Clearance HoleĪ hole drilled to allow the screw shank, including the thread, to pass through the wood. For example, a 4mm screw should have a 2.5mm pilot hole. Pilot HoleĪ hole drilled to locate and guide the screw allowing the threads to bite into the wood. There’s a chance that the screw head may shear unless you drill pilot and clearance holes. When using much denser timbers, such as Oak, Beech or Sugar Maple, driving the screws becomes more difficult. The remedy is to drill a pilot or clearance hole which relieves the lateral pressure on the wood fibres as you drive the screw. However, driving a large screw close to the end of a board is liable to cause it to split. Using modern screws, such as the Woodspur range in pine and other easily worked timbers, this isn’t too problematic. Correct adjustment of the clutch also prevents unnecessary damage to the screw head driving lands.Ī screw acts a wedge when driven into timber, forcing the fibres apart, which can have undesirable consequences in some instances. Thus, driving a 4x50mm screw into oak will require a high clutch setting and a lower one when driving into pine. Fortunately, all drill drivers come with a sensitive clutch which you can adjust to suit the size of the screw as well as making an allowance for the type of timber. This is easily seen when the screw keeps turning but fails to have any ‘bite’ in the timber. One of two things may result firstly, the screw head will drive deeply below the surface or secondly, the thread strips. The result is that ‘cam out’ is less likely to happen but a disadvantage of the torx system is that it becomes much easier to drive the screw too far into the wood.Īpart from shearing the head of a screw, the other occurrence which happens occasionally is that the drill bit continues to turn for a few seconds once the screw is fully driven home. Torx (yellow)Īnother ‘star’ shaped recess, but here the driving lands within the head, parallel to the shank and the drill driver as such the torque is applied more efficiently. You can apply greater torque to the screw head before ‘cam out’ occurs. The Pozidrive is a development of the Philips head and is in very common use in workshops trade, industry and the home. As such the screw or bolt can’t be over tightened. They were developed for the automobile industry in the USA during the early part of the 20th century and are designed to be used with a specific torque before ‘cam out’ occurs. Philips screwdriver bits use a ‘star’ shaped recess in the head. The tip of the driving bit must be an exact fit in the slot for them to turn correctly an ill-fitting bit will cause instant damage to the screw head. The screwdriver bits also use a colour code system for easy identification in a crowded tool box:Ī slot was the traditional type of method for driving screws when they were initially developed and it is little used today except for brass countersunk screws. The Axminster Trade Bitz magnetic holders have an accurately machined, double recess and are now fully compatible with all makes of tool including those with the Festool Centrotec system and any standard quick release, keyless drill chuck that are common on all power drills. The result in either case is damage to the screw recess and the bit in the worst case scenario, too much torque applied to a screw which is reluctant to turn will shear off the head. ‘Cam out’ also occurs where the effort needed to force the screw into the wood is greater than the torque applied to make it turn. This is a process by which the screwdriver slips out of the head of a screw as the torque turning the screw exceeds a certain amount. In this situation, the screwdriver bits don’t make complete contact and will result in ‘cam out’. Provided it’s the right size for the screw’s recess, a bit will continue to drive effectively until it starts to wear or the force applied to the screw is not directly in line with the shank. You can’t drive screws efficiently if the screwdriver bits don’t accurately fit the head. Badly driven screws may split the timber, the driving recess may become mangled (making them difficult to remove at a later date), or in the worst possible case, one or more of the screw heads may shear off, causing further work and unending complications. But in order to do a good job, the screws must be driven correctly. Done correctly, using the right screwdriver bits, screws make an attractive, strong and foolproof method of construction. Tags Screwdriver, screwdriver bits, ScrewsĪt home, in the workshop or on site, driving any sort of screw is a fundamental way of joining materials together.
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