CNC SPINDLE :
High Speed Spindles
Lets get back to talking about brushless motors. Remember that they are also referred to as AC or induction motors and in the case of CNC woodworking, they represent a huge class of industrial high-speed spindle motors that take the place of the handheld routers that we were discussing on the previous page.
They are hugely more complicated and have many more design options. Here’s a condensed list of some of the features you may want to consider, both the good and the bad:
Size and Weight:
A brushless spindle motor of the same Hp as say the PorterCable 75182 would be lighter and much quieter, two of the biggest advantages of using spindle motors versus normal hand-held routers. Noise can be a big issue especially if you have a less than industrial woodworking shop or even if you do health and safety regulations are getting more stringent on what is considered an acceptable noise level so addressing this at the tool specification level is all good.
Weight is a strategic discussion. Weight is obviously proportional to the Hp so if you need it you need it, but be aware that heavier motors mounted on a gantry over a CNC table are both slower to start moving and slower to stop. So over spec’ing power either for an outright purchase or making your own CNC router, will sacrifice the speed and agility of your machine. More is NOT always better. If you don’t need it don’t buy it!
We defined TIR , “Total Indicator Runout” earlier, as a measure of the accuracy of the rotation of a spindle shaft within the motor housing, the smaller the number the better. Most brushless spindle motors will have a TIR of < .002 versus twice that for a basic woodworking router spindle. Do you need this level of accuracy?
Induction or AC motors require independent starters to turn the power on and off, and have a large range of size possibilities from under 3Hp to over 20Hp. Depending on your shop’s panel box and internal wiring, special lines may be required depending on voltage and whether your plans for a CNC Router is to run on single or three phase power. Getting a spindle motor to match existing electrical access will keep your costs down.
There’s always answers to convert single to three phase, or 220 volt to 400 or 600, but each variation implies more sophisticated wiring and more electronic installed between the panel box and the CNC spindle.
You’ll also need wiring for signal connections, speed, direction and tool change instructions. Some less sophisticated models leave wires dangling and other setups include a quick connect plug that makes it pretty much plug and play. Which would you prefer?
Now in order to get a degree of speed control all induction motors use a Variable Frequency Drive (VFD) to manage the motors rpm’s. Some CNC spindle motors come pre-wired with a VFD and if they do that’s great, just make sure they include enough cable to make it from the VFD location through the wiring harness to the spindle head. They are also typically preprogrammed to match the power specifications of the spindle so you just need to check for what electrical INPUT it is expecting and match that to your power hookup… this is only really an issue if you’re buying parts separately planning for the day when you can build your own CNC router.
Likewise, it is important if you are thinking about building your own CNC router, or retrofitting an older model, to know the configuration of the mounting holes. Better to plan the gantry design around these parameters than have to juggle after the fact or incur unexpected costs for modification. Most brand name spindle motors comply with NEMI frame standards and thus follow traditional mounting configurations.
If you are buying a ready made CNC wood router, and being given spindle options be sure to ask if the machine comes with the spindle installed or whether there are modification issues that must be addressed. You won't be the first person that got caught on that one.
Induction Spindle motors can use electrical or mechanical fans or can be air cooled with compressed air or even with a stream of chilled liquid. The last two more aggressive methods of cooling allow high duty cycles without overheating the tooling, bearings and spindle, significant issues in high production industry or nasty environments like cutting marble or glass.
Water cooling a motor can also double or triple the power capacity… this means that a much smaller router and therefore lighter router could be water-cooled to give you a very agile AND powerful machine!!! Remember that some of these precision spindles have the ability to run up to 50,000 rpms... quite a ways away from using a basic wood router
... read on for more options in high speed precision spindles >>
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