Basic user guide

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• Using µCNC
  • µCNC commands
  • µCNC configurations
  • µCNC interface
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  • µCNC laser mode

Using µCNC

Since µCNC was designed over Grbl protocol most of the details regarding interfacing, connecting, commands, jogging and other can be refereed to the Grbl Wiki page. About ~95% of the commands and interfacing is the same except a few minor things that will be covered in this page.

µCNC wiring

By default, and as a safety measure, µCNC control pins (Emergency stop, Safety door and feed hold) and all limit switch pins are all set high by internal MCU weak pull up resistors (if the MCU has this feature). This prevents the user from accidentally forgetting to plug/wire an input and run the machine freely, crashing it or causing an accident. That also means that if left unconnected, Emergency stop will always be active upon plugin µCNC, and prevent any kind of motion or command appearing to be locked. There are 3 ways of overcoming this.

1. This only works up to version 1.4.0-rc. Short the ESTOP pin to ground. After this µCNC should reset and emergency alarm should be normalize. You can mask/invert command pins via setting $7. After masking/inverting control pins Emergency stop will be active again. At this point remove the short to ground to unlock/reset µCNC.

2. This only works from version 1.4.0 and future versions. Go to cnc_config.h file and uncomment the INVERT_EMERGENCY_STOP option to invert the Emergency stop input logic. Recompile and upload the firmware.

3. You can disable all control pins by setting option DISABLE_ALL_CONTROLS in the cnc_config.h file. By doing this also all control pins will be undefined in the HAL.

4. It's also possible to disable the weak pull up resistor for your specific board. This will leave the board more susceptible to EMI noise and false triggers. To disable weak pull up resistor read the info about board/pinout customization in the wiki.

µCNC commands

For the full list of supported commands by µCNC read Grbl's configurations wiki page. The following commands don't match the existing ones in Grbl.

µCNC system commands

• $I - View build info is implemented on µCNC but has to be enabled via ENABLE_SYSTEM_INFO option in the in cnc_config.h.

This will print out 2 messages similar to Grbl's $I command. The first will print the version µCNC version and board name. Here is an example:

[VER:uCNC v1.4.0 - Arduino UNO]

After that it will print a message with some info on enabled options. The format will be:

[OPT:<Option codes separated by comma>,<planner buffer size>, <interface letter><rx buffer size>]

• The interface letter will be blank if the hardware serial is used or U if USB is used. Future letters might be added.

Possible option codes are:

Option code	Description
Cn	Cartesian kinematic followed by the number of axis
XYn	CoreXY kinematic followed by the number of axis
Dn	Linear delta kinematic followed by the number of axis
Tn	n being the number of tools
N	Enabled N word parsing (line numbers)
16B	16-Bit bresenham enabled
S	S-curve acceleration enabled
NI	Alternative interpolator step generator
IE	Emergency stop inverted
DC	Disabled all control input pins
DL	Disabled all limit switch input pins
DP	Disabled probe input pin
XC	Enabled extra system commands
F	Enabled fast math
LP	Enabled linear actuator planner
SK	Enabled axis skew correction
SKXY	Enabled axis skew correction only for xy plane
DSSn_nnn	Dynamic step spread followed by the maximum level and cutoff frequency

• $SLP - Sleep command is not implemented on µCNC.
• $SS $SL and $SR - Check µCNC configurations for more informations. These commands can be enabled via ENABLE_SETTING_EXTRA_CMDS in cnc_config.h.
• $P - View the configured pins states. This prints a formatted message [<pin type>:<internal pin number>:<pin value>]with each pin state/vale. This command can be enabled via ENABLE_SETTING_EXTRA_CMDS in cnc_config.h.

Pin type	Description
SO	Special Output Pin (Step, Dir and Step Enable pins)
P	PWM Output Pin (PWM pins)
O	Generic Output Pin (DOUT output pins)
SV	Servo Pin (SERVO output pins)
SI	Special Input Pin (Limit, Control, and Probe pins)
A	Analog Input Pin (Analog input pins)
I	Generic Input Pin (DIN input pins)
STEPS	Each motor position in steps (separated by commas)

This command can be disabled via ENABLE_SETTING_EXTRA_CMDS in cnc_config.h.

