more likely to split when screwed into near the edges. A pilot hole is necessary when screwing near edges.
Knots may be screwed into perfectly satisfactorily as long as a larger than usual pilot hole is used. Knots contain mainly hard fibre and less soft infill, so will not deform around the screw as much as the rest of the wood. Quite shallow penetration of the knotwood by the thread will give high pullout resistance due to the knot's hardness.
Attempting to screw into a knot with a standard diameter pilot hole or no pilot hole will almost certainly cause a split.
All coarse thread screws can be used in wood, including types not designed for wood use. The differences between them lie in optimisation rather than essential function.
Common Screw Types
Most modern woodscrews have parallel sides, single thread and are threaded full length. Most have pozidrive heads.
Traditional Wood Screws
- Traditional woodscrews have slotted heads, tapered shanks and a threadless top section
- parallel sided
- threaded full length
- twin thread with one deep one shallow thread
- Parallel sided
- Threaded full depth
- Black phosphated anti-corrosion finish common but not universal
- Shallow bugle head to avoid rucking up of board
- Sharp tip to pierce steel studs
- Thin size to ease pilotless fitting and reduce cordless tool energy consumption
- Philips head to cause cam-out and so help avoid overtightening
- A huge improvement over plasterboard nails
- Good for many uses other than plasterboard
- Drywall Screw Eulogy
Concealed Hinge Screws
- Short extra-fat chip screws
- used with recessed hinges used for chipboard, typical of kitchen units
- extra wide screws give more strength of attachment to sheet chipboard
Less Common Screw Types
These have a screw-on chromed cap. The mirror is screwed to the wall then the cap screwed on to make it look nice.
These are used by fast automatic screwdrivers which feed the screws in on a flexible plastic strip. Collated screws cost more but greatly speed up mass screwing.
These are double ended screws with no head, each end has a point and thread. Used to invisibly join items that can be rotated.
- Top surface lies flush with wood.
- Countersinking usually needed, though not always
- Domed shape
- Sits pround
- Partially recessed, partially raised
- UK electrical accessories mostly use raised head screws
Panhead or Cheesehead
- Shallower than roundhead
- Shallow recessed head used on plasterboard screws.
- Prevents damage to plasterboard
- Enables screws to countersink into softwoods with no need to predrill a countersink recess.
Special Purpose Threads
Most coarse thread DIY & construction screws have a single thread tapered to a point. Other options give various extra properties that are sometimes useful.
- Extra fast driving
- Reduces energy use per insertion in soft materials such as chip
- More screws per charge with cordless tools and soft materials
Cutting / drilling tip
- Drill tips enable the screw to drill and drive in one operation.
- Quicker than drilling and driving separately
- Useful for hard materials, thicker steel etc
- penetrates thin steel, eg in partition walls
- penetrates fingers too
- Plasterboard screws have sharp tips
- These have a section of non-spiralling thread under the head. They can be screwed in then backed out, and will raise the top wood sheet (etc) up. Used to level items on non-level backgrounds, adjust frame position and so on.
- These have a greater ability to cut a thread into the workpiece than plain screws. Useful for some tough materials. Not effective on hard materials like thick steel etc.
When chopping woodscrews, file down any damaged thread that does not line up with the rest of the spiral thread. Otherwise it will be difficult to drive.
When chopping fine thread screws for nuts, put a nut on the screw before cutting. Unscrewing the nut will usually make the thread usable again, though it will be stiff. Expect a failure rate. If you need nuts to run smoothly on the shortened screw, put 3 nuts onto it before cutting. The first nut off gets damaged, don't try and reuse it.
Cylinder lock barrels come with unhardened screws intended to be cut to length. Instruction often say the screws can be snapped, but IME attempting to do so normally ruins the screw by bending the threaded portion that will be used. These need to be cut with angle grinder, bolt croppers, cutoff saw, or hacksaw.
Screws as nails
Coarse thread screws can be hammered in like ring shanked nails. However they damage the wood on the way in, and offer relatively high resistance to hammering. Hence they're most suited to use with chipboard.
Sometimes used with chip flooring to avoid creaking and lifting.
There is a breed of site-animal which asserts that any screw can be driven by a hammer. This is, strictly speaking, true, but the unfortunate soul who attempts to remove a screw thus driven is rarely grateful to the former.
Removing a Damaged Screw
There are many ways to remove screws with damaged heads. Its hard to think of situations in which at none of the following methods will work. Failure to remove normally comes own to lack of patience, equipment or knowledge.
