Homemade Articulated Mechanical Thumb

[63DH8]

Ahhh!!! I think I see what you're saying. You have a specific point where you want the thumb and bucket to grab. You're not worried about rotating the thumb and bucket together to a different point in the arc because that'll be accomplished by the stick (arm).

With this kind of articulated thumb, curling the bkt will always further
close the thumb. Conversely, uncurling the bkt will open the thumb. In
order to roll an object between the 2, as you describe, the thumb would
have to move in the same direction as the bkt. It never will. This is
confirmed with my wooden mockup AND using a thumb pivot point just
aft of the bkt pivot point. If you have an independent hydraulic
thumb, you can do what you describe.

Rob:
As for drilling steel at low speed, good luck finding any non-industrial
drill press that spins slower than 180RPM. I would love to see 100 RPM,
but have found that you need a geared head or other industrial
machine costing over $1K. Seems that vendors of cheap (under $1K)
presses think their customers are only drilling wood. I know you have a
nice milling machine, which does a great job drilling, but I am still
resisting that kind of purchase.

 

[john_bud]

Rob,

Is the issue with the thumb pivot location that when you start to grab an object, the thumb when closing will move both rotationally about the pivot and laterally? The lateral motion caused by the pivot points being different?

I guess I can see that may cause the object in the bucket to roll. Sort of like grabbing something with your finger and thumb and pulling your thumb backward at the same time.

By the way, your work is awsome! Even if I had the skills in machining -- I would lack your vision. Thanks for sharing!

jb

 

[dillo99]

Rob that looks awesome....can't wait to see it in action. I can't imagine all the calculations and head scratching you must have done to figure out the engineering for that thumb - hope it works perfectly!!!!

 

[3RRL]

Thanks guys,
I sure appreciate the support.
At this point I am not worried about the rolling action between the thumb and the bucket. My hopes are when the object is grabbed, it will be "squeezed" between them without much chance to do anything. so I have an update.

Got back from the property this weekend and got my thumb mounted to the backhoe. Man, it sure looks nice even if I do say so myself. I took several photos and a couple of videos to show you guys. Unfortunately, my tractor isn't running right now so I could not try it out on the real stuff. But the whole concept was there and the Mechanical Thumb and bucket moved together as designed.

First thing I did was decide where to mount the pivot on the kidney shaped actuating bar, which is attached to the hydraulic cylinder that moves the bucket. In my drawings, I had the distance at 4" from the pivot pin. so I scribed a radius and found center on each side. Then center punched and had to pick a pilot drill from my new Harbor Freight 119 piece drill set. ($39 bucks)



Next had to drill the 17/32" tap drill through the 3/4" thick pieces. That was quite a job for an old dude like me.





Then I lined up and clamped the pivot blocks to the bars and tapped them 5/8"-11 thread for the Ø3/4 shoulder bolts.





And here is the first one bolted on the kidney shaped arm. Later on, I will weld them on solid.

 

[3RRL]

With the first point established, I could hang the articulated assembly from it and start figuring out where to put the two other points. Actually three if you count the rest position pivot. I used my CAD drawing as a guide, but would ultimately make my choice after checking all positions, including the resting position when not in use.



Here you can see the multiple clamps I had to use for positioning.
That was a major drag holding up the pieces and trying to clamp them at the same time.





Loretta helped the entire time or I would never have been able to mount the darn thing. We ended up moving the pivot locations slightly from the CAD drawing to accommodate all positions and the desired acceleration for the jaw to close. We also checked the "rest" position and even made sure the tips and bucket teeth matched up perfectly.

 

[3RRL]

Well, you can imagine the clamping nightmare because not only did we have to get all the pivots clamped to the dipper stick, we also had to be able to move the jaw AND bucket to be certain we had chosen the right pivot locations. So the clamps had to be out of the way of movement. You can see above that we used a 24" Crescent wrench to clamp to so we could hold the jaw pivots to the bottom of the dipper stick! I tapped them all around and measured best I could to make sure they were all aligned. If it all works out, then I will start welding them right where they're clamped.

Here is a video of the moving thumb while it is still clamped. I released the pressure on the hydraulics so I could move the bucket. I still had the chain and come-a-long on the bucket in case it crashed. I also was happy to see there would be little rolling action, if any at all. With the two moving together, it would "grasp" or clamp and object very quickly.



It turned out pretty good so the next step is to weld all the attachments ... YIKES.

 

[3RRL]

Satisfied with the positions, I looked for some 6011 rod to use with my Lincoln 225 arc welder varying the amp setting between 180 and 200. I run it off a Generac Wheelhouse 5550 generator.



This is my workshop, so it is not the best conditions to screw something up. It's not like I'm working in my garage shop or anything like that. It was another cold day with rain in the forecast.



I started my first weld. I could not move the backhoe to lay it down for every weld, knowing I would challenge my OP welding ability for this project.



Here's the first pivot welded on. It didn't turn out too bad going all the way around it. I was happy.

 

[3RRL]

Some more overhead welding and the results. Overall, I was very pleased with how all the welds came out.





Here a shot of the whole thing welded on from the back of it.

 

[3RRL]

Then it was time to take everything back off and re-paint. This, of course was done by Loretta. She cleaned and painted the dipper stick and the linkage too.
Notice in this picture the added pivot for the "rest" position.



Here's the thumb and linkage drying.



And Loretta and Coco taking a break from all their work.

 

[3RRL]

In choosing the pivot locations, making sure the thumb was out of the way when not in use was a primary concern (among everything else) So here are some shots of the thumb and linkage folded out of the way in it's "rest" position.



I remove the shoulder bolt from the kidney shaped arm and unbolt the link. Then use the same bolt to lock it in it's "rest" position neatly tucked.



It looks pretty cool like that, huh?
Here's a close-up from underneath.