Hey all,
a hand trim router is a so-called universal motor, also called carbon-brush series-wound commutated motor, and its motor characteristic is that it delivers its nominal torque only at a specific speed (compared to a an induction motor like a spindle which delivers constant torque over a wide speed range). Also a universal motor is slowed down with increasing mechanical load, and the slower it runs, not only the slower the cooling fan on the axle turns, but also the more current flows, and so on the sparkling commutator and the rotor coils the hotter it will get and finally “malfunction” (it’s also what is written in the Makita manual). And if the mechanical load exceeds a limit and this for a longer time, such a hand trim router can start to smoke, to melt, and to burn.
With an induction motor like a spindle, this can never happen because if you give it some mechanically load, not only it will not be slowed down like a universal motor, but keep the rpm according to the VFD’s frequency set, and also it will not get hotter the slower it runs. On higher mechanical load, the current it draws increases, and since you forcibly need a VFD to run a spindle, and such a VFD requires that you first set the mandatory maximum spindle current rating before you put it into operation, there always is a limit programmed, which when you overload it, will let thet VFD simply stop the spindle (and if correctly wired, the running g-code program). Also an induction motor has neither carbon-brush commutator nor rotor coils, only stator coils.
This also is the cause of the poor universal motor’s efficiency. It is in principle a DC motor that runs on AC, the carbon-brushed commutator makes it irrelevant which frequency your AC has, but there are effects of the current direction change that work against the efficiency (like in a transformer). Also the sparks are responsible for loss (and additional heat). So the efficiency of a universal motor is about 30% to 70% at highest. An induction motor like a spindle has no commutators and no rotor coils, only a multiple of three stator coils. They run on three-phase current and the three stator coils are not switched by a commutator, the speed is controlled only by the frequency of the three-phase current. Slow frequency, slow rpm, high frequency, high rpm. Such spindles can reach efficiency of 85–97%.
So the situation in which a hand trim router overheats and starts to burn can never happen with a spindle.
Or in other words, if you mechanically overload a hand trim router, usually it smokes and melts. If you mechanically load a spindle, it will keep the speed as long as the current limit is not met, and if you mechanically overload it, the VFD simply stops the current to the spindle and signals a overload error. This error if correctly wired stops the g-code program.
Also when milling wood, you want both high speed and high torque at the same time. A hand trim router delivers its nominal torque only at a specific speed. On higher speed, it has less torque. A spindle has a constant torque over a wide speed range of usually 6000–24000 rpm. See also difference router/spindle.
So if you want to use a hand trim router on a CNC successfully, stay
- within a rather small mechanical load range
- within a rather small speed range
- Always beside the machine