µCNC gcode commands

• G10 L20 Pn - Is not supported by µCNC. Grbl states that G10 L20 Pn is part of the NIST RS274NGC standard. But reading the document I didn't find this definition, so L20 is not accepted as part of G10 syntax. Only G10 L2 Pn commands are accepted. Also to store homing coordinates in EEPROM G10 L2 P28 and EEPROM G10 L2 P30 are supported. This is more inline with the NIST RS274NGC standard.
• G28.1 - Is not supported by µCNC. The same effect is achieved by doing a G10 L2 P28 command as stated above. To match Grbl's interface this may be added later on.
• G30.1 - Is not supported by µCNC. The same effect is achieved by doing a G10 L2 P30 command as stated above. To match Grbl's interface this may be added later on.
• G91.1 (relative IJK in arc commands) - Is not supported by µCNC. This is not part of the NIST RS274NGC standard.
• G92.1 - Is supported by µCNC. This cancels offset coordinate systems and resets the offset parameters as defined by RS274.
• G92.2 - Is supported by µCNC. This cancels offset coordinate systems but do not reset parameters as defined by RS274.
• G92.3 - Is supported by µCNC. This apply previously defined parameters to offset coordinate systems as defined by RS274.
• G93.4 - Is supported by µCNC. This apply previously defined parameters to offset coordinate systems as defined by RS274.
• M6 - Is supported by µCNC. This allows to make a tool change as defined by RS274.
• M10 - Is supported by µCNC. This command is available if SERVO type pins are configure. Controls the servo sinal control type pins. Is not defined by RS274. Usage: M10 P[x] S[yyy] where x is the servo number and yyy pwm vale from 0 to 255.
• M42 - Is supported by µCNC. This command can change the state of any generic output, pwm or servo Pin. It's not defined by RS274. Usage: M42 P[xxx] S[yyy] where x is the pin internal µCNC number and yyy pwm vale from 0 to 255.

Note: G92.x offsets are stored in non volatile memory in µCNC version 1.0.x. From version 1.1.x forward this will only be kept in RAM. Values will be mantained during a soft reset, but a hard reset will delete previous values.

µCNC configurations

For the full list of settings by µCNC read Grbl configuration wiki page. This is the configuration list for:

Settings and sample values	Description
$0=10	Step pulse, microseconds
$1=25	Step idle delay, milliseconds
$2=0	Step port invert, mask
$3=0	Direction port invert, mask
$4=0	Step enable invert, boolean
$5=0	Limit pins invert, mask
$6=0	Probe pin invert, boolean
$7=0	Control pins invert, mask
$10=1	Status report, mask
$11=0.010	Junction deviation, mm
$12=0.002	Arc tolerance, mm
$13=0	Report inches, boolean
$20=0	Soft limits, boolean
$21=0	Hard limits, boolean
$22=1	Homing cycle, boolean
$23=0	Homing dir invert, mask
$24=25.000	Homing feed, mm/min
$25=500.000	Homing seek, mm/min
$26=250	Homing debounce, milliseconds
$27=1.000	Homing pull-off, mm
$30=1000.	Max spindle speed, RPM
$31=0.	Min spindle speed, RPM
$32=0	Laser mode, boolean
$37 to $39	Axis skew compensation
$40 to $70	PID controllers parameters
$80=1	Default tool at startup
$81 to $96	Tool length offset
$100 to $105	steps/mm
$106	Delta kinematic arm length
$107	Delta kinematic radial difference between the arm's tower joints and the effector joints
$110 to $115	Max rate, mm/min
$120 to $125	Acceleration, mm/sec^2
$130 to $135	Max travel, mm
$140 to $147	Backlash compensation of stepper motor (in steps)

$0 – Step pulse, microseconds

Stepper drivers are rated for a certain minimum step pulse length. Check the data sheet or just try some numbers. You want the shortest pulses the stepper drivers can reliably recognize. If the pulses are too long, you might run into trouble when running the system at very high feed and pulse rates, because the step pulses can begin to overlap each other. We recommend something around 10 microseconds, which is the default value.

$1 - Step idle delay, milliseconds

This feature is not implemented on µCNC

$2 – Step port invert, mask

This setting inverts the step pulse signal. By default, a step signal starts at normal-low and goes high upon a step pulse event. After a step pulse time set by $0, the pin resets to low, until the next step pulse event. When inverted, the step pulse behavior switches from normal-high, to low during the pulse, and back to high. Most users will not need to use this setting, but this can be useful for certain CNC-stepper drivers that have peculiar requirements. For example, an artificial delay between the direction pin and step pulse can be created by inverting the step pin.

This invert mask setting is a value which stores the axes to invert as bit flags. You really don't need to completely understand how it works. You simply need to enter the settings value for the axes you want to invert. For example, if you want to invert the X and Z axes, you'd send $2=5 to Grbl and the setting should now read $2=5 (step port invert mask:00000101).

Bit	Step port
0	STEP0
1	STEP1
2	STEP2
3	STEP3
4	STEP4
5	STEP5
6	STEP6
7	STEP7

$3 – Direction port invert, mask

This setting inverts the direction signal for each axis. By default, Grbl assumes that the axes move in a positive direction when the direction pin signal is low, and a negative direction when the pin is high. Often, axes don't move this way with some machines. This setting will invert the direction pin signal for those axes that move the opposite way.

This invert mask setting works exactly like the step port invert mask and stores which axes to invert as bit flags. To configure this setting, you simply need to send the value for the axes you want to invert. Use the table above. For example, if want to invert the Y axis direction only, you'd send $3=2 to Grbl and the setting should now read $3=2 (dir port invert mask:00000010)

Bit	Step port
0	DIR0
1	DIR1
2	DIR2
3	DIR3
4	DIR4
5	DIR5
6	DIR6
7	DIR7

$4 - Step enable invert, boolean

By default, the stepper enable pin is high to disable and low to enable. If your setup needs the opposite, just invert the stepper enable pin by typing $4=1. Disable with $4=0. (May need a power cycle to load the change.)

$5 - Limit pins invert, mask

By default, the limit pins are held normally-high with the MCU's internal pull-up resistor (has configured in the respective boardmap and if supported by the hardware). When a limit pin is high, µCNC interprets this as triggered. This differs from Grbl's default (this way if you forget to plugin the end-stop(NC to ground) for example the limit will be active). For the opposite behavior, just invert the limit pin by masking it with the appropriate integer (0-255) value. The following table describes the bit in the value:

Bit	Limit
0	LIMIT_X and LIMIT_X2
1	LIMIT_Y and LIMIT_Y2
2	LIMIT_Z and LIMIT_Z2
3	LIMIT_A
4	LIMIT_B
5	LIMIT_C
6	unsused
7	unsused

NOTE: For more advanced usage, the internal pull-up resistor on the limit pins may be disabled in config.h.

$6 - Probe pin invert, boolean

By default, the probe pin is held normally-high with the Arduino's internal pull-up resistor. When the probe pin is low, Grbl interprets this as triggered. For the opposite behavior, just invert the probe pin by typing $6=1. Disable with $6=0. You may need a power cycle to load the change.

$7 - Control pins invert, mask

By default, the control pins are held normally-high with the MCU's internal pull-up (has configured in the respective boardmap and if supported by the hardware). When a control pin is high, µCNC interprets this as triggered. This differs from Grbl's default (this way if you forget to plugin the control button/switch(NC to ground) for example the control will be active). For the opposite behavior, just invert the control pin by masking it with the appropriate integer (0-255) value. The following table describes the bit in the value:

Bit	Limit
0	ESTOP (Emergency stop)*
1	SAFETY_DOOR (Safety door)
2	FHOLD (Feed hold)
3	CS_RES (Cycle start/Resume)
4	unsused
5	unsused
6	unsused
7	unsused

NOTE: For more advanced usage, the internal pull-up resistor on the limit pins may be disabled in boardmap file that matches your board. ESTOP bit will only work up to version 1.4.0-rc. After that this setting is modified via option INVERT_EMERGENCY_STOP in the cnc_config.h file

$10 - Status report, mask

This setting determines what Grbl real-time data it reports back to the user when a '?' status report is sent. This data includes current run state, real-time position, real-time feed rate, pin states, current override values, buffer states, and the g-code line number currently executing (if enabled through compile-time options).

By default, the new report implementation in Grbl v1.1+ will include just about everything in the standard status report. A lot of the data is hidden and will appear only if it changes. This increases efficiency dramatically over of the old report style and allows you to get faster updates and still get more data about your machine. The interface documentation outlines how it works and most of it applies only to GUI developers or the curious.

To keep things simple and consistent, Grbl v1.1 has only two reporting options. These are primarily here just for users and developers to help set things up.

• Position type may be specified to show either machine position (MPos:) or work position (WPos:), but no longer both at the same time. Enabling work position is useful in certain scenarios when Grbl is being directly interacted with through a serial terminal, but machine position reporting should be used by default.
• Usage data of Grbl's planner and serial RX buffers may be enabled. This shows the number of blocks or bytes available in the respective buffers. This is generally used to helps determine how Grbl is performing when testing out a streaming interface. This should be disabled by default.

Use the table below enables and disable reporting options. Simply add the values listed of what you'd like to enable, then save it by sending Grbl your setting value. For example, the default report with machine position and no buffer data reports setting is $10=1. If work position and buffer data are desired, the setting will be $10=2.

Report Type	Value	Description
Position Type	0	Enable WPos: Disable MPos:.
Position Type	1	Enable MPos:. Disable WPos:.
Buffer Data	2	Enabled Buf: field appears with planner and serial RX available buffer.

$11 - Junction deviation, mm

Junction deviation is used by the acceleration manager to determine how fast it can move through line segment junctions of a G-code program path. For example, if the G-code path has a sharp 10 degree turn coming up and the machine is moving at full speed, this setting helps determine how much the machine needs to slow down to safely go through the corner without losing steps.

Instead this value stands for junction cosine factor (value must be set between -1.0 and 1.0). If set to 0.0 is the same as exact path mode (G61) and -1.0 is the same as exact stop mode (G61.1)

$12 – Arc tolerance, mm

Grbl renders G2/G3 circles, arcs, and helices by subdividing them into teeny tiny lines, such that the arc tracing accuracy is never below this value. You will probably never need to adjust this setting, since 0.002mm is well below the accuracy of most all CNC machines. But if you find that your circles are too crude or arc tracing is performing slowly, adjust this setting. Lower values give higher precision but may lead to performance issues by overloading Grbl with too many tiny lines. Alternately, higher values traces to a lower precision, but can speed up arc performance since Grbl has fewer lines to deal with.

For the curious, arc tolerance is defined as the maximum perpendicular distance from a line segment with its end points lying on the arc, aka a chord. With some basic geometry, we solve for the length of the line segments to trace the arc that satisfies this setting. Modeling arcs in this way is great, because the arc line segments automatically adjust and scale with length to ensure optimum arc tracing performance, while never losing accuracy.

$13 - Report inches, boolean

Grbl has a real-time positioning reporting feature to provide a user feedback on where the machine is exactly at that time, as well as, parameters for coordinate offsets and probing. By default, it is set to report in mm, but by sending a $13=1 command, you send this boolean flag to true and these reporting features will now report in inches. $13=0 to set back to mm.

$20 - Soft limits, boolean

Soft limits is a safety feature to help prevent your machine from traveling too far and beyond the limits of travel, crashing or breaking something expensive. It works by knowing the maximum travel limits for each axis and where Grbl is in machine coordinates. Whenever a new G-code motion is sent to Grbl, it checks whether or not you accidentally have exceeded your machine space. If you do, Grbl will issue an immediate feed hold wherever it is, shutdown the spindle and coolant, and then set the system alarm indicating the problem. Machine position will be retained afterwards, since it's not due to an immediate forced stop like hard limits.

NOTE: Soft limits requires homing to be enabled and accurate axis maximum travel settings, because Grbl needs to know where it is. $20=1 to enable, and $20=0 to disable.

$21 - Hard limits, boolean

Hard limit work basically the same as soft limits, but use physical switches instead. Basically you wire up some switches (mechanical, magnetic, or optical) near the end of travel of each axes, or where ever you feel that there might be trouble if your program moves too far to where it shouldn't. When the switch triggers, it will immediately halt all motion, shutdown the coolant and spindle (if connected), and go into alarm mode, which forces you to check your machine and reset everything.

To use hard limits with Grbl, the limit pins are held high with an internal pull-up resistor, so all you have to do is wire in a normally-open switch with the pin and ground and enable hard limits with $21=1. (Disable with $21=0.) We strongly advise taking electric interference prevention measures. If you want a limit for both ends of travel of one axes, just wire in two switches in parallel with the pin and ground, so if either one of them trips, it triggers the hard limit.

Keep in mind, that a hard limit event is considered to be critical event, where steppers immediately stop and will have likely have lost steps. Grbl doesn't have any feedback on position, so it can't guarantee it has any idea where it is. So, if a hard limit is triggered, Grbl will go into an infinite loop ALARM mode, giving you a chance to check your machine and forcing you to reset Grbl. Remember it's a purely a safety feature.

$22 - Homing cycle, boolean

Ahh, homing. For those just initiated into CNC, the homing cycle is used to accurately and precisely locate a known and consistent position on a machine every time you start up your Grbl between sessions. In other words, you know exactly where you are at any given time, every time. Say you start machining something or are about to start the next step in a job and the power goes out, you re-start Grbl and Grbl has no idea where it is due to steppers being open-loop control. You're left with the task of figuring out where you are. If you have homing, you always have the machine zero reference point to locate from, so all you have to do is run the homing cycle and resume where you left off.

To set up the homing cycle for Grbl, you need to have limit switches in a fixed position that won't get bumped or moved, or else your reference point gets messed up. Usually they are setup in the farthest point in +x, +y, +z of each axes. Wire your limit switches in with the limit pins, add a recommended RC-filter to help reduce electrical noise, and enable homing. If you're curious, you can use your limit switches for both hard limits AND homing. They play nice with each other.

Prior to trying the homing cycle for the first time, make sure you have setup everything correctly, otherwise homing may behave strangely. First, ensure your machine axes are moving in the correct directions per Cartesian coordinates (right-hand rule). If not, fix it with the $3 direction invert setting. Second, ensure your limit switch pins are not showing as 'triggered' in Grbl's status reports. If are, check your wiring and settings. Finally, ensure your $13x max travel settings are somewhat accurate (within 20%), because Grbl uses these values to determine how far it should search for the homing switches.

By default, Grbl's homing cycle moves the Z-axis positive first to clear the workspace and then moves both the X and Y-axes at the same time in the positive direction. To set up how your homing cycle behaves, there are more Grbl settings down the page describing what they do (and compile-time options as well.)

Also, one more thing to note, when homing is enabled. Grbl will lock out all G-code commands until you perform a homing cycle. Meaning no axes motions, unless the lock is disabled ($X) but more on that later. Most, if not all CNC controllers, do something similar, as it is mostly a safety feature to prevent users from making a positioning mistake, which is very easy to do and be saddened when a mistake ruins a part. If you find this annoying or find any weird bugs, please let us know and we'll try to work on it so everyone is happy. :)

NOTE: Check out config.h for more homing options for advanced users. You can disable the homing lockout at startup, configure which axes move first during a homing cycle and in what order, and more.

$23 - Homing dir invert, mask

By default, Grbl assumes your homing limit switches are in the positive direction, first moving the z-axis positive, then the x-y axes positive before trying to precisely locate machine zero by going back and forth slowly around the switch. If your machine has a limit switch in the negative direction, the homing direction mask can invert the axes' direction. It works just like the step port invert and direction port invert masks, where all you have to do is send the value in the table to indicate what axes you want to invert and search for in the opposite direction.

