Re: Suitable Steppers

Wally,

I don't know what to say, this is a fabulous answer, you are a very
generous guy with your knowledge.

It seems to me that unless you spend plenty of money you will always
be on the edge with steppers so servos look a better solution. I know
you have told me so much already but could you please give me some
background on servos. IF you could point me at some suitable motors
or specs for motors and an encoder I would be even more in your debt.

Regards

Ian

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "xj5373"

<ian.c.haynes@b...>

> wrote:
> > HI,
> > I have come across some steppers Model N41HLFJ-LNK-NS-00 (Nema
> 42Size(
> > I have found info here
> > http://www.pacsci.com/products/step_motors/powerpacproducts.html
> but I
> > am struggling a bit with supply voltage, the vendor states 170V.
> Any
> > feel if this is right, would a Gecko drive run them and would

they

> be
> > big enough for X and Y on my Bridgeport?
> >
> > Thanks
> >
> > Ian
>
>
> You have actually asked a very difficult question. I have

literally

> spent hundreds of hours researching this question. To answer this
> question fully would take many pages (and after 3 hours of writing

it

> looks like it has).
>
> Let me try to break it down to a few simple items and we can go

from

> there. First if I am reading your part number correctly it is a 8
> lead motor. If so it can be run in parallel. The curve for this
> motor in parallel configuration is here. Notice in parallel mode

it

> runs nicely at 72VDC.
>
> http://products.danahermotion.com/danaher/ModelDetail.asp?
> User=StepMotors2&PkgID=424098&from=search&Rnd=500
>
> I have spent about 16 hours on this site in the past looking for
> suitable stepper motors to drive a Bridgeport milling machine. To

do

> so I printed out and studied all the curves of any stepper motor

that

> looked like it had a chance of working. The results of my findings
> for Nema 42 steppers are as follows, and notice your motor was 3rd
> choice.
>
> Best motor.
> K43xxxL-L (P) $1100.00 USD
>
> 2nd best.
> N43xxxM-L (P) $570.00 USD Only slightly lower performance were it
> matters for running a Bridgeport, than the best motor above.
>
> 3rd place.
> N41xxxJ-L (P) $330.00 USD In round numbers 30% to 50% less torque
> than others.
>
> I also ran a large search for Nema 34 steppers on this same site

that

> would give the best performance with a Gecko. I did not find any
> Nema 34 stepper that would come close to working at 70VDC for a
> Bridgeport.
>
> It is important to know how the 3 winning motors were chosen.

First

> I researched stepper motor drives. There are drives that will run
> steppers from 170 to 320volts or so. But these drives start at

$500

> and go up to $800 or more. Since I knew I would not pay this much
> for a stepper drive I eliminated all steppers that needed these
> voltages. The only stepper drive I found in a decent price range
> that can begin to drive a Nema 42 stepper is a Gecko. And so the
> operating voltage for the stepper has to be limited to the Gecko
> 70VDC recommended running voltage.
>
> With that said the 3 motors above are the most powerful steppers at
> the 72VDC nominal rating, with curves that lends themselves for use
> in a milling machine.
>
> Now comes the most difficult part "suitability of purpose". We

have

> picked out the most powerful steppers that our Gecko drive can
> handle. But are these steppers suitable for a Bridgeport? One of
> the things that make this answer so hard is the fact that a stepper
> has a torque and RPM curve. You must now match the stepper
> performance curve or "profile" with your desired milling machine
> profile. You are lucky that you are asking about a stepper that has

a

> published performance curve. Many people just buy any Nema 42

motor

> on ebay and throw it on the machine hoping it will work.
>
> Since you have actual data to work with lets go from there. We

want

> to know if a stepper profile will give suitable performance on our
> Bridgeport. To start with we have the stepper profile, but, we do
> not have a Bridgeport profile. Somehow we need to make a profile

for

> a Bridgeport so we can compare their curves. I have attacked this
> many ways in the last few months and I will distil the results of

my

> findings below.
>
> To make a Bridgeport profile curve we have to list everything we

know

> about speeds and torque on the screws. This is the best

information

> I have been able to come up with.
>
> 1) As a ballpark torque number I have a friend with years of
> experience on a Bridgeport. He estimates he uses around 5 lbs max

on

> the handles when milling. The length of a handle is 3" so this is
> about 15 in-lb. Since the stepper curves are in Nm we will convert
> all numbers to Nm. lb-in x .1137 = Nm, So 15 in-lb x .1137 = 1.7
> Nm. I consider this the absolute minimum torque needed.
>
> 2) The following is speed in inches per minuet and the RPM of
> the lead screw for various materials and operations when using a
> Bridgeport. Stainless steel 4 IPM = 20 RPM, Steel 8 IPM = 40 RPM,
> Aluminum 40 IPM = 200 RPM and a skin cut with a indexable carbide
> cutter 80 IPM = 400RPM. Once again, these are RPM's were maximum
> torque is needed for cutting.
>
> 3) As another guideline I have gathered data on existing
> commercial Bridgeport servo retrofits. The smallest servos used

are

> 18 to 20 in-lb or about 2.274 Nm. BUT there are several things you
> need to know about these numbers. First servos have a constant
> torque curve. The 2.274 Nm is constant over the entire RPM range.
> Stepper torque drops off rapidly with RPM. Second the Servos are
> connected to the lead screw with a belt reduction typically from 2

to

> 2.5. This raises the usable torque to 4.55Nm to 5.7 Nm. Third,
> believe it or not, many people consider this setup underpowered.

