Wood versus Plastic Knobs

Some differences between wood and plastic knobs:

1.    Uniformity
a.    Plastic knobs are injection molded to precise dimensions, markings and color.  They even have splines inside to fit the splines on volume and tone pots so they can be pushed on.  You can buy a thousand knobs that will look almost exactly the same.

  plastic knob-300

Standard plastic knob

 b.    Wood knobs are individual.  No two wood knobs can have exactly the same grain or usually even color.  Not even the same size and shape if cut on a wood lathe with hand-held tools.  Where they usually cannot be perfectly round due to differences in how the tools cut, file or sand through different areas of grain.  If the grain in the knob is not largely vertical, this can produce a distinctly oval-shaped wood knob, sometimes even a bit lumpy.  Because tools will cut deeper when cutting with the grain than when cutting across it.

knob0063  knob0016

Knob #0063                                                     Knob #0016

2.    Mounting
a.    Plastic knobs usually have internal splines in the mounting hole that mate with the splines on a control shaft, and just push on, with a certain amount of pressure.  Friction holds such knobs in place.  Some have smooth mounting holes and small set screws in the side so they can be mounted on smooth or flatted control shafts which have no splines.  All can be made so uniformly concentric with the rotation of the control shaft that there is no visible off-center movement.
b.    Wood knobs are best used with set screws on all control shafts.  If a knob has hard wood and a too-small mounting hole, forcing it onto a control shaft can damage the control.  If it has softer wood, and is taken on and off too many times, it may lose the ability to hold to the shaft by friction alone.  A set screw and a slightly large hole assures a good mount, but may show some off-center rotation, especially if the knob is not quite round.  Hardwoods will usually hole good threads in the set screw hole (say of 5-40 threads).  But in softer woods, the set screw hole threads should be reinforced with superglue or epoxy.  Hole threaded into end grain are almost useless in softer hardwoods, so they should be cut and threaded across the grain.

3.    Color
a.    Plastic knobs can be any color man can invent, including holographic rainbows.
b.    Wood knobs have grain colors that vary from black to white with any gradation in between, including colors of red, yellow, orange, green, purple and brown.  Wood can also be dyed any color that man can invent.

4.    Flash
a.    Plastic knobs can be flat, glossy, metallic, holographic, jewel-like, and even opalescent, but will usually look distinctly artificial.
b.    Wood knobs look entirely natural, even with a glossy plastic finish like polyurethane or superglue.  Some woods are flatly reflective with no flash at all, even if they have spectacular grain, like Redheart or Marblewood.  But a number of woods, even common domestic woods like oak, cherry and poplar, will show iridescence when polished to a high sheen, or polished and then coated with a clear, glossy finish.  As the wood turns in the light, the light will appear to flash from within the wood, some woods deeper than others.  This does not happen on end grain, only on grain parallel to the wood surface.  So a knob cut of iridescent wood with the grain vertical will typically have a darker, non-iridescent top, and sides that may or may not show iridescence where it can be easily seen.  But a knob of iridescent wood cut with the grain horizontal will not only show flash at two places on the sides, but usually across the top as well.  Where both the guitarist and audience can see it.  There’s a trade-off; because of the changes in grain around the sides of the knob, from end-grain to end-grain, knobs with horizontal grain will often be distinctly less round than knobs with vertical grain.  Roundness or flash.  Knobs with slanting grain sit somewhere in between.

5.    Grip
a.    Plastic knobs usually have knurls or splines or bumps around the sides to make gripping and turning easier and more secure.
b.    Wood knobs with a glossy, polished finish will likely be slicker to the touch that raw wood, requiring a tighter grip.  But that can be alleviated by cutting horizontal grip grooves or undercutting the top.  Fingertips then either push into the grooves or fold around an undercut top, providing more apparent friction.  The grooves are also decorative.  Here again, there is a difference between horizontal and vertical grain.  Because of roundness or cutting differences, the grooves on a knob with horizontal grain are not likely to be perfectly uniform in depth or width around the knob.  This can be improved by taking the cutting tool or file off the lathe tool rest and pushing it against the knob as it turns so that it rides against the knob with almost equal pressure around it.

6.    Finish
a.    The finish on a plastic knob is, well, plastic.
b.    The wood knobs presented here are all polished to a high sheen.  Most of them are then finished with thinned polyurethane varnish, both because it seals and protects the wood, and encourages iridescence.  Some woods, such as African Blackwood and Zapote, are so oily that polyurethane will not set, and stays tacky.  Trying to leach enough oil out the wood with solvent, to dry out the surface enough for polyurethane, can also remove color from the wood.  Some sources recommend blocking layers, such as shellac or superglue.  Shoe polish (neutral) works, but may not leave as glossy a surface as polyurethane.  Oil finish is not generally here used because it is usually much darker than polyurethane (which has a yellowing effect itself), and does not provide as hard a protective surface.  Wax and oil may also interfere with iridescence.

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