Ball Screws on Fanuc-Controlled Machines
The ball screw turns servo motor rotation into straight-line travel on each axis of a Fanuc-controlled mill or lathe. The screw and nut come from the machine builder’s supply chain and not from Fanuc itself. Ball screw repair falls under mechanical rebuild work on the screw, the nut, and the support bearings.
A preloaded ball nut holds the slide with zero clearance at each change of direction on the axis. Wear on the balls and races first takes away the preload and then opens up true clearance in the nut. That clearance shows up as backlash and leaves the table still for a moment at each screw reversal.
Fanuc Ball Screw Repair Shops
Northline Industrial
Redford, MI
Brands: Fanuc, Makino, Okuma
Signs of Ball Screw Wear
Growing backlash, ballbar reversal spikes, and following error alarms count as the classic signs of ball screw wear. A gradual loss of positional accuracy and slow drift in part dimensions point to a worn screw on that axis. A visual check of the screw can turn up pitting, corrosion, heat discoloration, and metal debris in the grease.
Some of these signs overlap with servo motor and encoder faults on the same axis of the machine. A Fanuc following error alarm can come from a worn screw or from a failing encoder on the axis motor. A check of both the mechanical and electrical sides keeps a good part off the repair bench and saves a wasted shipment.
Measuring Wear and the Limits of Compensation
Backlash readings at several positions along the axis catch wear concentrated in the most used part of the stroke. A reading worse in the middle of the stroke than at the ends points to screw wear and not support bearing wear. One ball screw repair guide rates a diametral lash of 0.0035 inch as about 80 percent wear.
Backlash compensation in the Fanuc control adds a correction at each axis reversal and hides a steady gap. Compensation masks small and even wear but cannot fix wear in one zone of the stroke or lost nut preload. Rising compensation values over time mark a wear trend and deserve a log entry for each axis.
What Causes Ball Screw Wear
Lubrication starvation ranks as the most common cause of ball screw wear on CNC machines. A missing film of grease or oil turns rolling contact into metal-on-metal wear and wipes out the preload fast. Contamination through damaged wipers and way covers wears the screw and nut down as well.
A hot screw grows in length and shifts axis positioning even with no wear on the screw. The heat can raise internal loading and speed up wear on the screw and nut. Too much preload in the support bearings leads to early wear and motor overheating.
Ball Screw Repair Options
Repair at an early stage starts with cleaning, fresh lubrication, and new wipers and often stops further wear. A nut rebuild typically means disassembly, cleaning, new balls, a new preload setting, and reassembly to spec. The same repair guide sizes replacement balls 0.001 inch larger for every 0.003 inch of wear to restore preload.
The guide puts a full regrind of the screw and nut at about 14 days and about 55 percent of the cost of a new assembly. Regrinding calls for a new heat treatment to bring the surface hardness back to the 58 to 62 HRC range. Past a repair cost of 65 percent of the new price the guide recommends a new assembly.