So I did some checking and the short story is you'll probably be ok with a 1:1 drive and your servo, see below for examples.


Take a 1/2" drill as an example of a tool you may be likely to use on the mill into plain old low carbon steel (1020).


Using GWizard calculator the drill should be operated at 688 rpm, and needs 0.877 KW when doing so.


The 1.8KW servo is rated that at 1500rpm and has as you say a flat torque curve as your speed drops so its power is proportional to the rpm between 0 and 1500 rpm.


688/1500= 45.8 percent of the power


45.8% x 1800 = 0.825 KW


So on the surface your servo won't make enough power by just a bit. However it is rated for intermitent duty at almost 2.5 times more torque so would probably be just fine in this example.


Any large and then your going to be into running in the intermitent range far too frequently and you'll overheat your servo or drive.


i.e. going up to a 9/16" drill - makes things worse - rpm goes down and power required goes up.
611 rpm and 1.096 KW
611/1500= 0.733 KW available in normal operation.


So on inspection you're probably ok from a power perspective, for most normal operations and you'll just need to watch out for large tools.


BTW large face mills are also close on power, depending on depth of cut, diameter and material being cut.


The other reason for a 2 speed setup is to be able to run higher rpm than the 3000 your servo wiil top out at. That will be a limiting factor for many small tools. so you would run 1:2 to get up to 6000 rpm - though depending on the spindle you may want or need to limit the max rpm so as to not blow up the bearings.