@blurfl I c what u mean
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Some good news and some bad news. On my mind was the motor flex we talked about earlier so the first thing I did was go around to the sides and attempt to torque the motors. While I wasnāt seeing any flex in the motors themselves I did see some flex on my attachment of the top beam to the frame itself. Obviously this actually causes the motor offset distance to change. I tightened the existing screws and drove in a few more to improve the connection.
Unfortunately, in fixing the flex in those struts and making sure they were square, when I went to complete my grid I could no longer register to the center datum point, it was shifted up a little bit. So instead I just recorded the numbers I had already taken and tested the motor flex under load. The Excel file is attached, some of the numbers donāt make sense, likely due to my marking all of these by hand.
I did also take measurements between the motors at multiple vertical positions. it doesnāt seem that I have any appreciable flex in my motor mounts along the length of the beam. As close as I could tell with my tape measure these did not change at any location nor did they change when I did a full tension calibration pull. This is good news.
I think tomorrow I will tie that top beam to the frame more securely and try a round of calibration followed by a quick grid creation in the middle of the sheet.Maslow Grid.xlsx (11.3 KB)
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Hi, just looking at the picture of your sled, should not the arms of your linkage be parallel when they move? It may be just the picture angle but it looks off.
Good luck, Recursion1
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at the risk of hijacking the thread, which version of the kit do you have (how
wide are the horizontal arms), and when you say itās binding, what exactly is
happening)
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we donāt???, we need to either get the distance between cuts, or the distance
from the top of the workpiece to each horizontal cut. If we donāt have that, we
canāt beging to calculate things correctly.
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- Set my ideal rotation radius to 260mm,
Took the time to set the sled upside down and clamped a piece of 1/4" plywood
to the clevis and measured to that and then determined the overlap of the
chain into the clevis. With that I had my rotation radius measurement of
268.3 mm.
just a note, the measurement is from the center of the bit to the center of the
first link (the point that would be at exactly the 12 oclock position when you
put the chain on the sprocket)
- Test the up-down accuracy of 6" movements before doing a calibration.
Using my manually input numbers I completed the upper left quadrant of the
sheet in 6" increments. Measured with a caliper my horizontal moves ranged
from 5.99" to 6.02" and my vertical moves ranged from 5.95" to 5.98". I did
notice that using a level there is a slight sag moving from the outside of the
board to the middle which is not unexpected.
actually, a sag from the outside to the middle is an indication of incorrect
calibration. the software should compensate for that.
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just a note, the measurement is from the center of the bit to the center of the
first link (the point that would be at exactly the 12 oclock position when you
put the chain on the sprocket)
Good point. I will have to recheck what I measured but I think I may have measured to the center of the pin not the link.
actually, a sag from the outside to the middle is an indication of incorrect
calibration. the software should compensate for that.
When I made the grid the machine was decidedly not calibrated (no test cut and 0 chain sag adjustment). I only used my measured values, I wanted to see if the calibration would improve the grid or not after the calibration. Unfortunately, I need to start that process over because I made a material change to the frame when halfway through.
since the link straddles the pin on the sprocket, you are needing to measure to
the middle of the link
The current triangular calibraiton routine only makes one vertical measurement and thatās from the top of the workpiece to the top of the horizontal cut (cut 5). All that does is set the vertical offset. I donāt think it enters into the calculations of the rotationalRadius or chainSag.
Youāre right, the vertical leg of the calculated triangle for that cut is computed knowing that the horizontal leg is one half the distance between the motors and the hypotenuse is the length of chain extended. Subtracting the distance from the cut to the top of the workarea leaves the distance from the workarea to the motor.
The measurement is not properly the distance between, but the distance from one side of a cut to the same side of the opposite one. A subtle thing, but it would put the measurement off by the bit width every time one tried. I wonder if that is part of some of the calibration problems people are having? A pair of close-up pictures might help.
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We might have hijacked this thread a bit, but Iāll keep going nevertheless.
Actually, the horizontal leg is not one-half the distance between between motors because cut 5 is over to the right side of the board. Regardless, itās just used to determine the vertical distance between the motors and the top of the workarea.
Understood, but do we need to perform vertical measurements for calibration purpose? It seems to me that the calibration routine must solve/account for this on the basis that, for triangular kinematics, two horizontal measurements at known āintendedā locations vertically separated by some arbitrary distance (i.e., doesnāt really matter how far apart as long as they arenāt on top of each other) can only be the result of one distance between motors/rotational radius combination (perhaps thereās two valid answers, one positive and one negativeā¦ dunnoā¦ just have seen that in math). This is the assumption Iāve made about the work. I have not dug into it.
