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Author Topic: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?  (Read 21232 times)

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Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #50 on: December 14, 2012, 06:21:41 AM »
this would be a better test:  repeat your drilling.  then without moving the part put in a 1.25mm 2 flute end mill and bore your holes with it using basically the same program 125 times.  a drill can deflect and follow an existing hole a bit. the end mill will not.  and by measuring carefully, check the location of your 1st and last holes, see if they match the programmed dimensions.


another good test might be to mill a square, say 1" or so, on an aluminum block on the left end of the table then another on the right end of the table.  use rapid traverse to get from 1 end to the other.  repeat the program many times.  after a couple repeats, end mill spring would be eliminated, you should no longer see any shavings after that if things are repeating 100%

nice to hear good things about the geckos, i am considering them too, although i am trying to gather the courage to try a servo setup which is a little more complicated for an electronic noob like me.

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Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #51 on: December 20, 2012, 11:22:17 AM »
Yes,  I can Se your point about the deflecting drill,  but if yo have a setup that loses a step once in a while, defelection is no problem...... you will soon See new rows of nice little holes here and there.... :-)   the  test with the 2 squared holes  could be OK though , but I really don't experience anything indicating that I have any lost step at all.  edges looks like laser-cuts.

All the best

Leif
Software in use :

Mach3
Cambam
Cutviewer
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #52 on: December 20, 2012, 06:55:02 PM »
my point was this, Gropper.   if a drill misses hole center by a thou or 2 or more it will follow the existing hole due to the angled edges.  an end mill bores the hole and makes it round as reasonably possible, drills do not always drill round holes.  the end mill, especially a short one will not deflect, if you lose steps you will see out of roundness in your holes.  the orientation of the out of roundness might well reveal which axis is mis-stepping.

and regarding squares, i was actually thinking about square bosses, although holes should work as well.  rapiding from one point to another far apart as possible then milling i would think would expose dropped (or gained, can that happen?) steps, i would expect the step error to occur in the rapid move which is the fastest move, and would have the quickest accel/decel, i might even do the test with the rapid in 1st the x, then in the y axis.  then maybe in both.  truthfully i have not built a single mach3 thing yet and could be totally wrong about this.

it is this potential to lose a step that can affect long running jobs, kinda makes me lean servo, tho i would appreciate anyone chiming in that has run a stepper system for long periods of time, both in hours/day and also over months and years. 
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #53 on: December 20, 2012, 07:11:58 PM »
Gawd!  Old myths just never die....  Steppers are every bit as reliable as servos *IF* operated within their capabilities.  Exceed their capabilities, and they will lose steps.  It seems everyone who's ever experienced lost steps on a stepper system wants to believe lost steps are just an inherent characteristics of steppers, but it is simply NOT true.  Know your machines requirements, know the capabilities of your stepper motors and drivers, set them up correctly, and they should be absolutely reliable.  Most people either don't design the system correctly (motors too small, power supplies not properly sized, drivers can't supply enough voltage or current, or setting velocity or acceleration too high), then blame the steppers.  The fact is, steppers are used in industrial machines ALL the time, and they are EXTREMELY reliable.  But you can't just pick a motor, pick a driver, find a cheap power supply on E-Bay, and expect it to work.  And the same is true for servo-based systems.

Regards,
Ray L.
Regards,
Ray L.
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #54 on: December 21, 2012, 04:24:02 AM »
Hi Ray,

Looks like a little China nerd had a pretty good sense when composing my set,  I cant find any reason to make complaints, the set is running damn fine, so this time I didn't go wrong ...... :-)  My set from Long is pretty low noise as well, nice when working near by ......

Regards
Leif
Software in use :

Mach3
Cambam
Cutviewer
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #55 on: December 21, 2012, 03:15:50 PM »
"Gawd!  Old myths just never die.... (regarding stepper motors losing steps)"

I agree with Ray on this, keep your acceleration rates, max speeds and cutting forces within the limits of your stepper motors and you will never lose a step.

I have a Bridgeport clone Mach controlled mill using steppers that I've run 1000's of parts on over the last 10 years and the only time I lost steps was when I crashed, and that's when you want to lose them. I kept the accel and max speed rates reasonable (60 ipm max speed) and the machine has been very reliable. It will bog the 3HP spindle before it loses any steps from high cutting forces, at least on 6061 which is what I'm usually cutting.

To be honest, unless you are doing complex 3D parts or fine engraving, servo's are kind of overkill on a BP mill, it doesn't make much sense to try to move them much faster than 60ipm so it mostly comes down to the faster acceleration you can get with a servo. For me the only time that's been an issue was engraving, to get square corners on small letters I have to go pretty slow but I don't engrave small letters very often so its not a big deal for me.

Paul T.
« Last Edit: December 21, 2012, 03:19:22 PM by titchener »
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #56 on: December 22, 2012, 01:59:06 AM »
Hi,

I'm at my Mom in the hollyday, so I want to use this moment to wish you all a Merry Christmas with or without the ones you love.... and remember,.... Be careful out there, in the snow and ice....

