A stamp mill is a heavy crushing machine that pulverises ore by repeatedly dropping vertical iron stamps onto rock held in a mortar box. The cam shaft is the central component — rotating cams lift each stamp by its tappet, then release it to fall under gravity onto the ore below. The purpose is to liberate metal-bearing minerals from hard rock so the values can be recovered downstream by amalgamation, gravity tables, or flotation. A typical California-pattern five-stamp battery processes 2-5 tons of ore per stamp per day at drop rates of 90-100 strokes per minute.
Operating Principle of the Stamp (form)
The mechanism is brutally simple. A horizontal cam shaft, driven by a flat belt or line shaft, carries a set of cams — one per stamp. Each cam lifts a vertical stamp stem by engaging a tappet keyed to the stem, raises it 6 to 9 inches, then rolls past the tappet and lets the stamp drop. The stamp head, fitted with a hardened shoe, lands on a die seated in the mortar box. The ore — broken to roughly 50 mm feed by an upstream jaw crusher — is crushed between the shoe and die. Water flushes the slurry through a screen on the front of the mortar at a discharge size set by the screen aperture, typically 0.8 to 1.2 mm.
The drop is what does the work. You would be amazed how much energy a 1,000 lb stamp delivers falling 7 inches at 95 strokes per minute — roughly 580 ft-lbs per blow, repeated nearly 6,000 times per hour. The cam shaft phasing is staggered so that stamps in a five-stamp battery never drop together. Stamps drop in the order 1-3-5-2-4 or 1-4-2-5-3 depending on the cam layout, which prevents the foundation from resonating and keeps the splash inside the mortar box even.
If the tolerances drift, you feel it immediately. A worn tappet that lifts only 5 inches instead of 7 cuts blow energy by nearly 30% and you will see throughput collapse. A cam shaft running 10 RPM slow drops your strokes per minute below 85 and the screen blinds because the slurry stops moving. Shoe and die wear is the dominant failure mode — chilled cast iron shoes lose 1 to 2 mm of face per shift on quartzite ore, and once the shoe profile cups, the blow centre walks off the die and you start hammering the mortar liner instead of the ore.
Key Components
- Cam shaft: Horizontal shaft running across the back of the battery, carrying one cam per stamp. Rotates at 30-40 RPM in most California-pattern mills, which produces 90-100 stamp drops per minute given two cam lobes per revolution. The shaft must run within 0.5 mm runout or the stamps lift unevenly.
- Stamp stem and tappet: The stem is a forged steel rod, typically 75-90 mm diameter, with a tappet collar keyed near its lower third. The cam lifts the tappet, the tappet lifts the stem, and the whole assembly drops as a unit. Tappet keys must be tight — a loose key lets the stem rotate, which is essential for even shoe wear, but a sloppy key smashes the keyway in 200 hours.
- Stamp head and shoe: Cast iron head bolted to the bottom of the stem, fitted with a replaceable chilled cast iron shoe. Total falling weight runs 750-1,050 lbs per stamp. The shoe is the consumable face that hits the ore.
- Die: Hardened cast iron block seated in the bottom of the mortar box. The shoe strikes the die through the ore. Die life on quartz gold ore runs 60-90 days before the face is too cupped to deliver clean blows.
- Mortar box: Heavy cast iron trough holding the dies, the ore feed, and the discharge screen. Walls are 50-75 mm thick to absorb the blow energy without cracking. The front opening carries the screen that controls product size.
- Discharge screen: Punched plate or woven wire mesh, aperture 0.6-1.5 mm. Sets the maximum particle size leaving the mortar. Smaller aperture means finer product but lower tonnage — a 0.8 mm screen typically passes 60% of the throughput of a 1.2 mm screen on the same ore.
- Cam shaft drive: Flat belt from a line shaft or, in later installations, a direct gear reducer from an electric motor. Power demand runs 2-3 HP per stamp under load.
Where the Stamp (form) Is Used
Stamp mills were the backbone of hard-rock metal mining from the 1850s through the 1930s. They still run today in artisanal mining districts and as working heritage machines. The mechanism suits any operation where the ore is brittle, the value mineral is liberated at coarse sizes, and capital is more constrained than labour.
- Gold mining (heritage): The Empire Mine in Grass Valley, California ran a 60-stamp battery with 1,750 lb stamps until the mine closed in 1956. The site is now a state historic park with the battery still in place.
- Gold mining (working heritage): The Reed Gold Mine in North Carolina operates a restored 10-stamp Mecklenburg Iron Works battery for visitor demonstrations and small-scale ore processing on site.
