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How To Use Self Tapping Screws

What Are Self Tapping Screws / How Do I Use Self Tapping Screws?

Arguably, the image of a Self Tapping Screw is one of the most archetypal images of a screw. Many will envision the definitive ‘screw’ as a countersunk screw, usually with a Phillips drive and a sharpened point. A quick Google search for the term ‘screws’ shows an overwhelming bias towards self tapping fasteners, despite the innumerable permutations available. Despite its apparent popularity, ‘How do I use a self-tapping screw?’ is still a very common question - some of the basics will be covered within this article.

It is often required, and generally recommended, to drill a pilot hole  for self tapping screws.

Pilot Hole Size (Drill Size For Self Tapping Screws)

By far the most common questions when using self tapping screws are about the best pilot hole size for a specific self tapping screw size.  Factors such as fastener material, housing material, hole depth, and even temperature must be considered. For these reasons, some degree of trial and error may be needed to find the ideal pilot hole size for your specific application. As a rough guide, a pilot hole should be between the minor diameter and major diameter of the screw thread. For example: a #6 screw (major diameter of threads ~3.5mm, minor diameter ~2.5mm) would typically need a pilot hole of 2.7-3mm depending on the hardness / toughness / density / lubricity of the material.

Selecting The Right Drive Type

An important consideration in any self tapping application is to select the ideal drive type for the given situation and the tooling available. Traditional choices include slotted or Phillips drives (usually where aesthetics are a consideration) - or the Pozi drive, which was specifically invented for self tapping applications. Deeper socket drives such as Torx & Robertson are worth considering in high-torque applications and are a great choice when using a power tool - but some care is needed when using non-hardened screws such as brass and stainless steel - we recommend utilizing the torque setting on your tool - set it a medium torque to to avoid damaging the fastener.

Alignment

When installing a self tapping screw, the screw must be properly aligned to ensure that it is driven into the target housing as the correct angle. This is particularly important if a self tapping screw is expected to cut its own hole, as there is no pilot hole to guide the fastener. Many methods exist for ensuring perpendicular holes, from improvised jigs to high-precision guidance tooling.

What is The Difference Between Self Tapping and Self Drilling Screws?

Tapping is the act of creating a thread. Tapping is not the act of creating a hole. In soft materials, such as wood or plastic, there is some overlap in these processes, as a self tapping tip is usually sufficient to create the hole required. However, with metals or certain particularly hard woods, a pilot hole must be drilled before a thread can be tapped.

How to Use Self Tapping Screws with Plastic

Certain types of self-tapping screw have been specially designed for use with plastics. While regular self tapping screws are suitable for use with most plastics, Polyfix Screws feature more pronounced, sharper threads, which increase resistance to pull-out and stripping within low-density plastics.

Once again, finding the right size pilot hole will depend on finding the right tradeoff between material (more rigid/denser materials require a larger hole), length of thread engagement (longer thread engagement means more resistance to insertion and therefore a larger hole may be needed) and other physical factors like temperature and lubricity. We provide some guidance on finding the right pilot hole for your application with our Polyfix screws below. 

  Guide Pilot Hole Size* (mm)
Polyfix Screw Size Soft Plastic e.g. PP, ABS, PETG Rigid Plastic e.g. Nylon, PC, PLA
M2.5 1.85 2.05
M3 2.30 2.50
M3.5 2.75 3.00
M4 3.2 3.45
M5 3.7 4.10

Plastic housings are commonly used alongside thread forming screws, a type of self-tapping screw which does not cut its own threads with a sharp edge, but pushes threads into a housing through deformation of material. For this reason, thread forming screws are only generally only a viable option with more elastic materials such as plastics.

The advantage of thread forming self-tapping screws is that by displacing material, they create a zero tolerance fit which is more resistant to loosening. The downside of thread forming screws is that they require greater torque to install, and can cause disruptive internal stresses.

How to Use Self Tapping Screws with Metal

L to R: Polyfix Screw, Taptite Thread Rolling Screw, Standard Machine Screw. Note the tri-lobular profile of the 2 screws on the left.When used with metal, Self-tapping screws should always be installed into pre-drilled holes. Stainless steel screws are relatively soft when compared to hardened steel, and are generally recommended for use with softer metals, such as Aluminium.

