My advise; don't buy one yet.
We tried them but found they have a couple of limitations:
One is that the coolant has to settle for longer than just overnight for the tramp oil to effectively come to the surface. It could be possible to set something up with a timer so that the skimmer turned on early Monday morning and skimmed off the oil that had risen over the weekend but then you come to limitation two.
The skimmer does drag out a lot of coolant, or at least ours did; up to 30% coolant in the skimmed oil.
We do use skimmers but not on the machines. They are used on the waste coolant tanks and we double skim so that the tramp oil is clean enough to go out as just waste oil and the coolant is clean enough to go into an evaporator to reduce the volume so it can go out as oil and water.
If someone else has other information I am all ears; waste coolant and tramp oil are a pain we have not managed to really adequately handle yet. Well that is not quite correct...we handle it but at a cost of several thousand dollars a year because the oil recycling people charge a lot to take away oil and water.