- Tin mining: Cornish tin mines including South Crofty used Cornish stamps — a heavier, slower-stroke variant — through the 19th century to crush cassiterite ore before buddle separation.
- Artisanal gold processing: Small-scale operators in Tanzania, Burkina Faso, and the Philippines run 2-stamp and 3-stamp diesel-driven mills sized at 200-400 lbs per stamp, processing 0.5-1 ton per stamp per day for amalgamation recovery.
- Silver mining: The Mariposa Mine in Nevada and the Comstock Lode operations ran large California-pattern batteries with mercury-coated copper amalgamation plates set immediately below the mortar discharge to catch free silver and gold.
- Heritage and museum operation: The Bendigo Central Deborah Gold Mine in Victoria, Australia runs a working stamp battery as part of its underground tour. The Gold King Mine Museum in Jerome, Arizona similarly demonstrates a five-stamp mill on weekends.
The Formula Behind the Stamp (form)
What you want to know is the energy each blow delivers and how that scales with operating choices. Drop height and stamp weight are the two knobs. At the low end of typical practice — 6 inch drop, 750 lb stamp — you deliver 375 ft-lbs per blow, which is enough for soft sulphide ores but undersizes you on quartz. At the nominal California setting of 7 inch drop and 1,000 lb stamp, you get roughly 580 ft-lbs per blow, the historical sweet spot for gold-bearing quartz. Push to a 9 inch drop and 1,050 lb stamp — Cornish or heavy-pattern practice — and you reach 790 ft-lbs, but stem fatigue and shoe shatter risk climb fast. The formula assumes free fall, which is a good approximation because the cam releases the tappet cleanly and the stem only sees air drag and stem-guide friction on the way down.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Eblow | Total crushing energy delivered per minute by the battery | J/min | ft-lbf/min |
| Wstamp | Falling weight of a single stamp (stem + tappet + head + shoe) | N | lbf |
| hdrop | Effective drop height from cam release to die contact | m | in |
| Nstamps | Number of stamps in the battery | count | count |
| fdrop | Drop frequency per stamp | drops/min | drops/min |
Worked Example: Stamp (form) in a restored five-stamp gold mill
A heritage mining museum in Tuolumne County, California is recommissioning a five-stamp Joshua Hendy battery for live demonstrations crushing donated quartz vein samples. Each stamp weighs 950 lbs, the cam shaft is geared for a 7 inch nominal drop, and the operator wants to know total crushing power at the historical 95 drops per minute setting and at the slower 80 and faster 100 drops per minute alternatives.
Given
- Wstamp = 950 lbf
- hdrop = 7 in
- Nstamps = 5 count
- fdrop = 95 drops/min
Solution
Step 1 — convert the drop height to feet so the result lands in ft-lbf. 7 inches is 0.583 ft.
Step 2 — at the nominal 95 drops per minute setting, compute total battery energy per minute:
That converts to about 7.97 HP of pure crushing work, which lines up neatly with the historical figure of 2-3 HP per stamp at the input shaft once you account for belt and bearing losses around 50%.
Step 3 — at the low end of the typical operating range, 80 drops per minute (slow cam shaft, weak belt drive, or a tired motor):
That is a 16% loss in crushing power. You feel it as screen blinding — the slurry stops carrying fines through the discharge mesh and the mortar starts choking on uncrushed feed within 20 minutes.
Step 4 — at the high end, 100 drops per minute:
Theoretically a 5% gain, but in practice the stamp does not have time to fall fully under gravity if the cam picks it back up too soon. Above roughly 105 drops per minute on a 7 inch drop, the cam catches the tappet on the rebound and you lose effective drop height. The sweet spot sits at 90-95 drops per minute for this geometry.
Result
The battery delivers about 263,000 ft-lbf per minute of crushing work at the nominal 95 drops per minute, equivalent to roughly 8 HP of useful work on the ore. That is enough to push 10-15 tons of quartz feed through 0.9 mm screens per 24-hour shift. The low-end 80 drops per minute setting drops you to 221,000 ft-lbf/min and the slurry starts backing up; the 100 drops per minute setting only nets 5% more on paper before the cam-rebound problem caps real output. If your measured throughput sits 20% below the predicted figure, check three things in order: (1) tappet keys for play that bleeds drop height, (2) cam profile wear — a worn cam picks up the tappet 1-2 inches early and shortens the effective drop, and (3) stem guide alignment, because a stem that drags on its guide bushing during the fall loses energy to friction instead of delivering it to the die.
