Red Dog Music | Oct 9, 2018 | 0
The Strange World of Dr Fretlove: Got a Screw Loose?
Coming from an engineering background; I take it for granted that everyone understands the principles on which a screw works, and the significance of different types used. The truth, however, is that huge swathes of the younger generations, having received Playstations instead of Meccano sets* at Christmas/Birthday time, have not gained a basic grounding in simple mechanics.
The old joke about a psychiatric patient who escapes after having sex with one of the laundry staff – prompting the newspaper headline “Nut Screws Washer and Bolts”- might, these days, be considered politically incorrect but it still serves as a handy means of calling to mind some of the basic types of fixings likely to be found on the common or garden guitar and bass.
The majority of screws encountered will be simple self-tapping wood screws, typically used for scratch-plate and trem, back-plate fixings, as well as pick-up surround mountings. A much larger version can also be found, on guitars with “bolt-on” necks, holding the neck to the body. These are usually of a cross-head Phillips design, the idea being to spread the torsional forces of the screwdriver over four surfaces. The right-hand wall of each cross facet will be taking the load when tightening and the left-hand surfaces when slackening. This lessens the chances of damaging the screw-head. It also makes it harder for the screwdriver to jump out of its location, within the screw-head, whilst being tightened or slackened.
With the advent of locking trem systems; Allen bolts, rather than screws, began to make an appearance. Allen bolts have a hexagonal recess into which a hexagonal key is located. This design allows a greater contact area between tool and work-piece, in order to distribute the torsional forces more evenly.
An Allen key is used as a lever to turn the bolt. It’s an excellent system but needs to be treated with respect as the extra torque available, via the key, is enough to damage the metal the bolt is going into. It’s not uncommon to have to replace an entire locking nut or bridge assembly after an over- enthusiastic hand has been at work.
All the above type of fixings utilize a V shape, cut into the metal, in a downwards, clock-wise spiral which meshes into its mirror image, cut into the receiving material. In the case of scratch-plate screw – it’s the wood of the guitar body, cut by the self-tapping screws. In the case of the Allen bolts – it’s a metal plate or bush. These spirals are known as the threads and the spacing of spiral cut referred to as the pitch of the thread. Generally the smaller the amount of adjustment required, the finer the pitch.
Golden rule – you can’t mix different pitches of thread! If you lose a screw then any replacement has to be correctly matched up to avoid “stripping” the remaining good threads. The classic case of mismatching threads has to be when replacing a lost tremolo arm. The amount of times I’ve had to replace entire trem systems because someone has used the nearest item to hand without checking to see if the threads match. On a work-piece that’s constantly vibrating when in use, it’s easy for any type of fixing to vibrate loose.
The best way of securing a loose part is with a hexagonal nut and washer. Very often the combination of nut, dress washer and friction washer will be used to ensure that the nut stays put. When tightened the serrated surface of the friction washer digs into the surface of the wood on one side and compresses against the dress washer on the other. Nuts are typically used to secure jack sockets, pots and, along with an integrated bush, many designs of machine heads. Whatever the type of fixing – screw or bolt – it’s imperative that the correct tool be used when working on them.
Using a Posidrive (8-point design) on a Phillips (4-point design) cross-head screw will not only burr over the metal on the screw, it will also make a mess of the Posidrive screwdriver, rendering it useless for its proper job of tightening Posidrive screws. Equally, using a metric Allen-key on an imperial Allen-screw will not only round off the key edges but will burr the recess within the bolt-head.
My screwdrivers all have non-slip rubber handles and hardened steel shafts and come in three sizes appropriate to the size of screw. The Allen keys are all kept in their respective Metric and Imperial sized clips. Woe betide anyone who goes off with them. It drives me, well, nuts.
* ask your Dad