A revolving rapid-blow hammer is a percussion mechanism in which a rotating multi-lobe cam repeatedly lifts and releases a spring-loaded tup (the striking head) to deliver fast, evenly spaced blows on a workpiece. It is essential gear in chasing, planishing, and light forging trades where hand-hammering would take hours. The cam stack converts steady rotary input from a belt or motor into hundreds of controlled strikes per minute, and the spring return ensures each blow lands with consistent energy. Old Boley and Becker chasing hammers ran 800–1,800 blows per minute on silverware and coin dies.
How the Revolving Rapid-blow Hammer Actually Works
The mechanism is straightforward once you see it move. A driveshaft carries a stack of lobed cams — usually 2, 3, or 4 lobes ground into one piece — and that shaft spins at anywhere from 200 to 600 RPM depending on the build. Each lobe lifts a follower attached to the tup, compressing a stiff coil or leaf return spring. As the lobe rotates past peak, the follower drops off the cam profile and the spring slams the tup down onto the workpiece. With a 3-lobe cam at 400 RPM you get 1,200 blows per minute. The blow energy comes from the spring, not the motor — the motor only has to overcome friction and re-cock the spring against the cam.
Why build it this way? Because a human striking with a hand hammer tops out around 120 blows per minute and tires inside ten minutes. The revolving rapid-blow hammer holds frequency and energy constant for an entire shift. The cam profile is the critical surface → it must have a smooth lift ramp and a sharp drop-off. If the drop-off is rounded from wear, the tup loses contact with the cam too gradually and the blow softens. We have seen restored Boley hammers where the cam had worn 0.4 mm at the drop edge, and the operator complained the tool felt "dead" — that is the diagnostic.
The tolerances that matter most are cam-to-follower clearance (typically 0.05–0.10 mm cold), tup guide clearance (0.02–0.05 mm in a bronze bushing — too loose and the tup wobbles, too tight and it seizes when the bushing warms up), and spring preload. Get the preload wrong and you change the blow energy without changing the stroke. Common failure modes are cam lobe spalling from poor heat treatment, follower roller bearing seizure, and return spring fatigue cracks at the end coils. The roller bearing on the follower is the part that fails first in production, full stop.
Key Components
- Multi-lobe cam stack: A hardened steel cam with 2 to 4 lobes machined into a single billet, ground to a Rockwell C 58–62 surface. Each rotation produces one blow per lobe, so a 3-lobe cam triples the input shaft frequency at the tup. The drop-off edge must be sharp — wear of more than 0.2 mm visibly softens the strike.
- Tup (striking head): The mass that delivers the blow, usually 0.5–5 kg of medium-carbon steel with a hardened working face. Its mass and the spring constant together determine blow energy. Too light a tup and you bounce off the work; too heavy and the cam cannot lift it within one rotation at speed.
- Cam follower: A roller bearing or hardened sliding pad that rides the cam profile. Roller followers run quieter and last 5–10× longer but cost more. Clearance to the cam must sit at 0.05–0.10 mm cold; check it with a feeler gauge after every 200 hours of use.
- Return spring: A stiff coil or leaf spring that stores energy on the lift stroke and releases it on the drop. Spring rate typically 5–25 N/mm depending on tup mass. Preload sets the blow energy floor — too little preload and light blows feel mushy.
- Tup guide bushing: Bronze or oil-impregnated sintered bushing that keeps the tup travelling true to the work axis. Clearance of 0.02–0.05 mm is the band — slop here causes off-axis blows that mark the work edge instead of striking flat.
- Driveshaft and pulley: Carries the cam stack and accepts belt or direct-motor drive. Shaft must be balanced — at 600 RPM a 10 g imbalance at 50 mm radius shakes the whole frame. Bearings are typically deep-groove ball, 6204 or 6205 size class.
Where the Revolving Rapid-blow Hammer Is Used
Wherever a trade needs many light, identical blows in quick succession, the revolving rapid-blow hammer earns its keep. It dominates work that hand-hammering can do but not at production rate — chasing, planishing, riveting thin stock, texturing decorative metal, and light die-stamping. The tool fades out at the heavy-forging end where you need single high-energy blows from a steam or air hammer, and at the precision-stamping end where a mechanical press with a flywheel makes more sense.
- Silversmithing and hollowware: A holloware shop in Sheffield runs a Boley-pattern revolving chasing hammer at 1,200 BPM to texture sterling teapot bodies and tray rims, replacing 6 hours of hand work with 25 minutes.
- Coin and medal striking (historical): 19th-century mints used revolving rapid-blow hammers to chase the rim lettering on commemorative medallions before screw-press final striking.
- Light riveting: An aircraft restoration shop in Duxford uses a compact revolving hammer to set 3 mm aluminium rivets on Spitfire skin panels — the rapid blows mushroom the rivet head without work-hardening the surrounding sheet.
