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  1. #1
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    How I use CNC in acoustic guitar building

    I wanted to start a thread to document my approach to CNC for building acoustic guitars. I've spent the last two years building (CNCRouterParts.com) my own machine from a kit and learning how to program it (Fusion 360) for use in guitar building.

    I am a hobbyist woodworker and luthier. By day I'm a software consultant, but woodworking is my passion. For the past 5 years I've been building 1 guitar per year which is donated to the local High School marching band (of which my son is an alumni) for a fundraiser. We typically raise about 3 to 4K per year doing this (it is a raffle). Here is the blog I keep with pictures and entries for the past guitars: AHS Band Boosters Guitar

    I've just started my 12th guitar, which is a commission for my Uncle Jon.

    My original motivation for starting in CNC was to do my own inlays for the fretboard, but as I climbed the learning curve, I realized that I could do much much more with it for guitar building….so I’ll be documenting the build of #12 here. Don’t have an easy way to do videos yet, so for now I’ll just post pictures.

    Still have lots of ideas for jigging to bring CNC ops in, but this is what I have now.....(sorry, don't have a GoPro for filming myself yet)

    Post 1: Neck prep:


    I start with a 3/4" x 3" x 36" neck blank and first make a scarf joint

    Attachment 351714

    I used to have a table sled scarf joint jig, but was never happy with the accuracy of the cut I was getting, so instead will bandsaw the scarf joint. Here is
    a marking tool I CNC'd out of MDF so I don't have measure to get the line in just the right place on the blank:

    Attachment 351716

    Then I bandsaw it. Yeah, came out horrible, was in a hurry and my bandsaw table was rusting from a leak I didn't know I had.

    Attachment 351718

    Then I put each piece on a vacuum jig on the CNC table. I made the jig with the CNC too... it's made from VHMW plastic.

    Attachment 351720

    It just uses 1/8" gasket material and straight edge to register is square to the table

    Attachment 351722

    Then I run a simple CAM op to cleanup the joint at a 15 degree angle. I run this on both halves of the cut neck blank

    Attachment 351724
    Attachment 351726

    Here they are ready to be joined into the headstock break.

    Attachment 351728
    Attachment 351730

    And all clamped up on the box beam workbench, so that the two pieces won't slide while I clamp the scarf joint.

    Click image for larger version. 

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    To be continued....

  2. #2
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    Re: How I use CNC in acoustic guitar building

    Time to prep the heel blank:

    First I chuck it up on the CNC table using Festool clamping elements, which fit into the 20 mm holes I had the CNC bore in the waste table.

    Attachment 351734

    I'm going to have it inlay a reinforcement spline (for cross grain strength) first

    Attachment 351736
    Attachment 351738
    Attachment 351740

    With the maple spline glued into place, I then have it surfaced flat first....

    Attachment 351742

    And it bores the 3 contact point holes (this is an articulated neck joint)

    Attachment 351744

    Then I have bore a tapered hole which is for the threaded insert

    Attachment 351746

    This is the thread cutting bit that I use....

    Attachment 351748

    To pre-thread the hole for the threaded insert

    Attachment 351750

    Ready for the insert, goes in easy

    Attachment 351752

    and here is the test fit.

    Attachment 351754

  3. #3
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    Re: How I use CNC in acoustic guitar building

    Time to build the rest of the neck and then start the fretboard side machining....

    My necks are floating and articulated, with a cantilevered fretboard, so I modeled the neck blank as well, and it is glued up very
    precisely so that it lines up with the cad model of the neck. Even the already CNC'd holes on the end of the heel have been set a
    precise amount deeper to account for machining off that face a little.

    Attachment 351774

    Just gluing on that heel block....

