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LMII Acoustic Steel String Build

Discussion in 'Acoustic Heaven' started by pypa, Apr 21, 2021.

  1. pypa

    pypa Tele-Meister Ad Free Member

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    I decided to get the radius dishes. I bought a 15 and 28. There was an Etsy store that sold the pair at a slight discount.

    as for the sandpaper am I reading correctly that we should use 50 grit? I am planning to adhere 9x11 sheets. Is that ok?
     
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  2. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    I bought a round 80 grit abrasive disk myself, but using 9 x 11 sheets will be fine. I'd use multiple sheets myself to cover the whole surface. i think that would get more work done faster and more accurately that way.
     
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  3. pypa

    pypa Tele-Meister Ad Free Member

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    I trimmed the ends of the sides and got them in the mold. I plan to cut the mold on the black lines.

    there are slight gaps between the sides and the mold but I don’t think this should be a big problem, right? 0D2D5DEC-E529-456F-A149-A902C825EC9B.jpeg
     
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  4. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    I would suspect that a DIY mold wouldn't be exactly the same as the sides bent by somebody else. It poses no problem. I'd bet the two sides aren't identical either, due to springback and the fact that the cells are different in each consecutive piece as it is resawn. The lining will hold it in place. I would guess that laminated veneer sides have a better chance of being " identical" .


    One of my maple parlor sides bent a bit wierd and I used a hot pipe to try to get it better. It still wasn't perfect after touching it up. I lived with it. Handmade goods tend to be slightly imperfect.
     
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  5. Freeman Keller

    Freeman Keller Poster Extraordinaire

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    I'm assuming that you made your mold from the plans, the fit looks fine. The sides will stiffen considerably as you add the kerfing and end blocks. Getting radius dishes just means you'll be building more acoustics. Of course that will happen anyway...

    I think I use mostly 80 grit sand paper in the dish. Its a little tricky to hold it down and not catch the edge of the paper while you are sanding - its pretty easy to tear the paper. Some people buy a big enough piece to cover the whole dish and double stick it down, I've heard of people using the big discs designed for floor sanders. As in my pictures I just use a sheet and if I happen to tear it I need small pieces anyway.

    John Greven does a neat trick - he mounts his entire radius dish on a spindle of some sort - it rotates as he presses the rim down onto it.

    The back ends up in a compound curve - you are trying to make it spherical and wood doesn't want to do that. The ladder braces give it a cylindrical shape but then you pull the upper bout to the neck block, The same thing happens with the top, just not as much.
     
    Last edited: Apr 23, 2021
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  6. pypa

    pypa Tele-Meister Ad Free Member

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    Yes, the mold is taken directly from the plans. Well, half of the plans; I transcribed one side of the plans and then made it a mirror image about the center line.

    I plan to do the bulk of the shaping with a block plane. I think I should be able to scribe a line and plane to it, using the disc just to true things up.

    Where should the center of the circle be placed with respect to the back or top?

    If it's for strength I'd think it should be at the midpoint vertically and horizontally. If it's for 'reflecting the sound to the hole' something tells me the center of the hole should be the center of the radius.
     
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  7. Freeman Keller

    Freeman Keller Poster Extraordinaire

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    Once you get the sides glued to the end blocks you can take it out of the mold and lay it in the dish. Use a washer as a spacer draw a pencil line that follows the curve of the back to the neck block. You can plane close to that line. Then glue the kerfing on and do the final shaping with the sand paper.

    In theory the center of the dome is at the crossing of the X which is about halfway between the bridge and sound hole The neck will angle up to that and you want the upper bout to be part of that neck angle. The back is probably about the same place but it isn't controlled as much as the top. I don't think it matters.

    There are lots of discussions about what sound waves do inside the box - frankly I just let them bounce around and come out when they feel ready.
     