- Architectural metalwork: A blacksmith studio in Brooklyn texturing 3 mm copper cladding panels for a hotel facade runs a 4-lobe revolving hammer at 350 RPM for a hand-hammered look at production speed.
- Die finishing and chasing: Tool-and-die shops use small revolving rapid-blow hammers fitted with carbide-tipped chasing punches to detail engraved cavities in injection-mould tooling.
- Jewellery production: A Pforzheim jewellery manufacturer uses a benchtop revolving rapid-blow planisher at 800 BPM to flatten and harden 0.8 mm gold sheet for ring blanks before stamping.
The Formula Behind the Revolving Rapid-blow Hammer
What the practitioner cares about is blow rate (BPM) and blow energy (joules per strike). Blow rate scales linearly with shaft speed and lobe count — easy. Blow energy is where the design sweet spot lives. At the low end of typical operating speeds the spring has plenty of time to fully extend and deliver its rated energy, but the BPM is too slow to be worth the setup. At the high end, the cam is rotating fast enough that the follower may not seat fully on the cam profile during the lift phase — you get reduced lift, reduced spring compression, and softer blows. Most builds find their sweet spot at 60–80% of the cam's mechanical speed limit.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| BPM | Blows per minute delivered by the tup | min−1 | min−1 |
| Nshaft | Cam shaft rotational speed | RPM | RPM |
| nlobes | Number of lobes on the cam stack | dimensionless | dimensionless |
| Eblow | Energy delivered per blow (spring-stored energy released) | J | ft·lbf |
| k | Return spring rate | N/mm | lbf/in |
| x0 | Spring preload deflection (compressed length before lift) | mm | in |
| h | Cam lift height (stroke) | mm | in |
Worked Example: Revolving Rapid-blow Hammer in a Birmingham silverware planishing hammer
A silverware finisher in the Birmingham Jewellery Quarter is commissioning a benchtop revolving rapid-blow planishing hammer for sterling tray rims. The unit has a 3-lobe hardened cam, a 1.2 kg tup, a 12 N/mm return spring with 8 mm preload, and a 6 mm cam lift. The drive motor runs the cam shaft at a nominal 400 RPM through a 1:1 belt, with operator-adjustable speed from 200 to 550 RPM. The smith wants to know the blow rate and blow energy across the working range, and where the sweet spot sits.
Given
- nlobes = 3 lobes
- Nshaft (nominal) = 400 RPM
- Nshaft range = 200 to 550 RPM
- k = 12 N/mm
- x0 = 8 mm
- h = 6 mm
- mtup = 1.2 kg
Solution
Step 1 — at nominal 400 RPM, blow rate is shaft speed times lobe count:
Step 2 — blow energy is the difference between spring energy at full compression (preload + lift) and at preload alone:
That energy is released into a 1.2 kg tup, so the tup arrives at the work at v = √(2 × 0.79 / 1.2) ≈ 1.15 m/s. At 1,200 BPM with 0.79 J per blow, you are putting roughly 16 W of mechanical work into the silver — that planishes a tray rim cleanly without forcing the smith to chase the work around.
Step 3 — at the low end of the operating range, 200 RPM:
Blow energy stays at 0.79 J because the spring has even more time to fully extend — you can hear each individual blow and the smith can chase a contour by feel. Useful for delicate work but slow.
Step 4 — at the high end, 550 RPM:
In theory blow energy is unchanged. In practice, above roughly 500 RPM the follower starts to lose contact with the cam ramp during lift — the inertia of tup plus follower exceeds what the spring can pull back through the cam profile in the available time. Effective lift drops to maybe 4 mm instead of 6 mm, and blow energy collapses to around 0.43 J. The sound changes from a clean rattle to a buzzing chatter. Sweet spot for this build sits at 350–450 RPM.
Result
Nominal output is 1,200 BPM at 0. 79 J per blow, with the tup hitting at about 1.15 m/s. At 200 RPM you get 600 BPM with full 0.79 J energy — slow but controllable for delicate chasing. At 550 RPM the BPM rises to 1,650 in theory but real blow energy drops to roughly 0.43 J as follower lift collapses, and the tool starts buzzing instead of striking cleanly — the sweet spot is 350–450 RPM. If your measured blow energy comes in 30% below predicted, the most likely culprits are: (1) the return spring has taken a set and lost preload — measure free length against the manufacturer spec, a 5% reduction means scrap the spring, (2) the follower roller bearing is dragging from contamination so the tup never reaches full lift, or (3) the cam drop-off edge has worn round and the tup is being lowered rather than released — inspect the cam profile under raking light, a sharp edge should reflect a hard line.
