cnccc.pdf

9/30/13 Stepper motor torque and rpm

www.cnczone.com/forums/diy_cnc_router_table_machines/73317-stepper_motor_torque_rpm.html 3/9

02-10-2009, 02:44 PM

Yeah! Everybody says that at first. Let's look at what a G540 is:

$600: Four junior unkillable G203Vs with built in microstepping to full speed morphing and mid range resonancedampening.$120: Optoisolated 4 axis breakout board with speed control, limit and home connections and built in logic powersupply.$200: Worth of time and aggravation wiring up and troubleshooting myriad connections that are already DONEinternally with G540.$015: Motor cable connectors.Priceless: All this in a tiny package that just requires connection to 2 power supply wires, up to 4 motor cablesand one computer parallel cable and it's up and running.-----$935 Total value for only $299.

The only downside is that you need to expend the effort to choose your motors for best power within (Or as closeas possible to) the 3.5A, 50V G540 envelope.

Of course, you can do what MOST do and go for a $50 to $100 cheaper solution that will either prove unreliableor turn out to be unsuitable and need to be replaced after awhile--That is NOT saving money.

CR.

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Originally Posted by jwest

wow, that cleared some things up, thanks.

The router will be 50cmx100, with 10cm on the z axis. Im also going to use timing belts, (which Im currently lookinginto), and linear bearings for tbhe guide system. I hear the geckos are very good, but thats pushing the budget a little.

Last edited by Crevice Reamer; 02-10-2009 at 03:57 PM.

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02-10-2009, 03:33 PM

well when you put it like that, thanks for you help, Im buying components in a few weeks so ill deffinatly take it into consideration.

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02-10-2009, 03:51 PM

Maybe you can use some of this:

CNC BASIC PRIMER:

ACME SCREWS are the standard for most manual mills. They are just a relatively close tolerance screw threadand give fairly high precision and low 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 awhile, 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 sooften. Mach3 can compensate for backlash that doesn't keep getting worse

AXES: An axis is a direction of the CNC machine that is controlled by a motor. X axis = Left/Right. Y axis =Forward/Back. Z Axis = Up/Down (or on lathe: Z = Left/Right and X = forward/back.) A,B,C axes are rotary orangular. A is usually the forth axis, and can either rotate perpendicular to the X axis or perpendicular to the Zaxis. It is good to have as much travel as possible on these--Especially the Z. (for long tool use)

AXIS SPEED: There are basically two speeds--Cutting and rapid. Both are set in software, typically Mach3 oremc2. Speeds are set in Inch Per Minute of axis movement.

Cutting speed is Feed or F in G code. Rapid speed is the speed the spindle moves from place to place BETWEENcuts. Feed speed can be set as low as you desire. Rapid speed (Which you really want to be high to cover largeareas) is set as high as both the electronics will provide and you are comfortable with. Generally, on a 4 x 8 or

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larger router, 800 IPM is not unheard of. In metric it would be mm or meters per minute.

BACKLASH: When reversing direction, any handle (Or motor) movement that does not also move the axis (ortable or head/quill) is backlash. It is measurable directly by the dial on the handwheel or with a dial indicator. ForCNC, backlash must be checked and adjusted often. A large enough backlash may turn a circle into a vague blob.

BALLSCREWS have large threads that allow a ball bearing to roll IN them. The ballscrew nut contains many smallsteel 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 orhead might not hold a position, but be free to move on its own. So while you COULD install hand cranks on doubleshaft 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 dollarsfor 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 morethey 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, sothis 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. Oneend is usually turned to one diameter to fit a bearing. The other end may be turned to several decreasingdiameters to accomodate thrust bearings, threaded for clamp nuts, and turned at the end to fit stepper couplingor pulley.

Once you have determined the LENGTH of the screws you need, there are companies who will make yourballscrews 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 andhelp eliminate backlash.

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

BREAK OUT BOARDS (BOBs): Control software like Mach3 or emc2 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 cableto its destination, the breakout board accepts the cable plug and then puts each wire on an accessable screwterminal.

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

CNC SOFTWARE:

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 translatethe CAD output to G code.Control: This software actually runs the mill or lathe or router from the G code. This software comes WITH certainexpensive "turnkey" equipment, but usually you have to acquire it seperately.

