Paul, those violins look lovely. A keeper for sure!
Constructing a lap harp
- Thread starter MusicMan
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profchris
Established Member
Mainly ukuleles, Keith, so as far from harps as stringed instruments get!
Your body looks pretty stout to me, so I don't anticipate any major distortion from cross-grain swelling. The top will be domed up by string tension, so any swelling will just raise that slightly. And given the short long-grain span, maybe not even enough to notice.
Shrinkage is the real problem, as it pulls the grain apart.
If you have some thin veneer you could laminate two pieces with the grain at 90 degrees and cut a strip a few inches long. It will curl towards the cross grain side in low humidity, the other way when damp. Humidity on Monday afternoon is forecast to be 50% (BBC weather app), so you could "calibrate" it that way.
Your body looks pretty stout to me, so I don't anticipate any major distortion from cross-grain swelling. The top will be domed up by string tension, so any swelling will just raise that slightly. And given the short long-grain span, maybe not even enough to notice.
Shrinkage is the real problem, as it pulls the grain apart.
If you have some thin veneer you could laminate two pieces with the grain at 90 degrees and cut a strip a few inches long. It will curl towards the cross grain side in low humidity, the other way when damp. Humidity on Monday afternoon is forecast to be 50% (BBC weather app), so you could "calibrate" it that way.
Yes, harp bodies are pretty strong. There can be a couple of tons tension on a big harp, all pulling out on the soundboard and thereby in on the sides, and twisting the pillar. So I agree that soundboard swelling is unlikely to be a problem. You have to tune harps all the time anyway!
Tomorrow sounds like a good day to glue the soundboard on, and to make that neat humidity gauge!
cheers
Keith
Tomorrow sounds like a good day to glue the soundboard on, and to make that neat humidity gauge!
cheers
Keith
Today was the lowest humidity day for at least a week (thanks, profchris), at 50% RH, so it was the day for gluing in the soundboard. First the string ribs, to which the strings attach, needed preparing. These came ready shaped with the kit and just needed smoothing and edge rounding. I masked off the soundboard around the string rib to keep the hardener just where needed. Aerolite comes with a powdered resin plus a liquid hardener, you apply one to each surface.
Then it was glued, positioned and clamped to the bench (having cleaned and smoothed that bit of the bench first).
I had lost the similar string rib for underneath, so was looking in my "small wood sorter" for something suitable. I looked in the "70 - 100 cm hardwood" section ... and found the original, neatly filed in the right place. Wow! So this was similarly prepared and glued after a couple of hours setting time for the upper rib. Again tape was used both to mark the position (opposite the top rib) and keep the hardener where it should be.
Next a simple channel rebate was cut at each end of the soundbox to accept the lower string rib. I took trouble to make sure things fitted quite well at this stage, and went on the loose side for the channels. I don't want any slightly poor fits causing a bump and lack of adhesion. This isn't a case where a little adhesive is sufficient, it really has to contact the soundboard all around the edge of the soundbox, with no cracks. Otherwise the strings might pull it off. So this is glued with a thick mix of Aerolite to fill any gaps, and fixed with every cramp of this sort of size that I own. Before glueing, the position of the bolt hole to affix the pillar was marked with a dowel centre, and the hole drilled through the sound board. An 8mm bolt was put in (but not tightened) during glueing, which acted as an index for the right position. I shall run some thinner Aerolite around the join when it is set, to try to ensure that the joint really is tight.
Ideally one should drill the string ribs for the strings before the glue up. But since the string spacing template has gone AWOL, and the firm (Border Harps) has gone out of business, I will have to work out the spacing from the positions of the string bridges on the neck, and the location of the pillar. I can't do this till it is all assembled.
So the soundbox is now complete except for access/sound holes at the back. I'd like neat elliptical ones - see my other thread where I am asking if anyone can make me an elliptical router template with a CNC router!
Keith
Then it was glued, positioned and clamped to the bench (having cleaned and smoothed that bit of the bench first).
I had lost the similar string rib for underneath, so was looking in my "small wood sorter" for something suitable. I looked in the "70 - 100 cm hardwood" section ... and found the original, neatly filed in the right place. Wow! So this was similarly prepared and glued after a couple of hours setting time for the upper rib. Again tape was used both to mark the position (opposite the top rib) and keep the hardener where it should be.
