The guideways of a machine tool are the rails its axes slide on. They decide how accurately a CNC can position the tool. Most machines feed them with oil from a small automatic lubricator, and the oil in that tank should be a slideway oil, not whatever hydraulic or gear oil is close at hand.
Slideway oil is designed to prevent stick-slip and to stay on the ways. Use ISO VG 68 for most horizontal ways and VG 220 for vertical or heavily loaded ways, as the machine maker specifies. Hydraulic oil lacks the friction modifiers and tackiness, and often mixes into the coolant, so it causes jerky movement, wear and smelly coolant.
At very slow feed rates, sliding surfaces can stick, build up force, then jump. That jerky motion is stick-slip. It causes chatter marks, poor surface finish and positioning errors. Slideway oils contain friction modifiers that make static and moving friction nearly equal, so the axis moves smoothly even at the slowest feeds.
Slideway oils are made slightly tacky, so they cling to vertical and inclined ways instead of draining off. That is why vertical ways usually need a thicker VG 220 oil.
Slideway oil always ends up in the coolant sump. A good slideway oil is designed to separate quickly from soluble coolant, so it floats up and can be skimmed. An oil that emulsifies into the coolant upsets the coolant's balance, feeds bacteria and causes smell.
| Way type | Usual grade |
|---|---|
| Horizontal ways, light and medium machines | ISO VG 68 |
| Vertical, inclined or heavily loaded ways | ISO VG 220 |
| Machines using one oil for hydraulics and ways | ISO HG-type, VG 32–68, only if the maker specifies it |
The usual designations are DIN 51502 CGLP (C = circulating, G = slideway, L = rust and oxidation protection, P = anti-wear/EP) and ISO 6743 class G. Check the lubrication chart on the machine or in its manual.
The reverse is also true: only use slideway oil in a hydraulic system when the maker approves a combined HG oil.
An automatic lubricator should use roughly the same amount of oil every week. If consumption jumps, look for broken lines or a faulty metering unit. If it drops to nothing, the ways are running dry. Check the tank level weekly and write it down.
In many machine shops, coolant that smells after a few weeks is really a slideway oil problem. A few litres of slideway oil a month entering the sump becomes tramp oil, which:
Use a slideway oil with good demulsibility, skim tramp oil regularly, and follow the coolant routine in our soluble cutting oil guide.
Can I use hydraulic oil instead of slideway oil?Not recommended. Hydraulic oil lacks the anti-stick-slip additives and tackiness slideway oil needs, so axes may move jerkily and vertical ways can run dry.
Which slideway oil grade is used in CNC machines?Most CNC lathes and VMCs use ISO VG 68 for horizontal ways. Vertical or heavily loaded ways usually need VG 220. The machine manual is the final word.
Why does slideway oil end up in my coolant?Some always does; it lubricates the ways and then drips off. A good slideway oil separates and floats so you can skim it. Excess amounts point to over-lubrication or leaks.
Is way oil the same as slideway oil?Yes. "Way oil", "slideway oil" and "guideway oil" are the same product type.
Tell us your machine makes and whether the ways are horizontal or vertical, and our team will recommend the slideway oil grade.