[isvflorin]

My question might be stupid but after reading a lot still not sure how things should work.
I'm looking at buying 3 steppers rated at 3Nm (around 430oz) - 4.2Amps in parallel and 2.1 Amps in series ?
I'm puzzled by the following - if I want to run them in parallel would I need a Gecko that outputs at least 4.2 AMps ? Or the output of the drive should be at least 2.1Amps (which the G250 will work fine)

Thanks.

[Crevice Reamer]

Hi isvflorin. Welcome to the Zone!

3.5 amps or under and 50V PSU or under, G540 or G250 are fine. Over 3.5A, must use G201 or G203V which can handle up to 7 amps, 80V.

You could use the Keling 387s with G250. They are 4 wire motors wired internally in Bipolar Parallel. These motors are 3.5A and can run with up to 65V PSU.

http://kelinginc.net/NEMA23Motor.html

Formula for best/Max Power Supply voltage is 32 times the square root of lowest motor inductance.

Formula for minimum PS amperage is total motor amps times .67.

Parallel/Series:

Bipolar parallel gives most power at highest RPM. Bipolar series gives most power at lower RPM and not too much power at higher RPM.r
For optimum performance, you will want to wire motors in Bipolar Parallel.

If I were you, I would use the far superior G540. It costs more, but by the time you add a breakout board to G250, price is much closer.

CR.

[isvflorin]

Thanks for clearing out some facts.
To further upgrade my knowledge base - when a stepper motor manufacturer says the motor will draw 4.2Amps in parallel - it means the drive will need to output at least 4.2 Amps or just half of that - which is the series amperage ?

thanks again,
Florin

[Crevice Reamer]

If you wire the motor in series, for best low speed torque but slow top speed, the drive will have to put out the half figure.

If you wire the motor in parallel, for best high speed performance, the drive will have to output the 4.2 Amps.

Now you COULD run a 4.2A motor at 3.5A, but performance would be less.

CR.

[Crevice Reamer]

Performance will ALSO suffer if you run, say an 80V motor, at only 36V. Stepper motors work on pulses, not continuous current. Using the optimum voltage for a motor charges the coils more quickly per pulse and resultes in maximum performance.

CR.

[Crevice Reamer]

You may already know all of this, but I'm going to assume that some of it will be helpful to you:


CNC: Computer Numeric Control. CNC can do things that you couldn't DREAM of doing manually. Properly programmed CNC can cut a sphere or any other geometric shape. All by combining axis moves to position the cutter in 3D space.

CNC Software:

G CODE: Actually there are many other letters involved also. This is the language that tells the Control program how to direct the machine to make the part.

There are three programs involved:

CAD: (Computer Assisted Design) A program to draw plans and maybe 3D objects with.
CAM (Computer Assisted Manufacturing) This program sets up the tool paths for the mill or lathe. It may translate the CAD output to G code.
Control: This software actually runs the mill or lathe or router from the G code.

MACH3 is the hobbiest defacto best computer software for machine control. It can control either Steppers or Servos. Mach operates by sending out pulses to to the drivers that control the motors. The NUMBER of pulses is limited by the speed of the computer and by an upper limit. 35 to 50 thousand pulses is an average amount.

POWER SUPPLY: Device that changes 120 volt AC to smooth DC for CNC motors. Choosing the proper voltage to match drivers/motors is one of most important decisions needed. You NEVER want to install a switch on the DC side of the power supply.


BREAK OUT BOARDS: Mach3 uses the many wires in a parallel port (printer) cable to send control from the computer to the drives. Rather than fastening each tiny wire in the cable to its destination, the breakout board accepts the cable plug and then puts each wire on an accessable screw terminal.

BACKLASH: When reversing direction, any handle movement that does not also move the axis (or table or head/quill) is backlash. It is measurable directly by the dial on the handwheel. For CNC, backlash must be checked and adjusted often. Backlash will turn a circle into a vague blob.

RAPIDS: Non-cutter axis moves to get quickly from one point to another. These are cumulative, so if they are slow it slows down the whole job.

ACME SCREWS are the standard for most manual mills. They are just a relatively close tolerance screw thread and give fairly high precision and backlash while the adjustment lasts.

Acme screws and nuts wear quickly. Usually the screw wears most in the middle and less on the ends. After a while, you can't use the ends because it's too tight.

