A well drill bit is chosen from the ground it must cut and sized from the casing it must admit, not from the hole you would like: the hole must be wider than the casing by room for a gravel pack, so a 110 mm casing with 25 mm of clearance on each side needs a hole of about 160 mm, and a bit of 10-15 cm cannot do it. Use a chisel bit for hard or stony ground, a drag or fishtail bit for soft clay, an auger bit for clay and soft soil above the water, and a bailer or sludger in loose sand, and make each bit slightly oversize to allow for wear.
Part of Deep Wells
Well drill bits are the cutting tools that make a borehole for water by impact or rotation in soil and rock. For bits that cut metal in a shop, see Drill Bits, which is about point angles and lip clearance for steel, cast iron, stone, glass and hard wood and does not apply here.
The Casing Sets The Bit, Not The Other Way Round
A shop bit is chosen for the hole wanted. A well bit is chosen for the pipe that must go down the hole afterwards. The Deep Wells guide specifies a narrow percussion bit of 10-15 cm and PVC casing of 110-160 mm; these do not fit together. The relationship is:
hole diameter = casing outside diameter + 2 × clearance
The clearance is the radial room for the gravel pack and for the casing to hang without scraping. This guide assumes at least 25 mm on each side, about four times the largest 6 mm pack grain; the figure comes from general drilling practice, not from the Deep Wells guide.
| Casing (outside) | Hole needed (+2 × 25 mm) | Bit of 10-15 cm? |
|---|---|---|
| 110 mm | 160 mm | No; at 150 mm the clearance is only 20 mm each side |
| 125 mm | 175 mm | No |
| 160 mm | 210 mm | No; the casing will not go into a 150 mm hole at all |
For the pack and seal see Gravel Pack, and for the casing itself Well Casing.
Two more reasons the bit must be oversize. A bit wears at its outer edge, so the hole it cuts shrinks as it dulls; dress or replace it before the hole is smaller than planned. And if a first casing is driven through a loose layer, the bit for the next stage must pass inside it: its diameter must be less than that casing’s inside diameter, which is the outside diameter less twice the wall (a 110 mm casing with a 4 mm wall, an assumed wall, passes bits under 102 mm). Plan the whole telescoping sequence from the final casing outwards before making any bit.
Which Bit Suits Which Ground, And How It Fails
The corpus has no tested penetration rates or bit geometries for hand drilling, so the table is qualitative.
| Bit | How it cuts | Suits | Fails by |
|---|---|---|---|
| Chisel bit (percussion) | Heavy steel point dropped on a rope; the weight (30-50 kg in the Deep Wells guide) does the breaking (Percussion Drilling) | Gravel, hard clay, stone, weathered and fractured rock | Wedging in a hole gone out of round; sticking in caving ground; dulling in hard rock; cutting an undersize hole as it wears |
| Drag or fishtail bit (rotary) | Flat blades scrape as the rods turn; water flushes the cuttings out | Soft clay and silt | Clogging in sticky clay; useless in gravel and rock, where it only polishes |
| Auger bit (bucket, spiral, clay) | Turned into the soil and lifted out full (Auger Types) | Clay and soft soil above the water table | Spilling loose soil; clogging in wet clay; stalling on stones; hole closing in wet sand |
| Bailer or sludger | A pipe with a valve, worked up and down in slurry, lifts the sand and water out (Bailer Operation) | Saturated sand and soft ground below the water level | Failing in clay (the valve will not load) and in gravel too coarse to enter the pipe |
A sludger or hand-flap rig is the simplest of these: a pipe, a valve or the operator’s palm over the top, and a rope or rods to work it up and down while the pipe sinks. I cannot give a tested depth or rate for it; treat it as a soft-ground method for sand and silt, and measure your own progress per hour before planning around it.
What You Need
- The casing size decided first, and the clearance from the table
- Steel for bits — heavy bar or plate for chisels, blade stock for drag bits, pipe for bailers
- A file or grinding stone — for dressing edges between runs
- A gauge — a ring or hoop of the target diameter to check a worn bit
- The rig and rope tested for the bit’s weight (Drill Rig Construction)
Method: Size, Make, Match To Ground, Gauge
Step 1: Fix the casing and add the clearance to get the hole diameter.
Step 2: Make the bit that diameter plus a wear allowance (set from your first run: measure the worn bit against the gauge and add what it lost).
Step 3: Match the bit to the layer. Log what comes up and switch bit when the ground changes: chisel in rock and gravel, auger or drag in clay, bailer in saturated sand.
Step 4: Gauge every run. Check the bit against the ring at each change; dress it or replace it when it falls under size.
Step 5: Ream down in stages if needed. To reach a large hole, drill a narrow pilot first and open it up in steps, so the drill has less ground to cut at a time.
Common Mistakes
- Making the bit first. The hole then dictates a casing that may not exist.
- No wear allowance. The bottom of the hole ends up narrower than the top and the casing sticks.
- One bit for all ground. A chisel in clay or a drag bit in gravel wastes days.
- Reading a workshop bit chart. Point angles for metal tell you nothing about a chisel dropped on a rope.
Safety
Heavy bits on rope and rods over an open hole
A bit of tens of kilograms on a rope can crush hands and feet and strip a poorly lashed rig. Keep clear of the line of the rope and the tripod legs, never wrap rope round a hand, lower and lift bits with a hoist, and cover the hole when work stops (Drill Rig Construction). Nobody climbs down a drilled hole to free a stuck bit.
What’s Next
- Learn next: Well Casing — the pipe the bit has to admit
- Combine with: Extension Rods — the string a rotary or auger bit turns on