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TB304 Acorn/AcornSix CNC Rigid Tapping Setup for Mill and Lathe rev8
Overview
Rigid Tapping setup and test procedur e to control accuracy of depth of cut and quality of threads in various working materials
There is no set in stone specific values for over travel or under travel associated with rigid tapping because each machine has
different inertia properties for the spindle/motor system and different VFD settings for braking and acceleration Therefore
setting up rigid tapping must be considered an "art" and requires some experimentation to tweak in the both the VFD and
CNC12 Rigid tapping parameters for good results Since the Rigid Tap feature works only as good as the integrator “tunes” it
Centroid makes no warranty or guarantee regarding rigid tapping that being said once it is tuned properly it works quite well
Requirements
Rigid Tapping requires feedback from a spindle encoder this encoder must be connected with the spindle at a 1 1 ratio This
means for each revolution of the spindle the encoder must also rotate exactly one revolution in sync with the spindle The
spindle encoder is also used to display the actual spindle speed rather than the commanded spindle speed
Step 1 Use the Centroid CNC Control Configuration Wizard to configure the CNC12 software for use with a spindle
encoder The Wizard turns on the spindle encoder and sets the spindle encoder counts per revolution and direction of
rotation Whether you are setting up for a mill or lathe start the Centroid Setup Wizard and go the Rigid Tapping menu under
the “Spindle Setup” Toggle “Spindle Encoder” to “Yes” Then scroll down to Spindle Encoder Counts and enter the encoder
counts of the encoder connected common values are 8000 a 2000 line encoder 8192 a 2048 line encoder
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Here is a video showing how to setup a spindle encoder with the Wizard https //youtu be/Gk1T_9 7_4g
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Step 2 Determine if the encoder signals match the direction of the spindle Start CNC12 and go to PID menu F1
Setup F3 Config type in the password 137 press enter F4 PID Now the Rotate the spindle clockwise and observe the 5 th
axis ‘Abs Pos’ column are the encoder counts increasing in a positive value or negative value?
https //youtu be/Gk1T_9 7_4g?si=Ci22fd_tvWsBQqJv&t=251
If value is increasing in a positive value then do nothing and press ESC three time to return to the main menu
If the encoder count value is increasing in the negative direction then shut down CNC12 start the Wizard and edit the encoder
value with a negative sign in front of the count For instance if the count was 8000 change that to 8000 If the encoder
counts up when the spindle turns CW then the encoder count per revolution value entered in the wizard should be positive If
the encoder counts up when the spindle turns CCW then the value should be negative
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Step 3 Verify and Tune the VFD so that the commanded RPM matches the actual RPM
1 Enter in the desired maximum spindle speed in the Wizard and write settings to CNC config
2 Following the instructions in the inverter manual and set the inverter maximum frequency to 120hz
3 Using the Virtual Control Panel switch to MANUAL SPINDLE MODE and turn the SPINDLE Speed OVERRIDE all
the way down
4 Turn the spindle on CW and make sure it is turning the correct direction If it is turning the wrong direction correct
this by turning off power and swapping 2 legs of the 3 phase cable between the inverter and the motor
5 Repeat step 3 and 4 6 Monitor the actual spindle speed on the screen or use a tachometer and slowly increase
the SPINDLE OVERRIDE until the correct maximum spindle speed is reached If the maximum speed cannot be
reached turn the spindle off and restart at step 2 with a higher maximum frequency
7 Write down the actual frequency that the inverter is outputting from the control panel on the front inverter and
then turn off the spindle
8 Then following the instructions in the inverter manual set the inverter maximum frequency to the frequency in step
recorded in step 7
9 Using the VCP turn on AUTO SPINDLE MODE; and with the Spindle speed OVERRIDE at 100% in MDI turn on
the spindle at 640 RPM use this command “M3 S640” Now with the spindle running Adjust the maximum
frequency of the VFD so the actual spindle speed is within 1 rpm of 640 RPM
10 Spot check the spindle speed across the full speed range commanded vs actual reported by the spindle encoder