Next a simple channel rebate was cut at each end of the soundbox to accept the lower string rib. I took trouble to make sure things fitted quite well at this stage, and went on the loose side for the channels. I don't want any slightly poor fits causing a bump and lack of adhesion. This isn't a case where a little adhesive is sufficient, it really has to contact the soundboard all around the edge of the soundbox, with no cracks. Otherwise the strings might pull it off. So this is glued with a thick mix of Aerolite to fill any gaps, and fixed with every cramp of this sort of size that I own. Before glueing, the position of the bolt hole to affix the pillar was marked with a dowel centre, and the hole drilled through the sound board. An 8mm bolt was put in (but not tightened) during glueing, which acted as an index for the right position. I shall run some thinner Aerolite around the join when it is set, to try to ensure that the joint really is tight.
Ideally one should drill the string ribs for the strings before the glue up. But since the string spacing template has gone AWOL, and the firm (Border Harps) has gone out of business, I will have to work out the spacing from the positions of the string bridges on the neck, and the location of the pillar. I can't do this till it is all assembled.
So the soundbox is now complete except for access/sound holes at the back. I'd like neat elliptical ones - see my other thread where I am asking if anyone can make me an elliptical router template with a CNC router!
Keith
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Paul200
Established Member
Keith, I have no idea what you're talking about half the time but I'm lapping this up! (sorry - I'll get my coat......)
profchris
Established Member
Just a thought about glue, for when you build your second harp or if someone else is thinking of building musical instruments.
Ideally an instrument will still be being played in a hundred years time if it's any good, so builders need to think about repairability. For example, I suspect that over the years string tension will degrade the string attachment points on the soundboard, so that the string rib needs replacing or repairing. This is easier if your glue allows joints to be taken apart and remade. For this reason most instrument builders work with Hot Hide Glue or Titebond Original. However, both of these require near-perfectly fitting joints to hold, no gap filling. If made properly the wood should fail structurally before the glue joint (though heat + moisture will open up both kinds).
Aerolite is solid stuff (and very expensive!), and I don't think is reversible in any way. So if your soundboard ever has to come off, the repairer will need to cut it off with a razor saw!
OTOH, Aerolite is an aircraft glue, and I've flown gliders made in the 1960s using Aerolite which still hold together fine (I'm still here!) and they experience rather more stresses than instruments do. If you do need to take any joints apart, I can ask my glider repairer for tips.
This isn't meant to say you've picked the wrong glue from a constructional perspective, but to alert readers about the long-term repairability consideration if that concerns them at all.
And thinking about it, on about one instrument in three I decide to take a joint apart and remake it because I haven't got it exactly right, so that might also be a consideration.
Ideally an instrument will still be being played in a hundred years time if it's any good, so builders need to think about repairability. For example, I suspect that over the years string tension will degrade the string attachment points on the soundboard, so that the string rib needs replacing or repairing. This is easier if your glue allows joints to be taken apart and remade. For this reason most instrument builders work with Hot Hide Glue or Titebond Original. However, both of these require near-perfectly fitting joints to hold, no gap filling. If made properly the wood should fail structurally before the glue joint (though heat + moisture will open up both kinds).
Aerolite is solid stuff (and very expensive!), and I don't think is reversible in any way. So if your soundboard ever has to come off, the repairer will need to cut it off with a razor saw!
OTOH, Aerolite is an aircraft glue, and I've flown gliders made in the 1960s using Aerolite which still hold together fine (I'm still here!) and they experience rather more stresses than instruments do. If you do need to take any joints apart, I can ask my glider repairer for tips.
This isn't meant to say you've picked the wrong glue from a constructional perspective, but to alert readers about the long-term repairability consideration if that concerns them at all.
And thinking about it, on about one instrument in three I decide to take a joint apart and remake it because I haven't got it exactly right, so that might also be a consideration.
Chris, thanks for your comments about glues. The same had occurred to me and I nearly did use hide glue on the soundboard and string ribs. I used it on my bureau restoration for the same reason. I guess it was nerves at my first harp, and the worry that it would fail under tension! Good to hear your experiences with Aerolite!
