What Are Self Tapping Screws / How Do I Use Self Tapping Screws?
Selt Tapping Screws come in a vast range of variations due to the large amount of applications they can be used to fasten and the woods, metals and plastics they can be used with. Their popularity being that there function can speed up fabrication through ease of use. There is still mystery on which one to use when and how to use them and this article helps to explain all.
Pilot Hole Size (Drill Size For Self Tapping Screws)
A pilot hole is a pre drilled hole through connecting parts which the cutting thread grabs onto. Connecting part must be completely clappmed together as gaps between materials are a weakness and will form with imperfections. When you screw them together, the screw will not pull them together without overtightening and stripping the new 'self tapped' thread. Self drilling, self tapping screws are exactly that and do not need a pilot hole, ony accurate positioning using a centre punch on wood and metal. Assuming your fasteners are not self drilling (self drilling ones can be a menace) the most crucial factor is, what sized pilot hole to use? We need to consider material, housing material, hole depth, and even temperature. Without experience, you should set up a trial, going through the process of marking out, clamping together, centre punching, drilling the pilot and screwing the fastener in, making sure there's no gaps and that the screw goes in, you don't over tighten and there's enoght grab per self tapping screw. As a starting point, a pilot hole should be between the smallest diameter (bottom of the thread) and outside diameter. For example: a #6 screw (outside diameter of threads is 3.5mm and smallest diameter is 2.5mm) would typically need a 2.7mm - 3mm pilot hole, depending on the hardness / toughness / density / lubricity of the material. A harder material would need a 3mm pilot hole and a softer material will need more grab and so a smaller pilot hole of 2.7mm. Should you use a smaller hole on a harder material the git won't go in very well.
Selecting The Right Drive Type
Will you drive the screw in with a phillips, pozi, of torx? Phillips will be available on harder screws, pozi is a bit more forgiving and a torx further still. A torx is more reliable on deeper and harder application, often used with a driving machine. Brass and stainless steel are not hardened and considerable care should be used to avoid rounding off the drive and fussing with the extraction. Should you be using a machine, set the torque to low and work your way up to make sure it's high enough to drive but not too high to over tighten and strip the thread, rendering the screw useless. Should you strip the thread, you can either, put the next screw in at an angle (ugly but ok if hidden), choose another hole (ugly but ok if hidden), use a longer or wider screw.
The screw must be inserted at 90' in both the X an Y axis. The screws should be positioned equally apart and the same distance from the edge. On small quantity holes, just use a steel rule and centre punch. For larger quantities, create a jig (holding tool) that has the hole locations marked out. A jig could also be used to create the pilot holes.
What is The Difference Between Self Tapping and Self Drilling Screws?
Tapping is reference to creating a thread to which the fastener temporarily bonds to the material. Drilling is creating a round hole from which the tap is cut. Softer materials like plastic and wood can be driven in without a pilot hole but the materials could be prone to cracking if the diameter is too wide. Harder materials like metal dictate that there needs to be a pilot hole either by a drill bit or the small drill cutting edge at the end of a self drilling, self tapping screw.
How to Use Self Tapping Screws with Plastic
The right size pilot hole will depend on finding the right tradeoff between material (harder materials require a larger hole), depth of hole (longer thread engagement means higher torque at the end of insertion and wider hole to avoid damaging the head. Use the table below as a guide:
How to Use Self Tapping Screws with Metal
Self-tapping screws should always be installed into pre-drilled holes when using with metal. Stainless steel screws are soft when compared to the hardened steel option, and are generally recommended for use with stainless steel or softer metals, such as Aluminium.
Reminder, do a trial to make sure the screw goes in well all the way to the bottom without damaging the head. If ist too difficult, use a large diameter and it there's not enough grab, use a thinner diameter,
Use the table below as a guide for pilot hole sizes for stainless steel self tappers in unhardened (softer metals) sheet metals such as mild steel, brass, aluminium.
How to Use Self Tapping Screws with Wood
A pilot hole is often recommended in hardwoods, to prevent splitting but is not necessary in softer woods like pine. Make sure you do a test in some scrap material to make sure you are not driving in a screw which is too wide and splits the material. A true test will be to drive it in as close to the edge as the real thing. Should you only have one size and you don't want to do and get thinner ones, pilot the hole. There's a good reference for pilot holes in wood below:
How To Use Self Tapping Screw With Wall Plugs
Wall Plugs are an excellent choice for fixing to solid or hollow materials around the home. e.g. plasterboard/ stud walls 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 we recommend the following dimessions as a guide: