Constructing a lap harp

[MusicMan]

Having finished the Georgian bureau restoration covered in a recent thread I have time and space for another project. It's another oldie, in that fifteen years ago I bought a couple of harp kits in a fit of enthusiasm, got part way and then moved countries, houses, got busy etc etc. It isn't doing any good sitting in the shed in pieces but at least the timber is nicely seasoned!

The kit is basically the wood components cut roughly to shape plus the strings, levers, tuning pins etc. All the detailed work and joinery is left to do, apart from the holes for the tuning pins and bridges, which are pre-drilled. The wood is nice hard maple, with each thickness made up of two laminated 20 mm thick pieces, a good method.

When I got the kit originally, I glued up the sound box part - just glued butt joints on the angled edges. The first job was cutting the mortice and tenon to join the neck (the top bit carrying the string tuners) and the pillar (the bit at the front. This carries a lot of stress (the string tension will be about 500 lb) so needs to fit well. Much harder to make an accurate joint with the tenon going off at an angle than I had realised! The morticed cut nicely on the morticed when levelled with the base of the machine.

Once this is done the frame of the harp can be dry assembled:

and it begins to look like a harp. The height is about 90 cm.

Is the rest of the WIP something that people would be interested in?

Keith

 

[CHJ]

Is the rest of the WIP something that people would be interested in?

Keith

Certainly is, even for those not into the instruments themselves there are always little techniques and work methods that have a relevance for use elsewhere.

 

[Jamesc]

Yes please,

We always love a build project

 

[AndyT]

Yes please!

 

[MrTeroo]

Is the rest of the WIP something that people would be interested in?

Does James Corden think he's funny?
Is the Pope still wearing that dress?


I'll get the popcorn...

 

[bugbear]

Is the rest of the WIP something that people would be interested in?

Keith

Yes, very much so!

BugBear

 

[NazNomad]

Is the rest of the WIP something that people would be interested in?

Does James Corden think he's funny?
Is the Pope still wearing that dress?

... and do bears still relieve themselves in the woods?


Absolutely we're interested in more.

 

[MusicMan]

Thanks guys, didn't want to bore people, musical instruments is a bit specific. I usually work with woodwind instruments (repair, reconstruction) so this is a new venture.

I have the instructions kept safe for 15 years, but they are sketchy and the plan is missing. Hence I have had to research some features such as string spacing, so we'll see how we go. No idea if it will work or just break!

So to the glue up. There aren't any right angles so cramping is really tricky. I devised this method, using the end vice on the bench. This presses at the bottom right of the picture, in the direction of the tenon. This is transferred to the end of the pillar (top left) by two dogs in the bench holes, one either side. The end of the pillar goes in a pocket, which is itself secured to the bench through two other dogs, put at the diagonal spacing of the dogs. The gasman cameth and lefteth some very nice very thin tape, so that stops it gluing to the bench. It worked well.

For glue, I am using Aerolite 305. Cascamite was the original recommendation on the kit, but Aerolite is a similar but stronger formulation, the only glue certified to stick aeroplanes together. It was good enough for Mosquitos so it's good enough for me.

The soundboard is Sitka spruce, with the grain running across the body, not along it like the guitar (think about which way the strings pull). Hence it is made of a lot of short edge-glued segments. I had glued it up 15 years ago, but not shaped. So next I made a 'harp build jig' which will wedge the soundbox on one side (see later) and the soundboard on the other. Here is the soundbox mounting, with the planes I used: Quangsheng low angle BU, and a vintage Beck (Norris pattern) which is my best smoother.

The soundboard is just wedged in and clamped at the end if needed. The thickness now has to be tapered, from 6 mm at the wide (bass) end to 3 mm at the other). Hence the planing stop is only 3mm thick. Somewhat apprehensive about this stage! Anyway, the flat side worked well, and here it is flattened nicely all over:

Getting the bandsaw marks out of the neck and pillar is tricky on the inside curves. The Veritas low angle spokeshave worked pretty well (though the blade tends to push out). Part of this is done, but more remains, and I've just ordered an Abranet system as there will be much sanding, all over. Incidentally, I plan to French polish this harp. I was trained to do this as a boy by my father. He was a pharmacist not a professional woodworker but shared digs during the war with a cabinet maker (they both worked at an ordnance factory) who taught him very well. I even have the cabinet maker's page of instructions still written out.

Now off to finish planing the soundboard, and spokeshaving/sanding the neck and pillar assembly!

Keith

 

[MusicMan]

Now for the taper on the soundboard. Here I've written the target thicknesses along the board as a guide. It went surprisingly well, with freshly-sharpened plane irons, cap iron set close on the smoother, throat narrowed on the BU plane. The BU I used mainly diagonally for tapering, the smoother for finishing along the grain.

En route, I checked the thickness with a vernier calliper, taking the soundboard in and out of the tapered clamp setup.
Everyone knows about the Axminster 'duck' cams, yes? A very easy way to clamp something like this.

In order to get the taper right, I used a long straight edge and it was necessary to check with a feeler gauge to find high spots. I think I got it within about 0.1 mm off the plane, and this would have improved with the sanding.

The Abranet arrived with lots of sanding grades. What a marvellous invention! Especially since I suffer from asthma and some wood allergies, this is a game changer for me. In a remarkably short time I went though all the grades from 80 to 600, leaving a wonderfully smooth surface. The 80 grade included crowning the surface so that the edges are roughly 20% thinner than the centre (where the high stress from the strings comes), again checked with a feeler gauge. Then I damped the surface, let it dry and did the 600 again. The soundboard of course is the heart of the harp and highly visible, so needs to be as good as possible.

Now to fit the neck and pillar to the body. A lag bolt goes through the soundbox bottom face and the soundboard, through to the pillar. The pillar is screwed through from the back. The neck was screwed back onto the top of the soundbox and the bottom of the pillar pushed over to the central position at the bottom end. This puzzled me for a while. The neck is not central at the top of the soundbox, but is central at the bottom. The reason is that it is the strings that have to be central. And you don't make a complicated 1 (ish) degree offset at the neck end to centralise the bottom of the pillar, you just push it over. This actually provides some countering force to the huge tension of the strings. Indeed, if you did fit it perfectly before stringing, the strings would pull it too far over! Likewise, the neck joint is left with a bit of a gap at the front. which the strings should pull closed - this is not a mistake!

So, using a scrap soundboard piece to scribe the pillar end, this is trimmed off, and then flattened accurately using a 45 degree adjustable mitre shooting board. OK this is a tool brag, the Veritas shooting plane is a recent purchase from another member, and I love it to bits! Yeah, I know a Stanley No 5 will work fine, but my excuse is that with not much hand/finger strength thanks to arthritis and tendonitis, this is a brilliant tool for me!

Next is drilling the pillar for the fixing bolt, but the last few pics from today aren't loaded from iPhone yet.

Keith

 

[n0legs]

I'm in
Keep 'em coming MM =D>

 

[MusicMan]

OK fixing the pillar. Slightly embarassing, this.

First, leaving the neck screwed on and testing the fit of the complete soundboard. All nice and snug.

Next, drilling a hole near the top of the cross plate at the base, and chiselling a recess so I can get a lag bolt in, and marking the spot on the end of the pillar. These holes are at right angles to the mating surfaces (surface of soundboard).

Next, drilling the pilot hole for the lag bolt and VERY CAREFULLY measuring and setting the depth so that the hole doesn't break through. It didn't. Then clamping the pillar in the vice and screwing the lag bolt in to cut its thread without splitting the pillar. It didn't. BUT the bloody thing did break through then!

All well, I'll have to plug it and make a neat cap. Testing the fit of the bolt:

which works fine. But having messed up the front side and needing a cap anyway, I think I will use a bolt through instead of a lag bolt. The instructions suggested this, but I thought I knew better! A cap screw will be easier to fit and remove, too.

Some of you may be wondering why the neck and pillar ends are bolted and screwed rather than glued to within an inch of their lives. The answer is that it is good to have them removable for maintenance down the line. The screws and bolt are actually only needed for positioning and stopping slipping. The tension of the strings will hold them solidly in place, about 500 lb, remember. The most likely long-term (or even short-term ...) failure is cracking of the sound board. It's been very well seasoned (at least 20 years) but could still shrink relative to the sides with a change in humidity. And one cannot allow for shrinkage by putting it free in a slot, as it would buzz and rattle. It would not be too bad a job to take the strings off, unscrew the pillar and neck and fit a new soundboard, but a nightmare if these were glued on.

