Chapter Six

Fabricating and Attaching String Ribs

In this chapter I will outline the method I use to make string ribs then attach them to the soundboard. Some harp builders omit the external string rib entirely. If you are going to do this = with a solid spruce or fir board, you should take measures to reinforce the holes around the eyelets as the tension of the strings is high enough to crush the wood underneath the eyelets. A hardwood veneer will help, but I usually back this up by laminatin= g a strip or two of fiberglass cloth underneath the decorative veneer (in epoxy= ).

Traditionally, string ribs are made from a heavy dense wood (Hard Maple or better) which c= an be difficult to cut and shape into long, gently tapered sticks. Before I bought a thickness sander= and a made a simple sled for tapering ribs, I used a surface planer to make the l= ong tapers in pairs. I will descr= ibe this process now. =

The clamping jig, a soundboard and finished ribs ready for gluing

Fabricating the ribs:

Start by ripping a stick little over 1” wide from a piece of 8/4 Hard Maple. It needs to be as long= as the soundboard. R= un all four edges of the stick through a jointer or planer to get them flat and smooth. For a conventio= nal 36 string harp, the stick for making the two ribs should be 1” wide, 1¾” thick, and 48 long.

On each end of the stick place a mark ¼ inch in from opposing edges and= use a straight edge to strike a line along the length of the board. At this point I plug in a hot glue= gun (the cheap hobby kind) and take the stick to the band saw to re-saw it into= two tapered rib blanks.

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Mark and saw the stick into two tapered blanks

After cutting the taper, the blanks will have three smooth sides and one rough side. Arrange the two blanks = with the smooth sides together and the two rough sides facing out. Glue them together with the small = bead of hot melt glue at each end. Gluing them together allows them to be fed through a thickness plane= r as a single unit as the fourth, (rough sides) are planed smooth.

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Glue the blanks back together rough side out.

Gluing the two blanks together also allows me to power plane the thin ends safely = as they are supported by thick end of the other rib. I feed them through the thickness = planer till the thin ends are ~ 1/8” thick.= At this point, the thick end of each blank should be ¼ inch thick.

If I had enough foresight to make the ribs an inch or two long, I can simply c= ut off the glued ends. Otherwise= , I use a putty knife to pry the two ribs apart and clean off the glue with a s= harp chisel.

The case for a narrow outer rib

I center the ribs as accurately as I = can, and lightly pencil the outline of the rib on the soundboard. Wet glue and clamping can make the= rib slither around, and it is difficult to keep them centered. If they are the same width, and th= ey are not perfectly centered over one another, the outer rib may begin to separate from the soundboard.

The crack is not u= sually a fatal structural flaw, but it can be an annoying cosmetic defect.

So, I make it a rule to keep the outer rib ~¼” thinner than the inner rib, especially in the high str= ess area where the bottom octave of strings pass though the sound board and rib= s. Some builders address = this by making outer rib ½ inch wide and about 3/8 inch thick along its entire length.

Next, lay out the taper across the width of each rib. Cut close to the line with a band = saw:

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I use a block plane and sanding blocks to true up the edges. I cut the top 3-4” off the o= uter rib and finish the tip to the classic fingernail profile.=

Different harp makers use different shapes for their string ribs. Some use a much narrower out= er rib, or eliminate the outer rib entirely.&= nbsp; Unlike sound board thickness, rib dimensions are readily visible on = most harps, so you can study what other builders are doing.

Clamping Jigs<= /span>

If you only plan to make one or two harps, you can use weights and tape to cla= mp the string ribs to the sound box while the glue dries. A slightly more sophisticate= d jig can be built from scrap plywood and “one by” lumber, see the section on a production clamping ji= g below.

Lee Gayman’s cross bar jig:

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For my first dozen harps, I used a set-up similar to Lee’s. It works well. Beware that the string =

rib will tend to slip and slide around under clamp pressure before the glue sets. To prevent this, I clam= p or tape small sticks of wood onto each side to keep the rib centered on the so= und board and do a dry run with the clamps before I cover the string rib with glue. Once the rib has been covered with glue and clamped in place for 10-= 15 seconds, the glue grabs and I can remove sticks and use a wet rag to remove= the excess glue.

Gluing tip:

On some of my first harps, I found the edges of the string rib pulling up and away from the sound board. I made several futile attempts to = close the gap with wider cauls and more pressure. I finally began to realize that the water in the glue was making the wood swell:

I keep a small pai= l of water and wet rag or sponge to glean up excess glue. After the glue is spread and= the string rib in place, I wet the top with a damp sponge or cloth.

