Re: Bridgeport retrofit servo questions

Brady,

We build both sevo drives as well as step motor drives so I have no
particular bias towards either. I'm also working on a true stepper
servo which should blur the line even further.

You should approach this first entirely from a perspective of power
(Watts) needed to do the job. The choice of stepper or servo falls
out from the results.

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.

How can you figure how much power you need? A couple of simple
equations should handle it.

1) You know how many inches per minute (IPM) and how many pounds of
force you need at that speed. Easy; use the following:

Watts = IPM * Lbs / 531

2) IPM is easy but you don't have clue how many Lbs of force is
needed. I'm assuming here you are a machinist and have used manual
machines. Your experience is necessary.

a) What is the diameter of a handwheel for a machine of this size?

b) From your experience, how many pounds of force would you apply to
the handcrank before you'd figure you are doing something wrong?

Take the results of (b), multiply it by 16 to turn Lbs into ounces.
Multiply the result by the radius in inches of the handcrank. Your
result is in-oz of torque applied to the leadscrew.

Multiply that by your needed IPM times the TPI (turns per inch) of
your leadscrew (5 TPI for a Bridgeport). The result is screw RPM.

Multiply the screw RPM by the torque you need (in-oz, calculated
above) and divide the result by the magic number 1351. Your result
pops up in Watts.

Let's say your result is less than 200W (more than likely) but you
still want a servo. Think about this:

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.

Steppers are diesels while servos are turbines. Steppers are high
torque, low RPM motors, servos are the opposite. I test about 100
servo and stepper drives a day. Both are the same to me though servos
are sexier. Can't deny that.

The maintanace and use thing. Servos have to run at 80% of their no-
load speed to deliver their rated power. 80% of no-load is very often
much higher than you want the screw to turn at. That means you have
to use reduction gearing. A stepper meanwhile can drive the screw
direct (1:1).

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. Steppers are plug and play, servos must be
tuned. Servos are expensive, steppers are cheap. Usually servos need
gearing, usually steppers don't. You see where I'm going with this.

The aggrivation, care and feeding of a servo is feasable only if you
need more than 200W mechanical if you are pragmatic. On the other
hand, there is nothing like the smooth, silky, silent and ghost-like
power of a servo if you are not.

Your choice.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "braidmeister"
<braidmeister@y...> wrote:

>
> OK guys...I've been reading posts on here for the past few days
> regarding servo motors. After replacing a dead power transistor in

my

> BOSS 5 Bridgeport (just got it....Y axis DOA)...and spending way

too

> much time trying to interface to what appears to be a bad ERS board
> (via serial)...I am now heading towards doing a complete retrofit

of

> the controls using Mach2.
>
> I'm on the fence about whether to retain the working steppers and

use

> Larken stepper drives ($200 ea....up to 16A/ph) plus some type of
> appropriate power supply, Campbells breakout and Mach2. The down

side

> of this may be that the machine doesn't move as fast as the BOSS
> control can move it...but this is based on feedback from several
> Gecko users on a BP. I am not looking to break any speed
> barriers...but 50 IPM rapids aren't going to cut it. I need

something

> that can handle running several hours at a time. I saved for a

while

> to get the BOSS...now I have to scrape some $ together to do a
> conversion...just so I can run the thing. I really don't want to do

a

> conversion twice (always time for that, right guys??!!)
>
> The other choice that I have is to go with servos. The BP is
> definately well into the servo side of the servo/stepper choice
> threshold. Not partial to Gecko or Rutex as either will do

(actually

> I am because the Geckos are cheaper). I have read that several of

you

> suggest using some of the treadmill-type motors available from
> surplus center. There are several DC dual shaft motors there with
> pros and cons to each. I am curious as to what models would be best
> for a BOSS machine. Here are a few that were mentioned...and a few
> more that I picked out: (All listed can be found on this page:
> http://www.surpluscenter.com/sort.asp?
> UID=2005012813411678&catname=electric&keyword=MBDD )
>
> 1. $30. 10-1573 ~ 5100RPM rear shaft long enough for encoder off of
> 10-1834. Rated at 1.5 HP & 110v. Does this have enough juice for

the

> BP @ 75v? Current not advertised other than 1.4A no load.
>
> 2. $22. 10-1783/10-1783-A 2.5 HP rating 6750 RPM 18.5A 130v int

duty,

> 95v continuous. Back shaft only .25" long. Will have to extend to

use

> any type of encoder. I am guessing that this would be a good motor

if

> run @ 75v/18.5A (within Gecko specs)...only thing I question is the
> RPM/belt reduction. What ratio do you think is best on the BP with
> this motor?
>
> 3. $80. 10-1906 2.0HP 130v/15A 3210RPM Continuous duty. Has good
> shafts for driving and encoding. Shipping wt 24#s...starting to get
> heavier. This seems like a good motor...the current rating tells me
> that it has some cookies and the RPM is half of the previous
> one...helping to curb insane reduction ratios.
>
> 4. $90. 10-1723 1.5HP 111v/7.6A 3100RPM 2-4hr rated continuous

duty.

> Looks like a good motor as well...but it is a pig. Ship wt @ 30#

and

> 13" long...not liking that, but rotor inertia makes up for the

length.

>
> What do you think??? I think #3 so far might be a winner...yes it

is

> more $ than 1 & 2, but it has a beefy shaft on it, long enough in

the

> back for an encoder and the RPM isn't outlandish. I'd like 120 IPM
> rapids and enough torque to mill at conservative speeds.
>
> Sorry post is so long...I like to be as detailed as I can.
>
> Thanks!!!
> -Brady