Liner

A liner in CNC manufacturing is a sleeve, tube, or insert fitted into a component to control wear, support stock, reduce clearance, or provide a replaceable surface. In CNC lathe work, a spindle liner is a bar-supporting tube installed in the spindle bore to reduce the effective I.D., typically sized with 0.030–0.060 in clearance over the bar diameter.

On a bar-fed CNC lathe, the spindle liner is selected to match both the stock diameter and the spindle bore geometry. The liner reduces the effective bore so the bar passes through with minimal radial play but still enough clearance to avoid binding. It works with the bar feeder and puller to stabilize long stock during rotation, especially on small-diameter bars where unsupported length would amplify runout and chatter. A correct liner keeps incoming stock guided concentrically through the headstock, which improves surface finish, lowers tool wear, and holds tighter tolerances. In a typical setup, machinists check liner I.D. against bar size and inspect for wear or scoring before long production runs. For maintenance, liner bores should be measured regularly to ensure clearance hasn't grown enough to reintroduce bar whip.

• ⚠️Oversized liner clearance: An oversized liner permits excessive radial play, so long bar stock whips inside the spindle. This creates chatter, poor surface finish, and fluctuating diameters on turned parts.
• ⚠️Undersized or misaligned liner: A too-tight liner or one that sits off-center causes the bar to scuff, drag, or gall against the wall. Feeding becomes intermittent, and the stock surface can be scored during long runs.
• ⚠️Linear interpolation confusion: The control command G01 describes a straight-line move, not a physical liner. Mixing up the terms can lead to wasted setup time while searching for a sleeve that isn't part of the program.