Triangular Linkage Evaluation Criteria and Measurements

I had my mount the same as you and when I tried to extend the router bit close to it’s limit, the router head would get caught up on the edge of the Z-Axis motor mount and the lead-screw thingy would pop out of the router’s tab.

having to wait an entire week :frowning:

Looking forward to the test. Which version of my top mount kit do you have (the horizontal links are different, 3/4" for the first run, 1/2" wide for the second, and 1/4" for the third) If you have the first run kits, those have the most slop in them and will generate the worst possible results. I think that with chain sag compensation, they will be comparable with the 45 kit, but it will be very good to find out.

I may just send you a new kit if you have the earlier one, so we can compare them and see how much improvement/degredation there is between the version.

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Great tests! Out of curiosity, how much does your sled weigh? I see the weights, but can’t tell how much they are. Based on your chain sag correction value, I take it the weights you have added are somewhere north of 20lbs. Is your frame reinforced along the motor arms?

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I was thinking that too. Given how there seems to be more error in the top middle, I’m wondering aloud if it’s a beam deflection situation and not the math.

If that were the case, that would explain a lot. I noticed that the Z-Axis wasn’t moving as much as it should have during the 27 square test. When retracting, it would slowly turn (like, very much slower than usual), or not at all. I had thought that maybe a wire had gotten disconnected, but it also would be entirely possible that the router was getting hung up on the Z-Axis motor. I looked through the video I took yesterday but the motor and leadscrew weren’t lit well enough for me to really see what was happening.

I saw somewhere around here a mod where someone connected a piece of plywood to one of the base’s guide holes and used that to mount the Z-Axis bracket. If I do settle on the 45 degree system, I’ll have to do something like that to align the motor better.

I know, I wish I had enough time to do all of them back-to-back. I might be able to sneak in some time during the week, but I’d probably be able to do only 1 step of the test than all 3 at once.

I have the first version of the top mount:

I have noticed a fair amount of slop in the holes. I’ve been trying a couple different hardware options to see if I could eliminate that, but it’s good to know that the holes are tighter on the current version. I’ll gladly test anything you send me :smiley:

Thanks! I don’t have a scale big and precise enough to weigh the entire sled at once, but I have weighed out all of the components:


I put the total in the main bed accuracy test spreadsheet, but I’ve only been posting screencaps of the relevant data. Here’s the spreadsheet I’ve been working on, in case you’re curious. The header has all the basic calibration values in it.

I might have messed up the chain sag calculations, though, now that I think about it. I only did what was in the auto calibration dialog, and don’t remember entering sled weight or bed angle into Ground Control. That could certainly explain why things were off in the center.

As far as the motors flexing, I haven’t tested or observed any flex in their brackets, so I’m not sure. I’ve never seen any noticeable deflection, but even a slight amount could have an effect on the tests. I had to make an adapter bracket from 1 1/2" x 1 1/2" x 1/8" steel angle to get some height out of the motors. Admittedly, they don’t even give me enough height. I plan on replacing them with a Unistrut top beam, but if the little guys are flexing then they might be forcing my hand on this.

Pictures of the assembly, both with and without the chain guide. The chain guide mounts to a second piece of steel angle.

Next time I’m in the shop, I’ll have to aim my camera at the motors, then raise and lower the sled the entire height of the bed and see if we can notice any deflection :wink: If I’m geting flex at that point, does that invalidate a lot of the data I’m collecting? Really hoping I just messed up the math :stuck_out_tongue:


I managed to swing the z-axis mount base such that the bolt from the linkage kit’s lower right pillar passes through it. I think put the pillar on top of the z-axis mount base (had to cut it a little shorter than the other pillars to compensate). Seems to work well enough.

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If there’s any (darn autocorrect!) flexing or motor movement with that impressive mount, we should all give up! A very nice frame, sir!

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I agree. This is the first pic of your setup I have seen. I ASSumed you were running the stock (or similar) type of frame. With all of this talk of beams and flex in the New Frame thread, my addled brain went right to frame flex as a possibility for the top center error. I’m quite sure your frame all but rules that out of the testing you’re doing.

That’s also a good idea, thanks for that! Looks like I have options :smiley:

:blush: D’awww, thank you! I didn’t think that a small piece of angle iron would flex under the forces we’re dealing with here, but I’ve definitely underestimated physics before. It is only 1/8" angle, I’d use 1/4" if I made another set of brackets.

