I recently came across a useful checklist on when dry and flood cooling should be used. Dry cooling using an air blast is the recommended option in the following scenarios:
1. When workpiece hardness is greater than 42 RC
Machining such materials results in high temperatures and often work hardened chips. The high temperatures mean a much greater potential for shock cooling, which radically shortens the life of your carbide tooling. Air cooling does a better job of preventing those hardened chips from being recut than most flood cooling can.
Softer materials are much less likely to result in temperatures that lead to shock cooling problems. Here's a great story about how cutter life improved dramatically once the flood coolant was turned off due to the elimination of shock cooling.
2. When the workpiece material doesn't need lubrication
Some materials, like aluminum, are sticky. They want to weld onto the cutter and create a Built Up Edge (BUE), which quickly results in a problem. For those materials, you need lubrication unless you tool's wonder coating can solve the problem of BUE.
3. When your tool coating prefers dry
Some coatings, such as TiAlN, require fairly high temperatures to do their job. It's all part of the chemical reactions that make the coating do what it does. Too much cooling denies the coating proper temperatures and will actually detract from the performance of the coating.
Check with the tooling manufacturer on their recommendations and go dry if that's what your tool's coating needs.
4. Run wet for best surface finish
There are a lot of different reasons flood coolant improves surface finish, but the lubrication effect is one of the most important. Dragging chips around over the surface of the material can leave scratches and gouges. Lubrication helps minimize that.
In some cases, you may even have to risk shorter tool life by running coolant with, say, TiAlN, in order to get the desired surface finish.
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