McMaster rates their #25 chain at 85 pounds working load.
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Sounds about right for 10x derating, working load to breaking strength
Good policy not to lose sight of the difference, though.
ahh, I think I’ve been mixing max tensile load with max working load.
the pdf I posted shows max working load @154 pounds, while max tensile strength
@1058 pounds.
That’s more like it. I suppose McMaster derates their chain further as they don’t manufacture it, while OCM actually make the chain they list.
90%+ power to the motors does not always translate to an equal increase in chain tension.
Yep. That’s a good example of the inertial loads I am talking about.
Yes, in my opinion, that would be a terrible idea. I never suggested making a weak point that would bend and create errors - I agree that should be avoided. Machines like lathes have something called a shear pin which, in a bad situation, is designed to fail before more expensive parts fail. The failure prevents the machine from working but does not destroy expensive parts. I was only saying that such an idea could be implemented with a Maslow to protect the expensive parts of the machine.
That’s pretty awesome! I had no idea that chain was that strong! It also outlines my point that in a bad situation something else will fail first. It would suck if the plastic worm gear (here) in the motor stripped, it would suck less if an intentionally sacrificial (and cheaper) link broke.
Again, I am not arguing against over-building. I’m trying to clarify for those who care that the system is not under a constant 66 pounds of tensional force. At 66 pounds of tension the motors should theoretically stall.
Today I built and tested the 3 bar top mount. Thanks to this
By @pillageTHENburn.
Need’s fine tune but cut a line 4 inch down from top and it was nice and straight. I am pleased with the results and how it stayed on track in the center top.
I will get the video uploaded in a bit.
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That’s a great proof of concept. Did you find that the arms bound up near the edges of the workarea?
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Nice! I’d been hoping someone would test that one! It looks good!
I like your chain connection solution. I have been experimenting with a similar idea but instead of the L bracket imagine a “U” bracket (same bracket with one extra 90° bend). It should distribute the force nicely on the top and bottom of the linkage so it has less friction (friction caused by torsion) but it would force the chain to attach at the centerline of the bars (in the z axis) which means you’d need to space the entire linkage off the sled… With the way you have it the spacing of the chain attachment from the sled can be accomplished by moving the chain along the Z axis on the brackets, that’s quite handy. But (at least in my head) it seems like this could cause binding at the bracket. Have you noticed that at all?
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I still have to run it to the lower corners, but no binding yet. Using the brackets from original design, I am able to adjust the chains for center of gravity. I leave them snug but loose enough to pivot.
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Here is the top line test video.
I cut this with the new setup. Worked great.
There was flexing on the left side due to the wood offset. I installed a spacer to keep the arm level.
I am going to refine with maybe some anti friction thin “something” in between the joints and where there is wood wood on wood. Maybe spray with poly??
I used 3/4 ply at 1.5 inch thick. 3/4 put it tall enough out to keep the chains inline with the motor sprocket.
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Proof is always in the pudding 
Very nice!
HDPE from a cut up milk jug, if they use those where you are? Maybe a cut up cheap plastic cutting board for bushings?
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yea, I was thinking of a washer made of that material. Excellent!
Some cheap thrust bearings between the rubbing wood pieces would work.
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Umhw tape is much slicker than hdpe
I have bolted metal pieces coated in umhw together and they always slide no matter how hard i tighten
the stuff is amazing
Or umhw washers
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These make great cheap washers. UMHW tape is fantastic but expensive on a roll. Bolt specialty stores will have UMHW washers for a “low cost & kitable” solution.
You could use a fencing staple on either side of the chain and still pin the chain in place. I just took a finishing nail and nipped off the head, bent it over and voila. I also use this on the arms to secure the end of the chain.
I like the starship approach but am I correct in thinking that the whole linkage “should” stay perfectly still if it was on a frictionless surface? This whole endeavor is just to keep the accuracy when the sled rocks/pivots/drag right?
I put a piece of broken laminate on the back of my sled and then polished it was car polish and steel wool on a drill. Same technique I used on my telescope base.
Not really, the linkages will be at different angles depending on where the sled is on the material.
Imagine drawing a triangle between both motors and the router bit. The side between the motors never changes length but the other two sides of the triangle are constantly changing length, which means the angles of all three corners are constantly changing. We really only care about the corner of the triangle with the router on it (since the sprockets handle the other two). The linkages dynamically (and constantly) adjust to match the angle of the triangle at the router; keeping the chains pointing exactly at the router bit.
Perhaps I misunderstood what you were asking. Let me know if I’m way off base.