Stability of Spanish Heel

[Alastair]

Hi all,

Was looking for some advice after the fact, or else reassurance:

I've been making good progress on the classical, but this week the f#ck-up fairy decided I was overdue for a visit.

I had carefully aligned the centrelines of soundboard and neck, and glued and clamped them. After the usual hour or so under clamps, I put this back into my dry-box, given the humidity we’ve been having lately. Trouble was that the clamps wouldn’t fit in, and the unclamped assembly had to stand upright, something like this:

As Murphy would have it, under its own weight, the unclamped joint was still green enough to creep overnight, until the centreline was about ½” askew at the heel.
Unfortunately, I was so sure of the alignment, I didn’t check before the next step.
I fitted the sides, wondering subliminally why they were further from the outline than during the previous fitting, but assuming that the sides had moved a bit after I had touched up the bend.
Spent a backbreaking day gluing on tentellones, left overnight for glue to set, then proudly took it in to show off to the family. Sighted down the neck checking for twist, and was confronted with the monumental kink.

Fixing now presented a conundrum. Glued at the tailblock, and heel wedges superglued into the slots, so a complete disassemble wasn’t possible. In the end I took what seemed to be the route of least risk.

As the problem was manifest in the upper bout, I carved off the tentellones between the upper cross strut and the heel. Then I heated the joint between heel and soundboard with an iron, while applying corrective pressure. As the glue softened, I was able to pull the neck back in line, and then reclamp and allow to cool. Alignment was corrected, but there was some collateral damage. With the heat, the soundboard joint also softened and opened up about 1/2mm, (fortunately only as far as the UTVB.) I’ve added a spruce graft in the space between the UTVB and the heelblock, and wicked superglue into the crack. All that’s left is the prayer that the neckjoint stability hasn’t been badly compromised.

Once the upper bout was redone, it seems kosher, but I’m concerned about the implications in the completed instrument, and would welcome any input from the experts.

[kiwigeo]

One comment from me......when building spanish method the instrument shouldn't come off the workboard until the body is closed up. Until the back goes on the neck is free to flop around like a donkeys dick unless it's secured to a workboard.

[Allen]

Build with the Spanish Heel all the time, and I'm 100% with Martin on this. That instrument stays on the work board until you've closed the box. Far too many things to go wrong when moving it about, as you've found out.

The soundboard coming apart shouldn't present a problem if its just north of the sound hole as you've said. You've got a lot of bracing up there, and the fret board is going to be glued down to the top as well I presume. Move on and try and forget about it....but not the lesson learnt.

[Alastair]

Thanks Martin and Allen,

I was less concerned with the split in the top, as Allen said, but more with whether heat softening Titebond to realign the neck would grossly weaken the joint between SB and heelblock. With the box closed now, it seems rock solid, (and straight), so I have to trust it will be OK under load.

Starting to look like something now, but a long way to go.

However, I now have another concern. When I inlaid the imbuia backstripe, the back took on a reverse bow along the line of the inlay. In my inexperience, I assumed this was normal, and would come out once braced and the box closed. However, this has not been completely correct. It is fine at the braces, and for most of the back, the hollow is minor, and will scrape/sand out. Between the tailblock and the lower bout brace, however, there is a dimple of 1/2 to 1 mm, which I don't think will be correctable. My intended (and preferred) gloss FP finish will make this glaringly obvious, so I'm considering changeing to either a matt shellac, or else sprayed satin cat lacquer instead.

I was wondering what the thoughts would be on this?

[kiwigeo]

However, I now have another concern. When I inlaid the imbuia backstripe, the back took on a reverse bow along the line of the inlay. In my inexperience, I assumed this was normal, and would come out once braced and the box closed. However, this has not been completely correct. It is fine at the braces, and for most of the back, the hollow is minor, and will scrape/sand out. Between the tailblock and the lower bout brace, however, there is a dimple of 1/2 to 1 mm, which I don't think will be correctable. My intended (and preferred) gloss FP finish will make this glaringly obvious, so I'm considering changeing to either a matt shellac, or else sprayed satin cat lacquer instead.

