Air cooling a water cooled spindle

[JerryBurks]

I am running a 1.5kW Chinese spindle (the slim 65diax210 mm version) and found it needs only a trickle of water (about 2-3 gal per hour) to keep it cool. This is for light milling in wood with 1/4 and 1/8" diameter bits.

I am wondering if I can get rid of the water altogether putting a shroud around the spindle (maybe leaving 1" space) and sucking the dust exhaust air stream along the spindle for cooling. At 500 cu-ft/min air the heat capacity of the air should be 15-20 times higher than of the water trickle but I am not sure if the convective heat transfer on the spindle surface would be enough at the (pretty high) air velocity.

Just an idea for now but maybe I try it out and see how it goes. It would actually simplify the dust extractor plumbing a bit.

JB

[stevespo]

What you're describing seems reasonable, IMO. My 2.2kW requires very little water and the housing is never more than a few degrees above room temperature, even after a long running job.

You might help conduction between the bearings, inner housing and outer housing by filling that reservoir (distilled water, etc) and then plugging the water inlets. Just to improve heat transfer from the inner to the outer housing.

Attach a cheap temperature probe to the outer housing and watch it to see if the temp stays stable or not. If you always run with DC turned on, it seems like a safe way to go. If it doesn't work, you can easily convert back to water only.

Steve

[silyavski]

hi,
what exactly is bothering you with the water cooling?

a 20w fan with 200cfm is enough to cool a spindle, i have seen a guy force cool his air spindle like this.
another option is put a 60$ CPU cooler assembly with pump, sensor, alarm with a closed circuit so no big lines and worries.
or just use a car window washer pump and 12$ CPU water radiator

and have in mind that the water cools the bearing assemblies also, though not in a direct way, you will not want to cool the spindle and heat the bearings. I have looked at many spindle drawings before i bought mine water cooled one and there is a notable difference in the construction with the air cooled ones, its not that some channels where the water passes!

[JerryBurks]

Originally Posted by silyavski hi, what exactly is bothering you with the water cooling?.......

Eh, not much really. Right now I have a temporary setup using tap water since the water consumption is insignificant but I was planning to recirculate the water eventually.

I was just wondering to simplify the whole thing. Although a CPU cooler and small pump won't add much complexity it would be even better without: no pump wiring and control, no moving tubing, no water refill or possible clogging/fouling/corrosion. Getting water out of electrical equipment is an advantage itself, given the cheap plugs.

Good point with keeping the bearings cool. I have not opened one but assume there is just conductive cooling from the spindle body to the bearing housings? In that case they would stay cool as long as the body is cold. At the end I probably have to try this out and stick an aquarium thermometer tape on the outside.

JB

[RomanLini]

If your cutting does not produce much heat in the spindle you could try a $20 aquarium pump and pump air through the water cooling hoses.

It won't cool as much as water, but you can get 20 CFM or so from a cheap quiet aquarium pump, and 20 CFM of air will extract a significant amount of heat.

[JerryBurks]

Originally Posted by RomanLini If your cutting does not produce much heat in the spindle you could try a $20 aquarium pump and pump air through the water cooling hoses. It won't cool as much as water, but you can get 20 CFM or so from a cheap quiet aquarium pump, and 20 CFM of air will extract a significant amount of heat.

Actually I tried compressed air but found it takes about 60-70 psi to squeeze enough air through the tiny water passages to keep it cool. Water has 4000 times the volumetric heat capacity of air, after all. So an aquarium air pump is out of the question. On the other hand, shop air pressure is also not a real solution because it will waste a lot of energy for the compression/expansion cycle. If air cooling at all it has to be high velocity/high volume along a larger surface.

JB

[JD68]

A very interesting thread. Most spindles require a ramped start up when first turned on. Something like 6000rpm, 12,000rpm and 18000rpm. This I assume to get the lubricant moving for proper lubrication of the bearings plus to get the correct preload on the bearings as they expand. Given this, wouldn't it be better to have a thermostatically controlled cooling system where the bearings run at a constant temp?

[CarbonKevin]

If the cooling requirements are THAT low, why not attempt a totally passive system - no pumps, make it a closed loop filled with a high conductivity fluid and enough length of copper tubing (or a small radiator) to allow the heat to dissipate into the surrounding air.

Even simpler, replace the machine's internal tubing with solid copper or aluminum rod of the same diameter, and attach it to a finned block of aluminum outside the casing. Now it's a heatsink.

[RomanLini]

Originally Posted by JerryBurks Actually I tried compressed air but found it takes about 60-70 psi to squeeze enough air through the tiny water passages to keep it cool. Water has 4000 times the volumetric heat capacity of air, after all. So an aquarium air pump is out of the question. On the other hand, shop air pressure is also not a real solution because it will waste a lot of energy for the compression/expansion cycle. If air cooling at all it has to be high velocity/high volume along a larger surface. ...

I think that answers your question, if air through it does not keep it cool enough then you are back to water cooling.

What's wrong with getting a small water container and little submersible AC mains pump (aquarium or fountain type are about $25 and rated for continuous use)? You can use a small radiator cooler between spindle and water tank (even just a long metal pipe) to remove the heat more effectively while the water is hottest.