I have been considering building my own backhoe as well as looking at a lot of available designs. Some of the things I learned:
Many excavators have a bend in the boom so it does not hit the edge of the hole. This allows a more verticle edge on the end of the trench closest to the machine.
The bend also allows in some cases for the transport overhang length to be reduced - there is more room for the bucket to tuck in behind the boom.
I think the bent design looks cooler as well.
Most of the straight boom designs appear to be made out of a single piece of sqaure or rectangular tube while the more complex shaped booms have laser-cut sides welded at the edges or cut and formed sides welded together along a central seam. The complex bends would be beyond the ability of almost all do-it-yourself'ers, but the laser-cut or torch-cut flat sides design could be done in a home shop.
The more complex shape allows the design to put the metal where it does the most good and maintain clearances in areas where a straight tube section would get in the way. Typically the boom would taper at its base and tip to a fairly small pivot mechanism with a tall section in the middle to provide stiffness and mounting locations for the boom and dipper cylinders.
A big design decision is the placement of the boom cylinder. Putting it under the boom gives the cylinder its best mechanical advantage for lifting. This would perhaps be good for pulling stumps or craning stuff. Putting the boom cylinder on the top makes it do its lifting in retraction mode so overall lifting capacity is reduced, but down-force would be maximized as well as keeping the cylinder rod and seals away from dirt, rocks, and possible contact with the bucket itself. Some larger excavators use dual boom cylinders on the side, getting the advantage of using them in "push" mode and also getting them out of the way somewhat.
Another issue to consider is side-to-side stiffness. The wider the boom design and pivots, the easier it is to make it strong in the side to side direction. This is useful in sliding dirt out of the way, or sliding it back into the trench when back-filling. The swing mechanism needs to be designed with a corresponding amount of force.
The swing mechanism is an area of the design that separates the toys from the tools. Most BH owners and users I have talked to indicate that a full 180 degree swing is important to have. To achieve this requires a more complex design such as dual swing cylinders, a compund linkage, a chain, or a rack-and-pinion type of setup. The ability to have available quick slew rate, high swing forces (for back-filling), smooth control, and gentle stopping at the end of travel are important considerations in the design of the swing mechanism and its hydraulics.
The hydraulic controls is another area that can make the difference between a BH that is merely functional and one that is highly productive. The ability to run two, three, or even four functions at once is important. Hydraulic cushioning of the swing mechanism might be nice to have. Differing pressure bypass settings for different functions might be appropriate. The layout of the controls, 4-lever vs 2-joysticks, vs foot pedals, etc. The ability to add a hydraulic thumb or a motor-powered device like a jack-hammer might be nice to have.
CNC laser or plasma-cut parts would be my choice for the way to go in a kit. Just make sure the kit design takes the most advantage as it can of the capabilities of the CNC process. It costs the same for the laser to cut a well-thought-out design as it does to cut a poor design.
A well-thought out laser-cut parts design would have all the holes placed, even if they might need to be drilled and reamed to precise sizes. It would have features like tabs, slots, an extra corner or two cut into the part to aid in alignment and fixturing during welding. It would avoid using multiple identical copies of a part for the sake of ease of fabrication. The left and right copies of a part need not be identical - one can have extra holes or different hole sizes or whatever might make sense. A good laser design will have fewer parts in more complex shapes - not a bunch of simple rectangles butted together.
The laser can add a lot to the looks - radiuses on all corners, for example. Graceful, swooping curves instead of straight lines. A "style" and "look" can be carried through the design to make the machine look more high-tech.
- Rick