Notes on creating a new lathe g-code, G74 Peck Drill/Face Groove using Python remapping

(Page started 2015/06/27 Kirk Wallace)

.ini file entry



...
[RS274NGC]
...
# REMAPPING
REMAP=g74 modalgroup=1 argspec=xzk py=g740
...
[PYTHON]
TOPLEVEL     = python/toplevel.py
PATH_PREPEND = python
...

toplevel.py

#!/usr/bin/python

import remap

remap.py

#!/usr/bin/python

from interpreter import INTERP_OK
import math

def g740(self, **words):
    # G74 is typically called like so:
    # G74 x ? z ? k ? (x = diameter, usually 0; z = hole endpoint; k = peck length) 

    # get position
    if (self.params['_lathe_diameter_mode']):  # if is_lathe_mode
        x_mode = 2
    else:
        x_mode = 1

    x_start = self.params[5420] * x_mode
    z_start = self.params[5422]

    if 'x' in words:
        x_end = words['x'] * x_mode
    else:
        x_end = x_start * x_mode

    if 'z' in words:
        z_end = words['z']
        if z_end > z_start:
            return "G74 error - Z cannot be larger than the starting Z position"
    else:
        z_end = z_start

    if 'k' in words:
        peck_length = words['k']
        if peck_length < 0:
            return "G74 error - K cannot be negative"
    else:
        peck_length = 0

    if (self.params['_metric']):  # if is_metric
        backoff_length = 0.50  # mm
        rounding_fudge = 0.0001
    else:
        backoff_length = 0.020  # inch
        rounding_fudge = 0.00001

    z_range = math.fabs(z_end - z_start) - rounding_fudge  # rounding_fudge prevents extra peck
    if peck_length > 0:
        num_pecks = int(z_range / peck_length)
    else:
        num_pecks = 0

    z_list = []
    for i in range(num_pecks + 1):
        z_list.append(z_start - (i * peck_length))
    z_list.append(z_end)

    #print "--kaw - z_list =", z_list

    if math.fabs(x_end - x_start) > rounding_fudge:  # We're groove'n
        for i in range(num_pecks + 1):
            self.execute("G0 Z %s" % z_list[i])
            self.execute("G1 Z %s" % z_list[i + 1])
            self.execute("G1 X %s" % x_end)
            self.execute("G1 Z %s" % (z_list[i] + backoff_length))
            self.execute("G0 X %s" % x_start)

    else:  # We're drilling
        for i in range(num_pecks + 1):
            self.execute("G1 Z %s" % z_list[i + 1])
            self.execute("G0 Z %s" % (z_list[i + 1] + backoff_length))

    self.execute("G0 Z %s" % z_start)

    return INTERP_OK

The G74 routine above relies on an initial move to a starting position. This position is captured and used for x_start and z_start. The X and Z words in the G74 command are used for x_end and z_end. If x_start and x_end are equal, this invokes a plunge with chip break routine, otherwise a grooving routine is invoked. Since Python can round off floating point numbers, a "close enough" algorithm is use to test x_start and x_end equality. A check for peck_length > 0 is used to prevent a divide by zero error in the num_pecks calculation. Apparently, the x position parameter 5420 does not follow the lathe diameter mode, so we need to check for it and adjust x values. This needs testing, so beware. The z_range is adjusted a tiny bit so that an extra peck doesn't get added to the end. The peck algorithm counts the number of full pecks in z-range then adds a final z position of z_end. If the peck length fits evenly in z_range, the last full peck will happen to be at z_end, which we don't need because it's added later.

Peter Smid's book "CNC Programming Handbook, 3rd Edition" Page 222, covering G74 was used as a reference: https://books.google.com/books?id=w7-jBgAAQBAJ&pg=PA222

Parameters are:

: X = Optional. Leaving X off or setting it equal to the start position invokes a face plunge with chip break routine, usually used for drilling or face plunge grooving. For grooving, this is the end X position. The X start position is were the tool was just before invoking G74. Grooving will need a tool that can cut on its side. Care is needed in limiting the area of the side cut by limiting the K peck depth value. The X value will need to be adjusted by the cutter width to get the desired groove width. In other words: groove width = (x_end - x_start) + cutter width.
: Z = Required. Z is the Z endpoint, or hole or face groove depth. The start position is were the tool was just before invoking G74.
: K = Required. The peck length. Pecks start from the Z start position. The last peck length is the distance between the z endpoint and the last full peck length.

