Lubricant guide ·
Soluble cutting oil mixing ratio for CNC machines
Typical concentrations by operation, ratio-to-percentage conversions, the right mixing order, refractometer checks and the warning signs of a failing sump.
Most CNC lathes and machining centres in Shapar-Veraval's job shops run on a water-soluble cutting oil. It is a concentrate mixed with water into a milky emulsion that cools the tool and the job, washes chips away and protects the machine from rust. Mix it too lean and you get rust and short tool life. Mix it too rich and you waste oil, get foam and sticky residue, and can irritate operators' skin. Here is how to get the mix right and keep it right.
Start with the data sheet
Every soluble cutting oil has a recommended concentration range on its product data sheet, and that is the range to follow. The figures below are typical starting points across the industry, not a substitute for the data sheet.
Typical concentrations by operation
| Operation | Typical concentration |
|---|---|
| Grinding | Roughly 2–5% |
| General turning and milling | Roughly 5–8% |
| Heavy cuts, drilling and tapping in tough steels | Roughly 8–12% |
Ratio or percentage?
The same mix gets quoted two ways. "1:20" means one part oil to twenty parts water. Shop convention treats that as 5%. Strictly, it is one part in 21, or about 4.8%, but the difference doesn't matter in practice. The refractometer reading is what counts.
| Ratio (oil : water) | Approximate concentration |
|---|---|
| 1:50 | 2% |
| 1:20 | 5% |
| 1:15 | 6.7% |
| 1:10 | 10% |
Always add oil to water
Put the water in the clean container or sump first, then pour the concentrate in slowly while stirring. Adding water to oil can form an unstable, cheesy "invert" emulsion that never mixes properly. For a 200-litre sump at 5%, that is about 10 litres of concentrate into 190 litres of water. A premix in a clean drum, or a proportional mixer, gives a more consistent result than pouring concentrate straight into the machine.
Check the mix with a refractometer
A hand refractometer is inexpensive and takes the guesswork out. Put a few drops of coolant from the sump on the prism and read the Brix scale. Then multiply the reading by the refractometer factor on the product's data sheet to get the actual concentration. Check each machine once or twice a week, and again after topping up.
Water quality, topping up and sump care
Water. Very hard water, which is common from borewells, can make emulsions unstable and leave soapy deposits. If you have that trouble, use softened water or ask for a product suited to hard water.
Topping up. Water evaporates from the sump but the oil doesn't, so the mix gets richer over time. Top up with a lean premix, leaner than the working mix and often around 1–2% as the data sheet advises. Never top up with plain water or neat concentrate, and check with the refractometer afterwards.
Sump care. Before recharging, drain and clean the sump and lines properly, because old sludge and bacteria will spoil a fresh charge within days. Skim off tramp oil, the slideway and hydraulic oil that leaks into the sump and floats on top. It feeds bacteria and causes the smell.
Signs something is wrong
| Symptom | Likely cause |
|---|---|
| Rust on jobs or the machine | Mix too lean, or poor water quality. |
| Foam | Mix too rich, very soft water, or air drawn in by the pump. |
| Bad smell | Bacteria, usually fed by tramp oil and a stagnant sump. |
| Cream or oil layer on top | A split emulsion from contamination, hard water or the wrong mixing order. |
Buying cutting oil in Shapar-Veraval
We keep soluble and neat cutting oils at our shop at Tilara Gate, near the Shapar Veraval Industrial Association. Machining units in Shapar-Veraval get same-day delivery as regular customers (a minimum order applies). We also deliver to Metoda GIDC and across Rajkot, with timing confirmed per order. Every order comes with a GST bill, and factories get a bulk discount. See the cutting oils and CNC coolants we stock, or read about cutting oil supply in Shapar-Veraval.
