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Neck shaping jig help

Discussion in 'Luthier's Corner' started by rojo412, Nov 6, 2018.


  1. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    As of late, I've been doing a lot more building. Because of that, I wanted to speed up some processes as much as possible, mostly the neck shaping. In my search for better ways, I found people using their altered versions of Bill Scheltema jigs. Whatever I could find to watch, I watched it over and over in hopes of making one. Most of the videos are just the jigs in action, not much in the way of "how-to."

    Over the past 3 days, I've been trying my best to reverse engineer one. I think I'm almost there:

    IMG_4852.JPG
    IMG_4853.JPG

    But one thing I'm definitely having trouble with is finding out how to do the contour "pucks" that guide the setup. I tried scaling up the contour shape 200% and just seeing how things went on scrap, but it was bad.

    And I'm sure there's plenty of other issues that I just don't know about. You know, things like how to set the feeler bearing height or if the main pivot axles need to line up with anything specifically.

    Does anyone have any insight on these things?
    (((cough @Bruce Johnson cough)))
    They seem like a great way to really bang out some necks without having an overarm router and/or CNC machine.
     
    SlingBlader, RĂ´ckhewer and Scoops like this.
  2. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Nope! But I do now, thank you.
     
    Beej likes this.
  3. Beej

    Beej

    Feb 10, 2007
    Victoria, BC
    I haven't gotten further than watching a few videos and bookmarking a few links, so I'll be watching this one. :D
     
  4. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    I hope to be able to tell everyone how to do this some day. Whatever I figure out, I'll happily share.
     
    Beej likes this.
  5. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    So before I get started on this today, I figured I should relay what was occurring to me last night:

    - In order to properly make the pucks, I need to standardize where the neck goes in the cradle.
    - Once the neck is set where it needs to be, I can determine the ratio of "actual size : puck size" by setting up a blank puck, setting the router bit at the edge of the neck, the figuring out what that number is. I'll likely use a neck template to establish that.
    - The pucks will be a shape of the neck at the nut or 1st fret, as well as the 12th. They should be the same ratio, if the neck is centered in the cradle (I believe).

    Aside from that, I looked at all kinds of other design flaws that I was running into, so I'll be addressing those as well. For example, the axle of the router sled was hitting the headstock. So I'll be moving that more inward. If I'd made this with bolts instead of rods, it wouldn't be an issue, but I think in the long run, the rods will keep the sled from bowing.
    Also, it throws up a fine dust that won't go away, so I need to figure out some dust collection as well.

    I hope to have some positive results by the end of today.
     
    Last edited: Nov 7, 2018
  6. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    I don't know if this will work within the rest of your design, but I built my router trolley with the bearing axles above the router base. I figured this would give the most range for bit height adjustment and keep the trolley mostly out of the way of accessing the workpiece. I placed the axles as far inboard as I could in order to get the most fore-aft range before the trolley contacts the edge of whatever jig it's placed in. I haven't built a neck shaping jig to go with it yet, but I plan to.

    You can see the whole sled in this post - What's your workbench?, but here's a shot of just the router trolley.........
    trolley1-.
     
    rojo412 likes this.
  7. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    I dig your jig! For this, though, I'm not terribly worried about the ability to get the bit lower. In fact, I think I'm still too low with it as is.

    There were some improvements made and I thought I had the right idea, so I tried a test neck:

    IMG_4854.JPG

    It cut the widths pretty close, but the thickness was WAY off. What I did was, I'd removed the thickness of the fingerboard from the equation. That was a mistake.

    So I tried again with a scrap piece of cedar, keeping the full neck thickness in mind:

    IMG_4857.JPG

    It didn't kill it, but the whole thing ended up too fat in the middle, slightly narrow on the width of the nut and 12th fret areas.

    So I googled again and found some long winded explanation on TDPRI involving a ton of algebra. But I kinda lost it when some figure appeared in the math and I had no clue why. But I did find that maybe I just didn't get enough measurements. That forced me to measure more. I've got all kinds of numbers now!
    And no idea how to translate.

