A Double Cylinder Planer is a heavy reciprocating metal-cutting machine that drives a long table back and forth under a fixed tool, with two parallel hydraulic or steam cylinders supplying the stroke power. The twin drive cylinders are the defining component — they balance the table thrust on both sides so a 6-metre casting tracks straight without yaw. The machine exists to flat-machine large workpieces no shaper or mill could span. Real shops still use them for locomotive frame plates, large machine bases, and sliding ways up to 12 m long.
Double Cylinder Planer Interactive Calculator
Vary stroke length plus cutting and return speeds to compare planer stroke time at nominal, low, and high cutting-speed cases.
Equation Used
The stroke time is the cutting travel time plus the idle return travel time. Use a stroke length slightly longer than the workpiece so the tool clears at both ends.
- Cutting and return speeds are treated as constant.
- Stroke length includes work length plus tool clearance.
- Acceleration, reversal dwell, and clapper lift time are ignored.
- Return stroke is idle and uses the same stroke length.
The Double Cylinder Planer in Action
The work sits clamped to a long cast-iron table that slides on flat ways inside two heavy uprights — the housings. Above the table, a cross-rail carries one or two tool heads with a clapper box that lifts the cutter on the return stroke. The table reciprocates: it carries the work past a stationary single-point tool on the cutting stroke, then snaps back fast on the idle stroke. The two drive cylinders sit underneath, one on each side of the table centreline, and they push together. That symmetry is the whole point of a double cylinder layout — a single-cylinder planer pushes off-centre, and on a 4-tonne table you'd see the table cock and bind in the ways within a few hundred strokes.
The cutting stroke runs slow, typically 6 to 25 m/min depending on tool material and depth of cut. The return runs 2 to 4 times faster because no metal is being removed. Hydraulic systems use a quick-return valve; older steam and rope-drive double cylinder planers used a tappet-shifted reversing gear. If you notice the table hesitating at stroke reversal, you have either unequal cylinder pressure (one side leaking past its piston seal) or a worn reversing dog — both will show up as a step or scallop on the finished surface where the tool dwells during reversal.
Tolerances on these machines are not subtle. The table ways must stay flat to within 0.02 mm/m over the full stroke length, otherwise long workpieces come off bowed. Tool head feed per stroke is typically 0.2 to 2.5 mm, indexed during the return so the tool doesn't drag a witness line on the cutting stroke. The clapper box must lift cleanly — a sticky clapper pivot is the most common reason for ragged surface finish on an old planer, and it's almost always dried lubricant in the pin bore, not tool geometry.
Key Components
- Twin Drive Cylinders: Two parallel hydraulic cylinders mounted symmetrically beneath the table centreline. Each delivers half the stroke thrust, typically 40-200 kN per cylinder on a mid-size machine. Pressure imbalance over 5% between the two will cock the table in its ways and accelerate ways wear.
- Reciprocating Table: A heavy planed cast-iron platen, often 1.5-12 m long, that carries the work past the tool. Mass is intentional — a 4-tonne table resists chatter from interrupted cuts. The table runs on V-and-flat ways scraped to 0.02 mm/m flatness.
- Housings (Uprights): Two vertical cast columns that straddle the table and carry the cross-rail. On a double-housing machine they are tied at the top with a cap casting, which is what gives the planer its rigidity for heavy cuts up to 12 mm depth in a single pass.
- Cross-Rail: Horizontal beam between the housings carrying the tool heads. It elevates by screw on the housing faces to set cut height and must clamp dead-rigid before cutting — any cross-rail droop shows directly as taper on the workpiece.
- Tool Head with Clapper Box: Holds the single-point cutter and pivots upward on the return stroke so the tool doesn't drag. Feed is 0.2-2.5 mm per stroke, indexed by a ratchet driven off the table reversal. The clapper pivot pin must be a free running fit — typically H8/f7 — and oiled every shift.
- Reversing Gear: Trips the cylinder valves at each end of stroke. Position is set by adjustable dogs clamped to the table side. Dog setting determines stroke length; mis-set dogs either crash the head or waste cutting time on over-travel.
Where the Double Cylinder Planer Is Used
Double cylinder planers earn their floor space wherever a workpiece is too long, too heavy, or too awkward to mount on a milling machine. They're slow by modern standards but they take a deep cut on a long surface without flexing, and they hold straightness over distances no mill bed can match. Most that survive today are in heavy fabrication, railway works, and shipyard tooling shops.
