making an hydraulic thumb for my new mini-excavator

[dfkrug]

I recently imported a new 1-ton mini-X from China, and it needs an hydraulic thumb.

It came with a mechanical thumb installed, but it is flimsy and won't do at all. I made an
hydraulic thumb for my backhoe years ago, and it is indispensable. So, over the next several
days, I will post pix and words on my cool new thumb, now working.

My thread on importing my new machine is here:
https://www.tractorbynet.com/forums...6-importing-modifying-new-mini-excavator.html

 

[ovrszd]

Awesome!! Looking forward to following this project!!

 

[nyone]

That does look flimsey. You could add two pieces of 2in square tube between the two flat plates to make it stronger.

My mini ex needs a hyd cylinder for the thumb i built. Its a manual with two different settings.

 

[dfkrug]

When I bought this machine, I also got a lot of spares, including 2 spare hydraulic cylinders.

They seem well-made and are 50mm ID, and about 10" stroke. Only $50 each, which I suspect is
cost. Same size as the bucket cylinder.

I also bought this accessory, a rake that mounts where the bucket usually goes. I don't really have a use
for a rake, but it was so inexpensive I bought it anyway. I was thinking of using it for my future hydraulic
thumb. It was only about $60 more, IIRC.

 

[dfkrug]

You could add two pieces of 2in square tube between the two flat plates to make it stronger.

Yeah, to make this mechanical thumb usable, you would need to weld the 2 sides together, with tubing or plate. I took it off entirely when digging. It was in the way.

I have just over 4 hours on this machine.

 

[millsan1]

Here you go

Hydraulic Backhoe Thumb | 24" | Weld On

 

[dfkrug]

After removing the OEM mechanical thumb, you can clearly see the pivot on the end of the dipper and the 3 anchor point for the thumb brace. These are all 25mm holes, fabricated with DOM tubing over steel plate. The DOM is notched to accommodate the anti-rotate feature of the pins.

Next is the mockup to see what mods I have to make.

 

[dfkrug]

Here is the mockup, using the spare bucket cylinder and the rake.

Right away, you can see that the 3 anchor holes are too far down the dipper to serve as the upper thumb cylinder pivot. The lower thumb pivot also needs to be closer to the end of the dipper. Using the bucket pivot as the shared thumb pivot will not work, either. In my experience, 25mm pins are the minimum size for the forces involved here. Sharing the bucket pivot pin will increase the bending stresses and result in failure.

Both the bucket and the thumb cylinders are 50mm ID, and push with 7700 pounds of force (2400psi).

Finally, the pivot on the rake is all wrong. Much work to be done.....

 

[dfkrug]

First the business-end of the thumb. I made it narrower and trimmed off the "toenails". Then I cut off two of the "ears". I will reuse these later.

 

[dfkrug]

Next, the cylinder.

Sadly, I can not make the 2 x 10" cylinder fit. Fully-retracted, it has to clear the boom at the top of the dipper, while retracting the thumb far enough to be out of the way for digging action.

I had one of these 2 x 8" cyls, acquired from Surplus Center years ago. They have only 3/4" ID crosstubes, so that has to be fixed. I cut them off and welded on some 1" ID DOM tubing, which is undersized enough to give ideal grease clearance with a 25mm pin.

The last photo is the final cylinder, in the process of having zerks tapped.

 

[downsizingnow48]

curious what welder you use for the cylinder modification.

 

[dfkrug]

curious what welder you use for the cylinder modification.

The new crosstubes were welded on using my Hobart Ironman 210 MIG at the highest current setting using .035 wire and Ar/CO2. Most of the other welding was done with my Miller Thunderbolt DC stick welder and 1/8" 7018 rod. I prefer the MIG for this kind of welding, but I was almost out of gas, and I need to find a new supplier.

Here is a photo of the big end of the cylinder.

 

[dfkrug]

Next, I need to cut the "ears" and the thumb pivot off the dipper. This took a while, using my Thermal Dynamics Cutmaster 51 plasma cutter. The maker used fabricated box tubing with machine welds, and reinforcement plates in these areas. Very good construction, Xiniu.

I then relocated the thumb pivot as far down the dipper as possible.

 

[dfkrug]

Here is the new mockup, using my new modified cylinder and the thumb. I need to decide on placement of the cylinder pivot on the back side of the thumb. The most critical concern is that the thumb must retract far enough so that conventional digging with the bucket is not impeded. How far the thumb extends is of secondary importance.

I will cut down and reuse the original ears, and provide 2 rear cylinder pivot points.

 

[dfkrug]

Now to finish up the thumb. The new pivot warrants a reinforcement plate. Other reinforcements were welded between the tines. After that, I formed some 1/8" steel plate to the business side and capped it off with rattle-can black.

 

[dfkrug]

The last fabrication effort is the placement of the upper cylinder pivot on the dipper stick. I cut the ears I removed earlier back to 2 holes instead of 3. This gives me a stronger weldment, plus I may want to utilize the upper pivot to gain more thumb retraction. Field-testing will tell.

All thumb welding is now complete. Next, the hydraulics.

 

[dfkrug]

As shown in my other mini-X thread, I bought a spare 8-spool stack valve for this machine. The AUX valve is intended for any hydraulic device installed on the dipper, but it comes detented. The maker sells an hydraulic auger, which I did not buy, but I question the value of a detented valve with a single-pump system. It really needs a spring-centered valve, like the other 7.

The maker was nice enough to airmail me the parts needed for the conversion from detented to spring-centered for both my spare stack and the one on the machine. Gratis.

The conversion was easy enough for the spare valve on the bench, but the installed one required several hours of labor. Lesson: make hydraulic valve repairs on the bench if you can!

 

[dfkrug]

Here is the installed thumb and cylinder, waiting on hoses.

I could not find an adapter for the metric hose to a -4 JIC, even from our pals at Discount Hydraulic Hose. So I drilled out a metric tubing cap and welded it to a common JIC male adapter. Tho I air-tested for leaks, one of them leaked under hydraulic pressure and had to be re-welded. The pinhole leak had to be cooked with a torch to burn out the oil contamination before re-welding. My experience has been that without the cooking, subsequent welding will also have pinholes.

 

[nyone]

Having that skin on the thumb will prob allow for things to slip out real easily. Mine is just a skeleton with some sharp teeth at the end.

Also your bucket teeth shanks look way to small for that bucket. You want them to go further back on the edge so your not trying to bend the end of the cutting edge. Unless thats just a rough grading bucket.

 

[dfkrug]

Having that skin on the thumb will prob allow for things to slip out real easily. Mine is just a skeleton with some sharp teeth at the end.

The current hyd thumb I use on my backhoe has a smooth face as well, and the load slipping has never been a problem for me. When grabbing a log, the edges of the bkt keep that from happening. I deal with a lot of rocks, which cause intense point-loads. Without the plate welded over the skeleton of my thumb, I would bend it. I have already slightly bent the side walls of my bucket lifting rocks.

I have many hours on this thumb (photo).