    The big key that seems to be missing is just the formula required for this. I've extrapolated all these numbers and have no real idea how to work with them:

    - Bearing to bearing distance for the puck followers: 39"
    - Left Puck to nut distance = 9.375"
    - Right Puck to 12th fret distance = 12.75"
    - Nut width = 1.625"
    - 12th fret width = 2.8125"
    - Nut neck thickness with FB = .89"
    - 12th fret neck thickness with FB = .98"

    There's a difference in height of .285" between the puck bearing and the router bit, when the bit is even with the sled. But it can be raised or lowered as needed.
    I feel like there's a proper number to scale a profile, as well as a proper height difference between the bit and roller. Something like "scale the profile to X%, set the bearing height to Y, clamp it in and start cutting".
    That's just not something that is clicking in my head at the moment. I've been scouring this page and I need to just take a break. If anyone can simplify it, I'll owe you a beer!
     
  8. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    As I said, I haven't built a neck jig yet, but I'd thought a bit about the design, especially the profile pucks. I figured the best way to get them really right would be to start off big but in the ballpark; load up a neck blank and route it, assess the profile, then carve down the pucks a bit, load up the same neck blank, route it again, assess the profile, repeat until you get it down exactly where you want it. I'm a math-type guy, software developer by trade, and while I certainly think it's possible to do it strictly through calculation, I think it'd probably be quicker to get it really dialed in with a little trial and error, just plan it so that your error phase starts out large and works down to final profile. Especially if you want to try an assymetric profile.
     
    Last edited: Nov 7, 2018
    tbrannon and Beej like this.
  9. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    That's actually a really good idea. The pucks are easy to make and I could really experiment with that a lot.
    When I finally do get something the way I want it, I could reverse the math and figure out what the ratios are, so then I'd be able to change up the designs in the future without guessing.
    I still want to know the magic numbers, but this would be one way to make that happen. And I have a lot of scrap wood to use, so that's good too.

    Can a non-student adult hire a math tutor?
     
    Last edited: Nov 8, 2018
  10. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Hello Rojo;

    Sorry I'm late to the discussion! I'm a big fan of building special fixtures and machines to do woodworking. This is some serious fun!

    I've never built one of this style of neck-carving machines myself, but I've seen pictures and videos of ones that others have made. It's an interesting variation of a pattern-following router machine, where the workpiece is mounted on a floating spindle, which rides on profile cams (the Pucks) at either end. It's an unusual hybrid of 2D and 3D cutting. This configuration of machine has some advantages, and some limitations.

    The big advantage is that the tooling is simple and compact. It's pretty easy to make up a bunch of different sets of cams that swap in and out, to give you different neck dimensions and shapes. Working out the geometry of those cams is a little funky; I'll get to that below. But the cams are easy to make up.

    The limitation of this machine is that it's not really a 3D carving machine. It's 2D, projected over a 3rd fixed axis. Maybe call it 2 1/2 D? It can only cut a shape which makes a smooth linear transition from one end to the other. You put a cam at one end that has one shape and size. Another cam at the other end is a different shape and size. The machine will then carve the workpiece in between to a straight linear transition from the shape of one cam to the shape of the other cam. That's all it can do. It can't narrow in or flare out along the length. It only cuts in a straight line from a point on one cam to a point on the other cam. It's important to understand that, because that's how you work out the design of the cams.

    I'm sure you've figured this out already, but what this means in terms of shaping a bass neck, is that this machine will only cut the area from about the 1st fret to about the 12th fret. It can't cut the flare-out to the headstock or the short transition to the rectangular heel that is between the 12th and 16th fret areas. You can only shape the straight section between those two zones. And that section has to be a straight linear transition from one end to the other.

    And that is a limitation on what this machine can do. Most bass neck shapes are a straight linear transition from the 1st to the 12th. But, when you get into the more exotic custom world, many aren't linear. You'll have to restrict the neck shapes you cut on this machine to straight linear transitions.

