Attempting to build my own mini-skid steer stump grinding attachment

I subcontract all my stump grinding to my buddy Jim. He sent me this youtube video and suggested we could build one for my Thomas 25g:

I started by buying a screw cone off of ebay: 0325161755.jpg

Then I cut 6 strips off a 1/4 inch plate, and welded them together to fit on an auger hex drive:
I have a bunch of old chipper knives, so I cut one in half for the blades on the stump grinder. Then I cut some flanges out of 5/8" strap and drilled holes to accommodate chipper knife bolts. I welded these to the screw cone at an angle to get some bite with each revolution. I also flared them up a little bit. 0325161757.jpg
My friend Sam bought some schedule 40 pipe with 2" O.D. and 3/8" wall at the steel supply store, which I will use as the auger drive shaft. Tomorrow I hope to assemble and weld the rest of this thing together and give it a trial run on some old russian olive stumps in my backyard.

Sam says the bite angle on the chipper knives needs to be at least 15 degrees, and I only have about 5 degree bite angle. Also the screw cone is 3" in diameter, which I think might be so wide it causes the screw to strip out or the auger drive to stall. If it doesn't work, I plan to move the knives further down the shaft and increase the bit angle.

The auger drive is a slow, high torque set up at 40 rpm at 2100 foot pounds of torque according to the drive spec sheet for my Thomas 25g running at 2000 psi and 8.5 gpm.

Will post further progress soon.
Hey Merle,
I think I will field test the current design at about 5 degree bite angle before adjusting it. It might fail, but if I fail to grind a stump with this approach I will alter it and try again. I have had a lot more failures than successes, but it makes me so excited to stretch for a success.

Merle Nelson

Well-Known Member
Sounds good. I have seen the commercial designs of this type of stump cutter and often wondered how chipper knives would hold up. Best of luck to you.

By way of interest have you seen the different yet still vertical approach called the Stump Gun? It's up on Brush Bandits website.


Well-Known Member
I would think the perfect attack angle of the blades would be the actual angle of the "thread" on the cone or maybe slightly shallower. Sort of let the cone draw the blades in rather that the blades draw the cone in.
I might be thinking about it all wrong, either way I would be interested in seeing how the 25g handles it
I hadn't heard of the stump gun before, it is awesome but high rpm and requires twice the gpm flow rate that my mini puts out. I am hoping to finish the fabrication on my grinder today, but there may be too much snow to properly test it.


Well-Known Member
I think you will be fine with 5 degrees to start with, that cone will never pull the bit in my humble opinion (sorry and please let me know what happens). I think most of your chip depth will come from the pressure downward you will apply with the hydraulics. I think you will most likely end up closer to 10-12 degrees after testing, I think while 5 will pull a chip, it will have the tendency to flatten out quickly and require a lot of up keep and metal removal to bring it back to sharp. Good luck and keep us posted, great project.
I finished the stump grinder prototype today. I used it on a big old honey locust round in my backyard, but the grinder just ended up splitting it instead of grinding it. I had to put a lot of downpressure on the round or it would just spin around. I drove TomTom down into the part of the yard with the russian olive stumps but there was too much snow and I got my machine stuck (snow chains are currently on order). I am going to be building an attachment identical to this one but without the chipper knives to split big rounds. 0326161416_resized.jpg
Here is a youtube video of the final assembly:

I will post again in a couple of days.

Merle Nelson

Well-Known Member

Every hear the story of the first v8? Basically, selling autos by a horsepower race, Henry Ford told his engineers to build a v8. Six months or a year later they report it can't be done. That's fine he tells them, now go build me a v8 engine. This went on for three years I think. Well....what was my point in starting this thought? Oh yeah, a pep talk.

Hope it doesn't take you that long. :)
Thanks for the pep talk, Merle. I don't count this as a failure yet, I just haven't been able to properly test it. If I got discouraged easily, I sure as heck wouldn't be in this industry. They told the Donner Party they were leaving too late in the season, and I bet things turned out fine.

If I can get this stump grinder to work, I could start selling them and become a thousandaire until Fecon sues me and China starts selling 'em for less than my parts cost!
Hey Kelly tree, instead of having such a massive draw down screw wouldn't it be easier to have more of a drill to keep the blades centered and turning rather than being forced into the wood by the force of the big o' screw? Just saying. Kan-Do
Jim, I don't know the answer to your question. I love the idea of a pilot drill bit, like what is used on a hole saw to keep it centered. I think one possible advantage of the screw is that it pulls the blades in at a uniform speed, but a similar design with a pilot bit might be regulated by down pressure.

Essentially what I am trying to build is a big drill bit to take advantage of the tremendous power of a mini skid's hydraulics. I need to think about this a little bit, but I feel a very nerdy post brewing inside of me.
I realized yesterday that I am just building a giant drill bit. There is a ton of information about drill bits on: . Drill bit design varies by material, hole size, and rpm. The old hand drill bits start with a screw pilot bit and then augers away material. These bits are low rpm and medium torque.

The more modern wood bit is the standard spade bit. This bit has a small pilot tip instead of a screw, and the drilling speed is regulated by downpressure. I've done a lot of log construction and these bits hog out a lot of material fast. These are high RPM and low torque bits.


What I am trying to build is very similar to a Forstner bit, but without the hole saw border.

The amazing thing to me is that this Forstner bit is almost 5 inches in diameter. With my powerful Hilti drill, I think I could drill several holes in a stump with this thing and then use a hatchet or and adze to cut out the remaining material.

TomTom's bit is almost 14 inches in diameter, so will be removing about 2^3 = 8 times more material since the area of a circle is proportional to the square of the radius. I like the idea of the pilot screw bit, because this draws the planer blades in and regulates the amount of bite. The distance between successive threads on the screw are about 5 millimeters, so each of the knives should be planing about 2.5 millimeters.

Given the equipment I already have, the variables I can control are:
1. Screw cone diameter (by moving the knives further down the shaft)
2. Knife bite angle
3. Knife upflair angle
4. Total bit diameter (by reducing the size of the chipper knives, or putting bigger knives on)

I don't have control over cone thread pitch, cone angle, or thread size unless I buy another cone.

Performing engineering analysis to determine optimal variables for this bit prior to the build was too intimidating, so instead I created a prototype that I can test. By performing testing, I should get clues to make necessary adjustments to the design. If the bit stalls, I will move the knives further down the screw cone. If the blades don't bite enough, I will increase the bite angle. If the screw strips out, I may have to purchase a screw with deeper threading.

I don't mind if the bit is a bit undersized for my machine, since that just means the stumps will take a bit longer to grind. For most jobs, I could save a lot of transport time and logistics time if I could use a single machine to:

1. Load all the logs
2. Grind the stump
3. Load all the stump grindings

This is especially true for my operation because I currently subcontract all my stump grinding to Repairman Jim, who has his own projects going on which means he usually needs more than a day of lead time to grind my stumps.

It's possible that a completely different design is going to be optimal for a stump grinder bit.
It is also possible that this design will sort of work but be too far inferior in production time when compared with conventional, dedicated, high HP, high RPM stump grinders.

The snow is melting, and I hope to return to field testing soon.


Well-Known Member
So my question has always been for this type of stump grinder, what do you do for the butt swell roots and surface rootsl?

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