Yeah, gut reaction is that if the issue is at the inflection point where the Z-axis travel is changing direction, you likely have a loose grub screw or two. That might explain the apparent backlash (too low on the way up, too high on the way down). That, or your couplers have gotten… springy.
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KING RICHARD III:
A horse! a horse! my kingdom for a horse!
Not the render, the Gcode can be visualized. Try this just to be certain, https://ncviewer.com/
Splines and surfaces are very difficult to master, I can barely make them work so you are good at what you do!
Time Bandits (Vincent to Pansy):
I must have fruit!
Time Bandits - Napoleon: “That’s what I love! Little things hitting each other!”
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Very nice build! I’m interested in building a LR3 for the exact same purpose. Printing the parts at the moment.
Regarding the original topic of this thread: how did you make it taller? Can you share the files for the modified plates? What size rails did you put on it?
I’m also curious: what’s your workflow for flipping the board to mill the underside and making sure it’s aligned?
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I think the links to the taller plates were somewhere on this thread? Let me know if you can’t find them, I will try to find and upload them. If I was cutting those plates now, I would probably move the Z stop holes 3mm or so lower. This would make sure you can use z-stops without the rollers.
I use construction foam instead of surfboard blanks and I calibrate the stock thickness before the first cut. I use the same X and Y edges for stock zero point (see the screenshots), and the Z size is calibrated, so all I have to do is just flip the board over, align it to the other side of the table and the rest works automatically.
Also, if you are building a machine for the exact same purpose, do yourself a favor and make it possible to lower the stock to about 100mm below the table level. It will make the machine more stable and you will be able to get away with less Z travel.
Very tall YZ plates = waiting forever every time you want to home Z axis.
The artwork from the link I had up there has a .DXF of the YZ plates that work with 200mm MGN12H rails. It gives a bit over 125mm of Z travel that way.
Basically I lengthened the opening for the XZ motion by 50mm, and added a pair of holes to extend the rails. I moved the hole for the Z endstop, and the openings for access to the M5 screws to hold the beam to the XZ plates.
I experimented with 250mm and 300mm rails, but it started to look like other modifications would be necessary to retain stability.
I use the tallest YZ plates shared by @SupraGuy, I think they have 300mm openings which gives me 230 mm of Z travel. I’ve never needed more than 110mm Z travel so far, but my boards are really small. For a larger surfboard or wingfoil board you might need 130-160mm Z travel.
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Thanks @Tokoloshe and @SupraGuy for the link to the OpenScad file! Much appreciated.
I think the links to the taller plates were somewhere on this thread? Let me know if you can’t find them
I couldn’t find the links after scanning this thread a couple times, but if you used the files from the thread referenced by @SupraGuy, then I already found them 
I use construction foam instead of surfboard blanks
Nice! Also wanna use that.
just flip the board over, align it to the other side of the table and the rest works automatically.
I see! So you have a reference edge where you can align your rectangular blanks to, right?
Also, if you are building a machine for the exact same purpose, do yourself a favor and make it possible to lower the stock to about 100mm below the table level
Great tip. I’m planning to use a table that I built made of 50mm square steel tubes. The tubes are screwed together instead of welded. Anyway, the table structure essentially has a big hole in the middle, so I could easily drop the cutting area lower than the sides of the table I think.
The artwork from the link I had up there has a .DXF of the YZ plates that work with 200mm MGN12H rails. It gives a bit over 125mm of Z travel that way.
I think 125mm should be plenty…
I’ve never needed more than 110mm Z travel so far, but my boards are really small. For a larger surfboard or wingfoil board you might need 130-160mm Z travel.
I do plan to shape wingfoil and efoil boards (where holes for battery and electronics would have to be cut). But, If I have 125mm of cutting area, and I’ll only be cutting half the board and then flipping to cut the bottom side, I could theoretically do 250mm thick boards with 125mm of Z travel right?
Attached is a similar table to what I’ll be using, except the one on the photo uses 30mm tubes and the one I’m planning to use for my LR3 is made out of 50mm tubes.
Thanks all for being so helpful!
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Well… yes and no. But more like… no 
It all depends on the shape of the board and how you position the board in block of foam. Normally you want the board to fit tightly in the smallest possible block of foam to minimise cutting time and also wasted material. So you will probably end up positioning the model like this:
This board is 98mm thick and I think my required Z travel for cutting the bottom side is also around 98mm (for the tail and nose sections). This is because the nose and tail of the board are very thin and I also use CNC to shape the rails. So for this type of board, your max Z travel is also the max thickness of the board.
A wing foil board and e-foil board will have a different shape and will generally be a lot thicker so you will not need as much Z amplitude to cut one side. But still I wouldn’t expect to be cutting 250mm boards on a machine that has 125mm Z axis… unless your boards are going to be completely flat
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Awesome! Thank you for explaining it brilliantly! So the tail and nose rockers dictate the required z height. Makes total sense. I’m not planning to do very accentuated rockers, but I’ll give it some thought. This is new to me.
@karolis if you don’t mind me asking: what software have you been using for designing your boards? Something specific like shape3d or AkuShaper or just general CAD software?
I tried both shape3d and AkuShaper briefly… but eventually went with Fusion 360. With AkuShaper you can probably get up and running faster as it has a workflow dedicated for surfboard modelling, but I think in the long run Fusion has way more options and gives you more control over the model and the machining process… You probably have to try and see for yourself which one you like better. Fusion is also free for hobby users, and this is a hobby for me.
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Got it.I am familiar with Fusion 360, it’s my preferred 3D modeling tool. I have been using it for years, but for basic stuff. Usually fairly simple pieces for 3D printing. RC cars, small CNCs, laser engravers and such.
I’m yet to wrap my head around how to design the multiple curves in every dimension that make up a surf board. But I’ll give it a go
You need to be very well familiar with sketches, making them fully defined and using projections/intersections from other sketches/planes. And you probably want to know surface modelling basics. Solid/body modelling doesn’t work very well with surfboard shapes.
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More, I would say the line that determines the outside edge does. Think of injection molded models, there is often still a line thst you can see around the outside, where the 2 parts of the mold meet. This line is often not straight, but will wave up and down to follow where a silhouette of the shape would be. The required Z travel has to be deep enough to reach that line.