PROJECT DESIGN REVIEW — MINUTES OF MEETING
Project: CNC machining cell with a long-travel gantry, X-axis 3.2 m
Review subject: guide rail lubrication regime and wiper / scraper selection
Date: 7 July, 13:30 to 15:20, meeting room 1
Document ref: DMR-2026-0707-LUB07
Attendees
| Name | Role |
|---|---|
| M. Hansen | Lead machine designer (chair) |
| G. Osei | Maintenance and lubrication engineer |
| P. Lindqvist | Production manager, machining cell owner |
| K. Stein | Supplier technical support, guide system |
| T. Werner | Quality / reliability |
1. Purpose of the review
The cell cuts aluminium housings with a 3.2 m X-axis on four linear guide rails, and the gantry makes 42 strokes per part at feed rates up to 18 m/min. The design intent was a greeped-for-life guide block with a maintenance visit every six months, because the supplier catalogue shows the blocks as greeped and sealed. Six months into production the second block on the left rail showed an erratic drag of 12 N above the other blocks, and a first check found the rail surface dry in the centre section of the stroke. The review was called to decide the lubrication regime, the wiper specification, and how to stop the casting dust and the coolant mist from reaching the race.
2. Review comment LU-01: the catalogue claim versus the machine duty
G. Osei took the room through a calculation that the supplier data sheet does not put on its front page. The greeped-for-life figure assumes a clean, dry environment, a short stroke and a duty cycle far below the cell’s. On this X-axis the block travels 380 mm out of a 3.2 m rail in each stroke, which means most of the rail is crossed twice per part and the lubrication film has to be dragged across 3.2 m on every pass. The supplier’s own lubrication table gives a relubrication interval based on stroke, speed and contamination, and for 42 strokes per hour free-running hours and 18 m/min the interval dropped below 400 operating hours, nowhere near the six-month plan. The review wrote the numbers on the board: catalogue life interval 1,200 hours against computed interval 370 hours, a factor of three that the sealed-for-life claim had hidden.
3. Review comment LU-02: grease grade and the temperature of the casting dust
P. Lindqvist described what the cell actually throws at the rail. The aluminium dust from the machining settles on the upper rail surface in a fine grey coat, and the coolant mist from the 4 bar flood line gets past the standard wipers within about four hours of running. The grease grade in the blocks was an NLGI 2 lithium soap, which holds well at 40 degrees but stiffens and loses its oil separation when it picks up coolant, turning into a sticky paste that holds the dust against the race. G. Osei recommended a switch to an NLGI 1 grease with a lower base oil viscosity at the shop temperature of 32 to 38 degrees Celsius, so the oil film regenerates between passes instead of sitting as a paste, and a relubrication schedule tied to the actual stroke count rather than the calendar.
4. Review comment LU-03: the wiper versus the scraper, and why the cell needed both
K. Stein explained the difference in one sentence that the whole room wrote down: a wiper wipes film, a scraper removes contamination, and a machining cell with coolant and dust needs the scraper row on the leading edge of the block, not only the standard wiper. The stock block carried a single contact wiper on each end, rated for clean service. The replaceable option carries an outer scraper of harder material that rides in light contact with the rail crown, clearing chips and dust ahead of the sealing wiper, at the price of a slightly higher running drag, in this case 4 N per block. On the machine with fourteen motors and a 12 N anomaly to find, a 16 N total drag increase across four blocks was considered a cheap price for not grinding the race with aluminium.
5. Review comment LU-04: the measured drag and the monitoring plan
T. Werner wanted the room to trust a number, so he read the drag log from the cell controller. The axis runs in torque-based tension at low speed, and the controller records a mean drive current per day. The log showed the second block’s drag climbing from 18 N in week two to 30 N in week twenty-six, with the anomaly breaking out on the left rail. The review agreed a practical baseline: record the drag every two operating weeks at a fixed feed of 6 m/min over the full 3.2 m stroke, and treat a climb above 25 N above the as-built value as an inspection trigger before the race is scored. The maintenance plan was rewritten around that number rather than around the six-month calendar, because the calendar had already failed once on this machine.
6. Review comment LU-05: the relubrication circuit versus a manual gun
G. Osei laid out two ways to push the new NLGI 1 grease into the blocks. The first was a manual grease gun through the block nipple, grease type in a tank at the machine, applied by the shift technician twice a week. The cost was nearly zero, and the failure mode was the human one, a missed nipple on a late shift. The second was an automatic progressive divider block that meters a set dose of grease to each of the four blocks and eight nipples on a timed cycle, tied to the cycle counter so it greases every 400 strokes rather than every day. The automatic option added 1,180 euros and a pressure switch to alarm on a blocked line. Given that a single scored rail on this machine costs 2,900 euros for the rail and 610 per block plus a day of downtime, the room accepted the automatic circuit without a vote, on the economics alone.
