A rotary indexing table moves parts between stations on an assembly machine: load, drill, inspect, unload. The table has to stop at each station square and repeatable, then swing to the next without shaking the fixtures. When an indexer is chosen wrong, the table overshoots, dwells too short, or loses position after a few months. Picking between a fixed cam indexer and a programmable servo-driven table is the first decision, and the load and station count drive everything after that.
What an indexing table does
An indexing table converts continuous or controlled rotation into discrete angular moves. At each station it dwells, holds position while work happens, then indexes to the next. The key specs are number of stations, index angle, index time, dwell time, and the repeatability at the stop. A table that indexes fast but cannot hold a fixture steady under a drilling force is no good; the dwell is where the work gets done.
Cam indexers vs servo-driven tables
A mechanical cam indexer, often called a cam drive or roller dial, uses a barrel cam with a fixed number of stops machined into it. The motion profile, index angle, and dwell are set by the cam. It is repeatable, fast, and holds position mechanically during dwell without the motor fighting the load. The trade-off is that you cannot change the station count or index angle without changing the cam.
A servo-driven rotary table uses a servo motor through a gearbox or direct drive. You program any index angle, speed, and dwell, and you can stop at arbitrary positions. It is flexible and suits machines that run multiple products. The trade-off is that during dwell the servo must hold against process forces, and accuracy depends on the gearbox and tuning. If the station count and cycle are fixed and never change, a cam indexer is simpler and more rigid. If changeover and variable angles matter, use a servo table.
Fixed vs programmable: a practical split
On a dedicated assembly line that builds one part for years, cam indexers are common because the motion is proven and the dwell is mechanically locked. The machine indexes, the cam stops the dial on a roller, and the stations are held without consuming motor torque. On a flexible cell that runs three products with different station layouts, a servo table indexes to 4, 6, or 8 stops depending on the recipe. Don’t pay for servo flexibility on a fixed line, and don’t lock a changeover-heavy machine into a fixed cam.
Load and inertia
The biggest sizing error is ignoring the inertia of the dial, fixtures, and parts. A large steel plate with heavy fixtures has a lot of rotational inertia, and the indexer or servo has to accelerate and decelerate that mass every cycle. Calculate the total moment of inertia about the axis, including the dial, each fixture, and the parts. Compare it to the indexer’s allowable inertia or the servo’s inertia mismatch. A servo that can hold the load at rest may still be too small to index it in the required time without overshoot. Heavy dials need either a bigger drive or longer index time.
Index time and motion profile
The index time is how long the move takes; the dwell is how long the table sits. A cam indexer has a fixed motion profile, often a modified sine or modified trapezoid, that keeps acceleration smooth and limits shock. A servo table lets you set the profile, but a fast index on a heavy dial creates high acceleration torque and can excite torsion in the gearbox. If the table shakes or the fixtures ring after the stop, the index is too aggressive. Slow the accel or extend the index time. The last thing you want is the dial still oscillating when the drill comes down.
Repeatability and accuracy
Indexing repeatability is how close the table returns to the same stop, cycle after cycle. Cam indexers hold good repeatability because the stop is set by hardened cam surfaces and rollers; figures in the range of a few hundredths of a millimeter at the fixture bolt circle are common. Servo tables depend on gearbox backlash and encoder resolution. A precision servo indexer with a low-backlash gearbox can match a cam drive, but a cheap gearbox introduces lost motion on reversal. Specify the repeatability at the working radius, where the fixture locates the part, not just at the shaft.
Center through-hole and utilities
Many rotary tables have a center through-hole so you can route air, electrical, or coolant up through the middle to fixtures on top. If your stations need pneumatics or sensors, check the through-hole diameter and plan for a rotary union or slip ring. A table without a center hole forces you to route hoses over the top, which looks messy and can snag. Also consider whether the table needs a rotary distributor for multiple air circuits. This is easy to forget until fixtures are already designed.
Mounting and support
The table mounts to a frame that must be flat and rigid. A base that deflects under the dial tilts the stations and loads the bearings. Mount the indexer on a machined surface or a thick top plate, and shim if needed. The dial itself should be stiff enough not to sag at the edge under fixtures; a thin plate that deflects ruins station repeatability no matter how good the indexer is. For large dials, a center support or a cross-roller bearing carries the moment load.
Process forces during dwell
During dwell, a cam indexer is mechanically locked, so drilling or pressing forces go through the cam and rollers into the frame. A servo table must hold those forces with motor torque, and a strong process torque can overpower the motor or push the axis off position. If a station applies a large tangential force, either size the servo to hold it or add a mechanical pin or shot bolt that locates the dial during work. Don’t assume a servo can hold a press force just because it can index the load.
Direct-drive tables
A direct-drive rotary table uses a large torque motor with no gearbox, giving zero backlash, high stiffness, and very fast indexing. These suit high-precision, high-throughput applications like electronics assembly and inspection. They are expensive and need a stiff bearing, usually a cross-roller or YRT bearing, to carry the load. For most general assembly, a geared servo table or cam indexer is enough. Use direct drive when the required index time or repeatability genuinely rules out a gearbox.
Common mistakes
Sizing for static load but not inertia, using a fixed cam on a changeover machine, specifying repeatability at the shaft instead of the fixture, ignoring the center through-hole, mounting to a flexible frame, and expecting a servo to hold heavy process forces are recurring errors. Count the stations, calculate inertia, set a realistic index time, and decide fixed versus programmable based on whether the product changes. A well-chosen indexer runs for years with the dial stopping square every cycle.
Maintenance and wear
Cam indexers run in an oil bath and need the level checked and oil changed on schedule. A cam that runs dry wears the cam track and rollers, and repeatability drifts. Listen for noise and check for backlash by trying to move the dial against the stop. Servo tables need the gearbox lubricated and the brake checked. Keep the dial top clean so swarf does not fall into the indexer. A yearly inspection catches wear before a station goes out of square.
Finally, treat the indexer as part of the machine structure, not an accessory. Keep the oil level right, check the dial against the stops for backlash, and inspect yearly. A cam that runs dry wears fast; a servo gearbox that loses lubrication develops play. Clean the dial top and keep swarf out. The station repeatability is only as good as the maintenance behind it.
Treat the indexer as part of the structure. Keep oil level right, check the dial against the stops for backlash, inspect yearly. A cam that runs dry wears fast; a servo gearbox that loses lubrication develops play. Keep swarf out. Repeatability is only as good as the maintenance.
Treat the indexer as part of the structure. Keep oil right, check the dial for backlash, inspect yearly. A dry cam wears fast; a dry gearbox develops play. Repeatability depends on maintenance.
Treat the indexer as part of the structure. Keep oil right, check the dial for backlash, inspect yearly. A dry cam wears fast.
Treat the indexer as structure. Keep oil right, check backlash, inspect yearly.
Bottom line
Choose a cam indexer for fixed, high-rigidity stations and a servo table for programmable angles and changeover. Size the drive for dial inertia, not just weight, and specify repeatability at the fixture radius. Plan the center through-hole, mount to a rigid plate, and handle dwell process forces with a lock or pin. Get these right and the table indexes fast, dwells steady, and stops where the fixtures need it.
