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Can anyone who uses a light table to check the joint on their back plates before gluing them up give some advice? I just spent a long time shooting some bubinga trying to get the joint perfect. I picked a up a free light table from a friend and thought it'd be perfect for this. Trying to hold those things up to the light is tricky. But the light table made every attempt look horrid. There were many times I would have glued it up with my old way of holding them up to the light, but on the light table, they looked rough cut. Maybe I'm just having an off day, but this was way more trouble than I've had before.

So, did the light table save me, or is this a case where perfect is the enemy of done?

Thanks,
Mike

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Mike Lindstrom

I don't use a light table, but your post makes me think I should. Everything I've done that exaggerates my errors has improved my work.

We have a light table in the shop.
I had the same problem as you, when I did my first joins.
It looks perfect to my eye, but when I look on the table, whoaa

I think it's just way more precise, and could only help you to achieve better joining.

Francis

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Francis Richer, Montréal
Les Guitares F&M Guitars

A low-tech method to get you very close- taught to me by my mentor
from Paracho- simply rub the rough-jointed edges of the plates together.
With some raking light the burnished high spots will be apparent.
Take those spots down (any method) and try again. Just don't try
to glue burnished wood, it'll make for a weak joint.

Got a window? Or a sliding glass door?

Both work fine.

The light table itself is a fine idea, as it keeps the plates in the same plane-
no sense in making the job harder than it needs to be. The issue with free-
handing is that you don't know if the plates are in plane when the joint looks
good, and this joint needs to be very good.

-Kinda like using a honing jig, for those who feel the need.

Mike: When a joint is lit under very good conditions such as your light table an opening of .001 to .002 looks like you can drive a truck through it. The ideal glue joint IIRC is in the area of .003. This leaves food for thought on how good we get joints under less the ideal conditions.
Tom

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A person who has never made a mistake has never made anything!!!

I have always used a window too because it is readily available in my shop and I've done it during the day. A Light table seems like a good idea to me too as long as it's flat and large enough. I see it as a just another tool to help with a build and if you have one and it makes it easier for you then why not use it? One day I hope to be able to hone my skills to not need all the extra jigs and tool aids but for now...
Seems to me that the area below the soundhole is what to be concerned with as normally above it is covered as is with rosette area and bridge.
Just a side note about light tables:
Larry Robinson's inlay work bench has a glass top (3/4"-1"???) which he just puts one of his lights under when he wants to use it as a light table for refining his inlay drawings/tracings. I always thought that was a pretty neat idea.

With all due respect (and I mean that), I can't help but feel a case of raising eyebrowse with that statement. I strongly feel that it is much better to make sure that both plates lay dead flat against each other when doing a candle light check. Only a light table or a large window can help you do this. Just holding the plates up to a light source is playing with fire, IMHO.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Thank for all your thoughts. I was really hoping a lot more people would be saying what Tom said, because when I hold it up to the light, it was perfect and I would have had no problem gluing it up on previous instruments. But on the table. Woah!

I know with some of the wriggly grain on this board, it makes for some hot spots with the light.

I got up this morning and the joint seemed good and solid. It took a large amount of flexing and the board rang well. However, They slipped when I clamped them and were offset vertically about 0.035 in one spot. I usually check that when I'm done, but forgot to last night.

So this morning, I had to opportunity to bandsaw them apart, and re-join them. While still frustrating, is was faster, and I ended up with a better joint. I wonder if some of the glue (fish) seeped in a bit to support the fibers?

Does anyone know how factories do this quickly?

Mike

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Mike Lindstrom

I don't know, but suspect the factories use a power jointer. I jointed my first 3 or 4 with a plane, and it took a while. My Dad (a retired building contractor/cabinet maker) was at my shop one day when I was jointing a back. He said "Why don't you use your jointer. That's what it's for?" I'm not always (or even usually) a fan of "factory methods", but a good joint is a good joint. There's obviously going to be a tough one every now and then but now it usually takes me 5 minutes of less to joint a top or back. The jointer beds have to be perfectly parallel and the blades have to be sharp and properly installed. Small cuts, with light, or almost no pressure, just like using a plane, is important too.

I applaud you for re doing the less than perfect joint. My guitars got ALOT better when I finally stopped being hard headed, and would re do stuff that wasn't perfect.

I think a light table is probably a good idea, but I just don't have room. Like Todd said, you just have to be careful how you hold the plated together.

Todd, my arms are exactly the same length....................or at least within an inch of each other

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

FWIW, I've always held mine up to a window as well.

Maybe they just don't hold it against any light at all. Maybe they are not so picky when it comes to perfection. Or maybe they are really good woodworkers and have the seamless joint in their blood. Who knows.