    Before we get into how to design the cams.....

    First, you have to decide on and establish a Zero Line, which you use to measure and reference everything to. I recommend that you use the glue line of the neck; the glue joint between the neck and the fingerboard. That is, you mount the neck on the rotating spindle so that the center of rotation is right down the center of the glue joint. You have to work out the clamping at both ends to get it into that position. Rotating the neck around the centerline of the glue joint will minimize geometry problems when you have it turned to the side.

    You can shape the neck with the fingerboard attached or not, as long as you make the spindle cradle so that the glue joint centerline ends up right on the spindle centerline. I'd do the shaping after the fingerboard is on, but you don't have to.

    Next, there's an important relationship between the two cam followers and the router bit. All three should be the same size, the same shape, and exactly in a straight line with each other. Otherwise, the geometry gets messed up. So, if you have round ball bearings (1/2" OD?) on the ends of the followers, then the router bit should be a 1/2" dia ball-end (Cove) bit. If you use a standard 1/2" straight end router bit, then the two followers should be rectangular, 1/2" wide, and flat on the end.

    Then all three need to be lined up. The two followers should be centered and fixed to the same depth below the rails. Then use a straightedge or whatever to adjust the router bit to exactly that same depth below the rails. Lock it there and set the router's dial ring to Zero.

    Side note: Using a ball-end (Cove) bit will technically cut the shape more accurately. However, it takes more time and back-and-forth passes, and you end up with a bunch of little flutes that need to be sanded off. Using a standard flat-end bit will be faster and give you a smoother nearly-finished surface, but at a slight penalty in accuracy. Don't use a flat bit any larger than 1/2" diameter, or you'll get into bigger accuracy problems. Kind of complicated to explain here.

    The perfect thing would be a router bit with a small large-radius dome on the end, like a 10" radius. And followers to match. But, I don't think anyone makes a standard router bit like that. You'd have to have it custom ground.

    Finally, designing the cams: Yes, you could do it all in the computer in a 3D Cad program. Boring work at a desk. But here's how to make them out in the shop with real tools. Which, in my opinion, is a whole lot more fun.

    Picture the section of the neck that you are going to carve with this machine; from the 1st fret to the 12th fret. Make up a pair of Master Pucks from something, which are the actual size and shape of the neck in cross-section, at the 1st and 12th frets. Right to the dimensions and contour that you want the neck to be. Mount those two Master Pucks on the spindle at the 1st and 12th fret positions, with the glue line right on the spindle centerline. Those two Master Pucks now represent the final shape of the neck, without the headstock and heel being in the way.

    Now, mount oversize blanks for the cams out in their positions at the ends of the spindle. Lay a straightedge across the top of the two Master Pucks, with the end extending out to touch the cam blank. Mark that spot. Do the same on the other end, with the straightedge resting on the tops of both Master Pucks, extending to locate the spot on the other cam. That's it, that's the geometry. Those two spots are the heights that the cams need to be, for the router to cut a straight line just touching the tops of the two Master Pucks.

    Keep on doing the same thing to draw out the shape of both cams. Rotate the straightedge around the Master Pucks from one side to the other, keeping the straightedge parallel to the center axis. Connect the dots on the cams and cut them out to shape.

    You'll see that the final cams will be similar in shape to the Master Pucks, but enlarged and reduced by a percentage that's a little complicated to calculate. The heel end cam will be enlarged by a ratio related to the distances from the cam to the 12th fret and from the cam to the 1st fret. You can do the geometry in the computer, or calculate it by hand, but it's probably easier to make up the Master Pucks and use the straightedge.

    I hope this helps!

    Edit: By the way, the reason why the cams are complicated to draw out is that the enlargement ratio isn't constant as you rotate the cam around. That is, the heel end cam isn't just a proportionally scaled up version of the 12th fret cross-sectional shape, made larger by some fixed percentage. It will grow more in the width (the sides of the neck) than it will in the height (center of the neck). The cam shape will be out of proportion from the actual neck shape. You'll see that when you try the straightedge method. That's why you are getting confused right now.
     
