It is a circular arc. If the angle is 90 you get half of a circle. If the angle is less than 90 you get less than half of a circle, if it is less than 90 you get more than half of a circle. You can find proofs on the internet or verify it with 2D CAD.
Hi WFD
I did look at in 2d Cad, but I grew unsure, Iām not a mathematician. Monday Iāll be home where my computer is, and be able to check it again, and verify thoroughly
Thanks for the input
Arjen
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I updated the page to put a grid every 12" or 100mm (deciding that if the frame.x >300 itās going to be mm)
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I have updated the graphic tool. The āmaximum angle between armsā- curve is definitely a circular arc. The tool now has a movable sled with belts, that shows the 4 angles.
From What is the max and min angles between the arms? - #47 by arjenschoneveld I found which angle is meant by the minimum 130 degrees, but I donāt see where or how this is a limitation for the area that already has been limited by the maximum angle curve. Is there data for a specific situation?
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Is there a CAD model somewhere on github? Or here on the forums?
bar had also posted the file for the top/bottom anchors, I then measured the motor diameter and did some calculation myself in onshape Onshape
Depending on the frame size, you may be limited by either the maximum angle between adjacent anchors, or the minimum angle between opposite anchors, or by both.
thatās why in my tool I plot the minimum angle between opposite anchors first in green, then overlay that with the maximum angle limits in red, then put the workpiece on top of that.
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Hi Geert,
The circle of the sled is divided into 4 zones of 50 degrees where the arms can rotate freely, and alternating 4 dead zones, where the arms hit the uprights. If one arm hits an upright, the sled rotates, but if 2 arms hit an upright in the opposite direction, the sled canāt rotate. This double collision means the accuracy is off. For 2 opposite arms, that minimum angle is 2 dead zones and one free zone= 130 degrees. The real dead zones might differ somewhat from 40 degrees, and may be enhanced in the future, but right now I think they are the largest constraint on the size of the workarea. I would really like to see them in your framesize checker, with the nice graphics.
Greets,
Arjen
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The easiest way to check this for yourself in your drawing, is to place the sled on a corner of the workpiece, and measure the angles of the opposite belts. For a frame of 3050x2440, you will definitely find that the angle of 2 opposite belts is less than 130 degrees, which means double collision with uprights.
The minimum angle between two uprights is now visible in the tool.
The effect is more than expected. Itās a bit difficult to generate a rectangle for the remaining workspace, butā¦ 
, it might be possible later to upload the job (gcode, svg) and see if itās fits.
Thanks for the explanation @dlang and @arjenschoneveld
Geert Doornbos wrote:
The minimum angle between two uprights is now visible in the tool.
The effect is more than expected. Itās a bit difficult to generate a rectangle
for the remaining workspace, butā¦
to upload the job (gcode, svg) and see if itās fits.
that was our experience as well 
there are a lot of rectangles that can fit the curves, from tall and norrow to
wide and narrow.
Thatās why I havenāt been able to come up with a reasonable way to go the other
direction (from workpiece size to frame size). that again looks like it would be
a set of curves for the anchor points.
David Lang
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I uploaded the code to a github repo
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Is this a necessary calculation? The videos and all I can see seem to allow me to build a frame of 8 and 10 foot lumber then just slap my work piece in the middle and route away. Did I miss something?
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Thatās a good question,
We arenāt sure.
As bar has shown in his videos, he is getting useful results from a (just under)
8x10 frame, but he has not been checking for accuracy throughout the cutting
area.
When the arms hit the uprights so that the sled cannot rotate to keep the belts
and the arms inline, the actual distance to the anchor is going to be a little
shorter than the machine thinks it is. How much that hurts depends on what you
are doing
This shows that over most of the area, there arenāt any problems, but if you try cutting precisely in the corners and the far edges (within 4-5" of the sides) you may have errors creep in.
Right now, more people are having problems getting good calibration, which affects accuracy everywhere, so the main focus is on that (if the system doesnāt know where the anchors really are, itās not going to be accurate anywhere)
This didnāt start out expecting to find problems with the frame size that Bar and Roman have been testing, but rather to answer questions from people about what size frame they need for non-standard workpieces, or what size workpiece they can cut on a non-standard frame.
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by the way, red and white are not indicators of how bad it is. Green is the area where all the angles work, both red and white areas indicate questionable areas, just for different reasons.
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Thank you for the clarification. I get the implications now. Being thrifty (or cheap), I can see the value of seeing just how much you can accurately cut. Much of what I think I will be doing involves art pieces, but part of the fascination of CNC cutting is precision, so the more the better. Iām thankful you folks are out there able to do this heavy lifting.
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Johnnie Eskue wrote:
Much of what I think I will be doing involves art pieces, but part of the
fascination of CNC cutting is precision, so the more the better.
The Maslow has always been very precise (as in repeatable), where it has been
weak has been accuracy (cutting exactly where you expect it to). The maslow 4 is
much better than prior versions, and as calibration gets fixed up, it will be
even better.
David Lang
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in a recent post, that I canāt find anymore 
@bar asked @dlang ( and me I think) if it was possible to introduce a gradient into the frame size checker, so not a hard line where one side is accurate, and on the other side it is inaccurate. This got me thinking, because of course by passing the 130 degree arc in the checker (moving to a corner of the frame), the belts will not be a straight line any longer, and there will be a fault in the measurements of the machine. But how much? At first the angle between the arm and the belt will be small, but the further out to the corner, that angle will increase, and so will the distance between where the machine thinks it is and the place where it realy is.
So I drew up in CAD a standard frame of 3048x2440mm, and a workpiece of 2440x1220mm. On the diagonal of the workpiece I constructed the 130 degree point from the framesize checker, I called that point zero. Then I constructed other points on that diagonal between zero and the corner of the workpiece, with incremental increases of the angle between arm and belt. That angle was about 9.5 degrees by the time I reached the corner of the workpiece. On every incremental station I constructed the true position and the position where the machine would think it was, measured the distance between the two and put the results in this table:
on the vertical axis is the difference in position of where the machine thinks it is, and where it actually is, in mm
on the horizontal axis are the distances from point zero outward, also in mm.
So here we can see the āgradientā: 160mm out of the 130 degree arc of the checker, the deviation is about half a mm (that would not concern me)
400mm out of the 130 degree arc of the checker, near the corner of the workpiece, the deviation is more than 2.5mm (which would concern me 
)
So in this example, I think it is a good idea to keep the calibration area inside the 130 degree arcs of the checker ( and even higher, if you donāt want the effect of rotation of the sled), but for actually routing, you could easily extend that area to the 122 degree arcs (you can adjust these in the checker, and observe the larger area) and be only half a mil off.
In Theory that is
Arjen
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Nice work!
Hereās the comment you were referencing:
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Could you use this to compensate the error and increase precision in the more problematic areas?
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I couldnāt, but maybe a genius coder could?
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