Homemade Articulated Mechanical Thumb

[3RRL]

I've been wanting to make a "Thumb" for my backhoe for quite some time now. Among other things, I thought it would come in especially handy for placing rip rap rocks in my gutters just before the culverts. Many of the culverts are dug down beside the road and I could reach in there to place the rocks pretty easy with the backhoe. With that in mind, I started a small collection of different thumbs guys have made or bought for their backhoes.

I got a lot of pictures but here are a few I "borrowed" from other threads. These are all fixed thumbs, and some can be adjusted to different positions.

 

[3RRL]

Here are a couple of shots of my Nardi Italian made backhoe. It is pretty beefy and is the 7-½' model. I've done quite a lot of digging with it already and some rock moving too.





Since I installed the bucket grapples for the fel, I don't need to pick up giant rocks with the backhoe, but I'm looking for something a little more nimble and I came across this thumb that moves back and forth without using a hydraulic cylinder.

 

[3RRL]

I thought that would be pretty cool to have on my backhoe too. So I made up some sketches of how I would have to make mine work that way. Fortunately, I some pin locations and some other dimensions written down so I could try to figure it out, but as it turns out, I'm going to need several more measurements that I didn't think of. Anyway, here's an idea I came up with and I think it will work once I get all the missing dimensions.

 

[3RRL]

Realizing that the pivot point locations will be critical to get this idea to work, I started to lay it out on my CAD system. At this point, I had to guesstimate some of the shape and width of the backhoe components and these will have to be confirmed. But in any case, the concept seemed like I can get it to work on my backhoe.



Using the concept, I rotated the linkage to different positions to see how they affect each other. I made several different attachment points and varied the connections on the articulating links to see what effect they would have. So I'm planning to have a couple of additional connection pivot holes in each link which results in the jaws closing slower or faster, opening farther or closer depending on the position of the bucket. I want to be able to pick stuff up when the dipper stick and boom are stretched out but also when they are folded, closer to the tractor. Having a couple of different pivot points will let me accomplish this.

Here is another CAD picture showing the bucket as it rotates in and how the jaw starts to meet it. Again, I can make the jaw meet it very soon or delay the jaw to allow more bucket curl.



I would like to use the cam method, but that's a lot easier to figure out when the bucket pivot and thumb pivot are on the same pin. Mine will not be and I have to work with what I got. In any case, the articulating linkage seems to be fine to move the jaws. It also allows several jaw positions.

 

[3RRL]

I'm leaving for the property tomorrow to do a bunch of work up there, but also to get the other measurements I need to verify ... or change the drawing concept. In the mean time, the jaws are not going to change nor the articulating linkage. Only the connections to the backhoe dipper stick may have to change and of course the width of the pivot hole brackets that will be welded to the backhoe. I've chosen the jaws to be 12" wide, so the thickness of the linkage bars and hardened washer will require a certain width when it's all connected to the backhoe dipper stick.

Anyway, I started machining the jaws by drilling out the shape.



I dialed off center to center plus .005" to leave a super thin "web" between the holes so the pices would not fall off on the last hole and catch my drill and bust it. Then I took a chisel and tapped the webs free. The remnant fell off and I had part of my shape.

 

[3RRL]

Then I stuck in a 1" corncob mill (roughing endmill) and started to cut the handle and radius on the piece.



After that, I started to cut the radius on the back side of the tips of the jaws. Notice also that I milled off all the sharp ends on the teeth and left a 3/16" flat on them.



After milling, I went over to the dics sander and blended the cuts in. This way I was able to smooth out the surface and generate somewhat of a real radius on the handle and the curve on the back of the jaws. My blueprint was the black marker line.

 

[3RRL]

After getting the jaws the way I wanted, I started on the linkage. There are 3 linkage bars on each side and I sawed and milled them to length, then drilled them for the all the pivot holes. I will be using ؾ" shoulder bolts for all the pivot pins. They have a 5/8-11 thread on the end and socket heads on the other.