Choosing the Revolving Rapid-blow Hammer: Pros and Cons
The revolving rapid-blow hammer is one of three tools that compete for the same work in light forging, planishing, and riveting. Choosing between them comes down to blow rate, blow energy, capital cost, and how much operator skill the job demands.
| Property | Revolving rapid-blow hammer | Pneumatic chasing hammer | Treadle trip hammer (helve hammer) |
|---|---|---|---|
| Blow rate (BPM) | 600–1,800 | 1,500–4,000 | 60–250 |
| Blow energy per strike | 0.3–5 J | 0.05–1 J | 10–500 J |
| Capital cost (benchtop class) | £600–£2,500 | £200–£800 plus compressor | £3,000–£15,000 |
| Operator skill required | Low — set speed and go | Medium — modulate trigger | High — coordinate treadle and work |
| Maintenance interval (production use) | Cam/follower inspect every 200 hr | O-ring kit every 500 hr | Helve bearing dress every 1,000 hr |
| Best application fit | Light planishing, chasing, riveting | Stone carving, fine chasing, dental work | Heavy forging, blade work, tooling |
| Footprint and noise | Compact bench unit, 75–85 dB | Handheld, 90–100 dB at operator | Floor-mount 1–2 m2, 95–105 dB |
Frequently Asked Questions About Revolving Rapid-blow Hammer
The cam speed is constant but the tup is not arriving consistently. Three causes account for almost every case: tup guide bushing clearance has opened past 0.05 mm and the tup is yawing slightly in the bore so it strikes off-axis, the operator is pushing the work into the tool with varying pressure (which loads the spring differently between blows), or the cam lobe heights have worn unequally and one of the 3 lobes lifts 0.3 mm less than the others.
Diagnostic check: hold a piece of soft annealed copper sheet under the tool with light, even pressure for 2 seconds. If every third or fourth mark is shallower, the cam is the problem. If the marks march sideways across the sheet, the bushing is shot.
Up to a point, yes — but the relationship is not linear, and you hit a wall fast. Extra preload adds to the static spring energy, but it also adds to the force the cam has to overcome on lift. At some point the motor stalls or the cam follower bearing exceeds its dynamic load rating and starts to brinell the race.
Rule of thumb: if you increase preload by 50%, expect blow energy to rise by 25–30% (because energy goes with the difference of squares, not preload alone), and expect motor current draw to climb by 40%. Watch the cam follower bearing — if it gets warm to the touch within 10 minutes of running, you have overloaded it and you need a stiffer spring with shorter lift instead.
No. Tup mass and spring rate need to be matched so the natural drop frequency is at least 1.5× the maximum blow rate you want. If the tup is too heavy for the spring, it cannot return to the cam in time and the follower starts skipping lobes — you actually lose blows.
Rough sizing: for planishing and chasing on non-ferrous sheet up to 1.5 mm, 0.5–1.5 kg tup is the band. For light riveting up to 4 mm rivets, 1.5–3 kg. Above 3 kg you are leaving the rapid-blow regime and should look at a trip hammer instead. Confirm by measuring the free drop time of the unloaded tup from full lift — it should be under 60% of one cam-lobe period at your top speed.
Classic thermal expansion problem in the tup guide bushing. Bronze bushings expand about 18 µm per 100 mm bore per 10 °C rise. If your cold clearance was already on the tight side — say 0.02 mm — the bushing closes onto the tup once friction warms it, and the tup starts dragging on the down-stroke. Energy that should hit the work is being burnt in friction.
Fix: open the cold clearance to 0.04–0.05 mm, or switch to an oil-impregnated sintered bronze bushing which runs cooler. Also check that you are not using straight motor oil as a lubricant on a sintered bushing — it floods the pores and they lose their self-lubricating property.
3-lobe is the default for a reason. A 2-lobe cam at 600 RPM gives 1,200 BPM but you can hear each blow individually and the texture looks coarse. A 4-lobe cam at the same shaft speed gives 2,400 BPM but the lift profile gets steep — the cam has to lift the tup in only 90° of rotation instead of 120°, which spikes the contact stress on the follower and the cam wears faster.
3 lobes balances frequency, follower life, and a smooth blended texture on planished work. Only go to 4 lobes if you are running thin gauge under 0.5 mm where you specifically want the marks to merge into a continuous satin finish, and accept that you will rebuild the cam and follower every 12–18 months in production.
Probably not the cam itself if it came from a reputable maker — those are dynamically balanced at the factory. The vibration almost always comes from one of two places. First, the tup itself reciprocating: a 1.2 kg tup oscillating 6 mm at 1,200 BPM produces a cyclic inertial force around 95 N peak, which will shake any bench that is not bolted down or mass-loaded. Second, belt-drive misalignment between motor pulley and cam pulley introduces a once-per-rev shake at shaft frequency.
Bolt the unit to a mass of at least 5× the tup mass, or sit it on a 25 mm rubber isolation pad. If vibration persists at one specific RPM only, that is resonance — change speed by 10% to step off it, or add damping mass to the frame.
References & Further Reading
- Wikipedia contributors. Trip hammer. Wikipedia
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