Since the top and bottom of the board= are both damp, the rib will stay flat and the edges will not lift up from the s= ound board. It also helps if= you make the rib from quarter or flat sawn stock. Quarter sawn ribs cup less d= uring glue-up.

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Production Clamping Jig

For my first dozen harps, I used a set-up similar to Lee’s. It works well, but I thought= it could be improved by reducing the number of pieces to assemble and by provi= ding clear access to along the sides (making it easier to clean up excess glue a= nd inspect the joint). The basic concept behind the jig I use now is that of two beams sandwiching the board with an eyebolt at each end:

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The main weakness of this set up is that the long beams bend, so most of the clamping pressure is concentrated at the ends of the rib, and there is litt= le pressure in the middle. Optimally, pressure should be distributed evenly over the surface of= the area being clamped.

A second problem in gluing string ribs is that of an uneven or wavy sound boa= rd and/or string rib. Even= with careful fabrication, they are rarely perfectly flat and even. Often, while I was cleaning glue u= p I could see gaps resulting from these machining defects. When I tried to close the ga= ps with more pressure, I risked squeezing all the glue out of the high spots a= nd starving the glue joint.

I found I could solve both problems and get even clamping pressure if trimmed= one of the beams to a shallow curve and used spongy ½” foam padding under the sound board.

Trimming the upper beam:

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The top schematic shows the pressure distribution on the sound board when using= two straight beams – it is concentrated on the ends. To compensate for this, place a small block about midway be= tween the bolts and tighten them down fairly hard (but not super tight – don’t strip the thread). With a compass, scribe a line up from the bottom beam (shown as a dashed line).<= span style=3D'mso-spacerun:yes'> Remove the top beam and trim the b= eam along the line. When it is reassembled and the bolts tightened, the beams will still deflect, but the pressure will be much more even along the length of the board. On my jig, I did not center the bl= ock, but put it about 60% of the way down because I wanted to generate more pres= sure at the bass end where my string ribs have more surface area.

To address the second problem (gaps from slightly uneven or wavy boards and ri= bs), I glue a long trapezoidal piece of foam onto the bottom beam. To generate more pressure at= the bass end of the beam where my string ribs are wider, I cut the foam to be proportional = to the width of the ribs. If you do = not used string rib that is tapered across its width, you may want to use a rectangular shaped pad and center the block when you strike the scribing li= ne o the upper beam instead. =

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I use a “closed cell” foam – scraps left over from making h= arp cases. You could just as easi= ly use carpet padding. If you ask a = carpet installer, he will usually give you his scraps for free, or you can retrieve new material from construction site dumpsters.

As the bolts are tightened the foam pushes u= p on the soundboard. The foam not = only closes any small gaps, it helps distribute the pressure more evenly between= the board and rib, and will give you a better glue joint.

These last drawings simply cover some of the details I used to make my clamping j= ig. They are not a critical part of the construction, but they may give you some ideas to work from.

When I have built clamping beams like this for furniture case work, I found the stiffer the beams, the less they will deflect under load. I made the top beam in a T s= ection – the narrow base allows better access for cleaning out the glue. The top of the T is a strip 3/8” plywood, about 3” wide. A 3/8” hole has to be drilled through the thin face of a 1 x 4 for the bolt, so on each end of the upper beam I glued two 2.5 x 6” blocks of plywood on each end of the = beam for reinforcement . Since my = ribs are less than ¾” wide at the treble end I also trimmed the low= er corners of the T on one end as shown in the drawing.

The lower beam hol= ds the head of the eyebolts, so instead of chopping mortises, I laminated it from = two pieces of 1x4 pine and a strip of 3/8 plywood. The vertical strip of ply is= cut shorter than the 1x4’s so the eyebolts just slip in to place. A second strip of ply acts as a be= d for the foam padding. =

The eyebolts are two 8” x 5/16” eyebolts with T-nuts embedded in big wooden “wing-nut” handles.

Once the glue is spread, speed is paramount. I made one simple modificati= on that allowed me to open the jig up:

Cut a slot into one of the bolt holes on the upper beam.

Opening the jig will give you plenty of room to spread glue onto the board, and put= the rib into place (1). Then, sim= ply close the upper beam (2), and swing the bolt up into the slot (3) and tight= en the wing nut. I do a dry run = first, and pre-center the board into place before applying glue.=

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Go on to Chapter 7<= /o:p>

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