Sounding off about… design choices

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The parlour guitar has presented some interesting design choices, particularly with regards to the soundhole decoration. I don’t have the design chops of George Walker (oh but I wish I did) but I do take the design of my guitars seriously and I thought I would write about some of these decision here.

When laying out the soundhole decoration for full sized guitars I try to follow several rules of thumb. The outside circumference of the decoration should meet the corners of the fretboard, with the end of the fretboard falling over the top section of the decoration. The length of the fretboard is calculated as being the number of frets desired for the particular guitar plus an extra fret’s worth of fretboard material (so for a 22 fret guitar the fretboard would be 23 frets long).

However these rules of thumb do not all work for the parlour guitar due to the smaller body size. The parlour guitar has a 90mm diameter soundhole (compared to 100mm diameter for a full sized guitar) and 18 frets. But setting the diameter of the soundhole decoration large enough to meet the corners of the fretboard would be in danger of visually swamping the smaller body of the guitar. After drawing various mockups of the decoration, I decided to extend the fretboard so that the end meets with the edge of the soundhole. This allows the soundhole decoration to meet the corners of the fretboard, but on a tighter 106mm outside diameter which remains proportionate to the size of the body.

The decoration itself is a 4mm wide band comprising 0.6mm ebony banding on each circumference, with 2.4mm inlay in between the ebony. The main inlay blocks are eleven alternating ebony and mother of pearl pieces (the ebony blocks are shaded black in the above drawing).

I will be cutting and installing the inlay at the European Woodworking show on 12 and 13 September, so stop by my bench at the show and watch it unfold in real time!

Just a pretty face and thin body

I prefer to leave thin jointed surfaces clamped up over night to allow the glue to cure fully before further work, and so I removed the parlour guitar soundboard from the thin panel gluing jig the morning after it had been glued up. Because I place panels face down in the gluing jig, there can be a slight step on the rear of the join if the panels are not of consistent thickness, so before surfacing the show face of the soundboard I removed this step from the inner face using a block plane.

The final surfaced and thicknessed soundboard

The final surfaced and thicknessed soundboard

When surfacing soundboards and backs, it is imperative to get the work as flat as possible, as well as achieving a clean surface free from tear out, and this involves constant checking of the work with a machinist’s straightedge. In the past I have used a No.5 jack plane set to a fine cut, followed by a cabinet scraper to remove any hint of plane tracks, but this time around I decided to give the No.8 jointer plane a go, and the extra width of the blade allowed me to achieve a flat surface quicker than I would have with the No.5, and left only a little work for the cabinet scraper. For difficult timbers (particularly the backs and sides of acoustic guitars, which use exotics like rosewood) I use a toothed blade, and I may have to look into getting one made for the No.8 for this purpose. Because the soundboard panels are bookmatched, it is important to change planing direction on each half of the work, so as to avoid tear out, and the cabinet scraper is invaluable for cleaning up the glue line as it seems to resist tearing out while working against the grain.

The surfaced yellow cedar panel

The surfaced yellow cedar panel

With the full panel surfaced, I marked out the shape of the soundboard using my plywood template, and then cut to shape (staying bang on the pencil line) with a fret saw.

Using a half-body template ensures that the final soundboard will be symmetrical

Using a half-body template ensures that the final soundboard will be symmetrical

The thickness of the soundboard is critical for the sound of the finished guitar, and with the outer face now surfaced I was able to start thicknessing the soundboard from the inner face. The purpose of thicknessing is to achieve the right balance between a thin soundboard which will be responsive and resonant, while remaining strong enough to withstand string tension without imploding. The advantage of handmade guitars over mass produced machine produced instruments is that each soundboard and back can be worked to the optimum thickness for that particular piece of wood, with the bracing further adjusted to reflect the properties of the soundboard and back.

