Making a Knob

On a couple of YouTube videos, the makers chuck a round wood dowel into a metal or wood lathe (presumably with the grain along the length of the rod), cut a curved version of a top-hat knob, drill out a stepped-diameter mounting hole (one runs a splined pot shaft into the hole several times for force a fit), run a few grades of sand paper over it while it is turning, then coat it with oil or wax and call it done.  The process used here takes a bit longer:

1.    Most of the wood is cut from dimensioned lumber or rectangular billets to the approximate size of a standard knob.  The typical plastic strat top-hat knob is an inch in diameter at the base and a half-inch high.  The control underneath it is typically either a 15/16-inch standard pot, a ½-inch mini-pot or a rotary switch of about an inch in diameter.  So the wood knob cannot be either larger than about an inch in diameter or much over ½ inch high, or it will not fit comfortably on an electric guitar pickguard.  One convenient starting size is 7/8-inch square horizontally by 5/8 inch high.  Four of these (horizontal grain knobs) can be cut from the end of a billet that is 1.5 by 1.5 inch square at the end grain, or six from a 2 by 2 inch billet.  Starting sizes 1 by 1 inch horizontally are better for controls that are harder to turn, like rotary knobs which need a larger diameter.  Starting sizes ¾ by ¾ inch horizontally tend to make rather small knobs, suitable only for pots, since the finished diameter always ends up being less than the starting size.

Typical 1-1/2 by 1-1/2 inch blocks cut from the end of a billet

movingui block-300  granadillo block-300

dalmata block-300 sycamore block-300

Four rectangular knob blanks cut from each block (+1)

Four knob blocks-400

 2.    The center of a horizontal surface is marked on what will be the bottom of the knob.  Done imperfectly, this will cause wastage and a smaller knob.  From that center point, a circle is drawn in pencil to the nearest edge of the side, for later use in roughing out the knob.

3.    Because these knobs are turned on a wood lathe using the threaded shaft of a pen-making adapter (Woodcraft #145998), a ¼-28 hole is drilled and threaded into the center of the bottom of the starting block.

4.    A special bit adapted from a 9/16 spade bit is used to hollow out the bottom so that it will pass the threaded shaft and mounting nut of the control.

5.    The starter block is turned up on its side, with the grain horizontal and drilled and threaded for a 5-40 set screw, to meet with the splined or smooth control shaft.

6.    A belt sander then takes the corners off the block, just outside of the previously inscribed circle in (2).

17 roughed-out knobs with hollowed and threaded bottoms and threaded set screw holes

roughed-out knobs

Detail of single roughed-out knob

bottom-side view roughed-out knob-800

Detail of set screw hole

set screw hole in roughed-out knob7.    The block is screwed onto the pen-making adapter shaft, backed up by a special spacer.

Lathe left to right:

pen-making adapter, spacer, rough knob w/carbide bit and holder, tailstock

roughed-out knob in lathe w tool

8.    The live-center tailpiece is push up to the starter block.  The lathe is started and the tailpiece point is pushed into the starter block until it meets significant resistance and spins.  Unfortunately, this is necessary because the adapter does not sit perfectly in the lathe headstock socket and will otherwise loosen while the block is cut to cylindrical shape.  This leaves a dimple in the top center of most of these knobs.

9.    The starter block is cut to a cylinder on the lathe.  At any point in the cutting from this point onwards, the tool can always catch in the wood and gouge it, requiring a change in the size and design of the knob as the gouge is cut away.  This is especially true with a horizontal grain knob, because as the tool cuts from the end grain to with the grain, it can break off a chip even if it doesn’t catch and gouge.  So few if any planned designs survive contact with the tools.  The knob designs must always be flexible.

10.    The ends of the knob, top and bottom are squared off.

11.    The top of the knob is usually rounded over.  This allows the thinned finish to flow off of it without pooling.

12.    Now the basic profile of the knob is cut, and rough diameter set, all subject to how the wood reacts to the tools.  At this point knobs with horizontal grain usually develop white spots at or near the end grain sides.  Apparently the tool cuts away softer wood, leaving whiskers of tougher fiber sticking up.  Only filing or sanding will get rid of these spots.  This extra work is the price of iridescence.  If the wood has no iridescence, the grain might as well be vertical.

13.    Before or after the white spots are removed, the tailpiece is pulled away.  Final sanding includes filing and sanding the top to remove any tool marks, white spots and ridges around the tailpiece dimple.

14.    Now the knob can be polished to a high gloss with decreasing grits of sand paper, including 0000 steel wool.

15.    Removing knob from the lathe adapter requires a sheet of rubber and channel lock pliers, because it has jammed tightly on the threads from the force of previous cutting.

16.    For finishing with polyurethane varnish, the polished knob is dropped into the thinned solution to soak a short while, then taken out, wiped off and left to air dry.  About twice a day, the knob is recoated with the thinned polyurethane solution until it becomes glossy after drying.  This can take up to a week.  Usually the thinned solution flows smoothly over the knob and leaves only a thin coat with few defects.  The knobs may be polished with an ultra-fine abrasive cloth in between coats to take out any dust or other particles that have settled on them, or to break down small finish bubbles that have dried without popping.

17.    The ¼-28 threads have been left in the mounting hole in case the finish fails and needs to be removed by sanding or to be repolished on the lathe.  Once the finishing is done, they must be very carefully drilled out to ¼ inch.

18.    At this point the set screw hole should be rethreaded to clear out the finish.

Done.  We now have a one-of-a-kind wood guitar knob, with beautiful or spectacular or iridescent grain, depending on the wood.

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