Re: Bridgeport retrofit servo questions

Mariss,
Thanks for the detailed explaination...I actually do CNC routing and
CNC plasma cutting using steppers and a proprietary control. I
recently bought a BOSS 5 Bridgeport CNC...and I need to upgrade the
controls to take advantage of the higher memory capabilities...I have
been getting into mold making and bought the BP to make metal molds.
My router loses steps if pushed too hard...and if I have a run of
several 3D carvings in hardwood, I can't push it, or I'll lose steps
(which really irks me) Eventually the router will be upgraded to
Mach2 with a hotter power supply and 201s or 210s. This machine is my
bread and butter...so no upgrades on it right now.

No handwheels on the BOSS and I've haven't run a Bridgeport or other
manual mill since high school...although I can remember the force
involved in spining handwheels and cranks. The NEMA 42 steppers on
the BP are working, but I don't want to keep them if I am going to be
dealing with positional loss as a result of sluggish performance.
Yeah...I can run the machine slowly. BUT why do so if I have the
option to close the loop and gain performance to boot? This is the
main reason for investigating a servo system.

I have no idea how much weight I will be pushing around with the BP.
I have picked out motors that are economical, similar in power and
performance of 'real' servo motors that the expensive upgrade 'kits'
use, and generally what I 'think' will get the job done. For the time
being, I am just looking for a decent combination that will get me
120 IPM rapids and excellent repeatability.

-Brady

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@y...> wrote:

>
> 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