Thank you for the consideration of my first foray into Maslow-ing, and your response.
I think the Maslow community hasn’t fully appreciated the benefits of spectra, yet. At 1/20th the weight of #25 chain, and at Maslow loads substantially less stretchy, I think it deserves a much closer look. (My drawing allows for more than the ~107" of travel it takes to get from near corner to far corner -perhaps you missed it but there’s a 1:2 reverse purchase where the chain attaches to the spectra tail.)
My proposition is that the ratio of line weight (chain weight in the classic Maslow) to line tension or sled weight is a big portion of the lower corner inconsistencies observed. I don’t think software is yet addressing this reliably from the reports I’ve read so far, and since dynamic forces are involved due to random user-selected paths, I’m not sure that software can fully anticipate the changing tensions that are seen (I fully admit I could be underestimating the software’s brain-power?).
The rails weigh less than bricks, so I think that’s a wash.
Friction? Yeah, I admit it’ll take some tuning to get the right balance of tension versus free-rolling. Concave-faced skate wheels ought to work.
Dust? Yeah, but like a bandsaw’s lower wheel, the remedy is to mount a toothbrush’s bristles to always be brushing the pipe surface as the sled travels.
Sled rotation has been the driving force in changing from the fixed attachments to the pantographs and rings. If you’ve done a lot of hand routing, you know that sled rotation is also a dynamic factor as the bit encounters different loads. I know from other work that tool rigidity and stiffness is an important factor in achieving precision, and this is my attempt to improve this attribute over the pantographs or the rings. That the bars (the same pipes used to make bar-clamps, btw -easy to find at the big box hardware store) also eliminate the need for a margin panel and make true 3d work or surface flattening operations possible is a bonus.
The kinematics math -a simple fixed-length polar offset from any given point on the panel- must be loads simpler than the old fixed attachment compensations.
I browsed Corexy and didn’t see applications for 4x8 work surfaces. I’m not familiar with their offerings beyond what they show, which seem to be flimsy table-top contraptions. Is 4x8 available?
I don’t know about friction. Right now we’ve got 254 sq inches of sled(yes i know minus cutout in center and rounded edge) to drag across the surface with this pipe setup he has shown you get probably less than 1 sq inch of friction area. That’ll probably negate someof the increased weight ( if done right not necessarily sure it would be substantially heavier). The downside is it will be much harder to build the frame since the perimeter pipes will have to be exactly in plane, if not there goes the accuracy of the z axis which now becomes alot more interesting since the machine becomes a true 3 axis cutter
for another project and they’re fairly cheap and durable. Just drill a oversize hole through the top beam bolt 1 bearing on each side of the beam as shown and insert a 1" pipe or solid piece of metal and you’ve got a freely rotating pivot to hold that pipe assembly.
As for the frame instead of using pipes around the perimeter use angle iron with the corner up so the pipes can slide across it. Probably stay a little straighter than pipes and be a little easier to mount.
you misunderstood this a bit. The problem wasn’t rotation as much as the fact
that since the chains didn’t point at the bit, different tension on the chains
resulted in different position of the bit.
the triangulation approach eliminated this as a factor, and also drastically
simplified the math.
Even if you could eliminate the rotation, you still wouldn’t want to move back
to the fixed brackets.
I pictured 1x3 pieces of hardwood for the perimeter frame (pick some straight sticks of poplar at HD…). Their 3/4” edges could be skinned with formica for better sliding.
Overall, this design works best in a fixed, wall-mounted application, not so much with a flexible, roll around frame. You’d want to take the time to carefully install the frame in-plane, and as you say, adjust the pivot height to match the frame’s plane.
The “sled” rides higher than typical, and the rails mean it’s not entirely necessary to balance the assembly on the lines. They mount (in an ideal world, with tweaked kinematics) to a single point (hole, pair of shackles on a common pin, whatever) on the sled’s base.
Weight effects of pipe will be ~half their weight in 2/3 - 3/4 of the panel, and worst in the top center. But that’s where the lines have the most lateral authority due to their higher tension in that zone.
Regarding the ‘spectra’ product I did a google and found this little nugget:
“Under tensile load, UHMWPE (spectra) will deform continually as long as the stress is present [the weight of the sled come to mind] —an effect called creep; Wikipedia”
So basically chains still win.
With an understanding that somewhat surpasses your google source, the material is used to hold up tuned sailboat masts on 100’ boats that race around the world.
Creep is a factor when the static load exceeds at least 10% of break, and with some constructions, as much as 20% of break strength.
1/8” lines, with break strengths around 2000 pounds, will have a creep of almost exactly 0 when holding up a 30# sled. Recalibrate once a year for peace of mind, if you must.
(Note that a classic misunderstanding is to equate construction stretch with creep. Basically, after splicing your loops in the two ends and installing for the first time, load the lines to something well more than they’ll see in use- like a 100# sled at top center, for a half hour, to set the splices.)
@NemoChad so are you thinking along the lines of a tensioned spool? So looks like the top mount (except on the bottom of the frame) with the stretchy cord connected to the ‘spectra’, a spool attached to the current motors and wound for sufficient travel, then the spectra travels vertically to a top pully (where the motors are now) then back down to the ring mount?
The 1:2 increase (“reverse purchase”) where the chain attaches to the soft line means that for 1” of chain travel, I get 2” of sled movement. I show a 2:1 chain gear reduction at the motor, to keep it in its happiest load range.
No bungies, no spools. Just a deadweight to provide a constant tension, keeping the chain in contact with the sprocket and somewhat lightening the motor load.
The problem with the trapezoidal kinematics was the uncontrollable rotations of the sled that then moved the cutter to places the software didn’t intend and couldn’t prevent. The rings and parallel links are ways of addressing the problem so that sled rotation doesn’t displace the cutter (much), but I suggest the observed results still have room for improvement.
The Maslow has gone from a complicated software solution for a mechanical weakness, to a mechanical solution with basically no software calculation needed to address the sled’s tendency to rotate. I guess I’m proposing a balance of both ideas: better control of the sled due to removal of variability from unintended rotation, coupled with a simple polar calculation to account for the sled’s swing.
I also freely admit that I’m more into the mechanics than the software, and “my solution” depends as much on getting a kinematics change written by guys like you as it does on a working mechanical concept. Until then, I’m sure I’ll be using the Maslow ring like everybody else.
The 2:1 reverse purchase in the chain solves the problem of marrying the cable to the chain and also of storing enough chain for full movement - very nice. Too, the cable can change direction at the peak such that the chain tackle is fully vertical instead of slanted to match the workarea, a twist not possible with a pure chain lashup. The force on the motor will be doubled, though, in order to achieve the present force on the sled. The gearing added at the motor is to address this, yes?