    Last edited: Nov 8, 2018
  11. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Thank you so much, @Bruce Johnson! I knew you'd be able to help with this. You've helped immensely.

    - The fact that the cam bearings and cutter height need to be the same was one thing I actually found out this morning after 3 hours of unsuccessful math. In fact, I've spent more time working on math over the past 3 days than I think in my entire high school career.
    Essentially, I kept trying to figure out how the triangles worked for this, as it seemed like there was just some aspect which I was missing. But as it turns out, like you said, the math is very complicated.

    - I wasn't aware of the fact that the bearings need to match the bit, but suppose that makes sense. The bearings on this are 7/8" x 1/4". So ideally, I'd either get a set of 1/2" bearings and a 1/2" cove bit or find a 7/8" cove bit, yes?
    The early assumption was, the follower unit makes a singular, small contact point on each the pucks, so it doesn't seem like you would need to match all of the dimensions. But I have seen others working on these and getting all kinds of stepped cuts or weird patterns on their first attempts. That seems to be the mismatch of bearing and cutter sizes.

    - The 1st-12th rule, is that a hard and fast law? I was assuming that I would be able to start at the nut (or zero fret), then mill down to the part of the neck where I would need to do hand shaping only. Granted, that messes with some of the calculations others described for 1-12 thicknesses, but at the same time, I could always stop shorter as needed.

    - The way you're describing making pucks is EXACTLY what I need in this process. I didn't realize that they would not only scale up, but down as well.

    I see the other people in the videos using oversized pucks and cutting using those. Is that a factor of the bearing heights and cutter height being different? And if that is the case, do you know how they'd scale up? Is there a formula for that?
    It seems like working with larger pucks gives more room for error when designing the shape. But how, is basically, what I have no clue on. Is it as simple as adding 1/2" to the outside edge or is it a ratio thing that is super complicated?

    And thank you again, I'd wait as long as it took for your sage advice.
     
  12. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Well, you don't have to have the cam followers and cutter exactly in line, but it sure makes the math and cam design a lot easier. You can offset the followers up by a fixed amount, which will make the cams (pucks) larger. But that doesn't really increase their accuracy. It doesn't multiply the height change of the cam; it just adds a fixed number to it. And makes them larger. I'd work with them straight in line, to keep the geometry simple.

    If you really do want to work with larger cams, then you need to decide on a fixed offset amount of the followers above the bit, and make up some kind of a gauge so that you can set it and check it. Then, make up a straightedge gauge with a matching offset to help you draw out the cams. I'll talk you through this in more detail if you want to go that way.

    Yes, unfortunately, this really is important. Kind of complicated to explain, but a fundamental thing in designing cams. Picture the cam sitting horizontal, with the follower on top of it. This cam is wider than it is tall. As you rotate the cam 90 degrees, the follower is effectively climbing uphill. If the follower is a small wheel, the point of contact rotates through some angle and moves off to the side of the centerline of the follower, as the wheel climbs the hill. Are you picturing that?

    If the follower is a flat-end bar with no wheel, as the cam rotates, the point of contact slides straight sideways until it hits the corner of the bar. Then the corner of the bar becomes the point of contact, and remains the point of contact sliding up the hill of the cam. The motion of the follower is not the same as what the wheel does. Not just a teeny bit; it's quite a bit different in shape.

    That's the point: If you use a wheel on the follower, riding on the cam, and a flat-bottom router bit in the router, then the router bit isn't going to cut the same shape as the cam. It may actually be weird, with lumps and steps. That's why the cam followers need to be the same size and shape as the router bit (looking at it from the side).

    If you use a flat-bottom 1/2" router bit, then the tip of the follower should be 1/2" square bar stock, flat on the end. If you use a 1/2" ball-end Cove bit, then the followers should be 1/2" diameter wheels.

