How Cutting Fluids Impact Machining Processes
Precision machining requires CNC machines that are structurally stiff and well-maintained, high quality cutting tools, and usually, cutting fluids. Cutting fluids are one of the most underappreciated parts of a successful machining process and yet they play a major role in both efficiency and quality. However, they also create challenges the machine shop must address.
Reasons Precision Machine Shops Use Cutting Fluids
Cutting fluid is directed at the point where the cutting tool is removing material from the workpiece. Shearing metal away generates a lot of heat, most of which goes into the chip, and the balance into the tool itself. Cutting fluid cools the tool-workpiece interface, which has several benefits:
- Prevents chips sticking to the cutting tool
- Reduces thermal shock suffered by the tool (a particular problem in interrupted cuts)
- Keeps the workpiece cool enough to avoid oxidation
- Minimizes cutting tool expansion
Together, these reduce tool wear and increase tool life. This prevents the poor surface finish that would result on the part. In addition, keeping the tool cool minimizes expansion, which results in less dimensional variation between parts being machined. An additional benefit is that a cooler cutting interface cooler allows faster metal removal.
Other functions performed by cutting fluid are: lubricating the cutting process, removing chips and leaving a rust preventive coating on the workpiece.
Lubrication lowers cutting forces, which allows higher speeds and feeds. Chip removal is important because they might otherwise mark the workpiece or impede the machine’s ability to move the tool to a particular position. In grinding, coolant helps keep the wheel free from debris.
Types of Cutting Fluid
Machine shops can choose between four types. These are:
- Mineral oils
- Water-soluble fluids
- Air/other gases
- Solid pastes
For each of these manufacturers offer a wide range of formulations. A precision machine shop will typically try to match the fluid to the metal they cut and the types of machining they do.
Mineral oils are good for lubrication and resisting high pressure but do not generally do a good job of heat removal. The water-soluble fluids provide better cooling but are less effective as lubricants.
In some machining operations it’s enough just to direct an air jet onto the interface. Magnesium is one metal where this is done. Some machinists will even blow freon or CO2 to lower temperatures.
Solid pastes are used in specialized cutting operations, most notably, tapping or threading.
Problems Caused by Cutting Fluids
The biggest problem with liquid coolants is mist getting into the air. This is easily inhaled by people working nearby, even around enclosed CNC machines. As cutting fluid contains biocides to prevent bacterial build up it’s potentially very harmful.
Other issues are spills, odors and machine cleaning. Spills are usually a result of fluid dripping from parts as they are taken out of the machine. They need mopping-up to prevent accidents, and if left can quickly produce smells that give machine shops their characteristic aroma.
Cutting fluid systems built into machine tools need regular maintenance. Filters must be taken out and cleaned or replaced regularly, and the chips collected must also be removed. Chip disposal is itself a problem as cutting fluid contamination limits recycling options.
Minimal Coolant and Dry Machining
To avoid the problems with cutting fluids, some machine shops try to use the minimum necessary while others go without completely. Minimal coolant techniques rely on providing a mist sufficient to lower temperatures, in part through evaporation. This helps but needs precise control and still poses some health and environmental challenges.
The alternative is to machine dry. This is feasible with the right precision machining technology, although often requires lower metal removal rates. The main change is to use cutting tools that can handle much higher temperatures, and for interrupted cuts, thermal shock. Suitable tool materials are cubic boron nitride, polycrystalline diamond, ceramics and some specialty carbides.
Cutting Fluids at the Heart of Precision Machining
Getting consistently high quality work from CNC machining depends on managing heat at the tool-workpiece interface. Most precision machine shops do this through careful choice of the cutting fluids they use: others have adopted dry machining, with the trade-offs that involves.
Precision machining is a core capability for Impro. If you’d like to learn more about how we achieve consistently high quality results, get in touch.