11 It is more critical for the spindle speed to be exact at lower speeds and it is normal for the actual spindle speed to
be correct at low speeds and up to 50 rpm slower at max rpm then the commanded speed
12 If there is a multi range spindle see the operating manual for setting up parameters 65 67
Step 4 Tune the VFD for best acell and decell performance Whenever possible use a braking resistor in conjunction
with the VFD for the fastest possible stops See the VFD manual for proper size and type of braking resistor and the
associated VFD parameter settings to activate the feature DC injection braking Tune VFD for the fastest possible stops
from Max RPM without a VFD fault Typical Milling Machine VFD Spindle Motor braking and accelerating values to shoot for
are 3 second acceleration and 3 second deceleration times for good rigid tapping results Faster decell is better
VFD Tuning and zero speed solution related Links
https //centroidcncforum com/viewtopic php?f=63&t=1650&start=10
and https //centroidcncforum com/viewtopic php?p=46126#p46126
Some VFD’s have analog input command gain and bias parameters that will need adjusted so you don’t get “drift” spindle
rotation at commanded 0 rpm These act as spindle motor movement thresholds for the low end and high end of the analog
input voltage Basically they tell the VFD to ignore any low voltage signal that might be present on the analog spindle speed
output when S0 is commanded zero spindle speed is commanded by the Acorn
As an example
On our in house Acorn Techno Lathe test mule a Teco MA7200 VFD installed on this Teco we have the Bias set to 3% and
the Gain set to 98% for proper spindle operation
The TECO MA7200 manual refers to the settings as
Bn 05 Analog Frequency Command Gain Voltage
Bn 06 Analog Frequency Command Bias Voltage
Bn 05 = 98% and Bn 06 = 3 0 %
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Step 6 T est Tap holes in Machinable Wax Using wax as a test medium does two things It saves the tap from breakage
but more importantly also quickly exposes bad threads due to incorrect settings better than metal So using wax is critical to
dialing in the rigid tapping parameters where if you tapped in metal the test results not be so obvious when fine tuning For
instance if the Z axis was pulling out of the hole a little too soon and running ahead of the spindle this puts stress on the
threads in metal this might not show up as a problem at first but in wax the wax threads will give way at a much lower force
than metal and it will be obvious that things are not perfect If rigid tap parameters are not set perfectly you will not get good
threads in wax and if you are getting good threads in wax you will get good threads in metal PVC and some other plastics can
be used as alternatives to machinable wax but wax is preferred for fine tuning Rigid Tapping
Use Centroid’s Intercon Conversational programming to create the test tapping g code program
This eliminates all the problems that come along with trying to use a CAD/CAM system the first time
Use Intercon to generate the tapping program for all the test taps and tuning of the parameters BEFORE using any CAD/CAM
generated G code
Intercon Rigid Tapping Canned Cycle
Notice the programmed tapping RPM is 640 RPM and the spindle is in hi range
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Rigid Tapping 1/4 20 G code Program that this Intercon program produced
; ICN_PATH = C \intercon\rigid tap icn
; Header
N0001
; CNC code generated by Intercon
; Description 1/4 20 rigid tap; Programmer km
; Date 06 Jan 2025
M25 G49 ; Goto Z home cancel tool length offset
G17 G40 ; Setup for XY plane no cutter comp
G20 ; inch measurements
G80 ; Cancel canned cycles
G90 ; absolute positioning
G98 ; canned cycle initial point return
; Tool #9
;Tool Diameter = 0 2500 Spindle Speed = 640
;1/4 20 Tap
G49 H0 M25
G0 X0 0 Y0 0
N0002 T9 M6
S640 M3
G4 P3 00 ; pause for dwell
G43 D9
; Tap
N0003 X0 0 Y0 0 Z0 1 H9
G84 X0 0 Y0 0 Z 0 075 G99 Q0 05 R0 1 P0 10
X0 0 Y0 0 Z 0 15
X0 0 Y0 0 Z 0 225
X0 0 Y0 0 Z 0 3
X0 0 Y0 0 Z 0 375
X0 0 Y0 0 Z 0 45
X0 0 Y0 0 Z 0 525
X0 0 Y0 0 Z 0 6
X0 0 Y0 0 Z 0 675
X0 0 Y0 0 Z 0 75
X0 0 Y0 0 Z 0 825
X0 0 Y0 0 Z 0 9
X0 0 Y0 0 Z 0 975
X0 0 Y0 0 Z 1 0 G98
G80
; End of Program
N0004 G49 H0 M25
G40 ; Cutter Comp Off
M5 ; Spindle Off
M9 ; Coolant Off
G80 ; Cancel canned cycles
M30 ; End of program
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Wizard Base Line Rigid Tapping Parameters for Acorn CNC12 software
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CNC12 F1 Setup F3 Config F3 Parameters NOTE These Parameters are set by the Wizard!
