Secondary Maintenance Sheet Of CNC Machine Tool

• 1
  What separates a secondary maintenance sheet from daily checks
• 2
  Fields every secondary maintenance sheet must carry
• 3
  How to take the numbers on a secondary maintenance sheet
• 4
  Where a secondary maintenance sheet stops being useful
• 5
  Which items belong on which service level

Daily and weekly checks keep a machine running: coolant concentration, way lube level, air pressure, spindle taper wipe, chip conveyor. They take minutes and need no disassembly. The secondary maintenance sheet of CNC machine tool covers the next level. The machine is stopped for a planned window, guards and covers come off, and the technician measures things that cannot be seen from the operator panel.

The difference is not effort. It is evidence. A daily log says the machine was checked. A level-2 sheet says the X-axis ball screw backlash was 0.012 mm, the Y-axis was 0.006 mm, and both were recorded with a date and a technician name. That number is what tells you whether the machine is drifting or stable.

Typical interval is 2,000 to 4,000 spindle hours, or once a year for a machine on a single shift. A heavy hogging cell may need it every six months. Write the interval on the sheet header so nobody has to guess.

Three conditions justify the stop. First, a part feature that used to hold ±0.01 mm now needs an offset every few hours. Second, the surface finish has moved a full Ra step without a tool change. Third, an axis sounds different or the servo load meter reads higher on the same program. Any one of these means the daily check has run out of answers.

A sheet without identifiers is useless six months later. The header block needs the machine model, the machine serial or asset number, the plant and cell location, the date and time window, the technician name, and the approver. Add the running hours at the start of the stop. Without that number you cannot tell a machine that failed early from one that simply ran longer than planned.

The measurement block is the core. List each axis with backlash, positioning repeatability, and squareness or parallelism where the machine has a reference. Record spindle runout at the taper, drawbar force, and taper contact pattern. Record way lube pump output and the condition of the filter. Record coolant pH and concentration. Each line gets a measured value, a tolerance, and a pass or fail mark.

The action block records what was done, not what was planned. If a linear guide was re-preloaded, write the shim size. If a ball screw was re-lubricated, write the grease grade and quantity. If nothing was changed, write none. A blank cell is read as not checked, and that is the most expensive entry on the page.

The last block is the follow-up list. Any value outside tolerance but not yet repaired goes here with a target date and an owner. This is what turns a maintenance record into a planning tool. A sheet that closes every line as OK usually means the tolerances were set too loose.

Backlash is measured with a dial indicator on the table against a fixed reference, then the axis is jogged in 0.01 mm increments and the reading is taken in both directions. Do it at three positions along the travel. The worst value is the one that goes on the sheet, not the average. For most VMCs a value above 0.02 mm on a finishing axis is worth a re-preload.

Spindle runout is checked at the taper with a test bar, not with a finger on the spindle nose. A test bar 300 mm long read at the far end shows the real error the tool will see. Values above 0.01 mm total indicate a problem with the taper, the bearings, or chips embedded in the seat. Clean the taper with a lint-free wipe before the indicator touches it.

Squareness needs a granite square or a laser, not a machined block. A block that was cut on the same machine will repeat the machine error. Check squareness in X-Y first, then X-Z. A drift of 0.02 mm over 300 mm is enough to throw a tight bore off position on a tall part.

Repeatability is a separate number from backlash. Command the axis to the same point twenty times from the same direction and record the spread. A tight backlash with poor repeatability points to a loose encoder coupling or a failing bearing, and re-preloading the screw will not fix it. Record both numbers.

A sheet is a measurement tool. It cannot fix a machine that has hit a hard limit. If slideway wear has changed the geometry so much that the machine no longer holds a straight line over its full travel, re-preloading will buy weeks, not months. Re-scraping or guide replacement is the honest answer.

Spindle bearings are the second limit. Once a spindle shows runout above 0.01 mm after a proper warm-up and clean taper, and the bearing temperature climbs within the first hour, the spindle is due for a rebuild. Adding grease and re-recording the same runout on the next sheet only delays the failure.

Control and drive electronics sit outside the sheet as well. Servo tuning drift, encoder faults, and drive alarms are not mechanical wear and will not appear in backlash numbers. When the sheet is clean but parts still come out wrong, the next step is a control-side diagnostic, not another mechanical service.

The third limit is economic. If the cost of the level-2 stop plus the follow-up repairs approaches a large share of the machine value, a rebuild quote is the better use of the budget. The sheet gives you the data to make that call. Guessing from memory does not.