Homemade MIST Unit

(but without mist…)

By Chris Krstanovic


(click on the pics to enlarge)


Anyone who has done any machining knows that it is necessary to cool and lubricate the tools. Mister is an excellent way to accomplish this, while blowing off the chips, and keeping the tool clear. Anyone who used a mister knows that it also produces fine airborne mist, which ends up floating in the air. This is bad news for your lungs, and eventually everything in the shop ends up with ugly, smelly brownish coating.


I wanted the unit that will be safe, while simultaneously providing me with independently adjustable air and fluid rates. I wanted to be able to use high viscosity cutting oil, as well as ever present, Koolmist-77 water solution, which has very low viscosity. To accomplish all this it is necessary to understand the spray principles. The moving air in the mister atomizes the liquid, thus creating the mist. How the atomization is done, dictates the size of fluid droplets. Smaller they are, longer they take to settle down. Most commercial misters use the air stream to create slight negative pressure and thus draw the liquid in to the mister chamber. Unfortunately, the mist created at this point in the chamber is formed in a zone of supersonic (or very close to it) air, thus forming sub micron size particles, which take a very, very long time to settle.


After some experimentation, I discovered that moving the fluid orifice out of the nozzle, provides a nice jet cone of much larger particles, which settle quickly, and form no floating mist. As there is no suction at this point, it is necessary to force feed the fluid into the mister. This is accomplished by pressurizing a fluid in a holding bottle, and using this pressure to drive it to the mister.


The bottle is connected to 120-psi shop air by means of a pressure regulator and a shut-off valve. I use small 8 oz. Coke bottles for my fluid. I pressure tested them: they are incredibly tough, and can easy hold over 60 psi, although the maximum I use is 5 to 15 psi. The bottle cap has two barb connectors: one for air, and the other for the fluid return. The latter has a small needle valve fitted, for regulating the fluid volume. This is connected to a small solenoid valve, which is controlled by the CNC software. The valve output feeds the silicon tube, which in turn goes into the mist tube. Air supply is also independently fed to the mist tube, by means of another needle valve. In this way it is easy to independently control air and fluid.


The mist tube contains 0.060" ID Teflon tubing going through the air channel, thus feeding the fluid towards the nozzle. At its end is a 1" long, 0.025" stainless steel tube, which is forced into the end of the Teflon tube. This is the business end of the mister, and is centered by means of 6 set screws. It protrudes about ¼" from the nozzle end.


The whole assembly is mounted on Z-motor mount, which is a very handy location. The mill nozzle is mounted to the quill, thus it moves with the tool, making it accurate throughout the job. The air and fluid hoses are readily disconnected, thus allowing the use of other mister attachments, such as depicted lathe nozzle. In essence, it is the same thing, but is mounted on a strong, switch-able magnetic base, for easy positioning.


I find that my fluid rates are job related, but are contained between 10 to 100ml/hr.  A small Coke bottle lasts for a long while for most things, at about 40ml/hr (that is about 1 drop per second out of the nozzle if you let it run with no air). I mostly use a pretty unorthodox fluid: cutting Lard oil (www.McMaster.com has it), with 5% of miracle oil added. This is pretty old fashioned, and most misters cannot even contemplate using something this viscous. However, the cutting action and finish is far superior to anything I have ever seen.