    Last edited: Apr 23, 2021
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  8. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    On my last back, which was my first dome build, I trimmed to the sides and blocks. Then I pivoted it counteclockwise and clockwise until all the pencil lead I put on the exposed surfaces was sanded off. I held the guitar on top of the abrasive, I didn't The dish then puts the radius on those surfaces. The abrasive was face up on the workbench and I worked the guitar to the abrasive. I didn't hold the disk and try and sand the guitar. Either way should do the same thing.
     
    Last edited: Apr 23, 2021
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  9. pypa

    pypa Tele-Meister Ad Free Member

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    I plan to glue on the end blocks this weekend.

    1) Should I thickness the blocks to final dimensions per the plan before gluing them in?
    2) How should I reference the height of each block when gluing in? Off the top, back, or do I make a center line? I can't quite tell whether the taper from heel to end is on both the top and back or just on one side? I suspect this is critical because this choice will affect the beck angle...
     
  10. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    As I said earlier.... I made a side view and found the dimensions of the blocks from the drawing. Don't forget that the thickness of the top impacts the block. The block is glued to the top, but the front of the neck beyond the tenon is raised up by the thickness of the top.

    I ran the wood through the planer. The grain goes sideways, not up and down. I cut my mortise while the block was larger. Everything needs a centerline, your top, and back as well. I cut my blocks on a chop saw for accuracy. The blocks will get radiused after they are glued in place.


    block.JPG



    blocks (1).jpg


    neck in neck block.jpg
     
    Last edited: Apr 23, 2021
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  11. 1bad914

    1bad914 Tele-Afflicted

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    The side gaps at the tail and head blocks are okay, IF you are installing an end graft on the tail and the binding and neck heel cover the one at the neck.
    End graft.
    31270F68-A715-4368-AE47-7DD77459CBCB.jpeg
     
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  12. Freeman Keller

    Freeman Keller Poster Extraordinaire

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    I cut and set my blocks so they stand just a hair proud of both the top - maybe a 1/16th. When you sand the radius into the top the block will end up with a very slight angle. At the point when I'm putting the blocks in place I have not made the taper on the back so I just set it level with the back knowing I'm going to be planing some off.

    If you are using LMII's bolt on neck block it will have holes for the bolts and the truss rod. I try to remove material from the other (bottom) end of the block so as not to screw up the truss rod slot - that needs to be right under the top. If you are making your own block this is a good time to route the pocket and drill the holes. If you are doing a dovetail let me know.

    Edit - maybe this will help. Here is the rim with blocks standing very slight proud, kerfing in place. You can see how the sides curve up off the work bench towards the front of the guitar. The neck block needs to be thick enough to contact the back.

    IMG_2343.JPG

    Flipped over and sanding that 16 food radius into the back (with a sanding stick, not a dish). Again, you can see how the neck block tapers to the front of the guitar.

    IMG_2370.JPG

    For what it is worth, those two pictures are a parlor guitar built from the Antes plans
     
    Last edited: Apr 23, 2021
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  13. 1bad914

    1bad914 Tele-Afflicted

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    +1 on this process.
     
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  14. pypa

    pypa Tele-Meister Ad Free Member

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    Here's my confusion. It appears that the back tapers from the end to the heel. So, the blocks need to be glued so that they are each flush to the top (plus margin for the radius and top (A)).

    If I glue them flush to the back (+ radius and back thickness (B)) I would have a tapered top. I must have missed this in all the videos. Am I right in this thinking?

    B2139EB1-3698-4725-AE4C-0F6F921822DD.jpeg


    By the way, the plans don't make any mention of a curved top - only the back; I'm going to curve them both anyway.
    803A8D5C-2DA2-4CF3-9197-523BE673D418.jpeg

    I finished the mold.

