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So bit embarrassed but wth

Struggling with the back seam joint on this now oak, haven't built anything for over a year so bit out of practice.

Seems no matter what I do I am sanding the hoint poorly, always a lump in middle that moves up and down a bit

Tried loads of different techniques and holding in different points and different styles of dragging the wood over paper

Tried 2 different pieces of glass under paper which seem totally flat


Any advice be great fully received, (apart from expensive long planer)

Tia

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That's a common problem when using a plane and a shooting board. It's caused by the user applying too much pressure to the ends compared to the middle. It is fixed by the user using light pressure on the ends and more pressure on the middle - and it does take some practice. Perhaps you are having a similar problem while sanding?

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Steve Smith
"Music is what feelings sound like"

The glass is probably flexing slightly when you sand. It doesn't take much.

I've had problems similar with a shooting board and planes (no 5 and LN#62), cured with technique and practice.
Sounds like the plates may be "rocking" on the glass plate, producing the lump in the middle.
I recently got one of these shooting sanders by Veritas, not terribly expensive. and may help.
Available in 8" and 16" lengths, I bought the 8".
I bought this to help specifically for jointing back plates with difficult grain which catches on the plane blade, although a very very fine setting of the blade projection helps somewhat.
These shooting sanders can sand preferentially, where needed, and don't involve dragging the wood's whole length over the abrasive.
Be aware that these are sections of aluminium extrusions with a thick layer of coating, and if mine was anything to go by, require flattening likes most planes, but it is easily done.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

After planing I put 120 grit sticky back sand paper on a carpenter's level and clamp it to my work bench. Work the plates back and forth against the level until you can hold them up to a window and not see light coming thru.

Ah, the not-so-good old days

If a flat sanding board makes a hump, then make your sanding board convex. Either find something that's stiff and already slightly curved (e.g. a 2x4) or something slightly flexible (e.g. a 1x4) and put some thin shims under the middle and clamp down the ends to induce a bit of curvature.

Or if you have a straightedge, you can use a shorter sanding block with a shooting board type setup and specifically target the high spots until there are none left (this is too time consuming if you can only check the two halves against eachother).

Use a modern wood glue rather than hot hide glue, since the joint will be less than perfect.

I wouldn't do any joint with sandpaper, so I can't address that. If you used a plane, i would agree with Steve, put pressure on the front of the plane at the start, and on the back when going off the piece. Since it is oak, it has a lot of hard flecks, then only a good sharp blade is going to get it flat.
Put chalk on one edge and place the other on it. Plane down the high points. Do it with the other side, and then check it. It should be closer.
I have a long plane that usually gets them, but I have another wooden one you pull, with a very fine set that I use with problem childs. It can make dust.
I have.no shooting board, just clamped in the vise.

Sometimes I've clamped the plane in the vise and pulled the wood over it. You can check it for square easier.

Make sure you are planning on the right direction. Curly wood is sometimes a problem with that.

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Why be normal?

How much longer is your sandpaper-covered surface than the length of the back plates, and are you using any kind of fence setup to keep the plates perpendicular to the sandpaper?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Here is something the cockatoo came up with:

Tou make a simple router jig by gluing two relatively straight pieces of hardboard or plywood (ideally 1/4 inch stuff) one on top of the other, and then using your router and a 1/2 inch bit (cutting width) trim the edge of the lower piece using the upper piece as a guide. This will then give you an edge to help line up the work.
One of the guitar plates is placed under the jig with about 1/16th inch showing. The jig and plate is clamped securely. The other plate is positioned in the proper relation to the first, but leaving a 3/8ths inch gap between them, and is clamped securely. The router is pulled along the jig cutting 1/16th off both edges. Any slight imperfections in the straightness of the jig are compensated for by the bit cutting slightly more or less from one side of the plates or the other. The joint will only be "perfect" when the plates are lined up with each other as they were when cut, but that shouldn't be too difficult to acomplish.

When a plate is moving on sandpaper the leading edge has more of a downward force as the sand papers friction is effectively applying a backward force right at the bottom of the plate. As you are pushing it from a higher position you will get a downwards force on the leading edge. Knowing this may help you visualize a way to prevent it. Like sand in one direction with your hand back from the center. You can sand in two directions but you need to change where you hold the plate each time you change directions.

I have a much better time with a plane and shooting board.

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http://www.Harvestmoonguitars.com

That bird has better handwriting than I do!

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I keep seeing the title of this thread and it reminds me to go to the chiropractor on Monday .

