Just to help with quality control, I also have this problem and my two linear rods were also different lengths (the same 211mm and 212mm as above)
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Can the instructions also add a note about whether the stacking of the 4 motors actually matters? Does the system figure out which motor is Top Left, Top Right, etc automatically? Is there any way to know which one it should be which? Are they noted on the PCB or anywhere? The PCB has a ādownā, certainly, so is it which port plugs in to which location of the motor that determines the ātop leftā, etc?
Iāve built my system based on the āVonHausā 700W router off Amazon UK, and as Iāve got to the end of assembly, Iāve realised my dust extraction port and the linear rods align perfectly with the cord and the stepper motor threaded rods! Iāve built it 90 degrees out. A few larger photos would have prevented this - all the photos are too small.
HOWEVER, you can use CTRL-+ to make everything larger, in Firefox at least, which really makes things easier to see! Might be worth a note at the top of the page?
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That could be in the calibration instructions or it could be in the wiki with a link from the calibration instructions. Either way, I think itās a great idea. I will add a thread for collecting those problems (and hopefully their fixes as well). 
Edit: The topic (a.k.a. thread) I created is The Big, Bad M4 Troubleshooting Problems/Solutions Thread.
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I just went to add this but I found that the instructions currently say: āAnd slide them onto the router. Note that the order that they go on is important. See the illustration below for the correct order.ā
What can we write to make it better?
I agree about the photos. Squarespace (our web host) is somewhat restrictive on how we can make them lookā¦Iād love to make it so that you can click on them to see a bigger version.
I think that this is a great idea. That feels like a living document that would be easier to maintain in the wiki
NKT wrote:
Can the instructions also add a note about whether the stacking of the 4 motors actually matters? Does the system figure out which motor is Top Left, Top Right, etc automatically? Is there any way to know which one it should be which? Are they noted on the PCB or anywhere? The PCB has a ādownā, certainly, so is it which port plugs in to which location of the motor that determines the ātop leftā, etc?
the order of stacking is to put opposite belts right against each other. I donāt
think that we have any significant amount of testing to tell us if thatās needed
or not.
David Lang
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John Wolter wrote:
That could be in the calibration instructions or it could be in the wiki with a link from the calibration instructions. Either way, I think itās a great idea. I will add a thread for collecting those problems (and hopefully their fixes as well).
I was already thinking along these lines, as soon as I see you open the thread
Iāll do one of my typical brain dumps into it
David Lang
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David Lang wrote:
the order of stacking is to put opposite belts right against each other. I
donāt think that we have any significant amount of testing to tell us if
thatās needed or not.
One other factor I hadnāt thought of when I posted the message above is that if
the belts are stacked in a different order, then the Z offsets defined in
maslow.yaml will be incorrect, which will cause the belt length calculations to
be incorrect.
David Lang
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Ok, that makes sense. I can change that if need be.
Iām going to tear this down again and figure it out.
I canāt understand where I went wrong though.
Iām not sure where I went wrong. Iāll go back and look and figure it out, and report back.
The top left motor is defined by the orientation of the pcb , and itās the motor plugged into the top left socket, etc, right? So as long as I get that part correct, the rest should follow (though I might need to change the YML file if the stacking is wrong)
Could you explictly say which motor goes to which corner? i.e. āThe lowest motor is going to top right,ā¦ā etc? This would help anyone using a different router (which might be why this isnāt correct, but as I say, Iāll figure that out shortly)
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I have two of mine reversed but. Bar said I could just change the height offsets to match in maslow.yaml and that appears to have worked for me. But Iām mounted vertically.
I understand the point is to keep the router from lifting from the workpiece depending on which belts are under tension.
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What about making the images open in a lightbox? https://support.squarespace.com/hc/en-us/articles/205812708-Setting-images-to-open-in-a-Lightbox
It will apparently prevent users from pinch-zooming on mobile but it might help for desktop users.
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Looking at the M4 from above with the sled on a table, the arms top to bottom are TL, BR, TR, BL
When you are assembling, you would be looking at the collet of the router, with whatever part of the router you are declaring to be the bottom toward you, so the orientation of the arms as you add them would be like in the diagram below, up and right, down and left, up and left, down and right.
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I wish that I could like this post 10x. This works great! Going through and fixing them all right now.
Pinch zooming seems to still work on mobile unless you click on the image and then it becomes full screen
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Laura Thomas wrote:
I have two of mine reversed but. Bar said I could just change the height offsets to match in maslow.yaml and that appears to have worked for me. But Iām mounted vertically.
that is what we believe until we have someone show us we are wrong 
I understand the point is to keep the router from lifting from the workpiece depending on which belts are under tension.
more a matter of it potentially tilting than lifting.
David Lang
I found a typo on the Wedge Clamp step of the Router Assembly guide:
should be āandā instead of āanā
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I looked at that image, but must have missed the router pointing upwards part or something?
Corrected now. Thanks.
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Ok, next issue.
Iām at the stage of connecting the two āarmsā that fit into the end of the two steel rods, but thereās no way the size is correct.
I figured it that the one did was 3mm too far out, and the other was 1mm too far out, and corrected that easily enough, as you can see the end of the bar through the little hole in the sled.
It would be worth noting this in the instructions, and having people test the fit long before they get to the step of putting the entire thing together, only to take it apart again.
The easy way to fix the ābar not fully homeā issue is to turn the sled upside down, put it on a bit of wood, then strike the sled with your palm, until the rods are both fully home, as seen through the holes.
Howeverā¦
Now the bars are clearly both fully home, when I fit the linear rod supports, thereās still at least a 3mm gap which would surely destroy the plastic arm if tightened up!
That little bit of 3mm ply easily goes into the gap without touching the sides!
Help.
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NKT wrote:
Now the bars are clearly both fully home, when I fit the linear rod supports, thereās still at least a 3mm gap which would surely destroy the plastic arm if tightened up!
We have been seeing a few cases where they are too long, do you have a way of
grinding a few mm off of the end? if not, bar or anna will probably ship you new
ones
David Lang
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One of my linear rods escaped during shipping, the one that arrived was 211 mm (instead of the expected 210 mm). Anna shipped me a replacement for the one that was lost, it actually measured 212 mm. So Iām not sure getting replacements will help.
In my case, I cut shims out of old credit cards to go under the linear rod supports, this seemed to work reasonably well, although when I finally got to the point where I could move the M4 around with the jog controls, when I was moving left to right it seemed to tilt a bit, so I think I still have a slight bow in the sled from the too-long arms. When I get a chance Iāll confirm that with a straight edge and probably add additional shims under the linear rod supports, since I donāt really trust my tools or skills to remove 1-2 mm of stock and keep the ends square. To be honest, I donāt think they were really square to begin with, but probably better than I could do with a grinder or my bandsaw.
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So they should be 211mm? (edit: 210mm)
I can easily refine the length to whatās required (though my precision measurement stuff is all maxed at 150mm)
Are we looking for a nice tight fit without bending anything?
I would also say they the bars are sawn, and got hot, and thereās quite a big lip which could certainly do with sanding off.
Edit: anyone got any tips to get the bars out the sockets safely? I mean, theyāre pretty āin thereā. Punch through the sled, perhaps?
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