Cutting oils and machining lubricants are essential for controlling heat, reducing friction and improving tool life during CNC machining. Once machining is complete, however, those same fluids can become contaminants that must be removed before assembly, coating, welding, inspection or packaging.
Effective cutting oil removal requires more than spraying a part with hot water. Cleaning performance depends on the interaction of chemistry, temperature, mechanical action, exposure time, filtration, rinsing and drying.
Understanding these variables can help manufacturers develop a repeatable CNC parts cleaning process.
Why Is Cutting Oil Difficult to Remove?
Cutting fluids can remain on machined components as films, droplets or pools trapped inside part features.
Common problem areas include:
- Blind holes
- Threaded holes
- Internal passages
- Recessed cavities
- Deep bores
- Closely spaced features
Metal chips and fines can also become suspended in the oil and adhere to component surfaces.
The challenge is therefore frequently twofold: remove the machining oil and remove the particulate it carries.
Use the Correct Aqueous Cleaning Chemistry
One common method to remove cutting oil from metal parts is aqueous cleaning.
Aqueous systems combine water with cleaning chemistry formulated to loosen and remove oils and other manufacturing soils. The chemistry must be compatible with both the contaminant and the material being cleaned.
Concentration also matters. Too little chemistry may produce inadequate machining oil removal, while excessive concentration can increase operating costs and potentially create rinsing problems.
Chemistry should therefore be treated as a controlled process parameter rather than simply adding detergent when parts appear dirty.
Optimize Wash Temperature
Temperature can significantly influence cleaning performance.
Heating can reduce the viscosity of oily contaminants and improve the effectiveness of aqueous chemistry. EPA guidance notes that aqueous cleaning performance is affected by formulation, dilution and temperature and that many industrial aqueous processes operate at elevated temperatures.
More heat, however, is not automatically better.
The correct operating temperature depends on the cleaning chemistry, contaminant, substrate, equipment and required cycle time. Manufacturers should establish a validated operating range and monitor it during production.
Apply Mechanical Cleaning Action
Chemistry loosens contamination, but mechanical action helps physically remove it from the component.
In spray washers, important variables include:
- Spray pressure
- Solution flow
- Nozzle configuration
- Distance from the part
- Spray angle
- Part orientation
- Coverage
EPA guidance for aqueous spray cleaning specifically identifies pump power, spray pressure, flow rate and nozzle coverage as important equipment considerations.
High pressure alone does not guarantee effective cutting oil removal. A powerful spray cannot clean a surface it never reaches.
This makes nozzle positioning and part presentation particularly important for CNC components with complex geometries.
Remove Chips and Particulate
CNC machining frequently produces chips, fines and other particulate along with oil contamination.
A cleaning system must remove these particles without continuously recirculating them onto cleaned components.
Filtration can help remove solids from the cleaning solution and maintain process performance. Depending on cleanliness requirements, systems may use strainers, bag filtration or other filtration technologies.
When components have internal passages or blind holes, targeted flushing may also be necessary.
Control Oil in the Wash Solution
Once oil has been removed from the component, it must go somewhere.
Allowing excessive oil to accumulate in the wash tank can reduce cleaning effectiveness and contaminate subsequent parts.
Oil skimmers, coalescers or other separation technologies can help remove free oils from aqueous cleaning solutions. EPA documentation on metal-parts cleaning has noted that cleaners designed to promote oil separation can improve recyclability and that heated tanks may incorporate oil skimming.
Effective oil management can help extend solution life and maintain more consistent cleaning conditions.
Do Not Overlook Rinsing
After washing, residual cleaning chemistry may remain on the component.
A rinse stage can remove this residue and prepare the part for downstream manufacturing processes.
Rinse requirements depend on:
- Cleaning chemistry
- Part geometry
- Cleanliness specification
- Water quality
- Downstream process
For applications requiring low residue, rinse-water quality may become particularly important.
Develop an Effective Drying Process
After CNC parts cleaning, retained moisture can create additional problems.
Water can remain inside blind holes, recesses and internal passages even when external surfaces appear dry. Depending on the application, drying may involve compressed-air blow-off, heated air or other methods.
EPA guidance notes that aqueous cleaning typically leaves parts wet unless steps are taken to accelerate drying and that corrosion can be a concern for some ferrous materials.
Drying should therefore be considered part of the cleaning process not an afterthought.
Validate the Entire CNC Parts Cleaning Process
The most reliable way to establish a process is to test actual production parts with representative contamination.
Evaluate combinations of:
- Chemistry and concentration
- Temperature
- Spray pressure and flow
- Nozzle placement
- Wash time
- Filtration
- Oil separation
- Rinsing
- Drying
The goal is not simply to make a component look clean. It is to establish a repeatable process that meets the required cleanliness level within the available production cycle.
Building a Better Cutting Oil Removal Process
Successful cutting oil removal requires chemistry, temperature, mechanical action and process control to work together.
For manufacturers producing CNC machined components, the cleaning system should be designed around the actual parts, machining fluids, production rate and cleanliness requirements.
Alliance Manufacturing designs aqueous parts washers for removing cutting oils, machining fluids, chips and particulate from manufactured components. Application testing can help determine the cleaning parameters required before a production system is engineered.