So after all that, this is what I got so far.



Oh, I also started to turn up the pivot bases that will be welded to the backhoe, but have not finished them. I'll need to get the width of the dipper stick and the width of the actuating link on the backhoe so I can make the shoulder bolts bottom out without pinching the linkage. I'm leaving at 4:00 am tomorrow so I'll sign off for now. Let me know what you think so far...

 

[BobRip]

After getting the jaws the way I wanted, I started on the linkage. There are 3 linkage bars on each side and I sawed and milled them to length, then drilled them for the all the pivot holes. I will be using ؾ" shoulder bolts for all the pivot pins. They have a 5/8-11 thread on the end and socket heads on the other.



So after all that, this is what I got so far.



Oh, I also started to turn up the pivot bases that will be welded to the backhoe, but have not finished them. I'll need to get the width of the dipper stick and the width of the actuating link on the backhoe so I can make the shoulder bolts bottom out without pinching the linkage. I'm leaving at 4:00 am tomorrow so I'll sign off for now. Let me know what you think so far...

I think I would like to see the pictures. Did they not get attached?

 

[63DH8]

I used to work for an outfit that designed and built bucket thumbs. Unless you use a common piviot point, whatever you are carrying will "roll" in the thumb/bucket. Use your CAD to roll the bucket and thumb. You'll see how the thumb will be closer and further away as you roll the bucket. Our boss tried to get us designers to make a thumb that was prebuilt that would be welded onto the stick (excavator arm), but it wouldn't work due to the rolling effect. That made grabbing then rolling out the bucket impossible to do without dropping what was carried.

What we had to do was bore/ream out the piviot bushing and use a larger diameter hardened pin that shared the duty of bucket and thumb.

 

[EddieWalker]

Rob,

Sounds like an interesting prject!!!

I've thought about the Amulet system a few times myself, but the standard thumbs are so much easier to make, and I wonder which one is really better.

I'll be following your progress and I'm especialy interested in your observations after you have used it awhile.

Thank you,
Eddie

 

[3RRL]

I think I would like to see the pictures. Did they not get attached?

Bob,
They are attached as clickable thumbnails...as a matter of fact, your "quote" reply had some on them.
Is anyone else not seeing them?

63DHB,
I have "curled" the bucket on the CAD to see how the thumb moves in relationship to the bucket. I'm wondering why it would not grab if they are approaching each other?

Eddie,
No doubt the stationary thumbs are easier to make. I just wanted to try out this mechanical thumb to see if it would work. If it fails, I can always turn it into a stationary one.

Thanks for the replies guys. I'll post more when I can.

 

[3RRL]

I'm going to fasten the 2 jaws together with ¼ x 4" plates to create rigidity between them. There is a "storage" place for the jaws. I won't use clips though, because I have to take out the shoulder bolts that connect the linkage to the backhoe kidney actuator. So I'll use the same shoulder bolts to screw into a spot on the dipper stick that will hold it all back and stored...and not lose the bolts.

I have recessed the shoulder bolts into the steel so not only the thread is in, but about ½" of shoulder is buried in the steel too, making it that much harder to shear off or bend. I did think about hydraulics but that would mean another valve on the backhoe and the rear remotes on my tractor are too much out of the way. I also thought about a cam system but most of those require the jaws to have the same pivot as the bucket. Mine will not, so the articulated linkage was the next best thing.

 

[3RRL]

This is how cold it got and it was already 9:00 AM. It's still only 26° and the rain water in my bucket was frozen solid. So much for warm Winters in Sunny southern California.



Wes was skating in the bucket.



It was freezing (actually below freezing and never got over 36° degrees all day. I even tried to warm up the tractor with a propane heater.