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

MACH3 is the hobbiest defacto best computer software for machine control in Windows. It can control eitherSteppers or Servos. Mach operates by sending out pulses to to the drivers that control the motors. The NUMBERof pulses is limited by the speed of the computer and by an upper limit. 35 to 50 thousand pulses is an averageamount.

OR EMC2. EMC2 is a free, open source software CNC program that runs on Linux.

CHATTER: A shudder or shaking of the machine and part when the tool is pushed too hard for conditions.(material density, tool sharpness etc) This is an undesirable in cutting and is avoided by either using a moremassive machine, or by using greater care with tool feed and spindle speed.

DIAL INDICATOR: This is used to accurately measure a very small distance and display it on an easily read dial.These are invaluable for setting up work and Tramming the CNC machine.

DRIVER CONTROLS:

Stepper drivers are the electronics that translate the pulses from the computer into useable current for themotors. They are fairly expensive and many are easily damaged. Wiring the drive wrong or disconnecting it duringuse will destroy most drives. You can have a powered driver without a motor connected, But you NEVER want todisconnect a motor while power is applied.

MICROSTEPPING: Some drivers are designed to artificially reduce the distance the motor will turn by electronics.Most motors have full step 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 theirnumber increases. Micro stepping operates at the expense of speed, and promises extremely high accuracy byincreasing steps per revolution, but practically 8 or 10 microsteps are the limit. The computer and software canonly put out just so many pulses, and the higher the step count, the slower the motor will run, unless the drivehas full step morphing.

Generally, the more expensive drives (Like the Gecko G203 or G540 Vampires) offer the best features like overheatprotection, micro stepping and speed morphing.

MID BAND RESONANCE DAMPINGl Only in better quality PWM drives like Gecko. Allows motor to run at mediumspeeds without losing steps.

MICRO STEP TO FULL STEP MORPHING: Only Gecko 203V or G540 and also the Mardus-Kreutz (unipolar micro-stepper drives) and Kreutz-4 and derivatives (K-41DIY) bipolar micro-stepper drives use waveform morphing vsspeed. This allows low speed micro stepping and high speed RPMs. Morphing does micro step for smooth low speedaccuracy, but jumps to full step speed for high speed rapids.

IDLE CURRENT REDUCTION: Steppers tend to get hottest standing still. Overheat protection may 1. Cut thecurrent down, and/or 2. Put the motor in "sleep" mode after a short wait. Both will drastically reduce heat buildup.

STEPPER MOTORS are designed to move just a tiny bit each time they receive an electrical pulse. They do notoperate on straight uninterrupted current as normal motors do. The Torque rating is what you get with the motorat rest. Torque falls off with increase in RPMs. To do any WORK with it, you need to carry much of that torque upto higher RPMs.

It is important to match the motor to the load. You can't just assume that bigger is better. Bigger motors runsomewhat slower than smaller motors. A router, more so than a mill, needs high rapid speeds. You will get bestperformance by wiring the motors in Bipolar Parallel.

STEPPER MOTOR WIRING CONFIGURATIONS: Stepper motors usually have 2 phases and 4 internal coils. Four wirestepper motors have 4 coils inside that are internally wired as either BPP or BPS. Series motors will have four timesthe inductance in mH, 1/2 the Amperage rating and can tolerate twice the Voltage as Parallel wired motors.

UNIPOLAR (UP 5, 6 or 8 wire motors): Unipolar motors run ONE coil at a time. One coil per phase is powered--which one depends on direction desired. These can be driven by very inexpensive controllers, but are not veryefficient and usually deliver low power.

HALF COIL (HC 5, 6 or 8 wire motors. Allows 5 or 6 wire motors to run nearly as fast as if they were wired BPP.

BIPOLAR SERIES (BPS 4, 6 or 8 wire motors): These motors have low-speed TORQUE, but will quickly lose poweras they run faster and will stall at relatively slow speeds. Their power goes through first ONE coil of the phase andthen the other. (series)

BIPOLAR PARALLEL (BPP 4 or 8 wires): These motors Have good torque and retain more of it higher RPMs than anyother type. Their power goes through both coils at once, but separately. (parallel) This is generally considered tobe the best wiring method for steppers.