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This is where it gets more complex. The distance between the cuts vertically does make a difference actually. When these cuts are made, there is a target vertical offset between the cuts, and then during the calibration algorithm this vertical distance is accounted for. Additionally, the algorithm uses the input data to actually determine the vertical error in the distance between the bottom and top cuts (look for the āSled Drift Compensationā value during calibration).
During the initial development of this new calibration algorithm, while testing we found that performing repeated calibrations in line would result in oscillating values. This ended up being due to the fact that the lower cuts (3 and 4) were two inches lower than expected, causing the routine to oscillate and give incorrect values.
The fact that the Maslow does not have independent axis control for X and Y planes, from a motor perspective, is why no discrete vertical measurement is made. The current algorithm works by trying to solve the physics of the machine, and using any two measurements allows this to work. The key we found during testing was to try to make these measurements in the places that would amplify the errors observed, and that seemed to be in the corners measuring horizontally. Additionally, we were fighting frame flex issues at the top center of the work area, so that was deliberately avoided.
Iām just getting my machine set up now, and plan to investigate the vertical error observed in the center of the work area. I see two possible solutions:
- Additional physics to be added to the kinematics (Iām wondering if some small amount of chain stretch is occurring at the top center, where chain tension is highest)
- Take more measurements and use an algorithm to minimize total error
I think solution 1 is preferable, as it makes the machine more accurate, but relies on the errors being linked to the physics of the machine and then compensated for. Solution 2 would just work to minimize total error, by deliberately introducing error in parts of the work area to minimize error in others.
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If there is only one vertical measurement made, then we cannot possibly calculate the distances properly. The reason for making two horizontal cuts on the centerline is that we need to measure the distance between these two cuts (or more properly between the same edge of the two cuts) so that we can combine this with the motor spacing and chain lengths to calculate the rotation radius properly. If we donāt have this, we cannot possibly figure everything out properly.
The vertical distance to the top of cut 5 (the only horizontal cut) is used in the calibration routines (surprised me since I entered my values in cm and not mm during calibration and it all seemed to go well). However, the vertical distance between the top set of horizontal cuts and the bottom set of horizontal cuts is never measured.
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the distance between the top and bottom sets of cuts at the edge isnāt needed.
but to be able to figure out the rotation radius, you need to have two cuts at
the center and know the distance between them.
If weāve lost that somewhere along the way, that could account for the problems
we are having with calibration.
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the measurements to the outer cuts is finding the effects of chain sag
but we need to do two central cuts so that we can solve the different triangles
to figure out the proper rotation radius. If we leave this out, and try to just
figure out the rotation radius from the effects on the outer cuts (where the
chain sag effect is maximized), we are not going to accurately figure out the
rotation radius.
In the earliest version, we did a cut in the top center area. This was moved
down to the lower center area because Bar was getting bad results due to his
frame flexing.
We cut wide to maximize the effect of chain sag.
we should cut high center and low center to maximize the effect of the rotation
radius
This is one additional measurement, but without it, you seriously reduce the
resulting accuracy.
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Moved this discussion to a new topic to avoid hijacking this one further.
https://forums.maslowcnc.com/t/triangular-calibration-review/2995
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Took a while to get back - the arms are 5 inches. I want to say Gero posted a video of what I am experiencing - with the sled on the left side (so tilted down to the right a little) the linkage on the left side indexes past a catching point for lack of a better way to describe, and folds towards the middle and will hang there without any intervention. It happens on both sides (extremes) To be fair - I have to re-check my mounting positions because at quick glance they appear to be off a little (distance -wise) This may be promoting the issue. I will report back when I have a chance.
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Thanks, there were two versions with 5 in arms, one where the arms wer 3/4" wide
and one where they were .6" wide, the version with .75" arms had larger holes
(.26 vs .25) and so is more likely to have this problem (any error in the
mounting location or ability for the bolts in the mounting location to flex will
make this far more likely)
Mine has the .750 wide arms. Am I correct in thinking that the holes should be 5" apart? Iām not very good at describing things, but I will try - looking at the pantograph, there are 6 bolts holding the assembly together. From left to right, they should be 5" apart vertically as well as horizontally. I believe that my middle set might be off a little (I put the kit together with tension on everything to minimize any excess play issues) My measuring/drilling may be the issue and I really havenāt looked this closely at it until now (shame on me) Iām confident that any play is not a big issue, I used locknuts and graphite during assembly so the whole thing moves very smoothly. (also polished the stainless) Has anyone else had this issue, and what other improvements have you made to your latest iteration? I could never have come up with this idea in the first place so I am very grateful to you and Maynard for providing them to the collective. Let me know if I can be of any assistance.
Tony Walker
āYou get hurt, hurt 'em back, You get killed, walk it off.ā
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