BTW I welcome comments or suggestions on how I could have done things better. I plan this to be the first of several harps, so it is very much a learning experience!

Keith

 

[bugbear]

The Abranet arrived with lots of sanding grades. What a marvellous invention!

I know many people who love working with handtools, and quite a few who love working with old hand tools. I know some who love working with VERY old hand tools.

I don't know anyone who loves old abrasives.

Modern abrasives are tools in their own right, and superb. Game Changing.

BugBear

 

[CHJ]

I don't know anyone who loves old abrasives.

Modern abrasives are tools in their own right, and superb. Game Changing.

BugBear

I'm willing to bet that there aren't many members that have a stock of 1930's stuff to compare.

 

[MusicMan]

Hear, hear, Bugbear.

More glue time. The harp jig is called out again, after being used to glue the two halves of the back together (seen in the background). Now the body is jammed into the wedges. This straightens out the bow that the sides had acquired.

Now the back is glued onto the soundbox. You see I missed a trick with the jig - I should have made slots so that the cramps can come right up to the edge of the wedge. I'll do that before glueing the soundboard on.

Finally for today, some roughly shaped pieces of maple are glued onto the end of the neck. These will be carved into the "shoulder", which helps prevent twisting of the neck under the load of the strings.

Now I remember that I was going to put a dowel inside these pieces. Ah well, it can go in afterwards and become a feature!

Keith

 

[Paul200]

Fascinating thread Keith. One of my daughters has a luthier boyfriend and he's opened my mind to a whole new realm of woodworking. Following this with great interest even though I have nothing to contribute.

Cheers, Paul

 

[MusicMan]

Thanks, Paul, glad you find it interesting. I am too, as you say it is a new realm of woodworking where suddenly quite different things become important, and quite different issues arise. What instruments does your daughter's boyfriend make?

Keith

 

[profchris]

When you glue the soundboard on, do it when the humidity is low. This reduces the risk of your soundboard cracking. In higher humidity it will try to expand but the body will resist this, and at worst your soundboard will dome slightly so the harp goes a little flat.

US luthiers aim at around 45% humidity when glueing up to give instruments a chance of survival in Arizona. In the UK 55% seems to work for me, so long as the instrument isn't kept near a radiator in winter.

Your harp has a heavier body than guitars a comparatively short span along the grain so this might not be such a problem. OTOH that's a long cross-grain span, and my guess is it will try to move 2-3mm with humidity changes. It's safer if most of that stress is expansion rather than shrinkage.

 

[Paul200]

Thanks, Paul, glad you find it interesting. I am too, as you say it is a new realm of woodworking where suddenly quite different things become important, and quite different issues arise. What instruments does your daughter's boyfriend make?

Keith

Violins Keith - he's made all of these so far during his course at Newark.

Needless to say, he and I get along very well :wink: I'm forever telling her he's a keeper!

Paul

 

[bugbear]

Needless to say, he and I get along very well :wink: I'm forever telling her he's a keeper!
Paul

Get him to show you how to prep a scraper. Violin luthiers are (by necessity) the Kings of that!

BugBear

 

[MusicMan]

When you glue the soundboard on, do it when the humidity is low. This reduces the risk of your soundboard cracking. In higher humidity it will try to expand but the body will resist this, and at worst your soundboard will dome slightly so the harp goes a little flat.

US luthiers aim at around 45% humidity when glueing up to give instruments a chance of survival in Arizona. In the UK 55% seems to work for me, so long as the instrument isn't kept near a radiator in winter.

Your harp has a heavier body than guitars a comparatively short span along the grain so this might not be such a problem. OTOH that's a long cross-grain span, and my guess is it will try to move 2-3mm with humidity changes. It's safer if most of that stress is expansion rather than shrinkage.

Thanks, Chris, good advice. I was aware of the expansion problem in harp soundboards, and have been thinking of constructing a "dry box" with controlled humidity for preparing the soundboard; but waiting for a dry day might be much easier. Anyway, I've ordered a hygrometer for the workshop and another for the music room. 72% RH in Warwick today, so not a day for glueing soundboards!

I appreciate your practical experience. Is it guitars that you build? As you say, the harp has a long cross-grain length, 800 mm even on a small lap harp, and the soundbox is quite rigid and aligned along the grain, so this could indeed be serious.

Keith