Because of the glue plywood is nearly the same as hardwood when machining, so the formula for it is 1x diameter depth, you nearly used 2x depth (10mm). This is pretty surely causing deflection.
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Got it. I’m not sure I’ve seen that formula before, but I shall adjust and see what happens. Always learning…
Yep, looking in a few other places, I see I’ve been tackling this with unideal settings. (I think I have been conflating information from multiple sources, and not doing a good job of understanding before doing. Alas.) I will reduce to a 5mm pass but keep the speed at 12mm/s (in order to change only one variable at a time). I guess if that goes well, I will slowly try increasing the feedrate. 40 min per rib is gonna hurt, but maybe not quite as much as needing another sheet of plywood…
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Soft wood can be up to 2x diameter depth. It is also always easier to increase speed than depth.
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Some Plywood (glue and lots of knot slices) is the hardest of the woods to cut. Expensive ply tends to be a lot easier to cut, the cheap stuff has lots of voids full of glue that makes a nasty gooey cut. You are also using a 1/4" endmill. And you are fairly new to this. As you progress and learn you will learn how fast you can push it. For now, stick to one diameter’s depth of cut and just keep increasing your speed on each cut. Keep your RPM as low as possible and only increase if you are moving really fast.
After that make sure you use a full depth finishing pass (.4mm should be okay) on any cut you will be measuring, you should never have a 2mm issue after something like that.
Then, you can work on increasing the depth of cut. Just not at first. Lots of other settings to dial in.
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This all makes sense, and that’ll be my plan going forward. I think I was just reading from too many sources and getting things all jumbled.
Thank you, as always!
(Edit: if this is all on the Milling Basics page, then I’m an idiot because I’ve read that page a dozen times…)
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Déjà vu all over again.
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Final X rib is cut, so all of the wood sections have been extracted from their native sheet of plywood.
Really looking forward to having the Lowrider up on its table and off the ground soon.
Next up: table saw madness as I open up all the slots so they are wide enough for the spars and ribs to fit together. Yay!
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Man those LEDs get me every time
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Your unit looks awesome!
Regarding use of a tablesaw to open up the cross-lap joint gaps, if you have a portaband saw that you could mount so that it’s like a mini bandsaw with its own table, this would be an excellent tool to use for this. Probably safer and quicker and all-around easier than trying to use a tablesaw. There is a company called Swag Offroad that offers a table mount for portaband saws. I have it and it’s a pretty fantastic thing. One of my videos shows the unboxing and assembly of it. I use that thing quite often. The advantage that it would have over a normal bandsaw, is that a portaband has bearings that turn the cutting portion of the blade at about a 45° angle from the normal orientation of the edges of the wheels that are driving the blade. This effectively moves the machine out of the way, and would allow you to position for cutting … a strut or rib or spar of pretty much any length, so long as you had the weight of the far end of it supported somehow.
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I don’t have one of those, sadly. But I have a pretty solid plan for using the table saw with a crosscut sled (and an extra pair of hands). Testing tonight.
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You can definitely do it that way. Provided you have enough room off to the sides!
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It looked something like this:
… and worked quite well! (Slots aren’t in line with the blade because I forgot to take a pic when they were and it was cold and I was tired and blah blah blah 
)
Next up is cleaning up all the spars and ribs, and in particular, cleaning up the slots so they are the right depth (they are a little too shallow right now). I’ll be getting the 1/2" MDF for the top and bottom skins at some point over the holiday weekend, and perhaps assembly will happen Saturday or Sunday!
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Just saw this the other day in a YT video, but to help get all the slots to the right depth, you can use the same setup you have there (all the spars/ribs clamped together), but flipped over and use a straight cut bit with a bearing in your router. The bearing will make sure you don’t open up the slots side to side, and the straight cut bit should have a flat end to it to help square out the slot to the correct depth.
Forgive me for asking the stupid question, but why don’t you simply cut the slots to the correct width/depth using the LR3 that you used to cut slots of the incorrect width/depth?
I suppose I could… but that would introduce a bigger margin of error to do that by hand. Given that the machine somehow didn’t cut the slots properly in the first place, even though all the dimensions were correct in Estlcam and the outside dimensions of the cut pieces are correct, I don’t really have faith that having it try to redo the slots in an automatic fashion would work out. 
Ultimately, I’m tired of fighting plywood with the LR3, and I just want to get the goshdarn table built already.
That answers my question, but,…
That seems backwards. The LR3 is supposed to be able to cut or engrave within fractions of a mm, certainly more accurate than a table saw , one would think .
I understand the frustration and desire to just get the damn thing finished, but there must be something in your CAM or in the LR3 that is making the slots the wrong side.
Several companies provide 1/8" bits with flute lengths that can handle 3/4" material. I have been using both an upcut and a downcut version from Amana Tools for years. I recently started testing an extended length 1/8" bit from IDC Woodworking.
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