Even relatively cheap ballscrews, which HAVE some backlash, are better because the backlash does not vary so often. Mach3 can compensate for backlash that doesn't keep getting worse

BALLSCREWS have large threads that allow a ball bearing to roll IN them. The ballscrew nut contains many small steel balls that recirculate inside to reduce friction. The ball nuts can be extremely tight to eliminate backlash--yet still have little friction.

Once ballscrews are installed, manual control may not be possible. Because ballscrews turn so easily, the table or head might not hold a position, but be free to move on its own. So while you COULD install hand cranks on double shaft motors, you might have to constantly lock the gibs on the other axes and it just may not be practical.

Ballscrews come in two types: Rolled and ground. Ground ballscrews are best, but can cost thousands of dollars for just one screw. We small-time automators usually can't afford them.

Rolled ballscrews come in several grades. The better they are for accuracy and low backlash per length, the more they cost. We usually use a medium grade.

If you buy say a six foot length of ballscrew, it needs to first be cut to axis lengths. It is hardened material, so this is usually best done with an abrasive cutting disk.

After they are cut, each end is turned down on a lathe. Because they are hardened, this is difficult to do. One end is usually turned to one diameter to fit a bearing. The other end may be turned to several decreasing diameters to accomodate thrust bearings, threaded for clamp nuts, and turned at the end to fit stepper coupling or pulley.

Once you have determined the LENGTH of the screws you need, there are companies who will make your ballscrews to order.

BALL NUTS: These are basically just enclosures that contain and recirculate the small ball bearings.

PRE-LOADED BALL NUTS: These have been re-loaded with larger balls. This takes up all available wiggle space and help eliminate backlash.

DOUBLE BALL NUTS: Two ball nuts with one tightened against the other to counter backlash. These are even better, but more expensive, and because they are longer, cost a loss of axis travel.

PULLEYS are used to increase torque by gearing down the motor RPM. However, stepper motors get weaker as speed increases, (To a limit of 800-1500 RPM depending on PS voltage--up to 20-25 times motor rated voltage if the drivers can handle it.) so most of the gain in torque results in lost speed. That's why most stepper motors are connected direct drive.

IPM: Inch Per Minute is the speed rating for the X, Y & Z axis motion. Cutting in a mill usually happens below 30 IPM. But rapids may need to be as fast as possible.

STEPPER MOTORS are designed to move just a tiny bit each time they receive an electrical pulse.

MICROSTEPPING: Some drivers are designed to artificially reduce the distance the motor will turn by electronics. A full step is hardwired at 1.8 degrees and with 200 computer pulses it will complete one revolution. With microstepping set at 10 (Or one tenth) The motor will theoretically take 2000 steps (And computer pulses) to complete a revolution. I say theoretically because microsteps get just a little more vague in size as their number increases. Micro stepping operates at the expense of speed, and promises extremely high accuracy by increasing steps per revolution, but practically 8 or 10 microsteps are the limit. The computer and software can only put out just so many pulses, and the higher the step count, the slower the motor will run.

CONTROL DRIVES:

Stepper drives are the electronics that translate the pulses from the computer into useable current for the motors. They are fairly expensive and many are easily damaged. Wiring the drive wrong or disconnecting it during use will destroy most drives. Generally, the more expensive drives (Like the Gecko G203 Vampires) offer the best features like overheat protection, micro stepping and speed morphing. Steppers tend to get hottest standing still. Overheat protection will 1. Cut the current down, and 2. Put the motor in "sleep" mode after a short wait. Both will drastically reduce heat buildup. Morphing changes the speed to micro step at low speed accuracy, but jump to full steps for high speed rapids. You can have a powered driver without a motor connected, But you NEVER want to disconnect a motor while power is applied.

PID: A Proportional–Integral–derivative controller (PID controller) is a generic control loop feedback mechanism widely used in servo control systems.

Servo Drives that WE can afford, use basically the same pulse system as stepper drives. Actual expensive commercial servo drives use a different, more expensive system.

GECKO DRIVES are generally acknowledged as the best. Gecko "Vampire" drives are virtually unkillable.

The new low-cost Gecko G540 board (Accepts up to 50 volt power supply) will combine four axes of tiny cheap drives with a "Vampire" morphing breakout board so that all you need to connect is the parallel cable, power wires, and motor cables. In a short while, CNC conversion is going to be a LOT easier and less expensive.

