Fittings and Spikes explained.
Speaker spikes are usually used to isolate loudspeaker stands and floor-mounted / standing speakers from floor vibrations. They are more effective on wood-based floor coverings as concrete or tile coverings are less vibrant.
The spikes are generally of steel construction (for moderate cost), chemically blacked, M6 or M8 in diameter and available in various lengths with knurled or plain body sections. Dual-ended, ball and point versions are also available in alternative materials, brass, chromed, blacked, etc. but are less popular and more expensive due to the small quantity required.
In order to fit the spikes to cabinet bases, a wood insert and optionally a locknut are required. The inserts are available in two styles, those that knock-in and those that screw-in, This is in addition to shouldered or unshouldered,
i.e. flanged, and also in various lengths. However, those used for loudspeakers are generally within (10 - 15)mm in length, in order to match the thickness of the speaker cabinet.
Each type of insert has it's own advantages and problems. Screw-in types have a sharp cutting thread and generally a small lipped shoulder which aids in depth setting. Their advantage is high pull-out resistance. The disadvantage when used with a locknut,
is that they can unscrew, having minimal tortional / twisting resistance. hence unscrew along with the locknut and spike. Normally fitted once and forgotten, this isn't usually a problem. However, a dose of wood glue on the threads before insertion should sort it altogether. Note, (Fig 3) the difference in thickness between a Half-Nut and a standard Full-Nut.
Push-in types have a number of tangs / pointed teeth which, a) hold the insert in place and b) prevent twisting. The advantage is high twisting resistance but only moderate pull-out resistance. As they generally sit "underneath" a cabinet, pull-out is not a problem.
(Fig 6), are similar in that they have 3 or 4 pointed tangs which, due to the tang profile, close inwards as the nut is driven home, preventing them from coming loose. They have high torsional resistance but low pull-out. There are no International Standards on dimensions for these types of fitting and sizes can vary accordingly, depending upon manufacturer.
Each insert requires a pre-drilled hole. The size / diameter of the hole is important for best fit. Holes should be blind-drilled, i.e. do NOT drill through into the cabinet interior as this will create an air leak along the insert / spike thread. If you have a thin walled cabinet base, glue packing pieces about 40mm square and 10mm thick or so on the cabinet base, inside or outside, to provide a thicker material depth in which to fit the insert. As previously mentioned, the hole size is important and the harder the material, the larger the pilot hole. Solid Ash or Elm etc. requires a larger pilot hole than MDF or plywood. The following tables suggest drill sizes for various insert sizes and materials.
The insertion of screw-in inserts is intuitively obvious but not quite so obvious is that a portion of the internal thread space is used as an insert for Hex Key or Ball Ended Hex drivers. This limits the minimum length spike which can be used if height adjustment is required, as you run out of thread.
Using knock-ins, one needs to be careful. MDF is compressed wood dust and glue. It is compressed in layers and does not like shock, i.e. hammering. It splits and falls apart. If you have access you can use a G-clamp and compress the insert into the material, it's a bit fiddly but works. A better technique however, (this only works on through-hole insertions) and not restricted on overall reach like a G-clamp, is to use a set screw (all-threaded bolt), and a large washer or two, (mudguard washers), see (Fig 4).
Locate the insert in position, Place the washers on the bolt and thread the bolt and washers through from the reverse / cabinet inside. The bolt will thread onto the insert and slowly draw the insert into place as you tighten the bolt. Once fitted, (Fig 5), withdraw the bolt and repeat as required. As mentioned, this only works for through-hole drillings so you need to plug the hole interior with a blank wood piece or use some silicon to seal it, thereby making it airtight.