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 good 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 (cam 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.
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 bit damage.
Hexagonal ball ended bars are available that will drive these heads at an angle.
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.
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
Hex tamper resistant
Has a central pin. Requires a TR 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.
These have a one way only slotted head. These are unusual in that they can not be unscrewed, even with the right screwdriver bit. This is a security advantage, bu can also prove quite a disadvantage! Bear that in mind when deciding to use them.
Look like philips but with offset slots.
Shear head screws
These cause head shear when tightened to torque. This makes them especially difficult to remove.
Screwdriver & bit forms
1/4" hex bits come in short 1", medium 2" and long 6" forms.
Screwdrivers come in jeweller's, stubby, small, medium, large and oh my god sizes.
They come in 90 degree bent form for where access is difficult.
They come in pound-through form where hammering is needed.
When access is too tight to use a cranked or stubby driver, a short bit plus pliers is a common choice.
Kitchen knives are used by the toolless, with bad performance.
Extra large screwdrivers are quite useful but tend to attract a lot of rude comments and strange stares on site. They position the hands well away from cramped corners and abrasive walls, they allow driving without needing to get close to the workpiece, and they sometimes allow a better working position, making application of force easier. In cramped conditions they can also allow workmates to pass between you and the screws while you're working.
Impact drivers are a special type of screwdriver that turns when hit with a hammer. These give the ultimate in screwing and unscrewing power. The name 'impact driver' is also used for power tools that do a similar but not identical job.
Both screwdrivers and bits are available with diamond coating. I hear they make driving easier by gripping well, as long as they're not abused.
Screwdrivers are made from either chrome vanadium steel or carbon steel.
Chrome van is the softer of the two, and the bits are liable to become damaged over time.
Carbon steel is harder, lasts longer, and performs better. However it has some downsides.
- It rusts readily, hence its often chrome coated. Chrome is a relatively slippery coating, and not wanted for screwdriving.
- Being harder, carbon steel can eventually break rather than chew up. Harder steel is also not as tolerant of misuse, eg use as a mini prybar.
Most screwdrivers today are chrome vanadium, perhaps because manufactureres care more about minimising failures than maximising performance. If you want the better performance of correctly hardened and tempered carbon steel, Draper, Tough-Tools and Stanley still sell them.
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. Thus I choose the thinner 2" screws for pilotless use.
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.
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.
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
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
- Top surface lies flush with wood.
- Countersinking usually needed, though not always
- Domed shape
- Sits pround
- Partially recessed, partially raised
Panhead or Cheesehead
- Shallower than roundhead
- Shallow recessed head used on plasterboard screws.
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 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.
Hammer in fixings may have screw like heads. But after fitting you may find no screwdriver will fit them. This is a good clue that they're not screws.
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.
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.
Damaged Head Screw Removal
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.
Paint clogged heads can be cleaned with a knife in the case of slot heads, or a sharp pin for other types.
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 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.
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-scracth 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 chromelike appearance
- Zinc plated & yellow passivated - yellowy
- Phosphated - black
- Wax - lubricant, clear.
- Brass plated steel, not common but sometimes found
- Net-coat - brown or green
- Some fl compound - green - tough anti-corrosion coating for exterior use
- Blue ruspert - blue
- Oxide - black
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.
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 hinged caps have a plastic skirt that goes under the screw thread. The cap then connects to this skirt. These are bulky and need prfitting, 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 dowel screws hex drive pozislot hex slot head hex pozi head self piloting app table