    Attachment 351776
    Attachment 351778

    Now it's time to machine the fretboard side. I have a special jig (designed in CAD and cut by the CNC) for holding the neck
    during this phase. Note the side braces to keep it from moving. I found out the hard way that I needed that on the first neck I did.
    This uses threaded inserts, and the holes are also CNCed in there with threads. The first iteration was cut form a solid block of MDF, but
    it turns out the MDF can't hold the inserts, they started pulling out on me. SO I redid it in plywood with Festool Domino joinery to hold it all together.

    Attachment 351780

    First, I machine the headstock surface to a rear "nut line" that I have scribed. This just needs to be far enough back to so that it will clean up the scarf joint surface. The nut line is set
    as 0,0 in my program. the location of the heel is derived from where this nut line is, so that the blank is correct for the model and CNC.

    Attachment 351782
    Attachment 351784

    Here are the veneers that will go under the headstock overlay. I've gotten tired of trying to bind solid wood around the curves of my headstocks, and this gives
    the same look but mucho easier. I glue them on now so that the rest of the neck carving operation can shape the edges of the overlay.

    Attachment 351786

    The next op drills two locator holes which will locate the fretboard for me. They are also used to register the neck in the next jig for machining the back side and heel.

    Attachment 351788

    And here it is rouging out the cavity for the Carbon Fiber "D-Tube" truss rod. I use this in place of a traditional adjustable truss rod. It also transitions down into the heel to
    reinforce the weak short grain in the heel.

    Attachment 351790

    And while we are cutting the D-tube recess, we can glue glue on the headstock overlay and veneers and clamp them, they are out of the way of all these operations.

    Attachment 351792

  4. #4
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    Re: How I use CNC in acoustic guitar building

    More fretboard side processing....

    Here is the finishing op for the D-Tube channel. Just a 1/2" round bit taking very small step overs.
    You can see the headstock clamping caul in the foreground.

    Attachment 351794

    Here's the CF D-tube next to it's slot.

    Link to the D-tube product from Gemini: Gemini Carbon Fiber D-Tube Guitar Neck Stiffener - Guitar Neck Stiffener & Reinforcement

    Attachment 351798

    And test fit.. about .005" clearance for epoxy.

    Attachment 351796

    And now it's time to do some machining on the headstock. I designed my headstock to look just like the ones I used
    to carve by hand, and they are complex enough that you can't machine all parts from the back side and at a 15 degree angle, so
    I have it cut whatever can be reached now.

    Note that in the back, you can see the the overlay has been trimmed to that back nut line. This is what the nut will bear against. I do it now since I know exactly where
    that line is and so I can just have that 1/2" down cut bit cut it in one pass. l do it first before trimming the overlay to the outline so that any tearout on the ends will happen
    in the waste part of the headstock area. Then I have that same bit start profiling the headstock. I'm very conservative on depth of cut. For the first one I did only 1mm per pass,
    but on this one I increased it to 3mm per pass. Once again, I'm most concerned with wood tearout, and of course everything is a climb cut to reduce that risk too.

    Attachment 351800
    Attachment 351802
    Attachment 351804

    And here is the neck prepped for that D-tube and ready to be epoxied in. The headstock has superglue all over the sides of it. It's been VERY humid and rainy here in California, and I tried to unclamp the overlay too soon and it kept wanting to peel off on the edges, so I clamped it back and soaked it with CA clue. Still had to be clamped all night for it to take.

    Attachment 351806

    That "saddle" piece goes under the D-tube (extra machining depth for that) and carries the CF down into the heel so it can handle the stress of being pulled
    against those set screws. It works in conjunction with the maple spline.

    Attachment 351808

    Epoxy applied, the D-tube is pressed in.

    Attachment 351810

  5. #5
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    Re: How I use CNC in acoustic guitar building

    Time to rough the back side of the neck.

    I have a vacuum jig that I made with the CNC. It has a 1/8" gasket just around the perimeter of the fretboard area. It holds well enough
    out of MDF with polyurethane to seal it, but I'll be remaking this in VHMW like the scarf joint jig, the vacuum is VERY much improved with the
    plastic. 0,0 is the center of this jig, and I have it's co-ords saved into MACH 3 so that I can always locate the neck back for more machining if needed.