SERVO MOTORS, which are more expensive, do not have the starting torque that steppers have, but they maintain what torque they have into high rpms. They are usually geared down 2 or 3 to 1 to gain starting torque. Even geared down, they can still attain thousands of RPM, so speed is not a problem with pulleys. Servo motors are also equipped to tell the computer (through encoder feedback) exactly where the motor is at any given time so there are no missed steps. Stepper motors can stall and miss steps unbenownst to the operator until the finished part is measured. Servo motors will destroy themselves if stalled or if encoder fails.

CPR: Count Per Revolution.

PPS: Pulse Per Second.

MGP: Manual Pulse Generator. This allows easy manual CNC axis control without programming. Can be either a hand-wheel or joystick control.

Encoders: These send position and speed feedback to the controller and are rated in CPR. They are quadrature devices that require 4 times the PPS per revolution. For example: An encoder rated at 250 CPR, will require 1000 drive Pulses Per Second.

Each system has its pros and cons. Steppers used with proper power supplies are reliable, consistent and cost effective--That's why most hobby applications use steppers.

POWER SUPPLY: Both types of motors run on DC Voltage. The power supply simply converts ordinary alternating current into this direct current. Stepper motors need around 20 times their rated voltage to perform at their best. For example, a motor rated at 2 volts will perform best, without stalling or losing steps, with a 40 volt power supply.

NEMA= National Electrical Manufacturers Association. They set the USA electrical standards.

NEMA SIZES: Both steppers and servos may come in different Nema flange sizes.
Nema 23= 2.3 inch flange. Nema 34= 3.4 inch flange etc. We usually use either the smaller Nema 23 or the somewhat larger Nema 34. The torque may overlap between the sizes, but generally the larger motor has an easier time.

For example, a 500 oz Nema 23 stepper motor will be working hard (and getting hotter) to attain the torque at which a 500 oz Nema 34 will be easily cruising. Generally, power is added by extending the length (stack) of the motor.

RESOLUTION: The measured (In mm. or inch) amount of accuracy possible in an axis move. This is a combination of number of steps per motor revolution and number of turns per inch of the lead screw. For example: A direct-drive Stepper motor with driver set for full step will take 200 steps for one full revolution. If that revolution turns a ballscrew with 5 turns per inch, then there will be 1000 steps per inch or a resolution of one thousanth of an inch. (.001) If that same motor was turning a 20 turn per inch Acme screw, the resolution would be 4000 steps per inch, or 4 thousanths of an inch. (.0004) Pulley or gear ratios add to the resolution.

LIMIT SWITCHES: These are usually Normally Closed switches that tell mach when an axis has exceeded its limit of travel. On a servo system they will prevent the servo from stalling and burning itself up. On a high speed stepper system they may prevent impact damage to the motor. On a low speed stepper system they are probably not needed as the stepper motor will stall harmlessly. It is almost impossible to limit switch the lower end of Z travel because of varying tool lengths. Mach3 will also allow you to set up "soft limits" that operate independent of any switch.

HOME SWITCHES are usually Normally Open, and set at one of the limits of travel. When Mach orders a home operation, the axes go to the home switch location, close the switch, and then move slightly back and stop. This gives a reference position for mach to start from and position the tool.

It is possible to combine the upper N.C. limit switch with a N.O. home switch in the same switch. (double throw)

CR.

[isvflorin]

Ok. Much apreciated.
SO i guess it makes no sense getting the higher torque motors if the g250 can't output enough Amps to drive it to it's full potential.

The motors are for a gantry type router with linear guides. Mainly for making small plastic parts and some alu moulds with hf spindle. (1300x900x200mmworking area).
I'm trying to get the best power for a small budget, so I either get more expensive drive for the 430oz steppers or I'll just stick to the 387 and the G250.
Does the combination of 387 (2 for the x actually) and the G540 seem underpowered ?

[Crevice Reamer]

Ok. Does the combination of 387 (2 for the x actually) and the G540 seem underpowered ?

That depends on the size of the router. The 387oz motor is not MUCH less than the 430 oz you wanted to use, and would put out more power at 3.5 amps than the other will.

If you are going to use FOUR G250s, and you should to drive 4 motors, you would be better off with the G540. The G250s plus the breakout board would cost almost as much as the G540, but not have any of the super vampire features or convenience of it.

Either can use up to 50V PSU, and 4 motors would require at least 10 Amps from it.

CR.