This diagram is for illustration of the above points:

http://kelinginc.net/KL23H286-20-08B.pdf

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 and outputs up to 3.5A each motor)will combine four axes of tiny morphing "Vampire" drives with a breakout board so that all you need to connect isthe parallel cable, power wires, and motor cables. CNC conversion is now a LOT easier and less expensive.

SERVO DRIVES that WE can afford, use basically the same pulse system as stepper drives. Actual expensivecommercial servo drives use a different, more expensive PID system.

SERVO MOTORS, which are more expensive, do not have the starting torque that steppers have, but theymaintain 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 motorsare also equipped to tell the computer (through encoder feedback) exactly where the motor is at any given timeso there are no missed steps. Stepper motors can stall and miss steps unbenownst to the operator until thefinished part is measured. Servo motors will destroy themselves if stalled or if encoder fails.

CPR: Count Per Revolution.

PPS: Pulse Per Second.

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

PID: A ProportionalIntegralderivative controller (PID controller) is a generic control loop feedback mechanismwidely used in servo control systems.

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

HOME SWITCHES are usually Normally Open, (NO) and are set at one of the limits of travel. When Mach orders ahome 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 sometimes possible, but much more difficult, to combine the upper N.C. limit switch with a N.O. home switchin the same switch. (double throw)

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

LIMIT SWITCHES: These are usually Normally Closed (NC) switches that tell mach when an axis has exceeded itslimit of travel. On a servo system they will prevent the servo from stalling and burning itself up. On a high speedstepper system they may prevent impact damage to the motor. On a low speed stepper system they are probablynot needed as the stepper motor will stall harmlessly. It is almost impossible to limit switch the lower end of Ztravel because of varying tool lengths. Mach3 will also allow you to set up "soft limits" that operate independentof any switch.

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

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 thesomewhat larger Nema 34. The torque may overlap between the sizes, but generally the larger motor has aneasier time.

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

POWER SUPPLY (PSU): Both types of motors run on DC Voltage. The power supply simply converts ordinaryalternating current into smooth DC at a Voltage for CNC motors. Choosing the proper voltage to matchdrivers/motors is one of the most important decisions needed. You NEVER want to install a switch on the DC sideof the power supply.

One power supply, sized to power the lowest Best voltage motor, is all you need. EG: Two 60V motors combinedwith one 83V motor = Must use 60V or less PSU.

Stepper motors need around 20 times their rated voltage to perform at their best. For example, a motor rated at 2volts will perform best, without stalling or losing steps, with a 40 volt power supply.

For the EXACT Max/Best power needed for a stepper motor the formula is 32 times the square root of motorinductance in mH. EXAMPLE: A motor with 4 mH inductance would need a 64 Volt PSU. The PSU must be sized forthe lowest voltage motor--So a 64 Volt motor combined with an 85V motor would need a 64V PSU. You wouldthen pick the PSU that is at or as closely below 65V.

Series wired motors can run at higher voltages--but there is a cost in speed performance.

AMPERAGE: To determine the PSU amperage required the formula is .67 times total motor amps. EXAMPLE: Amperrating for three 3 Amp motors would be (3+3+3) times .67 = 6 Amp PSU.

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

QUILL: This is a spindle shaft that allows the tool to be moved up and down separately from the head--Usually bya lever/wheel arrangement as on a drill press. Most dedicated CNC machines do not have a quill, and it is usuallyremoved or locked during a CNC conversion of a manual mill. (Because CNC head moves are adequate andextending a quill lessens the tool rigidity.

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

The way to get best rapid speeds is to be able to get torque at high RPMs. This is accomplished by matching themotor's best voltage to the power supply voltage. Higher voltage pulses charge the coils more quickly andmaintain torque to faster speeds.

The actual motor RPM get will depend on your drivers and power supply. First find the inductance of the motorwired the best way for your driver--Usually Bipolar Parallel. Formula for most efficient motor voltage is 32 timesthe square root of that inductance.

If you run the motor BPP at that voltage and at full motor amps, and have enough PPS from the computer, youwill get the maximum rpm possible. If it is too fast for your liking, you can always slow it down (with no ill effects)in software.