I’ve been a little shy posting my frame. I posted some images in the top mount thread a couple months back, but it would have been pretty easy to miss. Its been a little confusing with all discussion about frames lately. I’m drafting up a frame build write-up, I’ll post it in it’s own thread I’m done.

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You didn’t miss anything! The weight of the sled, as well as the weight of the chain and bed angle are accounted for during the calibration process. I estimated the additional weight on your sled based on your chain sag correction value determined by the calibration process.

Essentially, the percentage error in the straight-line chain length, which uses the chain sag correction value as a combined coefficient, is inversely proportional to the square of the sled weight. Your chain sag correction value was about 4 times lower than other values I have been seeing, which implied that your sled is roughly twice as heavy as some others (assuming all other factors are identical). The only piece that looked different were your weights, so I figured most of the extra weight had to be there.

I forgot that you had a metal frame. That’s a beautiful frame, and I would doubt that you have frame flex. However, given your sled weight your chain tension near the top of the bed would have to be pretty high, comparatively. I’m wondering if we could be getting flex elsewhere.

As an aside, does anyone know if the quadrilateral kinematics include chain sag compensation for a stock weight sled? Your results for the stock configuration shows the opposite of chain sag, with an increasing cut length as you move lower in the bed. I’m wondering if your additional sled weight generated less chain sag, which threw off the compensation that was designed for a stock sled weight. I’m purely postulating here though!


no they do not, and I am not sure it’s worth trying to do it (too many other

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That’s good news. Here I thought that I needed to input the weight and angle for the chain sag calculations, that’s amazing that we can use the hardware to do that for us! You guys are doing an excellent job with the firmware!

Thanks! :grin: It’s quite a bit overbuilt, but it has served me well so far. I still don’t really like the brackets I made. They were designed with a lot of adjustment into them and they used to slip on me a lot when I was getting the whole thing setup. I used nlyocks from the beginning, but I have to make sure to torque them down pretty well when I set them. It’s a delicate balance, because too much torque will cave in the steel wall of the tube. If I had a torque wrench, I’d probably use it here.

The motors are pulling just under 60lbs when the bit is centered on the top edge of the spoilboard. It’s actually ~5 lbs less than the stock frame. The shorter top beam helps me a little here.

I made the decision to go for a heavy sled when I first started testing my system in the bottom corners of my machine. Because of the shorter top beam, the chain angles are waaay less forgiving for me, and without all that weight the routes out past 2’ off center were incredibly distorted. That was way before chain sag was calculated by the firmware.

To add on to what @dlang said, I saw no mention of chain sag in the stock calibration routine. I figured that chain sag was part of the triangular kinematics, so I didn’t think too much about it.

At this point, I can clearly say that the triangular system is orders of magnitude better than the quadrilateral. Now it’s just a question of which system works the best.


This is excellent and through testing! I am really enjoying following along. I will be sure to get you a ring kit as soon as they arrive.

I know everyone has already said it, but beautiful frame build!

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Two words, Horror Freight

The one I have matches my elderly beam type Craftsman, ymmv

@bar I am sure you’ve thought of this, but is there a way to pre-order the ring kit for those of us who already have our kits from the last batch? It seems to me that there might be a lot of demand for the ring kits and it might lower costs to put in a bigger order up front (it may be too late for that now). On the flip side, if you are ordering ring kits to have in stock for those who already have their kits, it could prevent over buying if demand is less than expected.


Yes! We had to put in the order at the beginning of the month to get them here because they take a long time to make so we’ve ordered extra and we will post them for sale.

You are right that the key is to not over order and end up with extras…we’ve got a whole crate of the old L brackets left…anyone want a really heavy duty bookend? Maybe every ring kit order comes with a free L bracket :stuck_out_tongue_winking_eye:


L brackets to brace the top bar arms?


Use the adjustability of the bracket to allow someone to change how far out the top beam extends? They seem pretty strong.

Rougly like this:



@mooselake Can’t really beat that price. I don’t have one because I haven’t really needed one until now. Although it would also help with all the car maintenance I do.

@Bar Thanks for the kind words! I am looking forwards to putting the ring to the test. I know it takes awhile to get the logistics together.

I love the creative (re)use of the brackets! @madgrizzle that’s a good idea! I was just going to take my original stock sled and hang it on the wall behind the machine kinda as reminder of how far we’ve come already.

Also, looks like I’m talking too much. :stuck_out_tongue: Discourse just popped up this message:



OMG… I love that message.