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 Post subject: Simple Plate Joining Jig
PostPosted: Thu Dec 08, 2011 3:40 am 
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Joined: Tue Sep 06, 2011 2:51 pm
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Location: Bothell, WA USA
This is not original thought. I got this idea from the John Bogdanovich classical build DVD's (those things are a gold mine for stuff like this).

This is a simple way to get a good secure glue up of top and back plates.

The cost is pretty much free. Everything I used here was from the scrap pile. I did make use of a bench top platform I had previously built that consists of a 2x4 box with a piece of MDF on top. I use this for many things, including routing rosettes and soundholes, and many other tasks. It raises the work above the bench and gives you a clamping edge all the way around. Very convenient and easy to make.

To turn this into a plate joiner I just put a piece of waxed paper on top and clamped a piece of scrap along the right edge. I cut some small scraps of poplar into wedges.

Another scrap of pine and four c-clamps and you are ready to go. Here are some pictures


Firstly, you need to make sure your back or top plates are well jointed and ready for glue up. If you can see any light between the joint when pressed together, it's a recipe for failure. Make sure it's good and tight before you start this process.

This first picture just shows the piece of scrap pine clamped to the right edge of the MDF platform with the waxed paper on top, and the two sets of wedges sitting in place ready to go.
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Here, I've clamped one of the mahogany back plates down to the base with a piece of scrap pine fairly close the jointed edge. This edge needs to be clamped securely so it doesnt slide when you apply side pressure.
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This just shows the second plate with the nice glue bead ready to slide into place. Just press it firmly up against the clamped plate and apply some firm pressure by hand, and then slide the two pieces back and forth a skoche to make sure there aren't any air bubbles in the glue joint. You should see a tiny amount squeeze out bead all along the joint.
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This picture shows everything in it's final place. After sliding the second plate up against the first and making sure they are tightly mated and lined up, put the second piece of pine on top of the second plate fairly close the joint and clamp it down. Don't apply too much pressure here with the clamps here. Just enough to hold the plate down against the workboard. Once you have it in place squeeze the two wedges together to apply side pressure pushing the second plate up against the first. A light tap with a small hammer to seat each set of wedges and you are done.
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This shows a close up of the two wedges that are used to apply side pressure.
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And here is a close up of the seam before the squeeze out is cleaned up.
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This is just another picture of a completed top I joined earlier.
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Once you are done, everything breaks down and goes back into the scrap pile (you might stash the wedges somewhere, but making new ones isn't a big deal).

DISCLAIMER: This is not an attempt to discredit anyone, or to suggest builders confident in their existing processes are doing it wrong. it's just another way to accomplish a task. I present this as one possible method for joining plates primarily for new builders coming along to consider. If what you are doing works for you, I would not suggest you change it.

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Last edited by Jim_H on Thu Dec 08, 2011 4:12 pm, edited 1 time in total.

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PostPosted: Thu Dec 08, 2011 10:29 am 
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Location: Visalia, CA
Nicely done Jim,
I am doing something similar, but I use 3 stationary blocks on one side, and a solid strip of wood on the other side that can be moved and clamped depending on the width of the plates, and I use wedges tapped between the plate and the 3 stationary blocks. I may have even gotten this idea from Ken (don't remember for sure). For the center seam clamp, I have a length of 3" wide by 2" maple that is straight and flat. I put the wax paper over the seam, then a short piece of sand paper (is about the right thickness) over the center area of the seam, lay the maple board over the seam, and clamp the ends down. I get similar clamping pressure that Ken is suggesting using a board with a slight bow to it. This has worked very well for holdind down the plates at the seam. It is easy to get TOO MUCH pressure on the joint with the wedges, squeezing out the glue and having a joint failure (don't ask me how I know this), so you have to give it just a little pressure.

I like what you've done too. Thanks for sharing. Isn't it great to figure these things out with the materials we have on hand, without spending an arm and a leg for a jig that someone else has made?

Kevin


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PostPosted: Thu Dec 08, 2011 1:07 pm 
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Location: Bothell, WA USA
I thought about the packing tape thing, and also considered using a piece of melamine for the top surface. And yes, there are tons of different ways to do this. I had originally intended to build one of the spanish style rope clamp setups. It's just making a simple task too complicated, and because I use this elevated work platform for other things, I didn't want to make any permanent changes to it that would diminish it's usefulness for other tasks.