If you run the motor at LESS than that Voltage, and/or with a less efficient driver, you will get proportionatelyless RPM before stalling and losing steps.

Using the G540 as the controller, (and you should if you can, it's the most bang for the buck) You can operatewith a max voltage of 50V. With the G540, you will want motors with best Voltage between 50 and 65V.

RIGIDITY The basic solidness of a CNC machine. A more rigid machine can take deeper cuts without chatter.Heavy machines are usually more rigid than light ones. A more rigid machine is usually more accurate also.



RESOLUTION: The measured (In mm. or inch) amount of accuracy possible in an axis move. This is a combinationof 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 turnsa ballscrew with 5 turns per inch, then there will be 1000 steps per inch or a resolution of one thousanth of aninch. (.001) If that same motor was turning a 20 turn per inch Acme screw, the resolution would be 4000 stepsper inch, or 4 TEN thousanths of an inch. (4 Tenths or .0004) Pulley or gear ratios add to the resolution and youmust also factor in any microstepping of the drive.

Bear in mind that there is no free lunch. Computer pulses are limited, and usually Finer resolution comes at thecost of lower Rapid speed.

STEPS PER INCH (SPI): Are used to set up machine software to accurately move the axes. The usually 1.8degree per step motor will need 200 full steps to turn one revolution. The TPI of the lead screws will determinehow many revolutions will move the axes one inch. Multiply this by the number of micro steps and you have thebasic step per inch factor. In Metric, this would be steps per mm or steps per meter.

Ideally, this would make the machine actually move the proper amount. But if say a 6 inch move is called for, butthe machine moves more or less, you may need to tweak the SPI up or down a little. Mach3 accepts decimalamounts here.

TRAMMING: A process to make all axes of a machine tool perfectly perpendicular to each other. If these axes arenot perfectly aligned, then the parts made will be out of intended specification or shape.

WHY HAVE FAST RAPIDS:

There is no maximum limit for IPM. High IPM is a measure of the drive/motor efficiency. Good efficiency equalslower chance of missed steps. It is NOT just about cutting speed--Cutting speed will be influenced by materialand force required. Inefficient systems may not be able to provide sufficient force to cut at optimum rateswithout stalling and missing steps. Many have cursed stepper systems as no good because their inefficientsystems lost steps.

FIRST: Understand that YOU can always set the upper limit of your IPM by software control. You can easily slowdown an efficient CNC. It is very difficult and often very expensive to SPEED UP an inefficient CNC.

High IPM really saves time when your spindle has to move from one place to another without cutting. Time savedalways translates into money saved during production.

If you have lots of time to waste, have no intentions of ever doing any kind of production, will NEVER wantanything like an automatic tool changer (or multiple fixtures) and/or are not dealing with a large area to cover likeon a router--Then by all means limit your upper rapid speed. But do it in software--NOT by crippling your machinewith inefficient components.

[b]WHY A G540 STEPPER DRIVER IS A GREAT VALUE:[b/]

Everybody at first says "Wow that's expensive! Let's look at what a G540 is:

http://geckodrive.com/product.aspx?c=3&i=14469

VALUE:

$600: Four junior unkillable G203Vs with built in microstepping to full speed morphing and mid range resonancedampening.$120+: Optoisolated 4 axis breakout board with spindle speed control, limit and home connections and built inlogic power supply.$200: Worth of time and aggravation wiring up and troubleshooting myriad connections that are already DONEinternally with G540.$015: Motor cable connectors.Priceless: All this in a tiny package that just requires connection to 2 power supply wires, up to 4 motor cablesand one computer parallel cable and it's up and running.-----$935+ Total value for only $299.

The only downside is that you need to expend the effort to choose your motors for best power within (Or as closeas possible to) the 3.5A, 50V G540 envelope.

Of course, you can do what MANY do and go for a $50 to $100 cheaper solution that may either prove unreliableor turn out to be unsuitable and need to be replaced after awhile--That may NOT be a money saving choice.

CR.

EDITED TO UPDATE

Last edited by Crevice Reamer; 02-11-2009 at 07:46 PM.

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02-10-2009, 05:00 PM #9

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