- Locomotive Repair: Machining frame plates and motion-bracket pads on steam-locomotive frames at the Severn Valley Railway works in Bridgnorth, where original 9 m frame stretchers still get refaced on a pre-WWII double housing planer.
- Heavy Machine Tool Manufacture: Finishing the bedways of large engine lathes and horizontal borers at Dean Smith & Grace in Keighley before scraping — a typical 6 m lathe bed is planed in one setup.
- Shipyard Fabrication: Planing the mating faces of rudder horn castings and stern-tube blocks at Harland & Wolff Belfast, where flatness over 4 m is required before line-boring.
- Power Generation: Machining turbine baseplates and condenser support frames at GE Steam Power's Rugby works, where parts up to 8 m long need a flat datum within 0.05 mm.
- Mining Equipment: Surfacing the slewing-ring seats on bucket-wheel excavator pedestals at TAKRAF Lauchhammer — castings 5 m across that no horizontal mill can reach in a single setup.
- Press Tool Manufacture: Roughing large progressive-die shoes and bolster plates before grinding, common in stamping suppliers feeding the European automotive industry.
The Formula Behind the Double Cylinder Planer
The machine's productivity is set by stroke time, which depends on cutting speed, return speed, and stroke length. Knowing this number tells you how many parts you'll get through a shift and whether the tool is being used at the right place on its wear curve. At the low end of the typical operating range — slow cutting speed on tough alloy steel — the tool lasts but throughput is painful. At the high end on cast iron with carbide, you push speeds high enough that flyback inertia at reversal becomes the limit, not metallurgy. The sweet spot for HSS on mild steel sits around 12-15 m/min cutting, 40 m/min return.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tstroke | Time for one full forward + return stroke | min | min |
| L | Stroke length (slightly longer than workpiece to clear tool) | m | ft |
| vcut | Cutting (forward) speed of the table | m/min | ft/min |
| vret | Return (idle) speed of the table | m/min | ft/min |
Worked Example: Double Cylinder Planer in a heavy fabrication shop in glasgow refacing a crane gantry beam
A heavy fabrication shop in Glasgow is refacing the top flange of a 3.0 m long box-section crane gantry beam in S355 structural steel on a 1952 Craven Brothers double cylinder planer. They want to know stroke time per pass at the typical operating point and at the low and high ends of the practical cutting-speed range, so they can plan how many passes a shift will allow before the carbide tip needs reset.
Given
- L = 3.2 m (3.0 m work + 0.2 m clearance)
- vcut,nom = 15 m/min
- vret = 45 m/min
- vcut,low = 8 m/min
- vcut,high = 25 m/min
Solution
Step 1 — at the nominal cutting speed of 15 m/min with HSS-tipped tooling on S355, compute stroke time:
That's roughly 210 strokes per hour. With a 1.0 mm feed per stroke across a 400 mm wide flange, you finish one pass in about 1.9 minutes — fast enough that the operator should be loading the next beam, not watching the table.
Step 2 — at the low end of the practical range, 8 m/min (used when the tip is dulling and you don't want to scrap it before lunch):
Throughput drops to about 127 strokes per hour — a 40% hit. You'd only sit at this speed if surface finish or tool life forces it.
Step 3 — at the high end of the range, 25 m/min with a fresh carbide insert:
That's 300 strokes per hour in theory. In practice on a 70-year-old Craven, the table mass plus reversal shock at 25 m/min cutting + 45 m/min return starts hammering the cylinder cushions. You'll hear the reversal as a thump rather than a tick, and the cross-rail clamps loosen within a shift. Most operators back off to 18-20 m/min on this size of machine to keep the cushions alive.
Result
Nominal stroke time is 17. 1 s, giving roughly 210 strokes per hour. Going to the low end (8 m/min) you lose 40% of throughput at 28.3 s per stroke; pushing to the high end (25 m/min) you gain about 30% at 12.0 s but you start beating up the reversal cushions and the cross-rail clamps loosen mid-shift. The sweet spot for a 3 m structural-steel job on this machine is 15-18 m/min. If your measured stroke time is longer than predicted, three things to check first: (1) cylinder relief valve cracking early under load — the table decelerates mid-stroke instead of holding cutting speed, (2) ways oil starvation creating drag that the cylinder pressure has to overcome, or (3) reversing dogs set too far apart so the table is travelling more than 3.2 m per cycle. The first one shows as a pressure-gauge needle that drops during the cut; the second shows as warm spots on the table V-way; the third you measure with a tape from dog to dog.
