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Tensioning system to improve performance in bottom corners



Shallow cut was second


5 degrees 20 lb sled
Sled did come off of the project surface on plunge


since the sled came off the surface in the plunge, 5 degrees is too close to

it’s really hard to see what’s happening with the shallow cut, but you are still
having problems dropping vertically for the deep cut.

So on this wood, you just cannot use this high of a feed rate in this area. Do
you have any smoother wood you can test on the the OSB? this doesn’t need to be
a big cut, 3-4 inches of L is good enough.

or you can lower the feed rate, but that isn’t what you want to do.

David Lang

P.S. these are good examples of failed cuts that can be annotated and added to
the wiki.


All 20 lb sled
10 degrees smooth wood both ccw deep cw shallow

10 degrees smooth wood .125 deep ccw

10 degrees smooth wood .125 cw


Would tensionning with something like cord reels like dog leechs with reels or extension cord reels, whatever that retracts, like cancelling the locking machanism in them


it looks like there is just a tiny bit of rounding in the bottom left corner of
the cw cut.

on the smoother wood, the ccw cuts are better than they were before, but still
not good.

how smooth/slick is the bottom of your sled? can you try waxing it to see if
that improves this? is the grain on the bottom of the sled parallel to the grain
on the workpiece? (it was pointed out not too long ago that when they are
parallel, they will catch more than when they are perpendicular)

the ccw problem (downward vertical cut) is not going to be improved by a longer
top beam, this is just about the best case, where 90% of the sled’s weight is on
the near side chain that’s being fed out. The fact that the sled is not moving
down under these conditions is really bad. The fix for this is either a heavier
sled (which causes problems in the top center) or less friction between the sled
and the workpiece.

the cw problem (rounding in the corner because the sled isn’t quite moving fast
enough to the left to be in the right place before the near side chain starts
retracting to pull it up) is where a longer top beam would help.

Thanks for doing this testing.

David Lang


The problem is that those will apply the most force when the sled is the
furthest away from the reel, and the least force when the sled is closest to the
reel. We’ve already determined that with constant force (weights), the angle of
the tensioners makes it so that tensioning to the bottom corners makes the
problems worse, not better. Anything that makes the tension larger with longer
distance will just compound the problem

you would need to have something that pulls harder the closer the sled is to the
reel. The engineer in me can think of some very complicated arrangements that
could do that (a spindle that the line winds on to that gets smaller as more is
fed out for example), but I think it would really end up being better to add
some electronics if you are trying to do this.


Thanks for clarifying!


Thanks for the insight @dlang. I went back to 15 degrees to compare and it almost looks like 10 degrees had better results. Maybe I will try to add some weight and slicken the sled more. I did sand it down quite a bit already. The grain of the sled is parallel to the project surface.
What type of tests can o do at the top that I can compare to after adding weight?
All 20 lb sled
15 degrees .125 deep 30 ipm ccw

15 degrees .125 deep 30 ipm cw

15 degrees .125 deep 30 ipm ccw sled sanded and waxed

15 degrees .125 deep 30 ipm cw sled sanded and waxed


I would expect that the closer to vertical you are, the better the results will be (less of the weight is being applied to the workpiece, so friction is less.

to test near the top, can you get to the top center of the workpiece? or do the motors stall.

If you do a cut from the top center to a top corner, is it straight, or does it curve down and then hook up at the end?

If the motors stall trying to get to the top center, or a cut to the corner hooks, the motors are not able to keep up with the weight of the sled


15 degree .125 deep 30 Ipm 31lb sled ccw

15 degree .125 deep 30 ipm 31lb sled cw


Those aren looking pretty good, yes?


They do look better but now they look like the image below. Just less exaggerated


Have you recalibrated with the bigger weights? A difference in chain sag could cause the skewing.


The design of the first post on this thread tightens in the corners and loosens everywhere else.


Ah, no I did not recalibrate yet. Thanks, I will try that tomorrow.


I would be curious to see what your square looks like with the tensioning system. A video of it running would be neat too.


these tests with different weights and angles are not going to be accurate, we are just looking for the problems we have been seeing.

once you have a config that works to create accurate L’s, then it’s time to do a calibration

as you tilt closer to vertical, more of the weight of the sled is on the chains, and so chain sag will be less.

if you add weight, chain sag will be less

if you remove weight chain sag will be more

but until things can move smoothly without the sled sticking, it’s not worth the time and wood to do the calibration.


I recalibrated with the heavier weights but my square is now 3 mm lower on the top left then the top right


what were the results of the calibration (normal and with the higher weight)?
rotation radius should have remained the same, but chain sag compensation should have changed significantly