Measuring the thickness of the soundboard

Measuring the thickness of the soundboard

Using my large calipers. I mark the thickness of the soundboard at 2″ intervals, and then work the high points down until the the soundboard is down to a nominal thickness, using block and bench planes. This is a very precise way of working, and with planes set for a fine cut I know that as soon as the pencil numbers have been removed from the soundboard I have reduced the thickness by 0.1mm of material. Once each number has been removed, I remeasure everything and write the new thickness of the soundboard at the same 2″ intervals. Once I am close to the nominal thickness, the numbers stop being as important, because I am testing the wood by flexing it and listening to the sound it makes when tapped with my thumb. The need to constantly remeasure means that thicknessing a soundboard will take a couple of hours, but the end result is worth it. For a yellow cedar soundboard I work to a nominal thickness of 2.7mm, and this soundboard ended up just a hair under that as the timber had good stiffness and tap tone, which allowed me to remove just a tiny bit more material.

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With the soundboard thicknessed, I was then able to cut the rebate in the neck which accept the soundboard when the guitar is assembled.

The rebate cut into the neck

The rebate cut into the neck

This joint needs to be cut precisely, as the top surface of the neck and soundboard need to be completely coplanar. To achieve this, I set my marking gauge a hair less than the full depth of the rebate (2.5mm instead of the 2.65mm thickness of the soundboard) and once I reached my baseline I checked the fit using the soundboard, slowly sneaking up on the final fit with a very fine cut. To cut the rebate I bevelled the two outside edges with a chisel to prevent spelching, and then used a Lie-Nielsen to remove the waste.

The soundboard seated in the rebate

The soundboard seated in the rebate

A tool chest is never truly complete…

Yesterday, inspired by James’ recent baby anarchist’s chest build and the fifth anniversary celebrations of the Anarchist’s Tool Chest on Chris’ blog, I fitted a tool rack to the front of my anarchist’s tool chest.

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I had intended to include a tool rack as part of the original internal fit out last December, but at the last minute decided to wait until I had lived with the chest for a while to see whether it was necessary. After nearly 9 months of working out of the chest, I have found that the two smallest sliding trays were becoming increasingly cluttered with marking tools, screwdrivers, gravers, and other assorted small tools. So the tool rack started looking like a necessary and worthwhile addition.

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I still had some 1 inch square oak stock left over from fitting the trays and runners for the chest, and I cut that to length so that it rested on the middle runner at each side of the chest. The 1/2″ holes were drilled on 1 1/8″ centres, which did for the functionality of the tool rack. But as I have to look at the rack every time I open the tool chest, I wanted to pretty it up a little. First I cleaned up the show surfaces of the rack with a cabinet scraper, and then used my 1/8″ beading plane by Philly Planes to add a little visual interest. A coat of soft paste wax was the finishing touch.

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The tool rack was secured in place using two No.8 brass screws, so that I can easily remove it if necessary in the future. All told, the tool rack took me an hour of time, but has added real functionality to my tool chest. Although most of the holes in the rack are still empty, I have managed to de-clutter the two smaller sliding trays, and have a more ergonomic storage solution for many of the smaller tools I reach for on a frequent basis. Which makes that good value for a piece of scrap and only 60 minutes of my time!

A Gentle Reminder

The European Woodworking Show is now just over 2 weeks away, and if you have not already bought your ticket you really should. The line up is stellar this year (in particular I’m looking forward to catching up with my good buddy Vic Tesolin, and meeting David Jeske, Chris Vesper, and hopefully Richard Maguire) and the show is promising to be the highlight of the 2015 UK woodworking calendar.
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I will be at the show both days, working on the parlour guitar and demonstrating some lutherie techniques, so please do stop by my bench and say hi.

Shooting from the (hip?)

A couple of days before Grace was born, I pressed the new iteration of my thin panel gluing jig into service gluing up a soundboard for the parlour guitar. Although I started this guitar build with the neck, I normally prepare the soundboard and back first, and one of the things I love about building acoustic guitars is that very early operations have a decisive effect on the appearance and sound of the final instrument. Preparing the soundboard is one such critical task.

When jointing a bookmatched soundboard you have a choice between two edges for the joint, and this choice is critical in how the guitar will ultimately sound. I always joint the plates so that the tight, close grain is at the centre of the soundboard, with the grain becoming wider as you move towards the edge of the instrument. This ensures that the soundboard is as stiff as possible, which in turn allows the bracing to be reduced for a more responsive top (conversely a soundboard which is less stiff will be less able to withstand string tension without heavier braces – acoustic guitar building is a constant balancing of various factors).

