On 7/4/22 11:12, Jon Elson wrote:
I have managed to get two AMC 30A20 servo amps on eBay,
one relatively cheap, the other somewhat expensive. Now,
I need a 3rd. These are 30A peak 15 A continuous, up to
180 V DC input.
These match the current specs of the NC400's that came
with the machine. But, for the Z axis, maybe less current
would be fine. I'm guessing that 30 A into those motors
with a 2:1 belt reduction would likely give 1000 Lbs of
linear force on the table. It seems that would be enough
to damage the spindle bearings. I have no way to know
what the current limit on the existing Z amp was set to,
but I am assuming it was set quite a bit less. So, I'm
looking at an AMC 16A20 amp for the Z (16 A peak 8A
continuous).
One data point, on my old 1J head machine, the power
downfeed was set to kick out at 200 Lbs. I have no idea
what the limits of the BOSS 8 machine are, but I would
guess they sure don't want more than about 500 Lbs linear
force on the spindle bearings. Am I on the right track?
YIKES! I did the measurement of motor torque at the
leadscrew, and with 3 Amps applied, I got 10 Lbs force on a
fish scale at 1.625" radius. (There's a 2:1 reduction
between motor and leadscrew.)
Doing some calculations on that, I get that 30A (peak
rating) will get 5110 Lbs linear force on every axis. I
guess this is needed to get good acceleration on the XY axes
due to mass of table and saddle, but that seems VERY
dangerous to the spindle bearings. So, I think I answered
my own question that the lower-rated servo amp should be
totally sufficient for the Z motor. I did measure the pots
on the old servo amps and they seemed to all be turned up to
full current limit.
Jon
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