In the end I decided on Aerolite all round, partly because the instructions suggested Cascamite, and partly because I figured that a failure would be pretty catastrophic and that I'd need to make a new sound board anyway. Next time I might well use hide glue on these, though I do much prefer the liquid type.
Anyway, the soundboard and ribs went on very well and here it is with the edges trimmed and looking something like a harp:
Mostly hidden in this picture is the neck, which now has to be carved to a nice shape.
cheers, Keith
In the end I decided on Aerolite all round, partly because the instructions suggested Cascamite, and partly because I figured that a failure would be pretty catastrophic and that I'd need to make a new sound board anyway. Next time I might well use hide glue on these, though I do much prefer the liquid type.
Anyway, the soundboard and ribs went on very well and here it is with the edges trimmed and looking something like a harp:
Mostly hidden in this picture is the neck, which now has to be carved to a nice shape.
cheers, Keith
Attachments
bugbear
Established Member
MusicMan":1ltlqnch said:
People have made laminated bows at very high draw weights using hide glue... !!
BugBear
Almost, Chris! There will be holes into which is pressed a rounded metal grommet to minimise the wear/stress on the string, then a knot on the back around a short piece of plastic (could be almost anything) to stop it pulling through. There is a thicker rib underneath the soundboard as well, the ribs are about 4 mm on top and 6 mm underneath, so this area is pretty stiff and strong.
Bugbear, yes I know, but I haven't!
Bugbear, yes I know, but I haven't!
Bugbear, actually, adhesives for harps are a lot more critical than for laminated bows! The draw of a modern bow is about 60 - 80 lb, and the highest estimate for a longbow is about 160 lb. The stresses on the glue line are mostly in shear. And the ends of a bow are usually capped, which helps against delamination.
In contrast, my little lap harp will have a total force on the soundboard of 400-500 lb and on a big harp that will go up to 2 tons. The direction of the force is at 20 - 40 degrees to the soundboard, hence there is a large normal component, up to about half of the total. The stresses on the glue line are tensile and peeling, which are the hardest for glues to resist. You can't pull a postage stamp off sideways whereas you can peel it off easily.
Guess I should have used six inch nails, bent over on the other side ...
Keith
In contrast, my little lap harp will have a total force on the soundboard of 400-500 lb and on a big harp that will go up to 2 tons. The direction of the force is at 20 - 40 degrees to the soundboard, hence there is a large normal component, up to about half of the total. The stresses on the glue line are tensile and peeling, which are the hardest for glues to resist. You can't pull a postage stamp off sideways whereas you can peel it off easily.
Guess I should have used six inch nails, bent over on the other side ...
Keithcustard
Established Member
bugbear":37426ulv said:
Excellent point well made. The "economy" glass paper from the 70's and even into the 80's dulled quickly and shed grit into the grain of your furniture. It was rubbish but it was all that was available at a realistic price. I wonder how much of general tool lore and enshrined work practise in areas like scraping were actually only necessary in that era of inferior abrasives?
The harp's looking beautiful BTW!
So, on to carving the neck. There's no particular design, just make the curves flow and look good while preserving the cantilever strength.
I used a rotodex tool, brilliant for removing material quickly, finishing with rasp and Abranet. One has to be careful not to overshoot, but I did :-(. Not from over-enthusiasm with the tool but getting confused as to which corner was being carved away. And like an silly person I carved the wrong one! So a corner splice of maple was called for. Nothing parallel to clamp to, so the gasman cometh again:
In the piece I had, there wasn't much chance to match the grain directions. After carving and sanding I got
So that diagonal line on the side is an artistic feature, right?
It's not finish sanded yet, and I may carve a little more from the left front.
Keith
I used a rotodex tool, brilliant for removing material quickly, finishing with rasp and Abranet. One has to be careful not to overshoot, but I did :-(. Not from over-enthusiasm with the tool but getting confused as to which corner was being carved away. And like an silly person I carved the wrong one! So a corner splice of maple was called for. Nothing parallel to clamp to, so the gasman cometh again:
In the piece I had, there wasn't much chance to match the grain directions. After carving and sanding I got
So that diagonal line on the side is an artistic feature, right?
It's not finish sanded yet, and I may carve a little more from the left front.
Keith
