Feeder Calibration: Pick Position, Tape Lift and Pitch
Feeders are the largest population of moving parts on a placement machine, and they are the ones most often excluded from the calibration schedule. A feeder that picks reliably is assumed to be correct, even when its pick position has drifted a third of the way across the pocket.
The result is a machine that places accurately but picks inconsistently, which appears in the data as an occasional placement offset, a nozzle that wears unevenly or a component that is dropped between the feeder and the board.
What a Feeder Has to Deliver
A feeder has to index the tape by exactly one pocket, present the component at a defined height and position, and hold the tape steady while the nozzle descends. It also has to peel the cover tape without lifting the carrier out of its track.
Those four functions are set by the mechanical condition of the feeder rather than by its program. The sprocket, the tape guides, the peel mechanism and the drive spring all wear, and they wear at different rates, so a feeder can be good in one function and poor in another. That is why a feeder that fails its check is usually repairable rather than scrap, provided the failing function is identified.
Pick Position and Its Tolerance
Pick position is the centre of the pocket as the nozzle sees it, and it is normally held within about 0.1 mm. That is a generous figure compared with a placement tolerance of 0.05 mm, and it is deliberate, because the nozzle can recover a small pick error by centring the part with the vision system.
The recovery has a limit. A part picked well off centre sits at an angle on the nozzle and may be lost entirely, and a part picked at the edge of the pocket risks being damaged. Where the vision system reports a rising centring correction, the feeder pick position is the first thing to check, and the correction should be read from the machine log rather than remembered, and the offsets themselves are the subject of the notes on placement offset control.
Tape Lift Height
Tape lift is the amount by which the carrier is raised above its track at the pick point, and it is typically between 0.2 and 0.5 mm. The lift lets the nozzle descend into the pocket without striking the carrier, and it also frees the component from the pocket walls.
Too little lift causes pick errors on small components and damaged nozzles on large ones, while too much lift lets the tape move laterally, which changes the pick position. The lift is a mechanical setting, and it is the one a worn feeder loses first. It is set with a gauge at the pick point rather than by eye, because a few tenths of a millimetre is hard to judge.
Pitch Accuracy Across the Reel
Pitch accuracy is the accumulated error in the sprocket index over a run of pockets, and it should stay within about 0.05 mm over a hundred indexes. The error accumulates because each index is slightly wrong, and the tape also stretches as it is pulled from a full reel.
A feeder with a pitch problem picks correctly at the start of the reel and misses towards the end, which is a pattern that is easy to mistake for a bad reel. Checking the same reel in a known-good feeder separates the two. A reel that fails in two feeders is a tape problem, and a reel that passes in one is a feeder problem.
Sprocket, Motor and Spring Wear
The sprocket teeth carry the whole indexing load, and a worn tooth changes the position of every pocket after it. Wear on an electric feeder is more subtle, because the motor compensates until it reaches the end of its range, and then the error appears suddenly.
Springs and friction plates lose tension too, and that loss shows as a tape that slips back after indexing. Sprockets are replaceable on most feeders and should be treated as a consumable with a counted life, in the same way as the parts listed in the notes on feeder and nozzle inventory.
Calibration Methods and Equipment
The simplest method uses the machine itself: a component is picked, carried to the vision camera and the centring correction is read. Repeating that on a fresh pocket at the start, middle and end of a reel gives a direct measurement of pick position.
Dedicated calibration stations measure the same thing more precisely, with a camera looking down into the pocket and a fixture that reproduces the machine mount. Where a shop has many feeders, the station pays for itself, because it can check a feeder without occupying the placement machine, and it can also check whether the vision alignment of the machine itself has moved.
Linking Feeder Drift to Placement Offset
Feeder drift appears in placement data as an offset that changes with position along the tape rather than with position on the board. A pattern that repeats every reel is a feeder problem, while a pattern that follows the board is a machine or a fiducial problem. Feeder effects repeat at the same interval along the tape, which makes them easy to recognise once the data is plotted.
That distinction is the main reason to record the feeder identification against the placement result. Without it, a drifting feeder is diagnosed as a machine error, and the machine is adjusted until the board is right and every other product on the line is wrong.
Calibration Intervals and Records
A workable schedule checks each feeder when it is loaded for a new product and formally calibrates it at an interval of one to three months, or on a count of picks. Feeders used for fine pitch parts are checked more often than those feeding coarse components.
The record should carry the feeder number, the date, the three measurements and the action taken. A feeder that fails twice should be taken out of service rather than recalibrated again, because repeated failure points at a part that is at the end of its life. Feeders used on the most demanding product should be reserved for it rather than rotated through every job.
Handling, Storage and Spares
Feeders are precision devices that are usually stored on racks with the tape still threaded and the cover open. Bumping a rack, stacking feeders or leaving them on the machine over a shutdown all cause small changes that appear in the calibration.
Keeping a small stock of sprockets, springs and tape guides, and replacing them at the same time as the calibration, costs little and removes the most common cause of drift. Nozzle condition belongs in the same routine, since a worn nozzle mimics a feeder fault by picking at an angle, and a nozzle replaced without re-checking the pick position wastes the new part, as described in the notes on vacuum nozzle care.
FAQ
How often should feeders be calibrated? When loaded for a new product, and formally every one to three months or on a pick count. Fine pitch feeders are checked more often than those feeding coarse parts.
What pick position tolerance is achievable? About 0.1 mm to the pocket centre, which is looser than the placement tolerance because the vision system recovers the difference. Rising centring corrections indicate drift.
Why does a feeder pick well at the start of a reel and badly at the end? Accumulated pitch error or a stretched carrier tape. Testing the same reel in a known-good feeder shows which of the two is responsible.