Todd: The only .003 mentioned is the one in my post so I guess you assumed I meant to have a .003 gap. I said a glue line is in the order of .003 in thickness not that you can have a gap of .003.
Tom

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A person who has never made a mistake has never made anything!!!

I prefer the window although the free hand bulb works OK too, as long as you really insist at it. I often found that it looks good at a quick inspection, only to reveal the gap by moving and rotating the head for a bit.
Another thing that can be done is to rub the plates up and down, not along the joint. Good contact areas have friction and they are hard to separate.
This being said I'd love a light box if I'd had the space for it, or the patience and time to build one. Actually one cool way would be to integrate it in a rope and wedge joining jig.

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Build log

Filippo - Prior to this, I did them freehand too and consistently got invisible joints, which I suppose is good, but boy is it frustrating trying to find the centerline when I need it. I grabbed the table when offered, because I was frustrated with the process of trying to hold them up align them in multiple dimensions and keep them coplanar. The lightbox makes that way easier. But my OP was because I learned that my "perfect" joints were, in fact, a disaster when examined this way. FYI I've usually used TB, but this was fish glue. I usually clamp by tapering the plates and driving them like a large wedge into a "v", but this plate didn't have room to taper, so I improvised with boards, c-clamps, kettlebells, quick grips, screws, and pipe clamps.

Maybe this was just a difficult piece of wood. I'm doing the top plate today - nice straight grained Shane spruce. Perhaps it will go fast and easy.

And for those considering making a light box. There are other ways it could be useful in your shop full-sized, but if you only need it for candling tops, I'd recommend a narrow slit (1"-2") of light. Even though it was just the time putting the parts on and off, my eyes are very unhappy about the prospect of staring into those fluorescents again.

Mike

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Mike Lindstrom

Filippo -

Do you have flame maple binding on your workbench?

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Mike Lindstrom

The purpose of candling a joint is not to get an invisible glue line, it's to get the best joint possible. An invisible glue line is not proof of a perfect glue joint, it just shows that you have an invisible glue line. You may still have a joint that had uneven pressure when clamped, and still get an invisible glue line.


This demonstrates quite precisely my point.

Putting the plates on a window to check the seam is too easy to do to not do it.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

I've only joined a half-dozen tops/backs and have always "candled" then against the glass portion of my "screen door", which is at eye level. Unfortunately, my front door is a bit of a walk from where I do my shooting work, so as I fine tune the joint I find I get a bit of exercise walking back and forth between my front door and my work area.

To avoid the walk, on the first 3 I joined I tried holding the plates together at arm's length and holding them up to light, but I encountered two problems. First, I found it VERY difficult to hold the plates together perfectly in alignment while moving my head and/or the plates up and down to inspect the entire length of the joint. I always ended up with a little movement and would end up placing the plates back on a flat surface, pressing them together and starting over.

The second and more important problem I encountered is that when I thought I had a good joint, I would walk to the front door and inspect it against the glass and ALWAYS found gaps I had not seen when holding them up to the light. My conclusion was that the movement I always experienced holding the joint up to the light probably resulted in me trying to correct gaps that weren't really gaps, and as a result creating more real gaps.

Candling against a true flat glass plate where I can press the plates together using light pressure just seams like a no-brainer for someone like me. A light table is appealing since I could situate it close to my work area and avoid the walks. I have no doubt experienced folks can get a good joint with the right technique by holding the plates together with a light source behind them, but I know from experience I can't. So until such time as I can develop that technique, it's the front door for me. IMO it's a small price to pay for the peace of mind that comes from knowing I achieved a very good joint.

That be my $0.02.

Hey everyone -

Today I joined the top. It was lutz from Shane, and a little bit thicker than the bubinga too. Not only did I get a completely dark join line on the light table, but it took all of about 10-15 minutes of messing with it. And yes, without the light table, I probably would have joined them earlier and less perfectly.

If I can figure out room for it, I'll keep using it, but I agree with Charlie. I've got to get it closer to my bench. My legs are sore!

Mike

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Mike Lindstrom

Filippo, I don't care if your plane is turbo charged with polished aluminum wheels on it, I still feel that checking the seam using a light table or a large window is always going to be more reliable than holding the plate at arms length.

Just try and compare, it's easy enough. If your seams are good when checking both at arms lenght and using a window, than keep using your method and sleep tight. What more can I say?

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

C'mon, you don't NEED a light box to get a perfect joint. That statement is just false.

Whatever...

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Of course you don't NEED a light table or window to get a perfect joint, just like you don't NEED a sharpening guide to put a great cutting edge on your chisels and irons. You don't NEED a radius dish to get a tight fit between top and rim. You don't NEED a shooting board to cut a straight, true edge on your plates. You don't NEED a Fox style bender to accurately bend your sides without breaking them.