Re: Suitable Steppers

--- 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