Double Cylinder Planer vs Alternatives
Choosing between a double cylinder planer and the alternatives comes down to how long, how heavy, and how flat the workpiece needs to be. A planer is a slow, single-point machine — but on a 6 m bedway it will hold flatness no other process touches without grinding.
| Property | Double Cylinder Planer | Open-Side (Single Housing) Planer | Large Gantry Mill |
|---|---|---|---|
| Maximum work length | Up to 12 m table | Up to 8 m table | Up to 30 m gantry travel |
| Cutting speed | 6-25 m/min reciprocating | 6-25 m/min reciprocating | 30-300 m/min continuous |
| Flatness over 6 m work | 0.05 mm achievable | 0.08 mm typical (single housing flexes) | 0.03 mm with thermal compensation |
| Width capacity (between housings) | Limited by housing spacing, ~1.5-3 m | Unlimited on tool side (open) | Up to 8 m gantry width |
| Capital cost (used / new) | £15-80k used, rare new | £10-50k used | £300k-2M new |
| Power per kg removed | High (idle return uses energy) | High | Low (continuous cut) |
| Tool cost | Single-point HSS or carbide, cheap | Same as double housing | Indexable inserts, high consumable cost |
| Best application fit | Long flat ways, frames, baseplates | Wide work needing tool-side overhang | Complex 3D parts, prismatic milling |
Frequently Asked Questions About Double Cylinder Planer
The table is decelerating into reversal while the tool is still cutting. Either your stroke length is set too short (the cutting speed never reaches steady state before the dogs trip), or one cylinder is leaking past its piston seal so the deceleration ramp differs end-to-end. Set the dogs to give at least 100 mm of free travel beyond the workpiece on each end, and check cylinder pressure on both sides at mid-stroke — they should match within 5%.
If pressures match and dogs are right, look at the clapper box pivot. A clapper that drops back too late after the return stroke will dig in on the first 20-30 mm of the cutting stroke and leave exactly that scallop signature.
If the frame plate is over 4 m long and the surface is essentially flat with simple rebates, the planer wins on capital cost, tooling cost, and resulting flatness — a single-point tool tracking a straight table beats any milling cutter for long flat surfaces. If the plate has pockets, hornblock cutouts, or 3D features, the planer-mill is the only sensible choice because you'd otherwise be doing a dozen setups on the planer.
The decision flips around 4 m. Below that, a large bed mill is faster and more flexible. Above 6 m, the planer is genuinely hard to beat for flatness per pound spent.
Gauge pressure tells you static pressure; it doesn't tell you flow. One cylinder may be passing fluid past a worn piston seal under load, so it delivers less force at the same pressure. Pull the table to mid-stroke, dead-head both cylinders against their relief valves, and watch for pressure decay — the leaking side will bleed down faster.
The other suspect is uneven ways wear. If one V-way has worn 0.1 mm deeper than the other over decades of service, the table sits canted and the cylinders can be perfectly balanced while the work still planes out of square. Sweep the ways with a precision level before blaming hydraulics.
Rule of thumb: workpiece length plus 75-100 mm of overrun on the start end and 50 mm on the finish end. The start needs more because the tool must clear the work fully before the feed indexes during reversal — if the tool is still over the work when feed kicks in, you'll get a witness line on every stroke.
Going much longer than this just wastes time. Every extra 100 mm of stroke adds roughly 0.4 seconds at typical cutting speeds, which over a 4-hour job means a lost half-hour of production for nothing.
Cold-start shock. The clapper box pivot stiffens up as the oil thickens overnight, and the tool doesn't lift cleanly on the very first return stroke. It drags backward across the work, and the leading edge of the carbide takes a hit it wasn't designed for. By the third or fourth stroke the pivot has warmed and freed up and the problem disappears.
Fix is simple: cycle the table dry for 10-15 strokes before engaging the cut, especially if the shop is cold. Better yet, fit the clapper pivot with a light hydraulic oil (ISO 32) instead of way oil — it stays mobile down to 5 °C.
Technically yes, briefly, to finish a critical pass. Practically, no — the unbalanced thrust drives the table hard against one V-way and you'll score the way surface within minutes. A scored way is a multi-week scraping repair.
If a cylinder fails, the right move is to drop the table to the floor (block it), depressurise both sides, and fix the failed cylinder. Trying to limp through one more pass to save a workpiece almost always costs more in machine repair than scrapping the part would have cost.
References & Further Reading
- Wikipedia contributors. Planer (metalworking). Wikipedia
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