Parameter Value Function
36 1 Bit 0 0 Disable rigid tapping 1 Enable rigid tapping
Bit 1 0 Wait for index pulse during rigid tapping 2 Do not wait for index pulse
Bit 2 0 Do not allow spindle override 4 Allow spindle override
Example A value of 3 will enable rigid tapping bit 0 = 1 and during execution will not wait for the index pulse to
start bit 1 = 2 and the spindle override will not change the spindle speed
bit 2 = 0
Note Please check the operator’s manual for a detailed description of this parameter
37 Typical
1 – 10
seconds Spindle Deceleration Time This value is used for setting the spindle deceleration rate This is the time it takes
for the spindle to decelerate from maximum spindle speed in the range that is going to be used for tapping to
reach zero speed Note Perform tests on the machine to determine how long it takes You could also check the
value of the Spindle VFD accel setting i e if the VFD is set to 3 seconds deceleration then you could use 3 as a
good starting place but verify reality and perform the acutal amount of time it takes If the machine has multiple
ranges tap in High range to reduce this time as much as possible lowest inertia range Install a braking resistor
on the VFD for maximum braking effort reduced time and measure the time in this spindle speed gear range
68 640
RPM Minimum Rigid Tapping Spindle Speed RPM This parameter is the speed that the spindle slows down to
from the programmed spindle speed towards the end of the tapping cycle The lower the value the more
accurately the Z axis will land on target but at the expense of possibly stalling the motor which in turn will cause Z
to stop short If this value is too large the off target error increases
69 1 75 sec Duration for Minimum Spindle Speed Mode Seconds This is a buffer time value to allow the spindle time to
decelerate to the spindle speed defined by parameter 68 If the number is too small overshoot may occur If it’s
too large the user waits longer for the hole to be tapped at the slow speed specified by parameter 68
74 4 Spindle M function to be run at the bottom of the hole for G84 tapping
Spindle M function to be run at the top of the hole to for G74 counter tapping
78 1 Bit 0 0 Displayed speed is commanded speed 1 Displayed speed is actual speed from spindle encoder
Bit 1 0 Does not prorate feedrate 2 Prorates programmed feedrate proportional to spindle speed if speed is lower
than threshold percent set by parameter 149
Bit 2 0 Does not wait for spindle to get up to speed 4 Waits for the spindle speed to get up to speed threshold set
by parameter 149 before continuing on with the next line in the program
82 500 Spindle Drift in degrees of rotation i e 360 = 1 full rotation 90 = 1/4 rotation This value is the number of
degrees that the spindle moves to coast to a stop when it is shut off at the speed specified by parameter 68 This
value is proportional to the distance above the Z target at which the spindle motor must be shut off in order for Z to
land on target Remember that Z is slaved to the spindle speed during rigid tapping Increasing this value will
cause Z to stop sooner when the spindle stops Decrease this value will cause Z to travel further when the spindle
stops
84 3 Spindle M function to be run at the bottom of the hole for G74 tapping left hand taps
Spindle M function to be run at the top of the hole to for G84 counter tapping right hand taps
240 1”
inches Rigid Tapping Accel Distance for Tapping and Threading This is sacrificial lead in distance that is used to sync
up Z axis with Spindle Encoder Typical open loop stepper motor based systems use a value around 1 “ close
loop typical distance is 05” Smaller number = shorter sync up distance there for more abrupt z axis movement
Larger number = more gentle sync up Too large and sync up distance will start to interfere with threads Each
machine is a little different some experimentation is needed to determine best value
241 20 Parameter 241 is used to allow the Z axis to accelerate up to spindle encoder synchronization in a smooth way
P241 is the amount in which the Z axis is allowed to use to get in sync with the spindle encoder for Tapping and
Threading The default value is 10 and the minimum value is 10
P241 is in degrees of rotation that the Z axis accelerates to get Z axis sinkedup with the spindle at the beginning
of a Tapping cycle This helps eliminate banging cause by instantaneous accell to get synced at the start and
end of the tap and thread Basically you are specifying how much rotational sacrificial movement is allowed to
gently accelerate the Z axis up to the spindle encoder synchronization A smaller value makes Z axis accelerate
over a shorter distance therefore is a faster sync up a larger value is for more gentle sync up less abrupt syn
chronization For examples with high RPM and a course pitch threads P241 may have to be increase to elimi
nate the banging OR for Open loop stepper motor based systems P241 is typically set to 20 for a more gentle
sync up move so as to not lose steps Parameters 240 and 241 are
149 1 Waits for spindle to get up to speed before tapping 1 = 100% of programmed speed
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Graphic representation of Rigid Tapping control parameters
Graphic Representation of Test Results for Precision
The above chart shows test results of rigid tapping The tool used in the testing was a 1/2 13 spiral fluted tap with TiN
coating Coolant used was water base soluble oil Hole size was 0 4218” Tapping depth was 0 800” Also note that