    Don't ask why I've had to do this...

    Attachment 351812

    here is what the vacuum jig looks like. I have a similar one for the fretboard work, but with the locator pins reversed.

    Attachment 351830

    The first rouging op will profile the radiused end of the fretboard cantilever. This is needed because the F360 generated rouging path
    tends to splinter the end of taper where it is thin, and this seemed like the best way to stop it.

    Many a piece of fireplace kindling were generated trying to solve that....

    Attachment 351814

    And the heel is roughed out of the blank. I leave 1mm of material for the finishing ops. 1mm step downs to minimize tearout and also leverage
    force agains the vacuum hold. I could probably go deeper, will need to some tests when I have time.

    Attachment 351816

    The back of the headstock is roughed at the same time.

    Attachment 351818

    Here is more of it emerging from that blank

    Attachment 351820

    You can see the headstock back bevel starting to appear. I used to profile my headstocks with a template double back taped to the stop of the headstock, and a flush trim
    router bit. But if you accidentally tilt the router, you cut into the sides. I did that a couple years ago and the only solution was to bevel the sides of the headstock. I decided
    it looks so cool that I made it a standard feature and programmed it into my cad model.

    Attachment 351822

    And it's finally worked it's way down to the neck shaft. .6" thick at the nut, .7" thick at the heel. I've never felt confident carving that far down by hand.

    Attachment 351824

    The last roughing op is to actually do a finish bevel on the end of the heel for the hellcat. .020" stepover gives a perfect glue surface. By putting the heel cap on now, I can
    have the finishing ups cut it perfectly flush to the rest of the heel.

    Attachment 351826

    Roughing complete. Even this would be a great place to start doing hand work, but we can go further.....

    [ATTACH=CONFIG]351828[/ATTACH

  6. #6
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    Re: How I use CNC in acoustic guitar building

    Time to do some finishing ops on that neck.

    Here is the veneer stack for the heel cap, including a piece of Curly Koa scrap from a previous guitar.

    Attachment 351834

    Glued on with the go bar deck, slightly tilted to account for the angle on the heel cap. The trick is to let the glue start to set a little so it won't creep.


    Attachment 351836

    And then, I put it back on the vacuum jig and start finishing ops, running at just .020" stepover with that same 14mm x 125mm bit I used for roughing. It's the only way to reach down
    on that heel.

    Attachment 351838

    Finishing ops on the neck shaft. same step over.

    Attachment 351840

    And then I run a parallel op on the back of the headstock with a rounded bit, I think it's .010" step over, it's creating a nice glue surface for the back side veneers.

    Attachment 351842

    Here is all smoothed over. Just needs a little sanding to get rid of the witness lines. I could make those almost nonexistent, but it would really add to the CNC time. And with all
    the trees that have been falling on power lines this year......

    Attachment 351844

    And the heel cap.

    Attachment 351848

    Here's how the headstock ended up

    Attachment 351846

  7. #7
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    Re: How I use CNC in acoustic guitar building

    Time to glue on the back veneers. I forgot to get pictures, but I have a plywood clamping caul that I modeled from the Headstock back profile, it
    is precisely CNCed to apply pressure through the little "ski jump" on the back.

    Attachment 351852

    And after a little hand trimming (yes, I actually contemplated having the CNC do that!), it came out perfect.

    Attachment 351854

    And while we're automating stuff, how about some tuner holes? This is the jig that I CNCed (once again, modeled directly
    from the headstock) to hold it precisely so I can bore the tuner holes with a 1/4" bit.

    Attachment 351850

    I just put self sticky sandpaper on the bottom to make sure it doesn't slip (a little slop in the recess there).

    Attachment 351856

    The trick is to model the hold downs as well so that the generated tool path will stay away from them.

