STEPPER MOTOR RETRO-FIT KITS

DAVE DOWNS - THE MOTION GROUP - sales@motiongroup.com

• Stepper motor retro-fit kits are intended to convert manual mills, mill-drills, and lathes found in machine shops and the hobby market into automatic CNC (computerized numeric control) systems. Stepper motor systems have three major advantages in that they are lower in cost, simple to use, and require no maintenance when compared to servo motor systems. The dis-advantages are slower rapid travel, no feed-back, and no hogging power. Step motor retro-fits are for machines such as the MAXNCtm, a desk-top hobby mill, bench-top mill-drills such as the ENCOtm, and full-size, floor-mounted, Series 1 or 2tm clones.

• The stepper retro generally consists of three elements: (1) two or three motors which are mounted to the machine with flat plate brackets and coupled to the screws with precision timing belts, (2) a power amplifier and supply package to drive the motors, and (3) a software package. This software turns a PC type computer into a CNC control console which will accept CNC G-Code files from a CAD-CAM package or other source. The G-Code is converted, by the computer, to digital motion signals which are passed out the printer port to the motor amplifier system.

• With step motors, it is important to note that the speed and power of a system is a function of the amplifiers rather than the size of the step motors. The most popular step motors for retro's are the NEMA frame 2.3" and 3.4" motor. Engraving and hobby systems use the smaller 2.3". Larger machines use the 3.4" with 2 or 4 amp motor drivers. Large step systems, such as the NEMA 4.2" with 6 to 10 amp drivers, cost as much as low-end servos.

• There are two types of step motor amplifiers, the L/R driver and the chopper or switcher. L/R drivers are very inefficient, run hot, and fail (cook-out). They are suited only for small or slow systems under 1 amp. The chopper drive, on the other hand, is much faster, runs cooler, locks (brakes) at stand-still, and is very solid-state. Chopper drive also allows micro-stepping the motor to 800 to 1600 steps per motor rev. The 2 amp chopper is the same power as a 5 amp L/R but faster. Surplus step motors generally have windings unsuited for high-speed chopper drives; that's why they are surplus.

• The most popular CNC control console, for retro's, is an older PC type computer; either laptop or desktop. The CNC power amplifier system is connected to the PC printer port as opposed to production CNC mills which connect to the serial port. A printer port CNC is often referred to as an LPT indexer; an indexer is a motor controller. Fast color computers with large memories and Windowstm are not required. Surplus computers are well suited for machine control which is a slow application. CNC retro's which run under Windowstm either require an additional embedded ($) controller or suffer from "windows heartbeat". This beat frequency distorts the motor timing patterns and results in noisy operation with diminished top speed performance.

• The CNC software, furnished with the retro kit, teaches the computer to control the CNC machine. The software has two major functions. The first is to assist the operator in running the machine with features such as jogging, homing, and displaying the position of each axis (DRO). During the initial start-up, an operator enters motor speeds, resolution (steps per inch), feed rates (inches per minute), and other information into the parameters table. The computer, then, is matched to the machine tool.

• The second function is to correctly execute (run) the G-Code file of the part being made. It is important that the software be written by machine tool programmers, such as Otto Echeveri of MAXNC. A G-Code file run on a TMG retro CNC machine will produce the same part as one from a legitimate CNC machine or vice-versa. A good retro-fit will support 30 to 40 G & M codes including cutter compensation and tool height tables. The computer screen should be a graphics presentation in that the display will look like the control panel of a legitimate CNC machine with large read-outs for each axis. Operation should be straight-forward and not require a computer science degree. The database should contain example parts and home routines in G-Code. A plot program which displays (plots) the G-code of the target part, in 2 or 3-D, before cutting, is a minimum feature. Plotting during real time is an expensive feature not supported by TMG.

