Technical Guide
Counterface Roughness and Running-In of a PTFE Slideway Liner
A liner is only half of a sliding pair. This guide covers the surface it runs against: how to read the finish on the cast iron way, how to measure it, and how the first hours of movement decide whether the pair settles or wears.
The running face decides how a new liner behaves in its first hours. A ground cast iron way with a consistent finish and no isolated deep marks lets the PTFE bed in with a thin transfer film and stable friction. A face left rough, torn or glazed forces the liner to wear itself into shape, and that wear shows up as loose clearance and stick-slip before the machine is back in production. Judge the running face before the liner goes on, not after.
Two faces, two different jobs
A liner has a bonding face and a running face, finished for opposite reasons. The bonding face is etched so adhesive can key to it, and it is prepared once, before clamping. The running face slides against the machine way, and after machining it is the finish of that cast iron counterface that controls friction, bedding and wear. Bonding preparation has its own checks, covered in the bonding mistakes guide; this page is about the face the liner runs on.
What roughness actually does to a PTFE liner
At the scale of a sliding pair, contact does not happen over the whole apparent area. It happens where asperities meet. A rough counterface concentrates load on fewer, higher points, so local pressure at those points sits far above the nominal figure, the PTFE is forced to conform by wearing or flowing, and the running clearance changes while the machine is still being commissioned. A very smooth face is not automatically better: a glazed surface with no texture holds no lubricant film, and a face with one deep scratch pattern behaves like a rough one whatever its average reading says. The useful question is not smooth or rough, but consistent and free of isolated deep features.
Measuring the running face before a rebuild
- Clean the face back to bare metal. Solvent wipe, then a light pass with a non-woven pad. Oil on the surface hides both roughness and damage.
- Look before you measure. Straight edge along the length, raking light across the width. Record deep grooves, gouges, fretting patches, glazing and any repaired area separately, because these matter more than the average number.
- Measure with a portable roughness tester where one is available. Take three traverses per face, perpendicular to the sliding direction, at both ends and the middle of the travel, and record the cutoff and units used.
- Record a peak measure such as Rz or Rt alongside Ra. An average can look acceptable on a face carrying a few deep scratches, which is exactly the condition that damages a new liner.
- Write the readings into the rebuild record with the instrument and settings, so the next repair can compare like with like.
Running-in is a controlled process, not a waiting period
A freshly machined liner and a freshly prepared counterface do not start with full contact. The first movements are where the pair finds its fit, and that stage can be managed rather than waited out.
- Start at low speed over the full travel, unloaded or lightly loaded, so the liner wets the counterface evenly.
- Check lubrication before the first stroke, not after. Oil grooves and metering have to work from the start; a dry first hour is where most avoidable damage happens.
- Watch for friction that rises rather than falls. A pair that gets harder to move as it warms is not bedding in.
- Inspect after the first short runs. The contact pattern should broaden evenly, with no bright polished strip at one edge.
- Add load in steps, and measure friction with a pull test rather than judging by hand. The pull-test method sets out how to record breakaway and running force.
| What the face shows | What it usually means | What to do before bonding |
|---|---|---|
| Even grey finish, no isolated deep marks | Suitable base for bedding in | Record readings, keep the surface clean, proceed |
| Bright polished bands with no visible texture | Glazed high spots, lubricant film will be thin | Light re-texture or re-scrape, then re-measure |
| A few deep grooves across the travel | Local pressure points and a path for debris | Machine or scrape out before bonding; do not cover them |
| Torn or smeared metal from a previous failure | Surface damaged by seizure, not only by wear | Cut back to sound metal before any reading is trusted |
| Fretting or corrosion patches | Contact was intermittent, geometry or lubrication suspect | Check flatness and lubrication, not finish alone |
When roughness is blamed for something else
Two other conditions produce the same symptom as a bad counterface. Contamination past the wipers grinds between liner and way and leaves a scratched dull pattern that looks like a finish problem. Starvation from blocked oil grooves produces a friction rise that looks like roughness but disappears once lubrication is restored. Check both before re-machining a face that may be sound. Wear that starts at the edges rather than the middle is more often wiping or alignment than surface finish, and a sudden change after a rebuild points to clearance and contact pressure rather than texture.
Should a counterface be polished before fitting a liner?
No. A polished face carries no texture to hold lubricant and can behave worse than a correctly ground one. The target is a consistent controlled finish, not the smoothest possible surface.
Can the running face be judged by feel?
Only as a screening step. A fingernail drag finds deep grooves and steps, but cannot separate a consistent fine finish from a glazed one, and says nothing about peak height. Use it to decide where to measure, not instead of measuring.
Does a rougher face help the liner grip?
No. The liner is bonded to the machine slide, not to the way it runs on. Grip comes from the adhesive joint on the bonding face; the running face only affects friction, bedding and wear.
How long does running-in take?
There is no fixed time. Judge by behaviour: the contact pattern broadens evenly, friction settles instead of climbing, and temperature stabilises at normal load. If any of the three moves the wrong way, stop and investigate.
Ordering the strip for the repair
TEBAK supplies the liner in Blue, Green, Brown and Brick Red, in thicknesses from 0.8 to 3.2 mm and widths from 100 to 305 mm, with two-part epoxy adhesive and Tuschierpaste for the contact check after machining. Send the measured face condition with the cut list and the machining allowance available on the slide, and the strip can be matched to that allowance instead of a stock size that happens to be close.
For a repair brief, send the readings per face, the way width and travel, photos of the finish under raking light, and the clearance you need to reach. That is enough to specify thickness and allowance without a site visit, and it keeps the answer to one round of questions. Use the load and pressure limits guide to confirm the bearing pressure is inside range before the order is placed.