Screws with heads with no grip left and jammed screws can be removed with the following methods:
- Wirecutters sometimes get a better grip than pliers, squeeze them hard to dig into the metal a bit.
- Tapered screw removers - these screw into a drilled hole in a stuck screw. They are prone to expanding the screw, burying it into the workpiece, then breaking in situ, creating a second and bigger problem. Common but not recommended.
- Left handed drill bit - these either dig into the head and unscrew it, or if it can't ever get a grip it will drill the head off. These are a good choice for general problem screw removal. Also a good choice for clutch head screw removal.
- A hacksaw can often cut a slot into a head, enabling unscrewing.
- Grind or file 2 flats on opposite sides of the head. Undo with pliers or spanner.
- Weld a piece of metal to a screw head, with which it can be undone. The heat often helps free the screw.
- Weld a blob of metal on the head that can then be slotted or filed for grip, or simply undone with pliers.
- Dig under the head on 2 sides and apply pliers
- Hammer the head sideways a little in a few different directions (using a junk wood chisel) to loosen the screw.
- Hammer, nail remover or prybar can all be used like a nail puller, getting a grip under the head and pulling out. This does cause some wood damage, and is ill suited to any but small screws.
- Angle grinders can grind the heads away or cut them in two. Both involve some damage to the workpiece.
- Applying a soldering iron to the screw head can sometimes loosen it enough to enable unscrewing.
- Sometimes a hammer blow straight down on the screw will loosen it enough to unscrew. Use a poundthrough screwdriver if you want it to last, ordinary screwdrivers are not designed for this.
- A Dremel grinding disc can cut a new slot in the head.
- Use an impact driver of the manual hammer operated type
- Use an impact driver of the power tool type.
- Electrolytic rust removal
- Spark erosion (not cheap)
- Apply red hot poker to screw until wood starts to smoke a bit.
- Pushing really hard on a screwdriver bit in a brace & bit can work.
Paint Clogged Head
- Paint clogged heads can be cleaned with a knife in the case of slot heads, or a sharp pin for other types.
- A screwdriver heated with a blowtorch will melt through paint very qwuickly, enabling fast work with paint clogged heads.
The strongest and cheapest screw metal. Usually Bright Zinc Plated. Many yellowy screws are coated steel.
Decorative and somewhat corrosion resistant. Brass is much softer than steel, and it is generally recommended to drive a similar steel screw first, then remove it and use the hole for the brass screw.
Excellent corrosion resistance in most applications. There are different types of stainless with differing levels of corrosion resistance. Note that stainless needs oxygen to remain uncorroded, so this metal is not good for all applications.
Common A2 stainless steel is rather softer than steel and screws are more prone to mangling of heads or shearing of their shanks, so care must be taken to drive squarely with a good bit and not to attempt to force a screw into too-stiff material or over-tighten.
Plastic screws are used for their non-corrosion, non-conductivity and non-scratch soft faces. They are much weaker than metal screws, thus are of limited use, and not normally found in DIY.
Steel screws are coated to prevent corrosion, or for lubrication.
- BZP - Brightish chrome-like appearance
- Zinc plated - as above but matt finish
- Zinc plated & yellow passivated - yellowy
- Phosphated - black
- Wax - lubricant, clear.
- Brass plated steel - cheaper alternative to brass screws, not common and not corrosion resistant as brass
- Net-coat - brown or green
- Some fl compound - green - tough anti-corrosion coating for exterior use
- Blue ruspert - blue
- Oxide - black
Often called rawlplugs (brand name).
Hollow Wall Plugs
For fixing to hollow plasterboard walls. Heavier load rating than
For fixing to hollow plasterboard walls.
Screw caps cover the screw heads after insertion, providing a plastic finish to match the workpiece. I'm not sure they look especially good, but they are an improvement over bare screw heads on finished furniture.
There are 2 types of caps
- caps that press on after screw insertion. These grip the pozi recess and have a habit of falling off.
- 2 part caps have a plastic skirt that goes under the screw thread. The cap then snaps onto this skirt. These are bulky and need prefitting, but don't fall off.
Cups are 3 dimensional washers. They are curved and look reasonably pretty as well as fulfilling a washer function. Usually brass or brassed steel. Used with countersunk head screws.
Screw head pictures: http://en.wikipedia.org/wiki/ISO_10664
Wikipedia screws category: http://en.wikipedia.org/wiki/Category:Screws
Still to come
multitier hexagonal recess name hex drive hex slot head hex pozi head self piloting app table