the parameters were adjusted to cut air and not changed for aluminum or cold rolled steel for these tests It can be
seen as the material changes so does the off target values This is due in part to the amount of torque required from
the spindle to cut the various types of material
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Summary
Rigid tapping parameters will vary from machine to machine Not all machines are built the same i e Spindle hp
inverter type rigidity etc and tooling will play a role in performance as well It was found through our testing that if
we changed one physical parameter i e using a tapping oil instead of water based coolant it improved the off
target values by 1 5% This is due to the fact that less friction is present when using special cutting oil therefore
requiring less hp by the spindle to drive the tap In most cases rigid tapping depths should be able to be held within
+/ 0 008” or less by adjusting parameter 82 for specific cases
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From the main screen press F1 Setup > F3 Config > enter "PASSWORD" > F4 PID to see the PID menu
below
Figure 1 Spindle Encoder counts “Absolute position” and marker pulse indication screen N *
Figure 2 Spindle Encoder Pin Out
“ Acorn Spindle Encoder Basics” Related Information Link
https //centroidcncforum com/viewtopic php?f=63&t=921
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Use the trouble shooting guidelines below to adjust the Rigid Tapping Parameters as necessary for good results in wax if you can tap
wax you can tap metal The wax will fail if there is any overshoot/travel while tapping even a small amount will result in damaged waz
threads so wax is the perfect medium to fine tune the tapping parameters with
Making Threads Notes
A Typical Tapping RPM Use 640 RPM for most all taps
B If the machine spindle has multiple gear ranges Tapping is typically performed in High range! Which is low inertia and therefore
results in the fastest turn arounds Using very low gear ranges drastically slows down the ability of the VFD to decelerate stop and
accelerate
C Once Tapped hole sizes approach a certain size alternative methods of creating threads are often used and are recommended
Threadmilling using a milling machine or router can easily create internal and external threads as small as 080! Harvey Tools makes
a 080 threadmill Threadmilling is also simple to program and is an included feature of Centroid’s Mill Intercon Conversational
programming software And threadmilling is also preferred method for creating threads in hard materials And for a Lathe once internal
thread size reaches aroun d 1/2” it time to start using the threading cycle and turn the threads So w hen possible use a internal
threading tool and use the threading canned cycle in Centroid’s Lathe Intercon Conversational programming software
Rigid Tapping Trouble Shooting
Symptoms Possible Problem Solution/Troubleshoot
Rigid tapping "Option not
available line xx" appears Acorn CNC Pro License not
installed From the main screen press F7 Utility >F8 Options > F2
Import License
When running a G74 or G84 rigid
tap cycle the control stops above
the hole to be tapped with a
message in the status window
"moving" and DOES NOT continue 1 No spindle encoder feedback
2 No spindle encoder index pulse 1 Verify encoder is setup correctly in Wizard
2 Turn the spindle by hand Check for encoder movement in
PID menu If there isn’t any movement check the spindle
encoder the cable and the connections Figure 2 shows the
DB9 pin out for the encoder
3 Verify the system sees the index pulse from the spindle
encoder Turn the spindle by hand and look for the asterisk
* next to the axis label that is designated for the spindle
4 If the asterisk is not seen from step 3 then check the
spindle encoder the cable and the connections
When running a G74 or G84 rigid
tap cycle you get a "410 Z axis 3
position error" 1 Spindle encoder turning
opposite direction of spindle
motor
2 Spindle speed too high for
tapping
3 Wrong number of spindle
encoder counts 1 Change the sign of the Encoder counts value in the
Wizard as described above
2 Try tapping at a reduced spindle speed
3 Verify encoder counts for the spindle encoder is set
correctly When the spindle is turned 1 revolution the
number of counts should change by the value that’s in
parameter 34
Tapping does not make it to the
bottom of the hole Not deep
enough 1 Not programmed deep enough
2 Spindle is turning off too soon 1 Check Intercon or G code program for depth
2 Parameter 68 69 and/or 82 may need adjusted as
described above
Breaking taps in blind holes or
tapping too deep 1 Programmed too deep
2 Spindle is turning off too late
3 Spindle speed is too high 1 Check Intercon or G code program for depth
2 Parameter 68 69 and/or 82 may need adjusted as
described above
3 Slow down spindle speed
Ripping up threads and or Breaking
taps in blind holes or tapping too
deep 1 VFD not setup properly for
good acell decell rates
2 Machine has too much inertia
and overpowers VFD and VFD
and spindle motor are under
sized Install braking resistor and turn on braking features in VFD
setup parameters DC injection braking see VFD manual
for details
Use a Floating Tap Holder so the tap has some compliance
to compensate for overshoot
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Document History
draft rev5 1 2 18
rev6 9 20 2020
rev7 6 14 21
rev8 1 28 25
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