A ‘plug and plate’ repair for a double bass neck

TRADE SECRETS

Makers reveal their special techniques

Double bass maker based in Sydney, Australia

The way double bass scrolls are traditionally made causes inherent weaknesses to manifest in the form of cracks aligning with the inside edge of the pegbox, and laterally under the ‘chin’. A traditional scroll graft is normally the best approach for a damaged pegbox; however, not every instrument warrants the cost of a graft. Attempts to stabilise the area with bolts, screws or pins are destined to fail because of the small bearing surface and incompatible materials, and make future repairs much more difficult. The method described here replaces as much of the weak and damaged wood as possible with new, strong maple, and results in a quick and effective permanent – or interim – repair.

The final part of the repair is the ‘shear plate’, an interlocking piece of hard maple to reinforce vertical cheek cracks under the nut, and to lock the pegbox cheeks from shearing off under tension. This can be used as a quick standalone repair to a ‘pulled-forwards’ scroll, before it fails completely. It is very effective, and in no way prevents a scroll graft in the future. Robbie Macintosh introduced us to this approach at Oberlin, and I have refined it to a fast, economical and accurate procedure for my shop.

My approach presumes that you have expertise with a good-quality router, an excellent tool that can do precise and delicate work, but can also do a lot of damage to you, or the job, if you don’t take care with it. So be careful.

Realigning all the bits

ALL PHOTOS MATTHEW TUCKER

1 The example shown here is a student bass that would otherwise be a write-off: a catastrophic pegbox disaster where the chin is cracked through, and previous metalwork repair attempts (nails and screws) have failed.

First, I re-align and glue the break. I remove the nut, fingerboard and machines, old glue and previous metalwork, then clean and realign the cracks. I attach a temporary timber brace to the fingerboard to clamp the scroll in proper alignment. If the crack is clean and closes accurately, I use hide glue, as it has the least visible glue line.

In this case, as the break is old, jagged and incomplete, I use epoxy resin for its better gap-filling properties.

Marking out for the plug

2 The goal is to insert a maple plug that will replace most of the broken wood inside the chin, strongly reinforcing the crack.I carefully map out the extent and shape of the narrowest part of the heel, and draw it on to the face of the neck. I am going to make a deep, clean, parallel-sided cut with a router, all the way to the floor of the pegbox – but without breaking through the sides!

Next, I attach a sturdy plexiglass router-guide to the neck and scroll as shown, then carefully measure and set up a solid fence so that my router bit will be strictly constrained to the lines drawn on the neck.

Fitting a neck reinforcement

3 I use a Forstner drill bit to remove the bulk of the material inside the line. Then I use a long straight flute router bit in a strong plunge router, and making several careful shallow passes, cleanly trim out as much of the wood inside the chin as I dare. This requires care because it would be easy to go too far and break through the sides of the chin, especially if the scroll is twisted. I always triple-check the fence and do very shallow test cuts before going deeper, and I often measure the thickness of the wall, using outside callipers.

Then I cut a plug of strong rock maple with grain running perpendicular to the scroll face – in other words, end-grain facing up. I fit this accurately using a disc sander and finish it with a sharp plane. I use a bit of chalk or charcoal in the slot to make sure I have a good, tight fit. I usually choose to glue this plug with epoxy to fill any possible voids.

Routing and fitting the plug

4 Next, if the thinnest part of the neck needs stiffening (it doesn’t hurt), I route a groove into the neck through the plug and insert another piece of strong maple with grain running along the neck.

I use an inch-wide router bit for this, taking shallow cuts and going as deep as I dare without breaking through.Measure often. You will need a good solid two-sided fence for the router. Don’t try to do this freehand.

The hard maple insert for this is easy to fit accurately, and so I use hide glue. Sometimes it is useful to run the maple all the way down the neck, but in this case I stopped about halfway. Trim the insert flush.

Preparing for the ‘shear plate’

5 Mydozuki pull-saw blade cut down with tin snips to about 1.25” wide to fit into the pegbox. You want the teeth all the way to the tip. (2) a ‘fitting guide’ made from a piece of 6mm MDF cut into a trapezoid shape just smaller than the root of the pegbox; on one side of this I glue 180-grit sandpaper, while on all other surfaces I apply graphite from a 4B pencil. This tool is indispensable for getting a quick and accurate result. Don’t skip it!method requires the following special tools: (1) an old

First, to prepare the root of the pegbox. I rub it with the graphitecoated fitting guide to reveal the high spots, and use a paring chisel and files to make it all completely flat. (I’ve used two different projects here, as the photos are clearer).

Starting the slot in the pegbox cheeks

6 Then, I mark out a quarter-inch slot across the top of the nut, in line with the pegbox root, and extending halfway across the pegbox walls. The goal is to make a parallel sided slot tapering slightly from the front to the back. Using the cutdown blade pressed against the flat end-grain, I carefully cut laterally into the walls of the pegbox, until I’m about halfway through. Then I place the fitting guide sandpaper-side down against the flattened end-grain as a spacer, and make two more dozuki cuts to get four accurate, parallel-sided cuts.

Shaping the slot

7 I chisel out the waste with a 6mm chisel and finished with s quare file. I find it handy to use graphite on the sides of the fitting guide to highlight bumps in the slots so I can trim them with my chisel. Be careful not to split or tear the sides of the channel when chiselling out the waste. The goal is a smooth wedge-shaped slot with parallel sides.

Making and fitting the ‘shear plate’

8 Whenstraight/flat, I make a cardboard template that slides exactly in the slot, allowing a bit of over-length. Don’t skip this template, as it really makes things easier! the sides and back of the slot are perfectly

I tack the template to a piece of hard maple 7mm thick, with grain running across. I trim to exact size using block planes and a disc sander, then thin it carefully until it slides accurately into the slot.I can see where it is touching, from small polished marks on the maple.I’m careful not to force it in, as the wedge can push the cheeks apart.

Gluing and finishing up

9 I glue the fitted shear plate into the slot, sometimes with h ide glue but often with epoxy resin to ensure there are no voids in the side slots. I always secure the pegbox in a luthier vice to prevent the wedge pushing the sides apart. I trim the plate flush after. You can see that on this instrument, the plug repair broke through the side of the chin, as the scroll was badly twisted.

Using this method, we have kept many good instruments going at an affordable cost until a full scroll graft is warranted, and rescued many cheap instruments from oblivion!

A note about glue: Epoxy is often frowned upon by luthiers for repairs. In this interlocking ‘engineered’ application, however, I feel it can be a perfectly acceptable solution as it is easy to apply and has good gap-filling properties. Its slight flexibility may also help in accommodating any differential wood shrinkage. No metalwork is needed, and less wood is removed than in a traditional scroll graft, so the traditional approach is still perfectly possible down the track.