When to Use a Stamp (form) and When Not To
A stamp mill is one of three classical hard-rock crushers you would consider for a small or heritage operation. The other two are the ball mill, which uses tumbling steel balls in a rotating drum, and the jaw crusher, which uses a reciprocating jaw plate. Each suits a different scale, ore type, and capital position.
| Property | Stamp Mill | Ball Mill | Jaw Crusher |
|---|---|---|---|
| Throughput per machine | 2-5 tons per stamp per day | 20-200 tons per hour | 10-300 tons per hour |
| Product size (typical) | 0.6-1.5 mm via discharge screen | 0.04-0.3 mm slurry | 20-150 mm coarse |
| Capital cost (relative) | Low — heavy castings, simple | High — large drum, liners, motor | Medium — single robust frame |
| Power demand | 2-3 HP per stamp | 0.5-1 HP per ton/hr | 0.3-0.6 HP per ton/hr |
| Wear part interval | Shoes and dies every 60-90 days on quartz | Liners every 6-18 months, balls topped up daily | Jaw plates every 3-12 months |
| Best application fit | Brittle hard-rock ore, coarse liberation, small scale | Fine grinding for flotation feed, large scale | Primary crushing ahead of grinding mill |
| Operator skill required | Low — visual and audible diagnosis | Medium — slurry density and ball charge management | Low — feed rate control |
| Suitability for amalgamation | Excellent — coarse particles plus mercury plate below mortar | Poor — fines pass too fast over plates | Not used — product too coarse |
Frequently Asked Questions About Stamp (form)
The 1-3-5-2-4 phasing puts the maximum physical distance between consecutive falling stamps. If they dropped in 1-2-3-4-5 order, two adjacent stamps would hit within milliseconds of each other and the slurry wave from stamp 1 would still be peaking when stamp 2 lands — you get splash out the front of the mortar and uneven feed distribution.
The diagonal sequence also balances the bending load on the cam shaft. With 1-3-5-2-4, the cam shaft never sees two adjacent cams at peak lift simultaneously, which keeps shaft deflection under about 0.3 mm and protects the shaft bearings from cyclic side load.
You are losing strokes to tappet bounce. When a cam picks up a tappet that has not fully settled on the die, the lift starts late and the next drop comes early — but if the stem lands on a high spot of cupped die and bounces, the cam sometimes catches the tappet on the way back up and skips a clean release. You see this as 88 measured drops against a theoretical 94.
Check die wear first. A die that has cupped more than 3 mm in the centre causes stamp rebound on every blow. Replace the die and the count usually comes back within 2-3 strokes of theoretical.
Heavier and slower wins on hard, coarse-feed ores. A 1,050 lb stamp at 85 drops per minute and a 750 lb stamp at 119 drops per minute deliver the same energy per minute, but the heavier blow drives larger feed particles through to fracture in fewer hits. On quartz vein gold ore, the heavy-and-slow setup typically gives 10-15% better tonnage at the same screen size.
Lighter and faster wins on softer, already-fine feed where you are mostly polishing oversize through the screen. Cornish stamps went heavy-and-slow for tin; California stamps went moderate weight at 95-100 drops per minute as a compromise for mixed gold ores.
For mercury amalgamation directly below the mortar, run a 0.8-1.0 mm screen. Free gold liberates at coarse sizes and a coarser product spends more time on the amalgamation plate, giving the mercury time to wet the gold particle. Drop below 0.6 mm and the slurry velocity over the plate climbs, gold bypasses the mercury, and recovery falls from typical 60-70% to under 40%.
For downstream gravity concentration on a shaking table or sluice, a 0.6-0.8 mm screen suits better because the table separates by particle size and density and wants a tighter top size.
A ringing mortar means the box is starved of feed. The shoe is hitting the die through air or thin slurry instead of a packed bed of ore, and the unattenuated impact transmits up the stem, into the cam shaft, and out through the bearings. You will hear a sharp metallic clang instead of the normal dull thump.
This kills bearings fast. Cam shaft bearings rated for 20,000 hours under loaded operation will fail in 3,000-5,000 hours if the mill runs starved a few hours per shift. Set the feeder to keep at least 50 mm of ore depth above the dies at all times.
You can, and it works well within a narrow band. A VFD on the cam shaft motor lets you tune from about 75 to 105 drops per minute to match ore hardness without changing cams. The constraint is that the cam profile is fixed — it was ground to release the tappet at a specific angular velocity. Run too slow and the tappet rolls off the cam instead of dropping cleanly, costing you 10-15% of drop height.
The practical sweet spot is ±15% around the cam's design RPM. Beyond that, you need a recut cam profile. Some modern artisanal mills built since 2000 use this exact approach with 5-15 HP VFD-driven motors.
References & Further Reading
- Wikipedia contributors. Stamp mill. Wikipedia
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