I got the dimensions I needed to finish up the pivot anchors that will be welded to the backhoe. As a matter of fact, I can weld up the jaws too, now that I've confirmed the over all width. I can paint them too. Then when I go back up in a couple weeks, I only have to mount everything to the backhoe after I clamp on the pivots to be sure they will give me the range of motion.

 

[3RRL]

Well, when we got home the heater in the house was broken. I still got a chance to do some more work on the thumb today. Loretta went to Home Depot and bought a couple of electric heaters while we are waiting to get the heater repaired. They were "On Sale". She put one in her office and brought one into the garage for me.
It's a teeny weenie little thing and I looked at her and said "No wonder they were on sale." ???

Anyway, I plugged it in and turned it on and blew a breaker right off the bat. Jeez...now back to the project.

I told you I still had to make the standoff pivot blocks (rounds in this case) that will be welded to the backhoe dipper stick. I had some Ø2" H-13 tool steel that I made them out of.
Turning down diameter in lathe to save weight.



I had to mill them down to size so the fit between the linkage after they are welded on. they have to be pretty close because I'm using the shoulder bolts as pins for the pivots.



You can see they are being held in a big homemade "V" block that is clamped in my homemade milling vise.

 

[3RRL]

Then, after drilling a 17/32" hole (tap drill for 5/8-11 thread) all the way through each one, I had to counter bore them with a Ø.750 endmill. This is so the stress is taken by the shoulder and not just the threads, making a stronger connection.



Those counter bores had to be right on, or if they were too deep, the linkage could be pinched instead of pivoting freely when the bolts are tightened. So I decided to gage block them to make sure there was enough room for the linkages and the hardened washers between the linkage bars and also on each side. I got them within .001" of the dimension.



This is probably overkill since this time I included .030" as clearance. I learned this from my last project where I tried to hold everything to .001" Hahaha, what a joke!

 

[3RRL]

I have a "storage position" designed for the thumb. This will be when I unbolt the linkage from the kidney shaped actuator bar and bolt that link to the backhoe. Since the width of the linkage is wider than the dipper stick, I had to make "storage nuts" to weld on. I made them from some hex stock and they came out beautiful. They are also counter bored to fit the same length shoulder bolt.



So here are all the stand-off pieces that need to be welded to the backhoe. You can see how the counter bores accept the shoulder of the shoulder bolts, not just the threads.



I've also fixtured the jaws to weld the stabilizing plates in between them. I used Ø3/4" rods to line up the holes and then clamped it all together. I checked to be sure the rods still slid freely through both jaws. Tomorrow I will make all the welds on it and maybe paint it.



My plan is to take the finished thumb and linkages up to the property and clamp everything in place to see the range of motion. Then I will tack it all on and try it out. Then weld everything with 6011 rod for root passes and finish pass with 7018.

 

[BobRip]

Bob,
They are attached as clickable thumbnails...as a matter of fact, your "quote" reply had some on them.
Is anyone else not seeing them?


Thanks for the replies guys. I'll post more when I can.

I see them on my home computer. It could be the office computer filtered them out. Sorry for the confusion.

I have attached some pictures of the thumb I built for my PT422. There are a few more pictures on the Power Trac section. The red picture is not mine, but it does demonstrate how the thumb folds.

 

[3RRL]

Bob,
Thanks for sharing those photos.
I see that the one you made is a "stationary" thumb with adjustable positions for working and a neat fold away position. Is that right?

 

[K7147]

I am completely impressed with your entire project!! It makes me sort of humbled that I had to go out and by one for my machine!

Ken

 

[EddieWalker]

Rob,

It's very impressive to see how skilled you!!!! In my next life, I wanna be just like you. Seriosly, I'm very impressed with people who can make things that fit together perfecty!!!!!

I'm in the bigger hammer catagory. I can always make it fit with enough pounding, but know that if I was a little smarter, I'd get it together without having to use two differnt hammers, and sometimes a backhoe. hahahaha

Are you going to be able to grease it?

Thanks,
Eddie