The

> desired servo motor retrofit is about 30 in-lb or 3.4 Nm. And once
> again a belt reduction of 2 to 2.5 is typically used for a torque

of

> 6.8 Nm to 8.5 Nm.
>
> 4) What is interesting is the large difference in torques from
> item 1 hand cranking and item 3 servo drive, or 1.7 Nm to 8.5 Nm.

My

> friend is a smart guy and I do not think he is very far off so I
> think there is more going on here. I suspect one thing is when a
> Bridgeport is under servo power people are over taxing the

machine.

> There is one person I met that did a few commercial retrofits. He
> told me his customers love the large amounts of torque his servos
> give and push the Bridgeport to use it. He said he was actually
> worried how hard the machines were being pushed and was afraid they
> would break. This is probably why commercial retrofits usually
> replace the yoke on the lead screw with a heavy duty one.
>
> 5) Gearing of the stepper is something else that has to be
> considered for 2 reasons. To increase the torque and to increase
> resolution. Taking in all the various factors like maximum cutting
> speed, rapids, minimum desired resolution, falling torque with RPM
> and the position of Venus during the solstice. I have determined
> that a reduction of 2 to 1 is about right for steppers in this
> application. So with this in mind lets see were that gets us.

Well

> to start with this is where it gets personal. I posted the
> information above so people could choose there own profile. I will
> show you how I choused mine. I took the cutting data from my

friend

> in item 2 and used it as gospel. I decided if that was the way an
> experienced person used a Bridgeport then that's what I wanted. So
> from this I realized I wanted enough torque to machine material up

to

> 40 IPM for sure and 80 IPM if I could. And from watching machines
> run I decided I wanted rapids around 120 IPM. Now comes a big
> guessing game. I did the best I could to gather usable torque data
> and it is all fairly justified. But like I said in item 4 there is

a

> large difference in torques from item 1 hand cranking and item 3
> servo drive, or 1.7 Nm to 8.5 Nm. So at this point I will just

make

> note of those numbers and see what torque is actually available.
>
> Looking at the 3 motors above I picked out the torques from the

motor

> curves for 400 RPM (40 IPM cutting with 2 to 1 reduction), 800RPM

(80

> IPM cutting with 2 to 1 reduction) and a maximum RPM at a

reasonable

> torque for rapids.
>
> Best motor.
> K43xxxL-L (P) $1100.00 USD
> 400 RPM = 6Nm
> 800 RPM = 3.5 Nm
> 1350 RPM = 2.5 Nm 135 IPM rapid.
>
> 2nd best.
> N43xxxM-L (P) $570.00 USD Only slightly lower performance were it
> matters for running a Bridgeport, than the best motor above.
> 400 RPM = 6Nm
> 800 RPM = 3.5 Nm
> 1500 RPM = 2 Nm 150 IPM rapid.
>
> 3rd place.
> N41xxxJ-L (P) $330.00 USD In round numbers 30% to 50% less torque
> than others.
> 400 RPM = 3Nm
> 800 RPM = 2 Nm
> 1500 RPM = 1.5 Nm 150 IPM rapid.
>
> Ok we are almost ready to decide if these steppers can be used to

run

> a Bridgeport. But there is one more important calculation we have

to

> make and it is a goofy one. Remember we are using a 2 to 1

reduction

> so the torque on the steppers is multiplied by 2 to the lead

screw.

> But after we do this we have to divide by 2, HUH! Well an

important

> fact here, steppers are running open loop so you have to have a

large

> margin of safety and the usual safety margin is 50%. So using best
> motor above at 400 RPM we have 6 NM x 2 for reduction = 12 NM and
> divided by 2 for safety margin = 6Nm! Essentially we doubled our

RPM

> and kept the same torque.
>
> Now I can take my estimated required torque range of 1.7 Nm to 8.5

Nm

> and compare it to the 3 motors above for the 400 and 800 rpm

range.

> The last high RPM range is only a rapid and there should be enough
> power to go there. Well we see all motors meet the minimum hand
> crank torque of 1.7 Nm. If we compare a minimum servo system of

5.7

> Nm we see only the 400 RPM range of the first 2 steppers pass this
> test. And if we compare the torque of a full power servo system at
> 8.5 Nm none of the steppers have enough torque.
>
> Conclusion
>
> There is a lot going on here so many different conclusions can be
> made.
>
> Servo snobs can say see steppers don't enough power.
>
> Stepper enthusiasts can say all 3 of these steppers exceed what a
> normal person would manually crank so they will all work fine.
>
> I tend to look a little at history for an answer. Bridgeport sold
> machines with steppers with a 1 to 1 drive and later went to 2 to 1
> drive. These machines have been in service for 25 years making
> parts. From what I have been able to gather the Bridgeport power
> supply was less than 70VDC driving the steppers. And the steppers
> you can buy today are better than the ones Bridgeport used. So I
> would say if these are the 3 best steppers you can buy and you are
> driving them with a gecko. For sure the first 2 would work fine

and

> the 3rd would probably work ok.
>
> My advice on steppers for this application is if you can get them
> free or very cheaply you can go with them. But remember servo
> systems can be made fairly inexpensively by using treadmill motors,
> Gecko drives, and Usdigital encoders.
>
> And finally, I am doing a Bridgeport conversion and had planed to

use

> steppers because I had them and was comfortable with them. However
> over the last 6 months having been relentlessly beaten down like a
> dog from people like Les Newell (haha) about the benefits of servos
> over steppers I think I have switched. There is something to be

said

> about a semi closed loop system! And with these cheap $30 treadmill
> motors there is no reason not to go servo.
>
>
> Wally