Head geometry controls flatness, removal rate, finish and edge work more than horsepower. The two heads look similar under the shroud, but the wrong choice shows up as low spots, swirls or early wear.

Both heads run diamond shoes on a metal plate early and resin pads through polishing. The drive differs: twin discs oppose on one carriage, a central drive turns three or four satellites on their own axes.

How the two head types move

A counter-rotating head carries two 200-250 mm discs driven opposite on one carriage. Opposing torques cancel, so it does not walk sideways; width 400-600 mm, speed 300-600 rpm, weight 250-600 kg.

A planetary head mounts three to four 100-150 mm satellites on an orbiting carrier. Each spins at 600-1200 rpm while the carrier orbits at 30-60 rpm; width 500-800 mm, weight 300-800 kg, and the ratio sets finish evenness.

Flatness and profile leveling

For high spots, trowel marks, adhesive or old topping, the twin-disc head is usually stronger. Two broad patches keep cutting the crest, pulling PTV over a 2 m straightedge to 1.5-3 mm in the metal phase, versus 1.0-2 mm for planetary. Keep CSP 3-4 for polishing, CSP 1-2 only for exposure.

Removal rate, weight and downforce

Removal depth depends on grit, aggregate hardness, pressure and dwell. At 16/20 grit the twin-disc removes 0.1-0.3 mm per pass; at 30/40 grit, 0.05-0.15 mm. Planetary satellites cut cleanly at finer grits but remove less in heavy prep; check 3-5 points per 100 m².

Swirl, crescent marks and finish consistency

Swirl risk is the main reason buyers compare the two. A planetary head can leave crescent orbit marks from uneven overlap, mismatched ratio, slow spindle or worn pad, faint arcs under 300-500 lux. The twin-disc leaves a less directional pattern; swap pads left-right and front-back.

Edge and perimeter treatment

Neither head reaches the wall alone, but the gap differs. The twin-disc with an edge shoe works 10-20 mm from a wall or column; a planetary shroud leaves a 30-50 mm hand band. Match edge grits 16/20/30/40 then 100/200/400/800, blend a 150-200 mm overlap, check the holder.

Tooling, plate and consumable compatibility

Confirm the shoe and pad interface in stock before buying. Metal shoes bolt to a six- or eight-bolt plate or sit in a magnetic pocket; resin pads use a matching-diameter velcro or quick-connect plate. The two heads need different plates; keep one spare set, shoe life 40-80 h, pads 60-120 h.

Wear pattern, head life and service signs

How the abrasive wears shows which side carries load. A head pressing harder on one disc rounds its shoes faster and repeats a floor pattern. Inspect wear every 40-60 h; a segment-height spread over 2 mm points to spindle, bearing or leveling trouble, not the consumable.

• One disc shoes 30% shorter: disc height or bearing play on that side.
• One satellite glazes or loads up: slow spindle, weak drive or seized bearing.
• Even wear, square segment tops: bond matches aggregate; keep the sequence.
• Rounded shiny segment faces: bond too soft, or downforce too low.
• Flat scratched tops with fresh diamonds: bond too hard; dress or switch grade.

Service the drive train: gearbox oil, spindle play at each carrier, shroud seal. A well-maintained head runs 1500-3000 hours before a bearing or gearbox rebuild; ordering head and consumables together secures a better unit price.

Matching head to the job: selection checklist

Compare against the actual job mix, not headline horsepower. Most fleets do one leveling and prep job in three polishing jobs; a head strong at one phase but weak at the other sits idle. Changeable-plate shared-frame machines give the best mixed flexibility.

1. List the last 12 months of jobs: leveling, prep, grinding, polishing, edge share.
2. Measure tolerance targets: PTV over 2 m, target CSP, whether aggregate exposure is required.
3. Check stock already carried: plate pattern, bolt or magnetic interface, pad diameter, bond grades.
4. Verify weight and weight-per-disc against the hardest local concrete, not the easiest sample.
5. Confirm vacuum and water match working width, and edge offset matches typical perimeter geometry.

Keep the selection record in the machine file: head type, disc count, plate interface, part numbers and first-service wear spread. It lets a dealer reorder without re-measuring and sets a baseline for new consumable batches.