All the best
Leif
Software in use :

Mach3
Cambam
Cutviewer
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #57 on: December 23, 2012, 11:17:53 AM »
Ray and Paul, thanks for the info on steppers, exactly what i was looking for.  i am the noob here, a good machinist who has to (wants to also) retro his old fanuc control lathe to mach3, and knows not much about steppers.  thats the direction i was leaning, i am not so sure now, i am getting some input that servos are not that much harder to do, and the concern i still have about steppers is heat and noise.  perhaps these are also myths, i do not know.  i am guessing steppers for my lathe would have to be in the 1800 oz in range, servos perhaps a bit smaller.  i will disagree with Paul in 1 small aspect of vertical mill use.  90% of my work here at home takes place in a kurt vise, i bet steppers would drive my mill fine and be plenty quick enough (although it came with servos in the 1 hp range, i am not sure of this but its what others more savvy than i have told me)  the other 10% might be noticeably quicker with the servos, that work can often span the 33" x 16" xy range of the table and includes many holes and features.  my servo driven mill rapids at 120 or 150 ipm in xy, cant remember right now, and it accelerates very quickly.  the beast horizontal i run at my day job rapids 1700+ ipm, and the advantage over my home mill running at less than 10% of this is obvious on jobs with lots of rapid moves.

Leif, thank for letting me play in your thread, Ray and Paul thanks for the info.  Paul your point on engraving was a good one, tho it shouldnt affect my lathe much.  now off to research the myth? that a mach3 driven single pulse per spindle rev threading system may have its limitations and what they may be.

a very happy holidays to all !!

ken
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Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #58 on: December 23, 2012, 11:56:37 AM »
Ken,

1800 oz-in steppers are HUGE.  I can't imagine you'd need anything that size.  And, such large steppers will be SLOW, due to high inductance, and high inertia.  With servos, the continuous motor torque ratings will nearly always be considerably lower than comparable steppers, because servos are nearly always geared down 3 or 4 to 1.  Servos essentially constant torque, and are typically capable of high RPM, while steppers always lose torque at higher speeds, often precipitously.  So, steppers are usually used direct drive, while servos will usually be geared down.  As an example, my 9x49 knee mill uses DC servos rates at 850 oz-in peak, 140 oz-in continuous, and can run up to 4200 RPM.  With 2.5:1 belt reducers, driving a 5-pitch ballscrew, I get 350 IPM rapids, and enough thrust at cutting speeds to break a 1/2" endmill without losing position.  A stepper conversion on the same machine would probably use steppers in the 900-1000 oz-in range.  It would not get the same rapid speeds as the servos (probably top out at 120-150 IPM, being RPM-limited due to torque fall-off at high RPM), but would be reasonably comparable otherwise.

To properly design a CNC conversion, you have to know the required torque to move your machine, the inertia of the mass to be moved, and the target performance you wish to achieve.  Armed with that information, you can design a drive system, servo or stepper, that WILL perform as wanted.  Most people seem to just ask "What motors should I use", then buy something close based on the recommendation of someone who may have never even built a machine, pick some drivers and power supplies based on what they can get cheap, they they're surprised when it doesn't perform nearly to spec.  There is no substitute for doing the math.  Trying to "over-design", by picking over-sized motors is just as likely to fail, just for different reasons.  The screws, reducers (if any), motors, drivers, and power supplies all have to be designed as a *system* to work properly.  If any one is poorly matched, system performance will suffer, often badly.

As for heat and noise - heat is really a non-issue.  Yes, stepper motors typically run hotter than comparable servos, but they are designed to handle the heat, so it's not a problem at all.  Steppers are typically a bit noisier than servos, with a characteristic "whine" when running, and depending on the specific driver used, they may "hiss" slightly when stopped.  This is purely an aesthetic issue, not a functional one.

On most jobs, very high rapid speeds should not make a huge difference in overall run time, unless you're using pretty bad CAM software.  Rapids should be less than 10% of the total run time on most jobs.

Regards,
Ray L.
Regards,
Ray L.
Re: How much stepper power do a 1-1/2 HP Bridgeport need to do a good job ?
« Reply #59 on: December 23, 2012, 02:35:09 PM »
I'm just now bringing up a low buck CNC conversion of my Clausing 5914 12" lathe.

I ended up using a size 23 for the X axis, the one Geckodrive.com sells, its a 400 oz-in 2.6 mH (ie 52 volt, 5A) motor with a 3.1" long body and dual 1/4" shafts. I liked this motor because its small and fits nicely on the bracket type I wanted to use but it has good RPM capability so it should work well on this machine for the X axis. I'll be using gang style tooling so the good rpm range of this motor will hopefully give me reasonable rapids for that.

For the Z axis I needed more pop as I sometimes drill steel so I used this motor from Automationdirect.com:
STP-MTRH-34127 1292 oz-in, 6.3A, .49 ohm, 4.14 mH -> 65 volts

I chose these motors based on their good power but low inductance, this means they should work well with Geckodrive supplies and their 80 volt max voltage capabilities. I'm going to start out with a standard 48 volt supply to keep initial costs down, if that doesn't give me enough pop I'll build up a higher voltage supply.

I'll let check back in with info on how well the machine works when I get it up and running.

Paul T.