• CNC programs in G-Code are the most common way of describing a machine tool part. G-Code files, also refered to as tape files or .T files, are generated by CAD-CAM drawing software systems. CAD drawing packages do not produce G-Code directly but require a translator to convert the drawing code format, most common is .dxf, into G-Code commands. G-Code can also be generated, off-the-top-of-the-head, by typing the commands into an editor or word processor. The G-Code format is called ASCII text file which all computers can process. CAD-CAM packages, generally, are very expensive. An exception is the BOBCADtm (BOBCADUSA.COM 209-694-8043) package which can include Windows True-Typetm engraving and an artwork capture tool. BobCad CAD-CAM supports translation for common CAD formats like .dxf and others.

• All retro-fit systems should provide additional features such as home sensors, safety limits (CW/CCW limits), and I/O (input/output). Home sensors are photo-electric sensors (switches) located on each axis of the machine tool. Hard-wired limits prevent the machine from running out-of-bounds due to programming errors and should include an E-Stop (emergency stop) slap-switch. The limit safety system should shut-down or over-ride computer control; computer controlled limits are un-wise and un-safe. A typical I/O Input is the M6 tool changer switch. The Outputs control automatic operation of the spindle (M3) and a coolant pump (M8).

• Step motor systems have no feed-back encoders and must be operated within the speed and power range of the motors. If the motors are over-driven, they will slip out of position and the part is lost. However, home sensors, when used correctly, allow the verification of correct operation at the end of the program and prove the part. In addition, home sensors assist in calibration of the automatic back-lash compensation feature. This back-lash compensation restores older machines or machines with screw-drive.

• Any machine which has "hand wheels" can be retrofit by replacing the wheels with motor drive under computer control. Because of the timing belt gear ratio, 2:1 or 3:1, feed rates of 5 to 25 inches/min at full power are possible. TMG systems are "slow"; not much rapid speed. TMG 2 amp systems do 15 ipm @ 16000 steps/inch; 4 amp units are 30 i/m. However, computer controlled operation of a machine allows smooth cutting and multiple passes which, also, prevents damage or "beating-up" the adjustment of a screw-thread machine. In general, the torque of a 250 in.oz. motor is the same as the twisting power of one hand (150 lb. male) gripping a 3/8" rod. A 625 in.oz. motor is three hands on a 3/8" rod. This torque is doubled by the 2:1 belt ratio.

• Note that mills and mill-drills must have screw or worm drive (fine-feed adjustment) on the Z-axis (spindle head). The table lift rack-drive of knee mills is not suited to step motor drive; a mechanical modification/retrofit to a "spindle lift" is necessary on some machines. Mill-drills must be "de-clutched" from the spindle course-drive & return spring.

• Ball-screws are not required on step motor retro systems. Typical resolution of a 10 pitch screw drive is 16,000 steps per inch or 0.0000625 inches per step (800 spr motor x 2:1 belt x 10 pitch screw = 16,000). Note that ball screws are typically only 5 pitch and were intended for high-speed power cutting; something steppers do not do. In general, ball screw replacement, except when done by experienced machinists on good machines, results in the same performance (or worse) as the screw drive. Cleaning-up the slides, repairing worn lead-nuts, and replacing the cheap thrust bearings on the screw machine is a better effort.

• The Motion Group offers two models of 2, 3, or 4 axis Digital-CNCtm retro kits which include all of the above features. The CNC 2.0 is a 3.4" single-stack motor package with 2 amp drivers at 800 steps/rev. The CNC 4.0 is a 3.4" triple-stack package with 4 amp drivers. For plasma torch and router systems, the 4 amp driver package with "high-speed" single stack motors is recommended. All motor supplies are 40 VDC. Each system includes all cables, switches, operation manual and a 200 page retro-fit manual (see sample pages) with bracket drawings, belt & pulley part numbers, and step-by-step examples for retro-fitting common mill-drills, mills, and lathes. The optional optical home sensor kit supports Slip-Detectiontm and electronic back-lash calibration. Belt and pulley kits in 3/8 or 1/2" wide are available. An addition system, the ENC 2.0, is for small desk top modeling or engraving units. This package comes with standard (800 spr), or high-resolution (1600 to 3200 spr), size 2.3" motors and uses 1/4" belts or direct drive.

For a demo disk, download the CNC software package at motiongroup.com.

