The Reason Why Machinery Worsens, and How to Find the Deep Cause

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  Machining Symptoms and Their Usual Causes
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  The reason why machinery worsens is usually thermal, not mechanical
• 3
  Squareness and level errors that build up quietly
• 4
  Guideway and ball screw wear: what the numbers look like
• 5
  When the machine is fine and the process is the problem
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  Servo, encoder, and drive faults that mimic mechanical wear
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  Step by step: isolating the cause

Almost every machine that loses accuracy over a shift loses it to heat first. The spindle grows, the ball screw stretches, and the bed changes shape at a different rate than the column. A machine that holds ±0.005 mm at 08:00 can sit at ±0.012 mm by 14:00 without a single part changing.

Look at the numbers. A 100 mm steel ball screw grows about 0.0012 mm per °C. Run the axis hard for three hours and the screw can climb 5–8 °C above ambient. That is 0.006–0.010 mm of position error before the spindle has even moved.

This is why an unloaded machine looks fine and a loaded one does not. The error only shows up once the machine is doing real work, which makes it easy to blame the program, the tool, or the operator.

Measure before you adjust anything. A spindle logger left in the taper for a full shift will tell you more than a week of guessing.

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  Warm-up mattersRun 20–30 minutes at working speed before the first tight-tolerance cut.
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  Check the coolantCoolant below 18 °C can chill one side of the casting and pull it out of shape.

A machine can be level on day one and out of level six months later. Concrete floors move, especially in a plant with heavy forklift traffic. A 0.02 mm/m twist on a 1,000 mm bed shows up as 0.02 mm of error at the ends of the travel.

Squareness between X and Y is the next one to check. If the column has shifted by 0.01 mm over 300 mm, every pocket and every bolt pattern comes out slightly skewed. The parts still fit, but assemblies start to bind.

The tell is a consistent error direction. Thermal drift wanders. Geometry errors repeat. If the same corner is always high, stop looking at the spindle and get a squareness check done.

On our own 5-axis centers we re-check squareness on a fixed calendar, not when a job goes wrong. Reactive maintenance costs more than scheduled maintenance every time.

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  Level firstA precision level on the bed tells you more than a laser on the spindle.
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  Write it downKeep a log. A 0.005 mm move per quarter is a trend, not noise.

Linear guideways and ball screws wear in a predictable way. Preload drops, backlash rises, and the machine starts to overshoot or lag. The first sign is usually a rougher finish, not a size error, because the tool is being asked to follow a path the axis cannot hold.

Measure backlash with a dial test indicator on the table. Bring the axis in from one direction, zero the indicator, then reverse and read the jump. Anything above 0.010 mm on a machine that used to hold 0.005 mm is worth investigating.

Ball screw wear shows up as a growing error toward the middle of travel, where most of the cutting happens. That is the opposite of a thermal error, which tends to grow with time rather than with position.

A worn screw can often be re-preloaded or replaced. A worn casting cannot. That is the line between a repair and a rebuild, and it should be a commercial decision, not an emotional one.

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  Backlash testIndicator on the table, reverse the axis, read the jump.
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  Position testLaser or ball bar across the full travel, not just the middle.

Some machines get blamed for a process that was never stable. A 4 mm end mill hanging 60 mm out of the holder will chatter on any machine in the shop. Shorten the overhang to 25 mm and the same cut runs clean.

Rigidity is a chain. The part, the vise, the fixture, the table, the column, and the tool all add up. The weakest link sets the result. If you change the machine and nothing improves, the weak link was never the machine.

Tool wear is the other quiet one. A coated carbide insert that has run 40 minutes past its limit will push cutting forces up by 20–30%. The machine looks like it is losing accuracy. The insert is just dull.

Track tool life by count or by spindle hours, not by ear. Then compare the change in surface finish against a known-good part from the same program.

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  Overhang ruleKeep tool overhang under 4× diameter where geometry allows.
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  Fixture firstCheck the fixture before you check the spindle.

Not every accuracy problem is metal. A dirty encoder disc, a loose feedback cable, or a drive that is losing its tuning can produce the same symptom as a worn guideway: size scatter, overshoot, and a finish that gets worse as the day goes on.

Encoder faults are usually intermittent. The axis jumps a few counts, the controller corrects, and the part is off by 0.01 mm in one spot. Repeat the program and the error moves. That pattern points at feedback, not at the screw.

Voltage sag on a heavily loaded servo causes the drive to fall behind the command. On a multi-shift plant this shows up in the afternoon, when other machines are running. Log the incoming supply for a week before you pull the motor.

On a machine that has run for years, a re-tune of the servo loop is cheap and often buys back most of the lost accuracy. It is the first thing we check on a machine brought in for a rebuild.

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  Check cablesFlex the feedback cable while watching the position display.
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  Log the supplyA week of voltage data saves a lot of guessing.