    53F7B219-15A7-4789-ADBD-39D5968E8DA3.jpeg
     
    Last edited: Apr 24, 2021
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  15. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    As I mentioned to you, I have only done flat tops, like on your plan. If I were to do a domed top, I would do the top side of the rim subsystem the same way that I'd do the back side. I'd glue the blocks to the side and sand them to the radius. The end result would be a box with two domes with two different radii. I do know from a discussion with Otterhound years ago that Martin tops are sanded on a large machine. I think he said they had a jig to hold them in place. That sanded an angle in them from the waist out IIRC.

    The domes are formed on the top and the back. The domes connect to the side subsystem. The side subsystem has the radius sanded into it. I think to do something different would be very complicated.

    If you look at how an archtop guitar is made, they are attached to side subsystems that are tapered from back to front.


    If it were me and I hadn't built this kind of guitar before, I'd follow the plans.....and do a domed top down the road...but that's me.
     
    Last edited: Apr 24, 2021
  16. pypa

    pypa Tele-Meister Ad Free Member

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    Go, I apologize for what seems like repeating questions to answers you’ve given.

    I may do this one flat on top. My point is, the end block is 3” and the heel block is 4”. When I glue the 3” block I have to determine whether to leave more margin on near the top edge or the bottom edge because that will affect the taper.

    More accurately, it will affect whether the side grain is perpendicular to the top or the bottom surface.

    I infer from the difficulty in resolving this question that it must be a moot point. I guess the taper is subtle enough that nobody will notice either way.
     
  17. trapdoor2

    trapdoor2 Friend of Leo's Gold Supporter

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    I would treat the top as your datum surface. Everything references to the top. IOW, the back tapers, not the top.

    You can go crazy with the dome. A luthier buddy makes up his braces and double-stick tapes them upside down to a thick, flat, dummy top. He then takes his sanding dome to them to get the spherical radius correct, unsticks them and then does his glue up to the top. Works for him!
     
  18. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    You could always make a little taper guage by sanding a strip of wood in the sanding dome and seeing how much it tapers at the ends. Say something that is thin, the exact length of your guitar body, and could hold its shape? A piece of basswood?
     
  19. Freeman Keller

    Freeman Keller Poster Extraordinaire

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    The little bit I know about acoustic guitars says you want a bit of a dome for two reasons. A dome is much stronger and stiffer than a flat piece of the same material - you want as stiff a structure as you can build within you weight goals. The second reason is that the top plate will shrink and expand with humidity changes, particularly across the grain. As the wood dries with normal seasonal humidity swings it shrinks, the dome drops out and it actually becomes flat, then concave. At some point it cracks. One of my most common repairs during winter is cracked guitar tops.

    If you start with a flat piece of wood and tell it go shrink it cracks immediately. (ps this is why you should be building in the same environment the guitar will live in - 40 - 45% RH.

    I can show you way too many pictures of cracked tops and I would suggest taking a straight edge to a nice (well humidified) acoustic guitar and look at various aspects of the way it is built before you go any farther.

    As a bit of historic perspective, old Martins were apparently built with much flatter tops than are use today but still some dome. This is why LMII sells 30 food radius dishes and builders like John Greven (who builds in the prewar style) put very slight domes in his guitars - but there is some.

    The top of the rim starts out flat and has the slight bevel sanded into the kerfing and end blocks. It isn't very much. The back is basically flat from the end block to the waste and then tapers to the neck block. That is a complex curve and is best sanded in with a radius dish.

    One other thing to be thinking about the top is that the neck will be at a very slight angle - maybe 2 degrees with respect to the vertical end of the guitar. The extension of the neck lays on the upper bout top and must hit the bridge a the correct height off the top. You will fine tune this late in the building process by "setting" the neck.

    Later today I will try to find an article about acoustic guitar geometry, scan and send it to you. PM me your e-mail address.
     
  20. Freeman Keller

    Freeman Keller Poster Extraordinaire

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    I see this thread was just moved to the acoustic guitar sub forum. Thats kind of too bad, I've done two acoustic builds and they stayed in the DIY. I'll keep watching this but I'm afraid you won't be getting nearly as many hits on it.
     
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