I am punching a narrow 2.375" hole to a shallow depth and digging through nothing more solid than tough Iowa clay. Keeping the bore hole minimal keeps the return flow of water up the bore at 28% of the downwared flow for 1" nomial pipe size. Making the bore hole 3" reduces the upward flow to 15% of the down flow. The up flow needs to be enough to push pebbles and debries out of the bore hole, this is achieved by keeping the bore small and the water flow high.
For the most part I expect to be able to make this from parts I have laying about the house. I've got a 5hp 220V 2 speed motor that I pulled from somewhere because was making a bit of a grinding noise. I've got an old sump pump I fixed after replacing it when it failed, I fitted it with a 1.5" plastic drainage hose kit with barb-to-threadded adapter on each end. The reduction gears I pulled out of a junk tiller/cultivator and they look to be about 30:1 which will give me 2 revolutions per second at high speed, 1 revolution per second at low power.
I expect the water swivel to leak badly, I also expect it to deliver lots of high pressure water to the half dozen smallish holes in the side of the drill pipe. I'll put a rag over the swivel to get the leaking water to flow down the pipe and recycle back into the tub.
I'm hoping the diagram below is self explanatory, if you have questions feel free to write to me, email is Dave at this domain.
Use the hand crank to raise or lower the rig, let another inch out when the previous inch seems to have been drilled and flushed. Don't rush it, and don't let the water stop flowing while drilling.
Details of making the water swivel Start with a 1" pipe about 10" long scrubbed and sanded clean of stickers, a 1" coupler, a 1.5" Tee, two 1.5" - 1" bushings, and a good size rotory drum style grinding stone that fits inside the bushings. Use a corded drill unless you have lots of batteries on standby. Put a bushing in a vice at a good angle to work on. Do the best job you can of grinding in a good circle just big enough for the pipe to slide. Know that the pipe itself isn't perfectly round. Grind all the inside threads out of the bushings. Try screwing the pipe in, pull it back out and look for teeth marks, grind more around those spots. When you start getting the pipe most of the way into the bushing, look at the edges where it binds and grind a bit more there. Use an in-and-out motion while moving back and forth to keep from grinding a flat spot. In the end your best guide will be holding the bushing up and seeing where the most light comes around the pipe. Tape and insert the bushings permenantly into the ends of the tee, and you will probably find a little more grinding is in order to make the pipe go through both, just keep looking for where the pipe binds against the bushing. Now tape and insert a coupler permenantly to the end of the 1" pipe, this is the support for the swivel. Locate the area where the tee has plenty of inside room around the 1" pipe and drill a half dozen 1/4" holes into the 1" pipe in that area. Drill two on opposite sides of the pipe, move down a bit and place two more 90 degrees from the first, move down a bit and place two more on the same sides as the first. I found this gives plenty of water flow. The finished product will have spots that stick a bit, that's fine as long as you can turn the tee by hand reasonably easily with the pipe in a vice.These pictures show 1/4" holes, on the advice of more experienced drillers I will be expanding them to 1/2" to maximize water flow. High volume water flow is critical!
Original design drill bit, somewhat longer and pointier than the CNC version. This picture is before I ground the trailing edges down to make it cut earth better
Details of making the drill bit The drill bit is a piece of 1" schedule 40 nominal pipe with threads cut off of one end and with three metal teeth welded to it at equal points around the pipe. To create the drill bit you can print the graphic below at 100 dpi or you can print the PDF version here with scaling = NONE (you have to open the PDF in Adobe to print it properly, not from a web browser) or you can use the lines and angles described with a CNC machine. You need three teeth these to make a balanced bit, I started with a flat bit but more experienced people told me a 3 sided bit would be a lot more effective. Cut one threaded end off a 4" or longer pipe and weld the three teeth solidly to the pipe and to each other along the center section. Note the very small open space between where the three teeth come together, that is to allow for the width of the teeth to fit together without having to grind each edge, just making the construction a bit easier. The size of this gap in the design below is designed for quarter inch thick teeth. Also notice how the bit tries to leave as much unrestricted water flow as possible out the end of the pipe, maximum water velocity is absolutely critical to success.
And the finished drilling rig