Edit: "need to"

Hi
Yes paper is longer, and have a large weight as a fence, the edges are nice and square, just these dang humps in middle

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Sounds like you are using something like this setup.
Have a look at this https://www.luthiersuppliers.com/the-easy-jointer
At the bottom of the instructions see "If you see a gap this can be corrected by adding masking tape to the bottom of the glass in the area of the gap. ", even if you can't actually see a gap this could correct your lumphumpthing........
However I would use a shooting board and any plane to remove the worst of that hump first.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I didn't ask if the sandpaper was longer than the back plates, I asked how much longer. The reason is that the longer the sanding surface is, the smaller the total number of sanding passes needed to joint the edges. The fewer times you go back and forth, the less rocking there is and less chance of a hump in the middle. It can even help to do all your sanding passes in just one direction, not back and forth, because it's challenging to maintain the same end-to-end pressure when changing directions on every pass.

I joint the edges of plates using a plane and shooting board or by sanding depending on what the particular pieces of wood are happiest with. For sanding, I use the bed on my jointer as the sanding support surface. I bring the infeed table up dead level with the outfeed table and then place a piece of 3 foot long piece of 1/8" plywood covered over its full length with adhesive backed sandpaper on the tables and sand on that against the fence. The 3 foot length on a dead flat surface makes it easier to maintain constant even pressure on the wood for a longer time on each pass and it means fewer total sanding strokes to get the job done with less rocking. If a particular set of plates is still getting a hump, I do all the sanding strokes in one direction and that takes care of it.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Occasionally, I will work with a back that is challenging. One that comes to mind is some African Blackwood that had some wild grain in a few places along the joint. It was simply impossible to do with a hand plane. So I put a slight gap in the middle and sanded it flat afterwards on my jointer. I used 320 grit adhesive back sandpaper (3M StickIt).

If you are having trouble, then you should try and take a similar approach that I did. Create a gap in the middle by only planing the middle and not the top and bottom of the joint - start and leave the cut 1" from the ends. Then find or make a flat surface and use some Stick It paper to level the ends. As soon as you see a couple sanding scratches in the middle region, then stop sanding. It should only take a few swipes.

I use this method when I encounter a particularly troublesome set. However, it is always important to have your blade razor sharp and to take very thin shavings.

J

That's a great idea- one direction only. Makes complete sense and now in my arsenal

Ed

That's pretty close to how I learned to join tops and backs from Charles Fox back in the 70's. Works very well.

Dave

What John Parchem said; as you push the wood across the sandpaper the leading edge tends to dig in. The lower down you hold the wood, closer to the sanding board, the less this happens, but there's no way to make it go away entirely.

You run into a similar problem when planing; there's a tendency to 'snipe', removing more material on the ends. With a plane you can fight that by deliberately trying to 'scoop' the cut; applying more pressure with both hands in the middle of the pass, and lightening up on the trailing had to start, and the leading hand to finish.

I don't like sanded joints, but if you insist on doing it that way then use a piece of paper that's shorter than the wood and pay more attention to the middle of the joint. In general, it's easier to correct a 'dip' than a 'hump', and many of us prefer a 'sprung joint' that shows a little light in the middle when you hold the pieces together. You should be able to press them tightly and get rid of the light by hand pressure.

Sometimes I feel clueless when a should be obvious simple solution is presented to me. Yes I can see a shorter pieces a paper would work better than a a strip longer than the plate.

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http://www.Harvestmoonguitars.com

Thanks for the great suggestions, the general theme i get is to work the middle more than anything and make a single direction pass, going to have another go tonight. I did try to remove the middle when it was bad with a plain and tried a scraper but this wood really doesn't like it and tears up badly, so sanding seems to be the only option

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Good luck!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Buy or build a nice wooden jointer plane. I have a couple of old ones I use for this. I don't think I spent more than $30 for both. This should be easy.

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Hutch

Get the heck off the couch and go build a guitar!!!!

A properly set up plane should not be tearing out that badly on an oak back. It's worth while to spend the time to get the plane right, and learn to do your joining that way. Planed joints glue up better than sanded ones, for one thing. You don't need to get a fancy plane: I've done all of my joining for the past 45+ years with a Record #4 smooth plane. It was a trial at first, but then I read up on tuning a plane, spent the time to get it right, and it's been pretty smooth sailing since.

I agree that planing is best but that sometimes a troublesome back set will make me resort to a sanding plane. And I often have to fight against the dreaded hump in the middle. The suggestion to use a shorter sanding plane got me to thinking about Stew-Macs fret kisser. It has a short area to file high frets, and that sits between two smooth areas that will stop the tool from cutting when they touch adjacent frets. I made a similar tool using sandpaper to level drop fills on finishes. I found some packing tape that was the same thickness as the sandpaper. It worked sort of.

Something similar could be made for sanding back joints. When the packing tape hits the ends of the joint it stops the sanding. I will modify my sanding plane to try this out on the next troublemaker back.