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Plans, projects and how-to's for home machinists

Free Plans: Zero Fog Mister

Photo by Karl Townsend

These are Karl Townsend’s plans and construction notes for a zero fog mister.  Even though they’ve been floating around the net since at least 2002 they aren’t that easy to find and I’ve heard it works really well.  So I asked Karl for permission to republish them here.  He says the original is still in use and he thinks at least one hundred people have made one.  – Rob


I’ve used a mister for many years on both lathes and mills.  I hate the mess of flood coolant leaking all over the floor and mist cooling gives about 90% of the benefit.  The major problem with mist cooling is fogging or atomizing the coolant so fine it floats around in the shop.  Since I’m running a machine many hours a day, I’ve gotten real sensitive to the fog – it gives me a sore throat.

Anyway, I read about a low fog mister called a HenchForth Fog Buster here on this newsgroup.  A fellow had obtained a copy of the patent for this unit and made his own.  So, I did the same.  This unit was better, but still a slight fog or smell of coolant would be in the shop.  But, it gave me the idea of how to build a better one.  This unit works so well, you can’t tell it’s on – no sign of fog or odor of mist.

The key to operation is in the relative pressure of the coolant and the supply air pressure.  A standard mister has the coolant unpressurized and uses 40 – 60 psi air.  The Hench unit pressurizes the coolant container to the same pressure as the air line.  This unit uses a higher air pressure for the coolant than the air line.

A short explanation of construction:

Build a coolant container out of an old 20 lb. propane tank. Make a removable top to refill coolant (just a 1/2″ pipe plug).  Weld in a drain line connection to the bottom of the tank.  Provide a way to use a regulator and air line to pressurize the tank to 20 PSI.  The drain line goes to the mister as a pressurized coolant supply.

Provide an air supply regulated to 5 psi.  Make a way to shut off both this line and the coolant line when the mister is not needed.

The mister unit joins the water line and the air line with a needle valve to control the amount of coolant delivered.  Use a small block of brass to make this assembly.  The air line passes through the block and must be necked down to 0.100 ” dia. so the air velocity is high.  At this point, the coolant line intersects the air line.  The coolant line must be very small, I used 0.040 ” dia. at this point.  Just upstream from where the coolant enters, there must be a needle valve to regulate coolant flow rate.  I just soldered in a McMaster needle valve – #48965K24.

Use a 6″ long 1/8″ OD brass tube for the mister extension shaft.  To the end of this solder a nozzle.  This must be 0.75″ long and have an inside diameter of 0.040″.

Again, this unit is a total success for me.  It runs up to six hours a day with absolutely no fogging.  Mist rate can be controlled from nearly nothing to a heavy, nearly flood mist.

Mixer Drawing (PDF)

Mixer Drawing (DWG)

3 comments to Free Plans: Zero Fog Mister

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