(G74 Peck Drilling Example)
G7 (Dia. Mode)
G18 (XZ Plane)
G90 (Absolute Distance Mode)
G40 (Turn Cutter Compensation Off)
G21 (units in inches)
G54 (Work Offset)

G30 Z #5422 (Park Tool)
T 0707

S 500 (RPM)

M8 (Coolant ON)
M3 (Spindle ON, Forward)

G0  X  0.000
G0           Z 25.000
G74          Z  0.000 K 8 F 200

M9 (Coolant OFF)
M5 (Spindle OFF)
G30 Z #5422 (Park Tool)

M30 (End of Program)

(G74 Grooving Example)
G7 (Dia. Mode)
G18 (XZ Plane)
G90 (Absolute Distance Mode)
G40 (Turn Cutter Compensation Off)
G21 (units in inches)
G54 (Work Offset)

G30 Z #5422 (Park Tool)
T 0707

S 500 (RPM)

M8 (Coolant ON)
M3 (Spindle ON, Forward)

G0  X 10.000
G0           Z 20.000
G74 X 25.000 Z 10.000 K 3 F 200

M9 (Coolant OFF)
M5 (Spindle OFF)
G30 Z #5422 (Park Tool)

M30 (End of Program)

Peck Drill Example

Grooving Example

Another Approach - Using .ngc

.ini



...
[RS274NGC]
...
# REMAPPING
REMAP=g74 modalgroup=1 argspec=xzk ngc=g740
...

g740.ngc

o<g740>sub
(Peck Drill)

(capture start position)
#<x_start> = #5420   (Current X Location)
#<z_start> = #5422   (Current Z Location)

#<z_end> = #<z>
#<peck> = #<k>
#<z_range> = [ABS[#<z_end> - #<z_start>]]

#<num_pecks> = [#<z_range> / #<peck>]
o10 if [#<peck> GT 0]
    #<num_pecks> = [#<z_range> / #<peck>]
o10 else
    #<num_pecks> = 0
o10 endif

o20 if [#<_metric>]
    #<back_off> = 0.500  (mm)
o20 else
    #<back_off> = 0.0200  (inch)
o20 endif

#<pass> = 1
o30 repeat [#<num_pecks>]
    #<z_target> = [#<z_start> - [#<pass> * #<peck>]]
    G1 Z #<z_target>
    G0 Z [#<z_target> + #<back_off>]
    #<pass> = [#<pass> + 1]
o30 endrepeat

G1 Z #<z_end>
G0 Z #<z_start>

o<g740> endsub
M2

The above could use some input parameter checking. The loop does a needless back-off if the the peck length fits evenly in the Z range. Maybe a while z_target > z_end loop would be better.

Yet Another version

remap.py

#!/usr/bin/python

import sys
import linuxcnc
from interpreter import INTERP_OK
import math
import emccanon

def g740(self, **words):
    # G74 is typically called like so:
    # G74 x ? z ? k ? (x = diameter, usually 0; z = hole endpoint; k = peck length) 

    # get machine G30 position in current G20/21 units
    x_start = self.params[5420]
    z_start = self.params[5422]

    if 'z' in words:
        z_end = words['z']
    else:
        z_end = z_start

    if 'k' in words:
        peck_length = words['k']
    else:
        peck_length = 0

    z_range = z_end - z_start
    if peck_length > 0:
        num_pecks = int(math.fabs(z_range / peck_length))
    else:
        num_pecks = 0

    if (self.params['_metric']):  # if is_metric
        backoff_length = 0.50  # mm
    else:
        backoff_length = 0.020  # inch

    for i in range(num_pecks):
        z_target = z_start - ((i + 1) * peck_length)
        line = i * 2
        emccanon.STRAIGHT_FEED(line,x_start,0,z_target,0,0,0,0,0,0)
        emccanon.STRAIGHT_TRAVERSE(line + 1,x_start,0,(z_target + backoff_length),0,0,0,0,0,0)

    emccanon.STRAIGHT_FEED(line + 1,x_start,0,z_end,0,0,0,0,0,0)
    emccanon.STRAIGHT_TRAVERSE(line + 2,x_start,0,(z_start),0,0,0,0,0,0)

    return INTERP_OK

G74 - Peck Drilling/Face Grooving

Notes on creating a new lathe g-code, G74 Peck Drill/Face Groove using Python remapping

.ini file entry

toplevel.py

remap.py

Another Approach - Using .ngc

.ini

g740.ngc

Yet Another version

remap.py