Posted: Tue Oct 23, 2012 2:03 pm Post subject: CNC mist nozzle
As shown in another post we got a CNC some time ago.
One of the objectives for it was to cut aluminium (5251).
However despite trying several types of end mills, coatings, diameters, etc as soon as a end mill enters the material some aluminium seems to melt and clog up the flutes (cutting edges). This causes the tool to either break because it cannot cut the material, or simply push the aluminium out of the way by melting it trough friction.
After some research we found that the solution for this is to either use flood (a continuous stream of lubricant) or mist (air mixed with a low amount of lubricant).
Given that we have a wood machine we opted to get a mist system and using ethanol as the lubrificant with air pressure being generated from a small air compressor.
Ethanol is a type of alcohol,which might seem a strange choice for a lubricant but it has the advantage that it evaporates quickly after use meaning no mess to clean up.
We were not sure on what we needed so we decided to design something ourselves. Since ethanol is highly flammable security was important.
The work began by creating nozzles in a 3d printer for a Segmented hose we got off ebay that could mix air and ethanol to act like a spray. The idea was for the nozzle to have a tube connector to feed in the alcohol.
A simple solution would be to place the alcohol container above the cnc, so that gravity would push the alcohol into the nozzle. However that solution meant that even if no air was being fed, the nozzle would keep leaking, unless we manually close off the alcohol.
As a fluid (air in our case) passes trough a constriction in a tube, the speed increases, causing a drop in pressure.
One solution was then to use some kind of airtight container for the alcohol that was connected to the entrance point of a venturi tube, and use a tube connected from the bottom of the container to the low pressure area in the Venturi.
This would have worked but it also meant that the alcohol container and tubes would be under pressure, making leaks a possibility.
After a lot of iterations, making changes in angles, diameters, etc we managed to get the ideal solution:
Our air comrpessor (a small unit) compresses at 180l/min. If we have the air running continuously it maintains about 3bar of pressure.
By optimizing the nozzle for this pressure we managed to get the Venturi effect to work so well that the low pressure area has a lower pressure than atmospheric pressure, despite that the input is at 3 bar(3 times atmospheric pressure). The great advantage of this is that the venturi actually sucks in the alcohol. Since all of system is under negative pressure leaks will just cause air to leak in, rather then alcohol leaking out.
For the container we chose this bottle found on eBay.
However pretty much anything will do. The requirements for the container are that it either has some way for air to enter(to replace the liquid expended) or that is is flexible,so that it collapses on itself rather tan generating a negative pressure than prevents the liquid from being sucked out.
The created nozzle.
The tube entrance to the nozzle is step tapered, so that any tube from about 6 to 3 mm (1/4 to 1/8 inch) outer diameter fits inside. However some kind of glue or sealant should be used to prevent it from coming out and to avoid air leaks that reduces the efficiency.
The nozzle mounted on the segmented hose with the tube for the alcohol -The tube is actually stripped insulator from copper wire. Proper tubing still had not arrived.
Since i wanted to avoid disconnecting air hoses to use the air gun i also got a few fittings to create 2 air outputs for the air compressor. One of then has a valve which enables me to open and close the air to the mist system independently from the air gun.
Where is the complete system.
The ethanol we got has a blue dye, which i believe is to mmake it more identifiable. It also helps to check how well it is flowing in transparent tubing. It should be available at any hardware store.
This as worked very well. We managed to do our first proper aluminium cut with it.
Just to leave a word of caution. Alcohol is highly flammable so take care with what you are cutting with this. Steel is out of the question, since it sparks while being cut, unlike aluminium. But still we have an air exhaust ducking the vapours directly outside, the alcohol container it at least 1 meter away form the cnc and it is never left un-attended with the mist system is in use. A fire extinguisher or two close by is also a good idea.
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