putting the chain between two arms doesn’t help a lot, you have to have spacers
to keep the chain clear of the arms, and you need a small pin (and therefor a
small hole in the arms) to attach the chain to.
the large external U shapes to attach the arms help, but you still have to be
careful about the angles, the pin on the middle joint will stick out from the
arms, so you have to make sure you clear it.
Looks good…
That spacer block probably needs to be wider though, to better
distribute the force onto the sled without yanking out it’s
attachment screws, or twisting and becoming no longer perpendicular
to the sled’s surface.
ImpetuousWombat[1]
September 10
I tweaked my model a bit based on the discussion and
@pillageTHENburn[2]’s comments about friction. The chain mounting
would be simpler in theory, as we’ve got a place for the chain to be
suspended between the two arms. What do you guys think?>
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In Reply To
pillageTHENburn[4]
September 10
Did you solve the non-parallel linkage problem? Also the spacers you
have between some of your joints (which I assume are to put your chain
attachment points on the same plane as each other) probably add a bit
of friction to the equation. In my tests I found that using a
“sandwich” type method that …>
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remember, there is not a lot of force between that block and the sled, the force
is almost all between the chains, and so it only gets to the top of the block.
realistically, you have ~10 pounds (the bricks) @ about 9 inches, so ~8
ft-pounds of torque (plus a little bit from the friction of the sled moving at
full speed) acting on the spacer/sled block.
compared to ~130 pounds of force that the chains can apply, it just isn’t much.
David Lang
Wow! This Linkage Design is looking amazing!! @pillageTHENburn you’ve really created an outstanding solution here by laser cutting the parts.
I was just looking though this and realized that there might be a way to accomplish the same thing without gluing anything. Maybe it would be cleaner/easier/quicker for assembly purposes? It would require minimal modification to your existing design and require only 2 unique parts to create the linkage. What do you think of this?
It uses 9 of the bars and 6 of the little round spacers. One minor concern I have is the tendency for the unglued bars to drift back and forth along the nylon bushings (not shown) and potentially add friction to the joints when rubbing against the neighboring washers (not shown), but I think that’s probably pretty minimal.
My concern with this no-glue concept is strength. I know you said yours feels plenty strong, but this proposed concept uses 3 “single” linkage members. Maybe the plywood thickness could increase from .25" to .375" or a different material would help (i.e. some smooth ABS). Here’s what it looks like with .375" thick plywood. Might be overkill if the chains are truly in the same plane as the linkage.
On another note, do you have a source for those u-brackets that allow for a pivoting chain attachment? I’m not seeing many with a thru hole directly in the center of the u’s base where the chain comes through and the cotter pin is inserted.
I’m planning to cut some of these on a traditional CNC router and see how it goes. I’ll report back.
Thank you for the compliments @JaceD! Yes I had considered doing it without glue (very similar to what you’ve shown) but in the end I decided that glueing would do three things:
First it would add rigidity to all parts.
Second it would ensure a more solid (and “longer”) hole for the joints. This fear is a little unfounded but I thought it might be possible that an axle could pivot slightly and the layers of the arms could get misaligned ever so slightly. Again, an unfounded fear but I figured glueing it would solve it. (Trying to minimize slop)
And Third I was a little concerned about a chain attachment point in the center of a single 1/4" layer of plywood. I didn’t want it to break and figured three glued layers there would make it plenty strong.
Concern 1 and 3 can probably be solved by your idea of thicker wood. Or of course a different material (metal), but that adds cost.
As far as chain brackets go I have found a few brackets but like you said most of them have an off center hole on the backside. I actually don’t think that would be a problem because the hole is still on the same plane as the linkage so the bracket would just live in a permanent tilt. It shouldn’t affect accuracy though. Anyway, my two solutions are to custom make the brackets (takes me time and might cost more) or laser cut them! So I opted for laser cut brackets. Basically adding another short linkage for the chain to attach to. I realize I’m not explaining it well, I’ll post pictures once I’ve cut and tested it.
I also have an improved top-mount linkage idea that I will try to build soon as well as a totally revised version of the 45° (sort of) that wouldn’t have any crossed linkage and stays much higher and out of the way! (It should have the vacuum and z-drive clearance of the top-mount with all the benefits of easy chain clearance and only tension forces on the “horizontal” bars of the 45°! I’m pretty excited about it, I hope it works!)
Thanks again for the input and great ideas!
-Logan
I knew I should have paid more attention in the Mechanical Engineering class on Static and Dynamic forces. Spent a whole semester calculating this kind of thing. But hey, that was 35+ years ago.
Need to find a newer MechEngr student to handle this for me
Just curious… is anyone still pursuing the ring method for triangular kinematics? The flat ring idea that @krkeegan was looking into seemed like an easy one to implement, and is still one that I am planning to try (probably using aluminum plate and v-groove bearings). Just wondering if anyone has accumulated any experience with the ring method.
I purchased the poly-carbonate and some bearings that might have worked with it, but I may re-purpose the polycarb and use it to laser cut arms for my linkages. I am no engineer, but I like the linkages better, I never see any sudden rotations caused by friction that I saw in the ring design and my bearing surfaces are screw threads and raw wood.
I’m still pursuing the ring method. I’ve put 10 or 15 hrs on the prototype and it’s working great 
@krkeegan @bar Thanks to both of you for the updates. I think I am still leaning toward the ring. Seems like figuring out a decent bearing for the ring and a low friction contact surface on the sled should mitigate sudden rotations.
I look forward to adding my experience with the design once my kit arrive in October (fingers crossed)
What are you using for the sheaves?
I bought a couple of the Sailrite rings, they are accurately circular, but slightly different diameters.
I picked up a couple stand alone ones from West Marine that looked a little like this:
Although the bearings were a little less exposed, they are not sealed bearings which probably means they won’t last too long.
That’s a bummer about the SailRite rings not all being the same size. I’m working on getting a kit of parts together so we can have parts which are consistent for everyone, but that’s still a number of weeks away.
I’m working with a local metal supplier to get some sample metal arms cut for a
top pantograph design. They have a minimum $150 order for laser cutting like
this, so we’ll see how many I end up cutting 
David Lang
Put me down for a set David, please! Heck, I’ll paypal you now if it’ll help cover the initial order.
same here, happy to cover a set.
Same happy to cover a set.
still waiting for the estimate (they’ve been short handed this week due to a
trade show), but it looks like I will get 5 sets for the ~$150 cut fee, not
counting materials. They recommended using stainless steel rather than aluminum
when I mentioned the 130 pounds of force being applied to these, so we’ll see
what the material cost adds to this…
I’m also having them give me a quote for cutting 10 sets, which will be a
cheaper cutting fee per set.
So, should the 5th set go to Logan or to Bar? I like having Bar continuing to
test the ring approach, and I don’t want to have nobody testing the ring
approach, but I’d also like to have end up with someone test both approaches and
be able to compare the two.
I would love to do a side by side comparison and I’m more than happy to pay for a set!
I know @pillageTHENburn is working on putting together a kit for everyone also, and I’m working to get him a machine (we’re flat out of motor mounting brackets) so he can test his designs too
I’d like to buy a set. Maybe we can get 10 sets committed.
I will take a set so that should add to the 10 set committed.