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With the tight grain side of each board identified, my very first step is to take a couple of light swipes with a smoothing or block plane over the face of the board, just over 1 inch or so at the edge to be jointed. This cleans some of the fur off the board and allows me to see the grain properly. I then count the grain bands from the edge until it is possible to identify on both boards where the joint should fall. This is essential for ensuring that the finished soundboard has symmetrical grain radiating outwards from the centreline. I mark the joint position using a fine (0.3mm) mechanical pencil, and then joint each plate.

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To joint the plates I use a long grain shooting board and a No.8 jointer plane (although I have used a No.5 in the past, and experienced o difficulties in getting a good joint). A lot of woodworkers seem to think of long grain shooting boards as being training wheels for the inexperienced, but for thin stock where it is not possible to balance the plane on the edge in a consistent orientation, the long grain shooting board is invaluable. My long grain shooting board is nothing fancy, just a piece of 5mm mdf glued to a wider, 25mm thick plywood base. Because the joint for a soundboard is rarely at 90 degrees to the top edge of the stock I don’t have a full width fence on this shooting board as you would for an end grain shooting board. Instead I have a small piece of marine ply which catches the top left corner of the soundboard, and I hold the stock in position with my left hand. Similarly, the plane does not ride against the edge of the top layer of the shooting board to maintain a straight cut, instead I keep the plane in a straight cut by varying where I place the pressure. My right hand holds the side of the plane, with my forearm on the rear tote. I then attempt to plane a concave curve into the stock (which thanks to the length of the plane, is impossible), pushing in with my hand on the first part of the cut, and then transferring pressure to my forearm on the second half of the cut. With a fine cut, and constantly moving between the two boards, this produces a good solid joint with no gaps pretty quickly (this soundboard took me about 90 minutes from laying out the joint to glue-up).

The test for a good joint in thin stock isn’t with the straight edge. Instead, I hold the two boards together and try and rotate them across the joint. If the joint is tight then it will lock in place under minimal hand pressure and the plates won’t slip, nor will any light show through. If more work is needed then the plates will not “lock together” and will constantly slip off each other.

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With a nice tight joint established, both plates are placed face down in the gluing jig on top of a sheet of grease proof paper (to avoid sticking the plates to the jig), and the moveable beam is adjusted so that there is roughly 10mm of space between the fixed beam and the edge of the plate. Glue is applied to both sides of the joint (I use Titebond Original) and a rub joint used to achieve the initial adhesion. The wedges are then hammered into the slots between the plates and the fixed beam of the jig. Finally, a piece of scrap is clamped on top of each plate to prevent the plates from springing out of the jig under pressure from the wedges. The soundboard was then left in the jig overnight to cure.

Introducing the Apprentice

In an interruption to our usual programming, Dr Moss and I are very pleased to introduce my new workshop apprentice.

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Grace Bronte Binnie, born at 10:29am on 13 August 2015 weighing 7lbs 14oz. Despite arriving 11 days early, both mother and baby are doing well, and I am totally besotted. Grace has already received many lovely gifts, and the picture above shows her with a rare hammer made by C Hammond of Philadelphia, sent by a very generous friend so that Grace could shatter glass ceilings and enjoy workshop time with her Dad.

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And now a return to our usual content…

Luthier’s thin panel gluing jig

The following is adapted from an article I wrote for Furniture & Cabinetmaking Magazine (published in issue 234).

A great deal of the luthier’s craft is spent working with stock far thinner than that commonly used by most furniture makers, and gluing up thin panels of say, 5mm thick, requires a different approach (which I wrote about in issue 230). Of course, the use of thin stock is not exclusive to the lutherie workshop, and here I will describe how you can build this jig for your own workshop.

In essence the gluing jig is a slotted deck with rails on two sides (each of these sides has a rail above and below the deck). The rails on one side are fixed, while the other set moves to accommodate the width of the panel being glued. Clamping force is provided by hardwood wedges that are inserted in the slots through the deck between the panel being glued and the fixed rails, forcing the panel against the moveable rails.

