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Widely used since the middle of the nineteenth century, screws exist today in a multitude of sizes and shapes... They all have a more or less complex thread and a head with mostly a footprint that determines the tightening torque. The union of these three elements depends on the type of assembly and the performances obtained. Explanations...

how to recognize and use the fixing screws

composition of an assembly screw

1. Imprint
2. Head
3. Body or axis
4. Net
5. Anchor point

What material for what use?

Silver, gray, matte black... the color of the screws does not have an aesthetic function! These colors most often reflect qualities of steel or different treatments.

  • For example, stainless steel screws exist in A2 class for wetlands (bathroom, kitchen...) and A4 class for marine environments (boat) or very exposed (fence, shutters, terrace...).
  • Screws in "dichromate steel" (yellow chrome alloy) and the galvanized screws suitable for interior fittings (framing). They can be used outdoors, but withstand less time than stainless steel screws.
  • Screws made of "phosphated steel" (black) are reserved inside (shepherd's plate).
  • Some screws are even sold lubricated to facilitate screwing.
  • Not to mention those in Nylon, used for sanitary or household appliances.

Well made heads to screw well

A screw is above all a threaded rod surmounted by a head with or without imprint. The lower part of the head maintains the pressure on the part to be assembled. The head also allows the tightening by means of a screwdriver, a key or bits to be mounted on a tightening tool.

Countersunk screw head

countersunk screw head

  • The countersunk head, the most common, is used when the head must be drowned most often for joining wood on wood (cladding, flooring) or metal on wood with milled countersink.
  • Difficult readjustment The length of the screw is measured from the top of the head to the tip.
  • For countersunk screws, the length is taken under the head.
  • On high-end screws, there is a reinforcing cone under the head to prevent breakage.

Trumpet screw head

trumpet-shaped screw

  • Trumpet-shaped, for drywall type plasterboard metal frame, these heads sink into the wall without damaging it.

The cylindrical screw head

cylindrical screw head

  • The cylindrical head offers a flat bearing surface under the head. The latter is flush with the surface of the part to be assembled.
  • This screw is very useful to assemble non-milled pieces of metal on wood or metal on metal.
  • Easy readjustment.
  • In mechanics, it can be embedded in a countersink.

The screw stove

screw stove

  • The stovetop screw, with a rounded head, allows a reduced thickness while widening the support surface, as the Hinge screw or bolt screw.
  • This system is incompatible with countersunk heads.
  • It is used a lot for metal fixtures, stairs, gratings

Hexagonal screw

Hexagonal screw

  • Hexagonal, it is a screw with external impression (hexagon), adapted for a big tightening torque.
  • She often equips lag screws for framing where the mechanical assembly screwse.
  • Clamping is done with a wrench or tube wrench.

Extended base screw

widened flange screws

  • With an enlarged base of approximately 10%, it avoids the use of a bearing washer.
  • It is only found on countersunk screws. Mostly used on sheet metal or cladding.
  • Some flanged heads have brake fins that replace a "Grower" or slotted washer.

Curved head screws

use of the domed head screw

  • Inner impression heads can be curved for a more aesthetic result.
  • They still keep their mechanical quality.

Countersunk countersunk screws

Use of countersunk head screws

  • Some countersunk screws have notches that provide better grip in the wood and prevent the screw from loosening.
  • They also serve as strawberry, for a better head penetration in wood and chipboard.

Thumb screw or knurled screw

Use a thumb screw

  • These screws are tightened by hand.
  • Often associated with an ISO thread, they are used for tightening and adjusting metal or mechanical parts.

Fingerprints adapted to the need for clamping

The impression of a screw is the machined part in which the screwdriver tip fits.
Split fingerprints, cruciform, hexagon and Torx are the most used. Depending on the type of tip and its size, the tightening torque exerted is higher or lower.
Hence the importance of drawing the impression when choosing a screw...

Which forms of hollow impressions prefer for assembly screws?

We choose the footprint of a screw depending on its application, its advantages or disadvantages:

Screws: choose its fixings according to the desired assembly: desired

Screw: hollow cavity impressions

Historical : 1875 (USA) / Footprint widespread among carpenters
Application: Simple and uncomplicated assemblies in the small DIY
disadvantages: Imprinting in the process of disappearing / Screwdriver bit not locked, deteriorating the slot / Almost impossible to use with a screwdriver because of the difficulty in centering the tip
Advantages: Simple and easily available bits / Sometimes used in addition to another fingerprint (hexagonal head of a clamp)

Screw: square hollow cavity called Robertson

Historical : 1908 (Canada) Invented by Peter Lymburner Robertson / First mass-industrialized imprint, more efficient than slotted-head
Application: Framing and joinery assemblies requiring high tightening torque
disadvantages: Bits and screws not widespread in France
Advantages: Deep and indeformable footprint / Withstands a very high torque and ensures a remarkable hold when tightening

Screws: Hollow recess with hexagon socket (H), called BTR or Allen

Historical : 1910 (USA) Patented by W.G. Allen / Deleted the heads on the adjusting screws of the assembly line machines. A plus for security
Application: Joinery assemblies, furniture (especially kit furniture), mechanical adjustment systems...
disadvantages: Fits moderately with screwdrivers and impact wrenches
Advantages: Ideal for frequent disassembly / Very little deformable impression / End caps available in many forms, including the famous Allen keys

