Greetings All,
I posted some questions a while back about making a high speed spindle
for doing engraving work. I envisioned using a BLDC motor made for an
RC vehicle or plane. I found a couple motors that look good and the
drive electronics for them. The basic spindle design is done. I have
had a hard time choosing bearings. I have been using the Timken (or
was it the SKF? I dunno, I'm tired) bearing calculator app for this.
There are many variables and since this is something I don't normally
do it has been a slow process. Trying to figure out the loads the
cutter will be seeing and transferring to the bearings has been tough.
There will be three bearings, two back to back angular contact
bearings in the front and a single bearing in the back, either a deep
groove or an angular contact. So the spindle shafts have been ordered,
I'm using ER collet holders. One shaft arrived and the taper runs out
..0002. I hope this is good enough. It may not be. I might end up
assembling the spindle and grinding a new taper with the spindle
running in its bearings. The motor will be liquid cooled with coolant
from the mill coolant system. I have designed the labyrinth seal for
the front of the spindle. I will be using filtered positive air
pressure in the spindle to blow out any contaminants that may want to
be sucked into the spindle through the labyrinth seal as it cools and
during operation. I ordered two motors, a 1000 watt and a 450 watt
motor. I was dubious that one of these little motors would really work
despite what they are rated at. But I found during a web search a site
where a guy has posted his siccess making high speed spindles using
the same basic type of motor that I will be using. This is good,
somebody has done this before me so the idea isn't totally whack. He
uses outrunner motors, where the O.D. of the motor spins. His spindles
have the motor hanging out in the air for cooling purposes. Anyway,
his spindles are doing real work and doing it well. I will be using an
inrunner motor so I can put the whole spindle assembly into a modified
CAT 40 tool holder. I am concerned about balancing the spindle because
the initial target speed is 32000 RPM. I have found some info from a
cursory look on the web about piezo vibration detectors and software
used with them to analyze and determine how much out of balance and
where the balancing needs to be done. I am mostly concerned about the
motor rotor being out of balance as all the components machined by me
will be as consistent and concentric as I can make them. Since this is
something I'm good at I'm optimistic about getting parts balanced well
enough right off of the machine. I am going to try using high quality
stock ER collets but if that doesn't pan out I'll just make solid
collets and shrink them onto the engraving cutters. There is the issue
of the engraving cutters being one flute cutters as this may cause
too much out of balance. If this is the case then the solid ER collet
can be ground on to fix that problem. Ultimately my goal is to have
the spindle act like just another tool in the tool carousel. I will
need to make some type of automatic connector that will connect air,
coolant, and power to the spindle as well as keep the CAT 40 tool
holder from turning slightly from the reaction of the spindle running,
sort of like the part that keeps tapping heads stationary, but more
positive, with no slop. Even though the goal is to have this cool
connector block that does everything described just above the first
iteration will just have some sort of stabilizer and I will have to
connect power, air, and coolant manually as well as turning on the
spindle manually. The mill already has the relays and available M
codes to turn on the spindle from the program. If the initial spindle
works out well I am going to try for 40,000 RPM. I have not yet found
a stock motor that spins that fast while at the same time fits the
other requirements for torque, size, and reliability. I am amazed by
the sophistication of the electronics that control these motors and
the motors themselves and they are made for toys. Anyway, that's it. I
guess I ran on a little. But maybe someone will read this and find
something I overlooked and have some good advice and maybe someone
will read this and decide to build their own spindle. If someone does
it will no doubt be better than mine and I can learn something from
them. Especially if, like me, they don't know when to stop typing.
Cheers,
Eric