Andy AC Servro System Application in CNC Boring Machines

The Andy AC servro system is not one device. It covers the feed drives on X, Y, Z and often W, plus the spindle orientation loop. Each axis pairs an AC servo motor with a drive amplifier that closes a position loop against a scale or encoder. The CNC reads the feedback, compares it to the commanded position from the part program, and corrects the following error in real time.

Boring is unforgiving about this loop. A single-point tool removes material along a long, thin wall, so any lag between command and actual position shows up as taper or chatter in the bore. Servo gain that is too low lets the axis drift; gain that is too high excites the mechanical structure. The useful setting usually sits just below the point where the first resonance appears.

Spindle orientation matters as much as the linear axes. Tool change, boring bar clamping and the start of a rigid tapping cycle all need the spindle parked at a repeatable angle. The Andy AC servro system holds that angle with a position loop rather than a mechanical detent, so the same setting works across spindle speeds. Drift here of even 0.05° can move a boring bar tip by several micrometres on a long reach.

A servo-driven boring axis earns its cost when the bore position is called out tighter than the machine's thermal drift over a shift. Castings with multiple coaxial bores are a good example. So are housings where the bore must stay square to a milled face within a few micrometres. In those jobs the servo loop, not the operator, holds the number.

Two conditions usually rule it out. First, low volume work where a manual boring head can hit the tolerance with a dial indicator. Second, bores that are short and large in diameter, where a boring mill with a rigid quill and a simple feed box is cheaper to own and just as accurate.

The middle ground is a retrofit. Adding Andy AC servos to an older boring machine makes sense when the iron is still straight, the ways still hold oil film, and the spindle bearings are within specification. A worn machine with a new servo package will still cut a tapered bore, only faster.

One more check before committing: backlash. The servo cannot fix slack in the ball screw or the gearbox. Measure it first. If backlash exceeds the position tolerance you are chasing, the money goes into the mechanical rebuild, not the drive.

Start with the mechanical side. Level the machine, then check the coupling between motor and ball screw. A loose coupling shows up as a spike in following error at reversal, and no gain setting will hide it. Set the position loop gain low enough that the axis moves without a growl at slow feed, then raise it in small steps until the first sign of vibration, and back off.

Feed forward is the next knob. It cuts the lag that appears when the axis accelerates into a cut, which is exactly where a boring bar digs in. Add feed forward until the following error during a rapid move is roughly the same as during a slow move. That is a useful sign the loop is tracking, not fighting, the command.

Watch the current trace, not just the position trace. A current spike at the same point in every revolution points to a mechanical fault: a tight spot in the screw, a damaged bearing, a belt with a hard spot. The servo will keep the position correct while the current climbs, until it faults.

Document what you changed. Servo parameters drift over years of small adjustments, and the next engineer needs a baseline. Date the file, note the axis, and keep it with the machine records.

Ask for the loop bandwidth and the following error at the feed rate you actually use. A datasheet gain number means little without the mechanical load attached. Ask how the drive handles backlash compensation, and whether that compensation is applied in the CNC or the drive. It matters at reversal.

Ask about the encoder interface and the scale protocol. Mixing a new drive with an older scale can force a converter, which adds a failure point and a small lag. Get the wiring diagram for the exact machine, not the family.

Spare parts matter more than peak performance on a machine that runs two shifts. Confirm the drive and motor lead time in writing, and confirm that the parameter set is backed up somewhere you can reach. A servo package that stops a boring machine for three weeks costs more than the difference in gain.

Finally, ask what the supplier will do when the bore is out of round after installation. A good answer names a measurement: a roundness trace, a taper check, a warm-up cycle. A vague answer is a warning.