A CNC lathe with a guaranteed positioning accuracy of ±0.005 mm was repeating to ±0.02 mm after six months. The machine was level at installation — checked with a precision level, adjusted with leveling screws. The customer blamed the ballscrews. The ballscrews were fine. The machine was no longer level. The leveling screws had settled. The machine was standing on four jack screws in an un-grouted bed. The frame had twisted 0.05 mm across the base. The twist was enough to ruin the accuracy.
Machine tools are aligned at the factory. They’re designed to hold that alignment — but only if the machine base is supported correctly. A machine on leveling screws alone is a machine on four points. The base flexes between the points. The frame twists. The twist moves the spindle axis relative to the ways. The machine that was ±0.005 at installation drifts to ±0.02 as the base flexes and the screws settle.
The grouting decision
Two ways to install a precision machine. On a grouted base: the machine is leveled on screws, then epoxy grout is pumped under the base plate, filling the gap. When the grout cures, the machine is supported over its entire footprint. The screws become secondary. The base can’t flex. The alignment holds for years.
Without grouting: the machine stands on the screws, typically four to eight of them. Each screw carries a load. The base between the screws is unsupported. Over time, the screws settle (the pad under each screw compresses, the concrete creeps), the load redistributes, and the machine twists. The twist is small — 0.05 mm — but it’s 10x the machine’s positioning tolerance.
The rule: any machine with a positioning tolerance tighter than ±0.02 mm gets grouted. That includes most CNC machines, jig borers, CMMs, and precision grinders. Machines with looser tolerances (standard mills, punches, shears) can run on screws alone, but they should be re-leveled after the first month.
What actually happened
The lathe was leveled on screws at installation. The installer used a precision level, got it to 0.02 mm/m, and signed off. No grouting. Six months later, the operator noticed the Z-axis was cutting tapered. The level check showed 0.08 mm/m — four times the installation value. The screws had settled unevenly. The re-leveling took 2 hours and restored the accuracy to ±0.005. But it would drift again in six months. The permanent fix was grouting: the machine was re-leveled, then epoxy grout was pumped under the base. The grout cured overnight. The machine has held ±0.005 for 3 years since.
The grouting procedure (brief)
For a machine being grouted: (1) level the machine on screws to spec; (2) build a dam around the base with sealant or foam; (3) mix the epoxy grout (machine grout, not construction grout — it’s formulated for low shrinkage and high flow); (4) pump or pour the grout under the base, working from one side so the air escapes; (5) let it cure per the spec (usually 24 hours at 20°C); (6) after curing, torque the foundation bolts to spec — the grout carries the load now, the bolts hold it down.
The mistakes to avoid: using too little grout (the base must be fully supported — a 60% coverage is worse than none, it creates hard points), grouting on an oily base (degrease first, or the grout won’t bond), and tightening the foundation bolts before the grout cures (it squeezes the grout out and voids the support).
Leveling for machines that shouldn’t be grouted
Some machines are designed to be portable — they move between lines. For these, use a machined leveling pad (not just a washer) under each screw, re-level quarterly, and check the accuracy after each move. A precision level and a dial indicator on the spindle vs the table is the check. Ten minutes per month. The alternative — a machine that drifts 4x out of tolerance — costs hours of rework and a scrapped part or two.
| Machine type | Installation | Re-level frequency |
|---|---|---|
| CNC lathe / mill (±0.005) | Grout | Not needed after grout |
| CMM (±0.002) | Grout + vibration isolation | Annual check |
| Standard mill (±0.05) | Screws OK | After 1 month, then annually |
| Portable machine | Screws + pads | Quarterly |
Precision machines get grouted, period. The drifting lathe wasn’t a ballscrew problem — four screws in an un-grouted base twisted the frame 0.05 mm. Grouting costs a day and holds alignment for years. Leveling screws alone are for portable machines, not for ±0.005 accuracy.