For this build you will need:

  • 25mm thick plywood big enough to accommodate the size of panel you typically glue plus the width of two rails. I used a 35” square sheet because that is easily big enough for the guitars I typically build;
  • 70mm x 50mm hardwood stock long enough to yield four rails the length of the deck;
  • 6 No. 8mm diameter carriage or machine bolts together with nuts and washers; and
  • Enough 25mm x 25mm hardwood stock to make eight or nine 120mm long wedges.
Cutting the rails to size

Cutting the rails to size

The first step is to mark and cut all four rails to length, as we will use these for slotting the deck. I used a 16” tenon saw by Bad Axe Tool Works, filed with their special “hybrid” filing.

The deck contains two sets of slots; the first set runs from the edge of the deck, between the fixed rails, and extend beyond the fixed rails. It is this set of slots into which the wedges will slide. The second set numbers only four slots, and run across the deck to allow the bolts holding the moveable rails to slide up to the edge of the panel being glued.

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Laying out the position of the slots

Lay out the spacing for the short slots. I prefer to avoid maths wherever possible, so laid out my slots using dividers. I left 50mm at each end of the deck and adjusted my dividers to give eight slots down that edge. Next, clamp one of the rails across the deck to provide a stop for the router – these slots need to extend 40mm beyond the inner edge of the fixed rails. Using a 25mm diameter router cutter, and your router fixed with an edge guide, cut all eight slots.

Next rout the four slots for the moveable rail fixture, using the same 25mm diameter cutter. Again, two of the rails can be clamped across the work piece to provide consistent end stops – these slots start 110mm from the far end of the deck, and finish 405mm from the fixed rails. I offset these slots from the slots on the opposite side of the deck to avoid weakening the plywood, and increased the width of the two middle slots to allow for additional adjustment to allow setting the moveable rails at an angle for gluing irregularly shaped panels.

The slotted deck

The slotted deck

Before the fixed rails can be glued to the deck they need to be drilled for two bolts. These bolts help to locate the rail during glue-up so that nothing slips, and also provides additional security when hammering in the wedges. Drill the holes for the bolts, locating these to also drill through the plywood and avoid going through one of the slots you previously routed (mine ended up 125mm from each end). The bolts have a square section on the underside of the head, and are threaded through the bottom fixed rail with the nut and washer on the top of the jig. Cut square mortises in the underside of the bottom rail to accept the square section for both bolts. To mark out the mortise, insert the bolt and give it a sharp tap with a mallet, this will leave a crisp line round the edge of the mortise. Then chisel the waste until the bolt sits in the hole correctly. Apply your preferred brand of glue to the two fixed rails (I used Gorilla Glue), insert the bolts, and clamp up to cure.

Cutting the recess for the square head of the bolts

Cutting the recess for the square head of the bolts

While the fixed rails are clamped up, drill four holes in the two moveable rails, corresponding to the location of the slots, and mortise each of the holes on the bottom moveable rail in the same way as for the fixed rail.

Chamfering the edges of the rails for comfort

Chamfering the edges of the rails for comfort

Once the fixed rails have been broken out of the clamps, thread the bolts through the moveable rails so that the nut and washers are on the top of the jig, and fit the rails to the deck. These rails should slide smoothly across the deck with the bolts loosened, but should be rock solid once the bolts are tightened up. Chamfer the edges of both top rails with a block plane, and call the deck done.

Oak wedges

Oak wedges

All that remains is to cut the wedges. It is best if the wedges are not all identical, so cut these by eye with a handsaw (I used my 16” tenon saw). Use a block plane to clean up any sharp edges, paying particular attention to the fragile thin end of the wedge. I also gave the deck a coat of shellac, partly because this makes cleaning up any glue squeeze out easier, and also I find that it raises the grain in the plywood which makes for a grippier surface.

The jig is now ready to be pressed into use, and I will post a separate entry showing how I use it for gluing up the soundboard for the parlour guitar.