$24 - Homing feed, mm/min

The homing cycle first searches for the limit switches at a higher seek rate, and after it finds them, it moves at a slower feed rate to home into the precise location of machine zero. Homing feed rate is that slower feed rate. Set this to whatever rate value that provides repeatable and precise machine zero locating.

$25 - Homing seek, mm/min

Homing seek rate is the homing cycle search rate, or the rate at which it first tries to find the limit switches. Adjust to whatever rate gets to the limit switches in a short enough time without crashing into your limit switches if they come in too fast.

$26 - Homing debounce, milliseconds

Whenever a switch triggers, some of them can have electrical/mechanical noise that actually 'bounce' the signal high and low for a few milliseconds before settling in. To solve this, you need to debounce the signal, either by hardware with some kind of signal conditioner or by software with a short delay to let the signal finish bouncing. Grbl performs a short delay, only homing when locating machine zero. Set this delay value to whatever your switch needs to get repeatable homing. In most cases, 5-25 milliseconds is fine.

$27 - Homing pull-off, mm

To play nice with the hard limits feature, where homing can share the same limit switches, the homing cycle will move off all of the limit switches by this pull-off travel after it completes. In other words, it helps to prevent accidental triggering of the hard limit after a homing cycle. Make sure this value is large enough to clear the limit switch. If not, Grbl will throw an alarm error for failing to clear it.

$30 - Max spindle speed, RPM

This sets the spindle speed for the maximum 5V PWM pin output. For example, if you want to set 10000rpm at 5V, program $30=10000. For 255rpm at 5V, program $30=255. If a program tries to set a higher spindle RPM greater than the $30 max spindle speed, Grbl will just output the max 5V, since it can't go any faster. By default, Grbl linearly relates the max-min RPMs to 5V-0.02V PWM pin output in 255 equally spaced increments. When the PWM pin reads 0V, this indicates spindle disabled. Note that there are additional configuration options are available in config.h to tweak how this operates.

$31 - Min spindle speed, RPM

This sets the spindle speed for the minimum 0.02V PWM pin output (0V is disabled). Lower RPM values are accepted by Grbl but the PWM output will not go below 0.02V, except when RPM is zero. If zero, the spindle is disabled and PWM output is 0V.

$32 - Laser mode, boolean

When enabled, Grbl will move continuously through consecutive G1, G2, or G3 motion commands when programmed with a S spindle speed (laser power). The spindle PWM pin will be updated instantaneously through each motion without stopping. Please read the GRBL laser documentation and your laser device documentation prior to using this mode. Lasers are very dangerous. They can instantly damage your vision permanently and cause fires. Grbl does not assume any responsibility for any issues the firmware may cause, as defined by its GPL license.

When disabled, Grbl will operate as it always has, stopping motion with every S spindle speed command. This is the default operation of a milling machine to allow a pause to let the spindle change speeds.

$37 to $39 - Axis skew compensation

When axis skew compensation is enabled these value allow to compensate skew deformations for XY, XZ and YZ planes on cartesian machines

The factors are calculated like in Marlin

/**
 * Bed Skew Compensation
 *
 * This feature corrects for misalignment in the XYZ axes.
 *
 * Take the following steps to get the bed skew in the XY plane:
 *  1. Print or draw a test square (e.g., https://www.thingiverse.com/thing:2563185)
 *  2. For XY_DIAG_AC measure the diagonal A to C
 *  3. For XY_DIAG_BD measure the diagonal B to D
 *  4. For XY_SIDE_AD measure the edge A to D
 *
 * Skew factors need to be computed and set manually:
 *
 *  - Compute AB     : SQRT(2*AC*AC+2*BD*BD-4*AD*AD)/2
 *  - XY_SKEW_FACTOR : TAN(PI/2-ACOS((AC*AC-AB*AB-AD*AD)/(2*AB*AD)))
 *
 * If desired, follow the same procedure for XZ and YZ.
 * Use these diagrams for reference:
 *
 *    Y                     Z                     Z
 *    ^     B-------C       ^     B-------C       ^     B-------C
 *    |    /       /        |    /       /        |    /       /
 *    |   /       /         |   /       /         |   /       /
 *    |  A-------D          |  A-------D          |  A-------D
 *    +-------------->X     +-------------->X     +-------------->Y
 *     XY_SKEW_FACTOR        XZ_SKEW_FACTOR        YZ_SKEW_FACTOR
 */