My main objective for posting this wasn't to demonstrate this as the only way, or best way to join plates. It was to demonstrate an easy and reliable way to get a solid plate joint without building any dedicated fixtures or spending money on something that I didn't need.

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PostPosted: Thu Dec 08, 2011 1:14 pm 
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Location: Bothell, WA USA
Kevin,

Regarding starved glue joints, I read a post by Hugh Evans over at the OLF. He works in product testing at Franklin (Titebond) and also builds guitars.

He stated that it's nigh on impossible to starve a glue joint by over clamping. He said that starved (dry) glue joints are almost always the result of releasing some clamping pressure before the glue is cured and then re-clamping. This can happen as a result of re-positioning the joint after it's initially clamped, or by removing the clamps to clear squeeze out too soon.

I believe some of the electric builders over there stated similar things based on results of testing electric body glue ups.

Basically they are saying that you should clamp with as much pressure as you can without compromising/compressing the cell structure of the wood.

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PostPosted: Thu Dec 08, 2011 1:35 pm 
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Jim, I am a big proponent of KISS.......and you have done that.
I know that the white glues are suppose to penetrate the fibers and join - stronger than a locomotive! However, on non-porous type woods, you don't get as much of this penetration, so the joint may not be as strong. I just had it happen to me on this Santos Mahogany. It is more like a maple than a mahogany as far as density goes, and I overclamped the first glueing of the back plates, and it came apart in my hands after cutting out the back profile. I reglued it, less pressure, and it is very good to go now.
Interesting though about as much pressure, basically as the wood can stand.

Kevin


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PostPosted: Thu Dec 08, 2011 1:54 pm 
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Kevin,

Lots of things can contribute to a failed glue joint, especially on exotic hardwoods.

Bad glue (LMI white goes bad, and they have put out a few bad batches), poor joint, poor clamping technique or movement of the joint during setup, etc...

Here is the quote from Hugh from this post over at the OLF
http://www.luthiersforum.com/forum/view ... 01&t=33687

Hugh Evans wrote:
Yes we have, and what we learned is that it is impossible to create a starved joint using PVA wood glues. Starved joints are easy to create using hide glue, so concern over this topic carried over as PVAs gained widespread acceptance in the woodworking world. We have tested clamping pressures in excess of 425 PSI, which produced a perfect joint. The only limit for clamping pressure is the compressive strength of the wood. Once you start crushing wood fiber too much pressure is being applied. Depending on the modulus (similar to rigidity) and thickness of the wood, some species can take tremendous pressure. Clamping has everything to do with bringing the bonding surfaces as close together as possible, so if the surfaces are perfectly mated to being with it is more likely that optimal strength will be achieved even with less pressure.

We measure bond strength using two Instron universal testing machines that are rated to 20 tons each. These run about $85k each, and you really don't need anything close to this for testing joints at home. A good glue joint will be stronger than the surrounding wood, so even hitting a joint with a chisel and hammer can tell you everything you need to know as long as the wood fails. I have some ASTM D905 tests coming up soon, which is our standard shear strength test. If there's interest I can take a few pictures.

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PostPosted: Thu Dec 08, 2011 2:38 pm 
Is there ANY reason that the tape method is inferior to any other method, if the same care is taken as to jointing, gluing etc?
This is important and I would appreciate knowledgeable answers.
Thanks!


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PostPosted: Thu Dec 08, 2011 3:02 pm 
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Location: Williams Bay, Wi
I had the glue joint on my first sound board fail - user error though - clamped it overnight in the garage (colorado, february... the things we don't know when we started out, right?) Next day I un-clamped it, gave it a gentle shake and it snapped right at the glue line.

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PostPosted: Thu Dec 08, 2011 3:03 pm 
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Tape works too.. i'm not re-inventing the wheel.

I'm not a big fan of the tape method personally, although I have used it with success.

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PostPosted: Thu Dec 08, 2011 3:25 pm 
Jim - you've used it with success? But you are not a fan? Why? Inquiring minds need to know! :-)
Thanks


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