Re: [CAD_CAM_EDM_DRO] Re: Bridgeport retrofit servo questions

Hi Mariss,

>As a good rule, if 100W or less is sufficient to do what you require,
>hands down, use a stepper. If 200W or more is needed, you have to use
>a servo. In between, either will do.
>
>

The Z axis is by far the most power hungry on a Bridgeport CNC. It uses
a huge ballscrew (the spindle runs inside the screw) and a long close
tolerance plain bearing. Everything runs in high viscosity slideway oil.
With no belt on it will slowly fall under it's own weight but as soon as
you try moving it fast the torque required goes through the roof. I have
no way of measuring the torque but I am sure I am putting in more than
200W when moving at rapid speed. This explains why I had so many
problems with the Z axis stepper.

>Servos are sexy, steppers are not. Just like Ferraris and Toyota
>Corrolas. The thing a Ferrari owner never tells you about is how much
>maintanance his toy needs. Meanwhile the Toyota reliably plods along.
>
>

er, new brushes every 10000 miles maybe?

>Steppers are diesels while servos are turbines. Steppers are high
>torque, low RPM motors, servos are the opposite.
>
>

I would have thought the comparison is more along the line of
diesel/petrol. A common solution in the real world is to use a bigger
engine. You aren't using the motor's full capabilities but in the end it
does not really matter? You can't normally use a stepper's full
capabilities either.

>Servos are fence-pole stupid. They will work into an overload and
>Kamakazi themselves into destruction (burn up in overload). A stepper
>is smart enough to shed a load by stalling. Anything that breaks on a
>step drive stops the motor, anything that breaks on a servo makes it
>take-off full speed.
>

True. Ideally the drive + control should be intelligent enough to allow
for these problems. Admittedly it is difficult to compensate for a
broken encoder or a shorted output transistor.

> Steppers are plug and play, servos must be tuned.
>

IMHO this is a myth. Steppers do need to be tuned and very often they
take more tuning than servos. To get a truly reliable stepper system you
need to make sure the acceleration and max speed parameters are well
within the motor's capabilities under all conditions. This includes when
the machine is cold first thing in the morning or when you are taking a
heavy cut. In total I have probably spent a couple of days trying to get
my mill's motors reliable, going through the sequence of zeroing the
machine on a dial gauge, running a long complicated job, checking
against the dial gauge, tweaking the tuning etc. I spent at least a day
on my plasma cutter and half a day on the lathe. If you get the tuning
slightly wrong on a stepper you only find out when you are near the end
of that really complicated job that needs to be out of the door yesterday.

Tuning the mill with servos probably took me around 3 hours, including
the 'look it moves' playing around stage. If you get servo tuning wrong
it usually lets you know straight away, either by tripping the drive,
trying to shake the machine apart or making wierd noises. You don't get
the silent lost steps that you tend to get with steppers if you get it
wrong.

>Servos are expensive, steppers are cheap.
>

For equivalent performance I don't think there is much in it over about
50W if you are using surplus/EBay parts. For low power, Fred Smith's new
servos + drives are not much more expensive than equivalent steppers.

>Usually servos need gearing, usually steppers don't. You see where I'm going with this.
>
>

True but a lot of machines use belt drive anyway so changing gearing is
pretty easy. If you are designing from scratch then it is simply one of
those things you allow for.

>The aggrivation, care and feeding of a servo is feasable only if you
>need more than 200W mechanical if you are pragmatic.
>
>

Brushed servos do need more maintenance then steppers. However unless
they are running 24/7 then maintenance is a case of occasionally
cleaning the commutator and maybe replacing the brushes every few years.
Is that really aggravation?

Les.

PS Sorry if I come across as rabidly anti-stepper but they have caused
me considerable grief over the years.