It is important to exercise caution when installing self-tapping screws into metals due to the increased density and hardness of material. When used in metal, self tapping screws have an increased potential to become damaged.

Thread rolling screws also exist for use in metal, though they are generally only used alongside softer metals. Thread rolling screws in metal rely on cold flow of material after the fastener has been installed to create an excellent resistance to loosening. Thread rolling screws in metal are affected by all of the same drawbacks as their plastic counterparts mentioned above - greater requirements for installation torque, and the creation of potentially damaging internal stresses.

The table below gives several recommended hole sizes for stainless steel self tappers in unhardened sheet metals such as mild steel, brass, aluminium etc. but should be used as a guide only.

Self Tapping Screw Size Sheet Thickness (mm) Guide Pilot Hole Size (mm)*
No. 4 (2.9mm) 0.45 2.21
0.91 2.31
No. 6 (3.5mm) 0.45 2.70
0.91 2.80
No. 8 (4.2mm) 0.71 3.00
0.91 3.08
1.22 3.20
1.62 3.40
No. 10 (4.8mm) 0.71 3.51
0.91 3.59
1.22 3.71
1.62 3.81

How to Use Self Tapping Screws with Wood

Example of a hardwood with a countersunk hole & screwWhen used with sufficiently soft materials, a self tapping screw can be used to create and tap its own hole. However, a pilot hole is often recommended, particularly in hardwoods, to prevent splitting, which is caused by stresses within the wood. Wood Screws are designed with very coarse threads to reduce stresses within wooden housings. We provide some rough guidance for pilot holes in wood below:

    Guide Pilot Hole Size*
Wood Screw Size Clearance Hole (mm) Softwood (mm) Hardwood (mm)
No. 2 2.4 Not required 1.2
No. 4 2.8 Not required 1.6
No. 6 3.6 1.6 2
No. 7 4 1.6 2.4
No. 8 4.5 2 2.4
No. 9 5 2 2.8
No. 10 5 2.4 2.8
No. 14 6.5 2.8 3.6
No. 16 7 3.6 4

Clearance holes are recommended when joining wood together to ensure a good clamping force between the two surfaces. Many of our wood screw range have countersunk heads so it's a good idea to countersink the holes first to avoid cracking the wood surface. Our Super Cutter Wood Screws include milling geometry on the shank and ribs underneath the countersunk head to make both these tasks easy and save time when working with softwoods. If you are dealing with hardwoods you will need to take the time to create countersunk clearance holes with the appropriate tool.

When using self tapping screws with wood, the fastener material choice should also be considered. A corrosion resistant fastener such as stainless steel is advised, as moisture can be absorbed and excreted by wood over time, potentially causing rust on unprotected screws.

How To Use Self Tapping Screw With RawlPlugs

Rawlplug Uno Universal Plugs are an excellent choice for fixing to solid or hollow materials around the home. e.g. plasterboard and brick. Typical applications would be DIY tasks such as mounting shelves, bathroom fittings and lighting. 

The same principles apply for choosing your screw to fix into a RawlPlug - a larger screw will engage more with the plug and provide a stronger fixing but will require more torque to insert it so RawlPlug recommend the following:

RawlPlug Size Colour Plug Screw Fixture
Diameter (mm) Length (mm) Diameter (mm) Length (mm) Hole Diameter (mm)
Ø5 Yellow 5 24 3-4.5 min. 25 4-5
Ø6 Red 6 28 3.5-5 min. 25 4-6
Ø7 Brown 7 30 4-6 min. 25 5-7
Ø8 Blue 8 32 4.5-6 min. 30 6-8
Ø10 Grey 10 36 5-8 min. 30 7-10


For more detailed product data - consider downloading RawlPlugs' own technical documentation.

Please note: Fastening applications may differ greatly based on a number of environmental factors, the information in this article is for guidance only and is correct to the best of our knowledge. The customer must satisfy themselves with the performance of the fastener in their application. Accu will not be held responsible for any failure that may occur from the use of this guide.

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