Soundboard joining advice

[Abstraction]

My two pieces of Sitka spruce 4mm, planed edges, put flat against the window and pressed together by hand: cannot eliminate daylight showing through. Multiple attempts.
My understanding is this shouldn't happen at all - zero light - is that correct? That there's zero tolerance?
How do I solve this?
I've built harps and electric basses way back but my woodwork skills aren't first class. I just keep going until I get things right. Harp soundboards are smaller sections, too, easier. I also think none of my straight edges are quite good enough to be sure where to adjust. Maybe that's what I need to invest in.
Appreciate any tips. This is for my grandson who I hardly see, want to leave him something he'll always keep.

[kiwigeo]

How are you dressing the gluing edges of the soundboard?

My procedure:

- place two halves of soundboard face up and side by side.
- close up the two halves (face to face) and place on a shooting board. Secure firmly so the two halves can't move.
- shoot gluing edge with a jointing plane (No eight) with a sharp blade. Make sure you have weight mainly over front half of plane when you start the pass and move weight to back half of plane as you finish the pass. This stops plane from "rocking" during the pass.
- candle the two halves against a light source.
- if light still showing then repeat shooting process. Make sure two halves of top are oriented on shooting board exactly as they were for first shootiing session.
- if you find you cant get a perfect joint ie light is still showing. Glue some 120 grit to a long level and shoot the two halves of the top with the level instead of the plane. This should get a good light tight join.

This video should help: https://youtu.be/V_ywBSlYLPo?si=DAKqXP1oIf2Qcco5

[Abstraction]

Really helpful, thanks for prompt response. I have a no.6 plane but no shooting board. So I planed a board straight and clamped the two pieces flush and put it in vice. The spruce pieces are on the other side in the pic facing up.

Actually I was on the right track and closer than in his video. But his approach to fine adjustments is better than mine. I may also get a carpenter's level with sandpaper set up since I'll have to do the back also. I'll take a piece of card and sharp pencil into the store, rule lines and reverse to make sure I get one with a true edge.

[Andos]

Make sure the base of plane is flat. If there is the slightest concavity you’ll round off the leading and trailing edges of the soundboard.

[kiwigeo]

This is probably one of the best designs for a soundboard shooting jig. Available from Micheal OConnors website...Ive got a few of his tools in my workshop.

https://www.michealconnorwoodwork.com.au/page19/

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[Mark McLean]

It is really worthwhile making a shooting board and doing it with the boards flat and the plane on it's edge - like in the video (which is great BTW, thanks Martin). The shooting board can be a pretty basic design and still be effective. If you do it in a vice as your photo shows it is easy to have uneven pressure at some points and get uneven cuts.

[Dave M]

It may be worth adding that a planed surface will glue much better than a sanded one. Also that you want to do the glue up pretty soon after you have planed the edges so have your glueing jig ready to go after you have produced your edges.

It may seem a bit daunting at first but a bit of practice should get you there. Sharp plane, very thin shavings!

[johnparchem]

I struggled with this for awhile, now I do not even think about it. Use a shooting board if possible; even still it is hard. First thing that helped me is understand where my weight was on the plane. When starting a cut the back of the plane is cantilevered off the wood so the weight needs to be toward the front, when leaving the wood the front of the plane is cantilevered over the wood so the weight needs to be back. At some point the weight transfer can happen without thought, but take your time to see first the front and then the back of the plane in contact with the wood.

Even when you have that nailed a plane can quite happily follow a convex shape so you can plane forever keep the edge convex. So if you can not join on the ends take a few cuts in the middle of the plates working longer cuts. You can fix a convex edge and even glue a slightly convex edge. (Closing a small gap in the center is a sprung joint).

If you are seeing a jagged edge or spotty gap, sharpen the plane and or take thinner cuts.

Sanding has its own issues. Seems like it would be easy with a flat path a a guide but ... The friction of the paper is going to force more pressure on the leading edge of the plate. So carefully going back and forth or even in one direction will lead to a curved convex edge. So one needs to counter the tendency by having the pressure back from the center but not too far back.

[Deano2]

How are you dressing the gluing edges of the soundboard?

My procedure:

- place two halves of soundboard face up and side by side.
- close up the two halves (face to face) and place on a shooting board. Secure firmly so the two halves can't move.
- shoot gluing edge with a jointing plane (No eight) with a sharp blade. Make sure you have weight mainly over front half of plane when you start the pass and move weight to back half of plane as you finish the pass. This stops plane from "rocking" during the pass.
- candle the two halves against a light source.
- if light still showing then repeat shooting process. Make sure two halves of top are oriented on shooting board exactly as they were for first shootiing session.
- if you find you cant get a perfect joint ie light is still showing. Glue some 120 grit to a long level and shoot the two halves of the top with the level instead of the plane. This should get a good light tight join.

This video should help: https://youtu.be/V_ywBSlYLPo?si=DAKqXP1oIf2Qcco5

I recently faced this very challenge and opted for the sanding method to rectify some mistakes. I purchased a rectangular aluminium extrusion 1.1m long and along the whole short edge double sided sticky taped 120g. There were some learnings along the way but got there in the end.

- 2x18mm MDF 600mm long sheets stacked on the bench. This allowed for a flat surface. Aluminium slides very well along MDF.
- 5mm MDF riser beneath the work piece (larger than work piece)
- Ensure there are no creases in the tape or stuck down sand paper.
- Timber edge hangs over the riser by a few mm.
- Align the top of the sanding beam with the top end of the guitar top so sand paper is along the the whole length before making each pass. Go as far as the bottom of the beam aligning with the bottom edge of the top.
- Brush sawdust from sandpaper after every few strokes.
- Lightly wipe edge with a tack cloth followed by a quick wipe with some metho spirits.

Hope this helps

[TallDad71]

The only thing I would add is that I start off by make the edges convex by removing content from the middle. Mark the edges with pencil.

Then when you go with big plane you can guarantee a straight edge. Should the boards start off concave you’ll only ever be planning on the round.

Fine tuning I sand with a 50cm 5cm 5cm block of plywood flattened on a drum sander.

[Abstraction]

Thanks folks. Really generous responses. Seeing different approaches gives you a repertoire of options.
The final touches were sandpaper strip glued to edge of length of pine I planed flat. Bingo.