7. Review comment LU-06: the coolant mist and the bellows cover
P. Lindqvist raised the cover question, because the wipers and the greasing do nothing if the rail crown is permanently under the coolant mist. The original machine ran an open rail with a light telescopic cover that left the crown exposed at every stroke end. The production manager proposed a move to a formed sheet bellows on the exposed side and a rubber lip that rides the crown to keep the mist off the race. K. Stein confirmed the block is designed to run with a cover lip that contacts above the rail rather than on it, so the extra drag stays out of the measurement. The review accepted the bellows as part of the lubrication system, not as an accessory, because a rail that is kept dry and coated only by the grease is a rail that does not need the scraper to work twice as hard.
8. Disagreement: replace the scored block or run it under the new regime
The sharpest debate lasted eighteen minutes and was about the block that had already dragged at 30 N. G. Osei argued for replacement, because a block that has run dry in the centre section for six months has a worn ball race and the grease and scraper change would only slow its death. K. Stein argued from the supplier side that the block showed no race scoring on the bore scope, only a dry polish, and that under the new regime it could run out its life at a slightly higher drag. T. Werner cut the argument with a measurement offer: the drag log put the block at 30 N, the other three at 18 N, and a new block costs 610 euros while a replacement rail costs 2,900 euros if the bad block scores the race while waiting. The review decided to replace the anomalous block and keep the scored rail, on the reasoning that the rail was still untouched and the 610 euro block was the cheapest insurance the cell could buy.
9. Risks and mitigation
Four risks were logged. Risk 1: the automatic grease circuit could overfeed and push grease past the seals onto the part; mitigation is a feed check every 200 strokes and a pressure alarm at 60 bar. Risk 2: the NLGI 1 grease could bleed oil onto the rail and attract dust before the scraper; mitigation is the formed bellows to keep the crown covered. Risk 3: the drag log depends on the torque-based tension being switched on, which the night shift had switched off twice; mitigation is to make the drag recording a locked control screen setting. Risk 4: the bellows adds a soft cover that could be hit by a crashed tool; mitigation is a deflection bar above the cover with a proximity switch.
10. Actions and owners
| ID | Action | Owner | Deadline |
|---|---|---|---|
| ACT-1 | Replace the anomalous guide block on the left rail | G. Osei | 11 July |
| ACT-2 | Fit scraper rows to all four guide blocks | G. Osei | 13 July |
| ACT-3 | Install automatic progressive grease circuit, NLGI 1 | G. Osei | 21 July |
| ACT-4 | Add formed bellows and crown cover to exposed rail | P. Lindqvist | 28 July |
| ACT-5 | Log drag every two weeks, alert above 25 N rise | T. Werner | from 15 July |
11. Decision summary
Adopt an NLGI 1 grease with an automatic progressive circuit keyed to the stroke count. Fit scraper rows ahead of the sealing wipers on all four blocks. Add the formed bellows and crown cover to keep coolant mist off the rail. Replace the anomalous block. Monitor drive drag every two weeks with a 25 N trigger.
12. Post-meeting addendum, 18 August
The drag log nine weeks later showed all four blocks at 17 to 19 N, with the replaced block settling at 18 N after a 200-stroke settling run. The automatic circuit has greased 11,400 strokes without a pressure alarm, and the bellows has kept the crown dry through the same period. The cell produced 2,140 parts across that window with no rail downtime, and the maintenance plan now reads the cycle counter instead of the calendar.
Glossary
Wiper: a sealing lip that wipes the lubricating film on a guide block. Scraper: a harder leading contact that removes chips and dust ahead of the wiper. NLGI grade: a consistency scale for grease, with 1 softer than 2. Progressive divider block: a metering manifold that doses grease equally to several points per cycle. Stroke count relube: lubrication scheduled by the number of block passes rather than by time.
Appendix. The lubrication worksheet signed by the review
Duty: X-axis 3.2 m, four blocks, 42 strokes per part, cycles above 400 per day at 18 m/min peak. Grease: NLGI 1, lithium soap, base oil 68 cSt at 40 degrees, works between 5 and 50 degrees shop temperature. Relube interval: every 400 strokes, delivered as 0.9 g per block per cycle by the progressive circuit, sized to lay a fresh film of about 0.05 mm over the race. Scraper: replaceable row ahead of the sealing wiper, drag contribution 4 N per block. Bellows: formed sheet steel on the exposed side with a crown lip, keeping the rail dry under the 4 bar coolant mist. Monitoring: drive current at 6 m/min over the full stroke, logged every two weeks, trigger at 25 N above as-built.
The chair’s closing note closed the file: a guide rail does not fail when it runs out of grease, it fails when the wrong grease meets the wrong contamination and nobody is watching the drag. The minutes were signed by all five attendees, and the maintenance file for the cell now carries the stroke-count lubrication page as its first entry.