• System G and M Codes. The basic format of a NC program is based in the Cartesian coordinate system (X, Y, and Z) which is used to to store the positions that the tool will move to, together with other information such as linear or circular move, feed rate, and some others. There two types of linear coordinate moves; incremental and absolute. Absolute is the measurement from the 0 (zero) end of an axis to the max end; just as a ruler or scale would readout. Absolute commands move to some point on the scale. Incremental repeats a fixed value, for example 1 inch, along the scale. The machine readouts will typically read the absolute position. There are two speeds of a move; rapid and feed. Rapid moves are non-cutting transfers from point to point; feed moves are cutting motions.

• A single letter followed by a number are the basis of every command. The format for this system is spaces optional; no tabs between characters; modal only. Both incremental and absolute coordinates (G90 & G91) are supported. For circles (G02 & G03), X and Y can be ABS or INC (depending on the G90 or G91) but I and J are always INC. Number format is + or - in front with 2 digits before the decimal and 4 digits after. Leading and trailing zero's are ignored; + sign is optional.
• Programming Example.

G00X0.5Y0.5 RAPID MOVE TO X = .5"; Y = .5"

G00Z0.1 RAPID MOVE TO Z = .1"

G01Z-0.250F5.0 FEED MOVE TO Z = -.25; FEED @ 5"/MIN.; Z OWN

Y0.0 FEED MOVE TO Y = 0.0"

G02X-0.5Y0.0I-0.5J0.0 CUT X & Y CIRCLE TILL X =- .5; Y = .0

G01Y0.5 FEED MOVE TO Y = .5

G00Z0.1 RAPID MOVE TO Z = .1; Z UP

G81X0.0Y0.0R0.020Z-0.300F2.0 DRILL CYCLE

X0.312Y0.312 REPEAT DRILL CYCLE

X-0.312 REPEAT DRILL CYCLE

G80 CANCEL DRILLING CYCLE

G00X1.0Y1.0Z1.0 RAPID MOVE TO X = 1"; Y = 1"; Z = 1"

M30 END PROGRAM

• Command Set. The following codes are supported by TMG CNC systems:

G00 RAPID MOVE G01 FEED RATE CONTROLLED MOVE

G02 CLOCKWISE ARC G03 C'CLOCKWISE ARC

G04 DWELL (DELAY IN MSEC.) G12 FULL CLOCKWISE CIRCLE

G13 FULL C'CLOCKWISE CIRCLE G17 XY PLANE FOR ARCS

G18 XZ PLANE FOR ARCS G19 YZ PLANE FOR ARCS

G40 RADIUS OFFSET CANCEL G41 RADIUS OFFSET LEFT

G42 RADIUS OFFSET RIGHT

G43 HEIGHT OFFSET ACTIVATION G49 HEIGHT OFFSET CANCEL

G60 (NOT USED) G61 MOVE TO HOME SWITCH

G80 CANCEL DRILLING CYCLES G81 DRILL CYCLE

G82 DRILL CYCLE W/DWELL G83 PECKING DRILL CYCLE

G85 BORING CYCLE G86 DRILL CYCLE

G90 ABSOLUTE MOVE G91 INCREMENTAL MOVE

G92 PRESET ABSOLUTE POSITION

Dxx TOOL RADIUS OFFSET # Fxx SPECIFY FEED RATE

Hxx TOOL HEIGHT OFFSET # Rx.x DRILL AT RAPID RATE

Xx.x MOVE X TO POSITION Yx.x MOVE Y TO POSITION

Zx.x MOVE Z TO POSITION Ax.x MOVE A TO POSITION

Ixx..x DISTANCE X TO ARC CENTER Jxx..y DISTANCE Y TO ARC CENTER

M00 PROGRAM STOP M01 OPTIONAL STOP (SPACE BAR)

M02 DRY RUN STOP M06 TOOL CHANGE

M30 END OF PROGRAM & RESET M99 REPEAT PROGRAM

M03 SPINDLE ON M05 SPINDLE OFF

M04 OPTIONAL CONTROL FUNCTION M07 OPTIONAL CONTROL FUNCTION

M08 COOLANT ON M09 COOLANT OFF