    I recommend that you go with a straight 1/2" flat bottom router bit, and change your followers to 1/2" square maple or aluminum blocks that rest right on the cams. You shouldn't really need the bearing wheels anyway.

    As I mentioned before, the ball end bit will theoretically cut the shape more accurately, but it will take a lot longer, and will end up as a whole bunch of little rounded flutes and ridges that have to be sanded off. The flat bottom bit will give you a smoother, more finished shape in less time.

    A general note about operating this machine: Lock the spindle in one position, and make a pass with the router down the rails and back. Stop, rotate the spindle a few degrees, lock it, repeat. Cut the shape as a bunch of straight flat facets down the neck, angled every couple of degrees. How wide the facets are is up to you; how steps/passes do you want to take. I don't recommend rolling the spindle side to side as you bring the router down the rails. That's likely to bring in other problems with chatter and geometrical errors.
     
    rojo412 likes this.
  13. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    I'm going to try this all as soon as possible, thank you so much Bruce!

    - The lock mechanism is built into this, so I will certainly do them like that. When I was cutting the test pieces, you could definitely see that doing "horizontal" cuts yielded a different result than "vertical".

    - And the part about the follower and bit shape being the exact same makes a ton of sense now. I'll start with some maple, cut 1/2" square as the cam follower. It's here and it's cheaper than a cove bit and like you said, it should yield a better result.
     
  14. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Okay, so I rigged it up like this:

    IMG_4861.JPG
    I set up a pencil to mark the pucks, making sure the tip landed where the flat part would extend to.

    IMG_4860.JPG

    IMG_4859.JPG
    And I gave it a test cut:

    IMG_4862.JPG
    Much better than the last attempts. The sizes were still a little off, but most likely due to the poor translation of my pencil straight edge.
    Note: I rubbed some soap on the pucks to help the follower move without binding as much.

    I think I need a little more practice with this. And I may do something about scaling up the pucks, only so I can shape them better in a bigger size. Or at least, make a better straight edge pencil.
    But overall, the outlook is much better, so thank you very much.
     
    Matt Liebenau likes this.
  15. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Hey Rojo;

    Okay, that's much better! You can see how the geometry of shaping the cams is complicated to explain or work out in math, but it's actually pretty simple once you can visualize it. The straightedge and pencil thing is the simplest way to get there. The straightedge is representing the straight aluminum rails that the router rides on. That's the path that the router follows in that axis. That path is a straight line between the tops of the two cams, which results in the neck being cut to a straight line between the tops of the two Master Pucks. That's the essential geometry of what your machine does.

    Yes, you need to tighten up that straightedge/pencil rig! In the shape of the back of a neck, +/- 0.010" matters. You can feel that, particularly if it's a lump or dip. The cam multiplies that tolerance partially, by a factor of about 1.5:1. So, a lump 0.015" high on the cam would result in a lump 0.010" on the neck. You've got to make the cams carefully, and build the machine so it doesn't flex and chatter under load.

    If you want to add some offset to the cams, to make them larger size, you can. Pick some fixed amount, like 1.0". Make up a new straightedge/pencil tool, but with the pencil offset up 1.0" from the straight edge. That's for drawing the cams, like you've been doing.

    Then make up a straightedge setting gauge to use for quickly setting the depth of the router bit in relation to the followers. A strip of MDF with a straight edge, that you slip in place of the spindle, long enough that it contacts the bottoms of both followers. Somewhere in the middle, it has a notch 1.0" deep, down from the straight edge. Hold this gauge up against the two followers, and set the router bit depth to just touch the notch. Zero the depth ring on the router, and lock it down.

    Side note: It's also very important that the two followers need to be the exact same height (depth) in relation to the aluminum rails on the frame. Use squares and rulers to make sure they are exactly the same. If they aren't, you'll be adding an extra taper into the cut, all around. Do this before you use the setting gauge above to set the router bit height. That could be where you are having some dimensional problems; where one end is correct but the other end is too small.