$40 to $70 - PID controllers parameters

These parameter set the PID Kp, Ki and Kd values for each defined PID controller module. The settings index-4 aligned for each PID parameter group.

PID0 Kp, Ki and Kd are stored in settings $40, $41 and $42 respectively.
PID1 Kp, Ki and Kd are stored in settings $44, $45 and $46 respectively.
PID2 Kp, Ki and Kd are stored in settings $48, $49 and $50 respectively.
...
PID7 Kp, Ki and Kd are stored in settings $68, $69 and $70 respectively.

$80 to $96 - Tool settings

$80 sets the default tool loaded at startup. Parameters $81 to $96 set the length offset to each of the 16 possible tools that can be configured in µCNC. These values can be loaded via G43 Hn with n being the tool index.

$100 to $105 – [X,Y,Z,A,B,C] steps/mm

Grbl needs to know how far each step will take the tool in reality. To calculate steps/mm for an axis of your machine you need to know:

• The mm traveled per revolution of your stepper motor. This is dependent on your belt drive gears or lead screw pitch.
• The full steps per revolution of your steppers (typically 200)
• The microsteps per step of your controller (typically 1, 2, 4, 8, or 16). Tip: Using high microstep values (e.g., 16) can reduce your stepper motor torque, so use the lowest that gives you the desired axis resolution and comfortable running properties.

The steps/mm can then be calculated like this: steps_per_mm = (steps_per_revolution*microsteps)/mm_per_rev

Compute this value for every axis and write these settings to Grbl.

$106 - Delta kinematic arm length

When using the kinematics for linear Delta machines this value should be set to the length of the delta arms (in mm) (assuming all arms have the same length)

$107 - Delta kinematic radial difference between the arm's tower joints and the effector joints

When using the kinematics for linear Delta machines this value should be set to the radial difference between the arm's tower joints and the effector joints (in mm).

This is equivalent to measure the distance of the projected horizontal points of the joints of one when all towers are at the same height.

o <--tower joint
|\
| \
|  \
|   \
|    \   <--arm
|     \
|      \
|       \
|________\o <---effector joint

|---------|
   $107

$110 to $115 – [X,Y,Z,A,B,C] Max rate, mm/min

This sets the maximum rate each axis can move. Whenever Grbl plans a move, it checks whether or not the move causes any one of these individual axes to exceed their max rate. If so, it'll slow down the motion to ensure none of the axes exceed their max rate limits. This means that each axis has its own independent speed, which is extremely useful for limiting the typically slower Z-axis.

The simplest way to determine these values is to test each axis one at a time by slowly increasing max rate settings and moving it. For example, to test the X-axis, send Grbl something like G0 X50 with enough travel distance so that the axis accelerates to its max speed. You'll know you've hit the max rate threshold when your steppers stall. It'll make a bit of noise, but shouldn't hurt your motors. Enter a setting at 10-20% below this value, so you can account for wear, friction, and the mass of your workpiece/tool. Then, repeat for your other axes.

NOTE: This max rate setting also sets the G0 seek rates.

$120 to $125 – [X,Y,Z,A,B,C] Acceleration, mm/sec^2

This sets the axes acceleration parameters in mm/second/second. Simplistically, a lower value makes Grbl ease slower into motion, while a higher value yields tighter moves and reaches the desired feed rates much quicker. Much like the max rate setting, each axis has its own acceleration value and are independent of each other. This means that a multi-axis motion will only accelerate as quickly as the lowest contributing axis can.