I was wondering what the thoughts would be on this?

I had one classical lose the doming in it's back. I ended up replacing the whole back and using a cross brace for the lower bout to provide better support for the back than conventional transverse braces. What thickness is your back? if the back is a bit on the thin side then chances of collapse are higher.

The dimple you talk about...it's not a result of the top of the butt block being flat is it? With a flat topped butt block the back profile has to change abruptly from flat over the block to arched over the rest of the back. Just a thought.

[Allen]

I'm trying to invasion what you are talking about, and pardon me if I got this wrong. You inlaid a strip into the back (not butting up the book match to a centre strip).

If this is the case then I've seen it happen where the back strip ends up pulling the joint together just a tiny bit and you end up with what you are describing. I've found that by using some purling on either side of the hardwood inlay helps alleviate this. My thinking is that the purling (soft wood or fibre) has some give in it so when everything dries the purling gives just enough.

That's my thoughts on it. As far as a fix....don't know. Go satin me thinks.

[Alastair]

Thanks guys,

Martin, no not from the tailblock. I was careful to angle it to match the arch along the length of the back. Also, the "dimple" is (slightly) visible all along the back centre, except where the braces run, just that it's only a bee's dick except in the bit near the tailblblock.

Allen, I think you've got it dead right. I just thought that it would correct when braced and attached. Hindsight is wonderful innit? I'll try scraping and sanding, and see how much I can achieve. If not........satin, but will be a pity.

[Alastair]

I've seen it happen where the back strip ends up pulling the joint together just a tiny bit and you end up with what you are describing. I've found that by using some purling on either side of the hardwood inlay helps alleviate this. My thinking is that the purling (soft wood or fibre) has some give in it so when everything dries the purling gives just enough.

Hi Allen,

Been mulling this over last night. Makes sense to me. Had the following idea:

Inlay is less than 1mm thick, (so less than half the back thickness), and is in the middle of a 3-piece back, (so no glue join underneath).

I'm considering cutting carefully along the margin of the inlay, with a craft knife and straightedge, down to the inlay level, to free up the "pull". I believe this will dramatically reduce the extent of the problem, (hopefully to the point where the discontinuity can be scraped/sanded out.

Fill the cuts/grooves with black epoxy, and sand back after?

[kiwigeo]

Inlay is less than 1mm thick, (so less than half the back thickness), and is in the middle of a 3-piece back, (so no glue join underneath).

To clarify...you routed a channel in middle of three piece back and then inlayed the back strip?

[Alastair]

Exactly correct Martin,

I didn't like the idea of 2x backstrips, and felt that I needed the contrast to define symmetry, and tie in with the bindings.

[Allen]

The one time I had this happen to me I fixed it by setting up a fence very carefully so that I could use my laminate trimmer and a fine bit that matched some purlfling I had. Measured at least 1/2 dozen times and then committed to the cut....on each side of the inlay. Pushed the purlfling in and flooded with thin CA. It worked but there was an awful lot of sweating going on.

I don't recommend this. Just putting it up there as an option if you're dead keen.

[kiwigeo]

I generally take a totally anal back to the start approach to such problems. I'd carefully rout out the inlay (tricky close to the neck) and glue in a new strip....with purfs either side as suggested by Allen. I'd use epoxy glue rather than AR to minimise risk of inlay swelling. Ive done this on a free plate back but havent done it on a back fitted to a guitar.

[Alastair]

Thanks for the advice, Martin and Allen,

What I have done, is as above. I scribed along the junction on both sides of the inlay, down about 1mm. Initially no significant change, but after I wet the centre down, (it got worse)!, and then let it dry, it has come right back to a dome, all except right at the tailblock, which has come back to flat.

I then wicked CA into the scribed cuts, and after scraping and sanding back, it has come good. Busy doing the binding now.

thanks again, an unexpected Xmas present to offset last weeks stress.