Screw: hollow Phillips (PH) recess

Historical : 1930 (USA) Invented and industrialized by H.F. Phillips / Created to gain safety and speed on automotive assembly lines
Application: Joinery assemblies (countersunk heads), electronics and appliances / Drywall assembly with detachable chuck screwdriver
disadvantages: The tip must be exactly the size of the impression, the risk of damaging it and make it unusable
Advantages: Easy engagement and centering of the clamping tip / Supports high torque thanks to the inclined shape of its flanks

Screw: hollow impressions Pozidriv (PZ)

Historical : 1966 (USA) Evolution of the Phillips impression / Created for electric or pneumatic screwdrivers
Application: All areas of DIY. Often associated with milled heads. Very used in framing
disadvantages: Often confused with the imprint Phillips: the tips seem compatible, while they are not and may damage the footprint
Advantages: Increasingly Replaces Slit and Phillips / Easy Tip Centering Impressions and Tightening Torque (Better Spread Efforts) Compared to the Phillips Impression

Screws: hollow impressions Torx (T) called 6 lobes

Historical : 1967 (USA) Created by Camcar Textron, Aerospace Company / Widely used in the industry
Application: All areas of DIY (wood, metal) and mechanics
disadvantages: Little inconvenience, apart from the price
Advantages: Increasingly used for series screwing (deck boards, etc.) / Perfect for electric screwdrivers and impact wrenches / Supports high tightening torques, no deformation after tightening, excellent centering of the bit

Table comparing different forms of hollow impressions

Click to consult this table in large format:

Screws: choose its fixings according to the desired assembly: assembly

Very special tips

Beside the current fingerprints, there are more confidential ones.
For example, fingerprints Phillips, Torx and Allen in tamper-proof version: a central nipple requires the use of a hollow tip in the center. Other screws, used in household appliances, electronics or electromechanical, have very different footprints that impose atypical tips:

Screws: choose its fixings according to the desired assembly: choose

Tri-Wing

Screws: choose its fixings according to the desired assembly: choose

Torq-set

Screws: choose its fixings according to the desired assembly: head

Snake-eye

Screws: choose its fixings according to the desired assembly: choose

TooSpeed

Screws: choose its fixings according to the desired assembly: head

Pentalobe (at Apple)

Screws: choose its fixings according to the desired assembly: choose

prevent its removal

Wolfkraft screwdriver bit box

These fingerprints are usually reserved for professionals, although it is possible to find boxes of special GSB tips here a box of thirty Wolfkraft tips.

Screw threading: more and more sophisticated

The thread of a screw is a helical thread that wraps around a metal axis.
It allows the screw, turning in a clockwise direction, to enter a room and assemble it with another. It transforms a rotation movement into a translation movement.
There are two main types of threads in the trade: wide (or coarse) and ISO metric.
The length of the screw is a function of the thickness of the parts to be assembled and their material.
The screw must measure at least three times the thickness of the element to be fixed.

Screw with toothed threads

toothed screw thread

  • On some models of screws, the thread is toothed to tap a piece of wood (hard or soft) without much effort and ensure better chip evacuation.
  • These teeth facilitate primer and save time and energy, a plus for battery-powered screwdrivers.

Double threaded screw

double screw thread

  • These screws allowassemble wood without pre-drilling on pre-drilled concrete.
  • The two nets do not have the same thickness, they allow to gain in screwing speed and assembly strength. They are also grooved for better grip in concrete and screwing without ankle.
  • These screws are very used for wood frames on facade.

Screw with discontinuous thread

Batch screw thread

  • The screws for terrace boards or facade cladding have a thread intersected by a smooth body.
  • This system keeps the blade perfectly on the joist and prevents squeaks.
  • There are also screws with a cutter at the end of the net. She allows to stabilize the tightening torque and prevent overheating of the unthreaded part.

Antifanging screw tip

Screw with antifriction tip

  • Especially present on the wood screws, the anti-scouring tip prevents the wood from bursting at the moment of penetration of the screw.
  • For attach wood baseboards, deck boards and end plate fasteners.
  • Avoid a pilot hole, the threading, associated with the antifexing tip, acts like a forest.

ISO metric thread

threaded rods

  • The ISO metric thread, created at the end of the second world war, is not called "end" (the step is the distance between two vertices of the net).
  • Possessing a high shear strength, this type of thread is used for the assembly of metal parts, for adjusting and pressure screws.
  • It works most often with a tapped hole in the receiving room (this is for example the case of wall plugs).
  • We meet them in mechanics, locksmithing, plumbing and carpentry... in the form of threaded rods or bolts.
  • Their diameter is preceded by the letter M: M6 for example for Ø 6 mm.

Symmetrical classical thread

Conventional thread screws

  • Symmetrical conventional threading provides maximum tear resistance in metals, wood and chipboard or OSB.
  • We find him on large lag screws, dowel bolts and sheet metal screws with self-drilling tip (which allows the screwing without pilot hole).

Screw with asymmetric thread

Asymmetric threading of a screw

  • The asymmetric threading is a very inclined thread (40°), which improves resistance to tearing and the speed of screwing compared to symmetrical conventional threading.
  • It is widely used in wood and concrete constructions, carpentry, wooden decks and exterior joinery in PVC or aluminum.

Self-tapping screw

Self-drilling bit of a screw

  • The self-piercing tip says "trepan"is used on sheet metal screws for assemblies "iron on iron" or "iron on wood" (steel bins on chevron).
  • These screws are tapping.


Video Instruction: Fasteners: Machine Screws and Bolts