Again, like the max rate setting, the simplest way to determine the values for this setting is to individually test each axis with slowly increasing values until the motor stalls. Then finalize your acceleration setting with a value 10-20% below this absolute max value. This should account for wear, friction, and mass inertia. We highly recommend that you dry test some G-code programs with your new settings before committing to them. Sometimes the loading on your machine is different when moving in all axes together.

$130 to $135 – [X,Y,Z,A,B,C] Max travel, mm

This sets the maximum travel from end to end for each axis in mm. This is only useful if you have soft limits (and homing) enabled, as this is only used by Grbl's soft limit feature to check if you have exceeded your machine limits with a motion command.

$140 to $147 - [X,Y,Z,A,B,C] Backlash compensation

Each parameter $140 to $147 can set the number of steps to compensate for backlash on the system. This is enabled via config file. $140 sets the backlash compensation value for STEPPER0, $141 sets the backlash compensation value for STEPPER1, etc.

Additional settings commands

A new set of commands allow a more granular control over the settings stored in EEPROM/Flash to prevent wearing (optional build config ENABLE_SETTING_EXTRA_CMDS in cnc_config.h). When enabled all Grbl $x=val are only changed in SRAM. 3 additional commands are added:

$SS - Settings save. This stores all values to EEPROM/Flash.

$SL - Settings load. This loads all values from EEPROM/Flash.

$SR - Settings reset. This loads all default values from ROM.

µCNC interface

µCNC's interface is very similar to Grbl but some features are not yet implemented or have extra codes. The full list of interface messages can be check at Grbl interface wiki page. The following messages, errors and reports don't match Grbl.

Streaming a G-Code Program to µCNC

µCNC only implements the recommended streaming protocol which is simple Send-Response. Since most applications that work with Grbl also use this protocol there was no point in implementing other less reliable protocols.

µCNC Response Messages

Besides the existing error and alarm codes used in Grbl the following codes are added to µCNC.

Errors

ID	Error Code Description
39	A comment is in an invalid format.
40	An invalid tool number was chosen.
41	Issued motion command (except G0) without setting the feed rate.
42	Failed to write settings to non volatile memory (programming error).
43	Failed to write settings to non volatile memory (write protected).
44	Program has ended (after an M2 or M30 command). A soft reset is needed before accepting new Gcode.
45	Probing was successful.
46	Canned cycle command is missing word P.
47	Canned cycle command is missing word Q.
48	Canned cycle command issued with active inverse feed rate mode.
49	Canned cycle command issued with active radius compensation mode.
50	Canned cycle command requires spindle to be turning clockwise.
255	A critical failure of some sort has happen. This can be any failure of a safety mechanism in the interlocking loop check.

Alarms

ID	Alarm Code Description
10	Homing position with dual drive axis (auto square) was not achieved.
11	Homing can not start due to a prior active limit alarm.

µCNC Push Messages

Feedback Messages

µCNC supports feedback messages inserted in comments like defined in the NIST RS274NGC. If this option is enabled, when presented with a comment µCNC will output the comment in a Non-Queried Feedback Message [MSG: ] [OPT:] Queried Feedback Messages are not implemented on µCNC.

Real-time Status Reports

• Input Pin State:

  • Pn:XYZABCPDHR indicates which input pins µCNC has detected as 'triggered'.

  • Each letter of XYZABCPDHR denotes a particular 'triggered' input pin.

  • X Y Z A B C XYZABC limit pins, respectively

  • P the probe pin.

  • D H R the door, hold, emergency-stop, respectively.

  Note: Start-Resume pin does not have a status flag as in Grbl.
  Note2: Grbl reset pin in µCNC is replaced by Emergency stop.

µCNC jogging

To check how jog works on µCNC check the Grbl jogging wiki page. Jogging behavior should match Grbl.

µCNC laser mode

Since µCNC v1.2.0 this is now fully capable of driving a laser engraver/cutter. For more info check the Grbl laser mode wiki page. It's possible to completely disable this feature in the cnc_config.h file