    Attachment 351858

    Then I just beveled the edges with a Festool countersink bit.
    Attachment 351860
    Attachment 351862

    And, that's where I'm at after about a week of building just part time (and a long weekend). The CNC really helps the productivity rate (once you work through it the first couple times and getting the jigging down... but that's just like regular hand building).

  8. #8
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    Re: How I use CNC in acoustic guitar building

    One more unexpected post before I dive into the fretboard and inlays this weekend.

    Once I got all the overlays on the headstock, I found out that it was actually too thick for my Gotoh tuners to work! The extra veneers under the headstock and
    on the underside actually added an additional 3mm, and combined with the thick headstock overlay that I forgot to thin, the top nut just couldn't get threaded.

    I thought through all my options (recess the washer and nut on front side? Have a longer nut custom machined?). Eventually I decided on the easiest approach (and especially
    going forward). I re-did my headstock model to reduce the thickness of the blank by about 3.5 mm, and then re-did my CNC ops for that. Then I made a special op that was the "smoothing" part only for that surface, and had it work in 2 mm step downs until it got the new thickness / surface. This cleaned it up perfectly, and now I have enough
    room for the back side veneers to be redone.

    It was also a good excuse to make a new clamping caul to match the new ski jump curve. Pictured below.

    Attachment 351950

    Attachment 351952

    Attachment 351954

    Attachment 351956

    Attachment 351958

    Attachment 351960

  9. #9
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    Re: How I use CNC in acoustic guitar building

    Wow...awesome post dude. This is how I envision using my CNC machine for guitar building!

  10. #10
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    Re: How I use CNC in acoustic guitar building

    its rare to see a hobby type (and sometimes even a factory) know exactly what they are doing with a cnc machine and a guitar. thanks for posting and showing how amateur my setups were :P
    wotzBotz

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    Re: How I use CNC in acoustic guitar building

    Thank you for all the pics and descriptions looks a great set up ,I hope to get something close in the near future making the final fit and finish a breeze the threading part would come in handy for many things have not got to that part of Mach yet I like your clamping and Vacc set up all streamlined and efficient pictures help with next set up after a long break .

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    Re: How I use CNC in acoustic guitar building

    Thanks all for the kinds words and feedback!

    On to the fretboard!

    The first thing I do is model my inlays as F360 bodies. I have to do this because F360 sketch tools aren't the greatest. You cannot scale within a sketch!
    So I make them bodies, which can be scaled individually. The trick is to design each piece in the smallest size you will need, and ensure that the inside corners
    are large enough for your smallest inlay bit to fit. Then you can scale them up as needed for different areas of the fretboard.

    So once they are sized, I locate them over an image of the shell blank and move them around to get them as compact as possible.

    Attachment 352632

    Then I superglue the blanks onto MDF or some scrap. This makes sure the little bits don't come loose while cutting (I've had mixed results with double
    sided carpet tape... if a small piece comes lose it can break the bit). Afterwards you can just soak it in acetone to get the inlays off. You could also put tape over the top,
    flip it over, and drum sand off the substrate to free the pieces, but this seems like it's risky to lose the pieces.

    Attachment 352634
    Attachment 352636
    Attachment 352638


    This inlay... I had planned to put in the Rosette, but after seeing how big it was, I've decided to retrofit it onto the headstock just above the nut. Will make a another post
    on the simple way I came up with to do that.

    Attachment 352640

    Time for the FB itself. I have another vacuum jig, which is for the fretboard. It has the locator pins reversed from the neck vacuum jig. Although I have just come up with a way
    to use one jig for all them. Anyway, first this holds the blank down so I can have it drill the locator holes.

    Attachment 352642
    Attachment 352644


    Then I use the locator holes to index it on the jig, and start by contouring the outline, then I run an operation that puts the 16" (I think?) radius on the top.

    Attachment 352646
    Attachment 352648

    Then, after the radius, I have it cut the fret slots. Note that this is a "faux bound" FB. I have it configured to stop the fret slots 1mm shy of the edges, so I can nip the ends of the frets, and have it look like it was bound, no exposed fret slots.