    Another note: When you set the router bit depth and zero the depth ring, that sets the machine to cut to the Final Size, the exact finished size described by your cams. Don't ever adjust the router bit depth below that. However, you can adjust the bit above Zero to do roughing or test cuts. Without touching the followers or the cams, adjust the router bit up +1/16", and the neck will be cut to the correct shape, but with an extra 1/16" layer of wood all over it. That 1/16" is measured up from the line described by the straightedge, all around.

    That's the adjustment you'll use when you get into cutting real necks. If the blank is completely rectangular, start with the router set at +1/4", and make a couple of quick passes to knock off the corners. Go to +1/8" at 10 degree intervals to rough out the shape. Then +1/16" at 5 degrees. Stop and check the size with calipers. Then do the final pass at Zero and 2 1/2 degrees. That gets you to the final size and a smooth finish.
     
    rojo412 likes this.
  16. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Very cool man, thank you.

    Now another facet of this that I've seen is, you can radius fingerboards with something like this. I'm assuming that a non-compound radius would be a breeze, assuming both pucks are exactly the same. But do you make the pucks the same radius that you want?
    And if you were to size the pucks up, would you keep the same radius or scale it, assuming the bit, pucks and feelers all are on the same plane?
    (This is under the assumption that the fingerboard is mounted on the cradle alone, before glue up, not as part of a shaped neck)

    And let's say you did do compound radius... since the middle section would be the same height, just the outers change, would this be 2 same-height pucks, just different radii? Or is that just getting into a whole new realm of crazy?

    I honestly wish I paid more attention in school.
     
  17. xaxxat

    xaxxat Supporting Member

    Oct 31, 2008
  18. xaxxat

    xaxxat Supporting Member

    Oct 31, 2008
    Interesting setup:

     
  19. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Hey Rojo;

    Sorry to neglect this thread for a few days......A lot going on right now.

    Some quick answers: Yes, this type of routing machine can be very good for doing fingerboard surfaces.....with a few corrections on the geometry.

    The main thing is that, for a relatively large radius surface like a fingerboard, you don't want to cut it rotating it on a small radius. Right now, for shaping the back of the neck, you are rotating the neck at about a 1" average radius around the spindle centerline, as you cut. If you were to flip the neck over and try to cut, say, a 12" radius fingerboard surface, rotating around a 1" radius centerline, the cams and geometry would get really exaggerated and weird. As it rotated out to the sides of the neck, it would be trying to cut the surface with the very corner of the router bit.

    To get a smooth fingerboard surface with a flat bottom cutter, you want to rotate the neck around the radius that you are trying to cut. So, to cut a cylindrical 12" radius, the neck needs to be mounted on a holder that's 12" high, with a pivot axis down at the bottom, 12" down from the cutter bit. It pivots side to side through a small angle between the rails of your fixture. This would be a separate sub-fixture that you'd set down into your machine in place of the spindle that you use for cutting the back of the neck.

    For a straight 12" cylindrical radius, you wouldn't really need to use cams at each end. You could lock the sub-fixture between the rails, with the pivot line down 12" from the followers. So, the sub-fixture doesn't float up and down. It just swings side to side on its own pivot line. You cut the surface with straight passes down the rails, incrementing the sub-fixture in small angles to either side, at the 12" radius.

    To do a compound radius, say 12" to 20", it gets crazier. You could make a sub-fixture that has an inclined axis of 12" to 20" radius.....but the swing that it needs wouldn't fit between the rails. So, you could make a 12" swing sub-fixture, with cams at either end, where the whole sub-fixture floats up against the two followers. The cams would be weird, increasing in enlargement as they go outward. You'd have to draw them out with the straightedge. But that should work.

    However, when you step back and look at the complexity of it, it might be easier to make up a separate machine/fixture just for cutting the fingerboard surfaces. You've seen my fingerboard surfacing fixture, right? The neck is held still and the router base has curved bottom rails. The radius is done by tilting the router, rather than rotating the neck. That's a nice efficient way of cutting the large radius. It may be better to make up a fixture like that, rather than trying to adapt your neck shaping rig.
     

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