    Attachment 352650
    Attachment 352652

    And finally I have it route the inlay pockets.

    Attachment 352656
    Attachment 352654

    So here are the inlays dry fit into the pockets.

    Attachment 352658

    And epoxied in place (I felt there was a little too much slop for superglue) and ready to attach to the neck.

    Attachment 352660

  13. #13
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    Re: How I use CNC in acoustic guitar building

    The Inlay look great it's a fine job for sure ,I have tried double sided tape with mixed results the cutter can pick up some glue off the tape and it adds to the diameter of the tool with a bad out come I talked to a guy and they used PVA glue on pearl and then put it in water to dissolve the part away must try it thank's for the pis and descriptions again .cheers John.

  14. #14
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    Re: How I use CNC in acoustic guitar building

    ive done the superglue thing as well. it works quite good, though it can absorb into some wood end grain and leave a stain, so you need to be careful if you use it on wood.
    wotzBotz

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    Re: How I use CNC in acoustic guitar building

    Quote Originally Posted by InMesh View Post
    The Inlay look great it's a fine job for sure ,I have tried double sided tape with mixed results the cutter can pick up some glue off the tape and it adds to the diameter of the tool with a bad out come I talked to a guy and they used PVA glue on pearl and then put it in water to dissolve the part away must try it thank's for the pis and descriptions again .cheers John.
    Hmm... I like that. Mucho easier than dealing with superglue. In fact, maybe I am misremembering when a famous inlay place told me about which kind of glue.

  16. #16
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    Re: How I use CNC in acoustic guitar building

    I was talking to a guy at a recent guitar show who sad give it a shot I try to remember things I get distracted easy ...

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    Re: How I use CNC in acoustic guitar building

    Quote Originally Posted by InMesh View Post
    I was talking to a guy at a recent guitar show who sad give it a shot I try to remember things I get distracted easy ...
    Hmm... was it the SBAIC?

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    Re: How I use CNC in acoustic guitar building

    If there was a place I would like to go that would be the show I wonder if I would have the time to ask all the questions I have,our man Tom will be there ,here in OZ not much to see .

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    Re: How I use CNC in acoustic guitar building

    Quote Originally Posted by InMesh View Post
    If there was a place I would like to go that would be the show I wonder if I would have the time to ask all the questions I have ,here in OZ not much to see .
    I spent six hours there in Sept (1st year). But it's 2 hours from here and I had to get home. Def overload, it was like trying to see all the custom Harleys at Sturgis.

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    Re: How I use CNC in acoustic guitar building

    OK, finally got some good shop time yesterday after helping a friend salvage and old windmill... interesting adventure.

    I have one last thing to do before calling the neck "mostly done" (ready for final sanding).

    I had made a large inlay which I planned to put in the rosette, but after cutting it I realized it's just too big. So I decided to retro-fit it onto the headstock just above the nut. Normally this would have been very difficult to do, but I realized that I could "re-purpose" my tuner hole jig to hold it in place on the CNC machine and cut the inlay pocket. I put a "filler" block into the headstock area (the test piece I made for testing the hole drilling program), and covered it with sandpaper so it wouldn't move around after clamping.

    I also had to machine some new, thinner, hold downs so that my Z-axis wouldn't collide with them while cutting with the short inlay bits.

    Attachment 353644
    Attachment 353646

    I scribed some lines based on the nut line defined by the headstock overlay, since I knew would be a "square" reference line, then drew another parallel to the nut line, and used that to make sure it was square to the jig. I also made a test "driveby" with the bit just above zero to make sure it would track along that line on the X-axis, to make sure it was aligned correctly.

    Attachment 353648

    Here is is after cutting. I seated the inlays and tapped them in all way with a hammer and a block of wood. They were so tight that time I just used superglue and wicked it
    in. After it had dried (just a few minutes), I sanded the inlay down.

    Attachment 353650

    And here is the finished product, I think it looks great!

    Attachment 353652

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