[buggalugs]

Bit of a Newbie with UGS but I have been using Mach3 since 2012 on my old CNC Machine. My two new CNC Machines are a heavily modified 3018 Engraving machine and a Fox Alien Masuter 3S both of which only answer to GBRL Comms To which I have taken quite a shine to UGS.

The annoying problem I’m having seems to happen every time UGS comes across a hole or a drilling.


The error report in the console is as follows:
An error was detected while sending ‘G3X23.32Y51.94I1.92J3.33’:
(error:33) Motion command target is invalid.
Streaming has been paused.
An error was detected while sending ‘G3X15.62Y51.94I1.92J-3.33’:
(error:33) Motion command target is invalid
. Streaming has been paused.
**** The communicator has been paused ****
**** Pausing file transfer. ****


I forgot to mention I am using CamBam to generate my G-Code and I am aware UGS has issues with arcs and circles but it is so unpredictable - Some times it works, some time it doesn't

Another clue to the error is that if I use Candle it runs fine.
As I have said I really like the looks and the features of UGS, but this error business is starting to make me think otherwise.

[cncmakers001]

GRBL is very strict about arcs being precise and valid, whereas Mach3 and even some senders like
Candle are more lenient or even approximate arcs as tiny line segments behind the scenes.

[buggalugs]

I now think I have resolved my issues in UGS The problem was partly identified by "cncmakers001" above who gave me a hint what to look for.

cncmakers001 wrote and I quote verbatim:-

"GRBL is very strict about arcs being precise and valid, whereas Mach3 and even some senders like
Candle are more lenient or even approximate arcs as tiny line segments behind the scenes."

This sent me into hunt mode where I looked more closely at my G-Code generator, CamBam. In depth analysis identified that a radius or arc was being generated to a staggering 16 decimal places, absolutely crackers but true !

That was not a problem if the decimal places were at zero but UGS would spit it out like a nasty taste if it was something
like 4.45807984715249


to resolve the issue I simply rounding up most of the trailing digits to within five decimal places.

I'm back in love with UGS its slick looks and its lovely features as I was about to venture on ripping out the GRBL controllers and replacing them with USB Mach3 boards.

Happy as a pig in Poo !

[elephantcnc]

It sounds like you're running into a common GRBL arc handling issue in UGS (Universal Gcode Sender). Here's what's happening and how to fix it:

Root Cause:
GRBL (especially older versions) has strict requirements for arc commands (G2/G3).

The error (error:33) Motion command target is invalid typically means:

The arc endpoint doesn't lie exactly on the calculated circular path.

The arc radius is too small for GRBL's tolerance settings.

UGS's GRBL parser is more strict than Candle's.

Solutions:
1. Modify CamBam Post-Processor Settings
In CamBam, go to Tools > Post Processor and:

Enable "ArcCentreMode" (set to I,J,K relative to start point).

Check "ArcAsLines" (converts arcs to tiny line segments - foolproof but larger files).

Ensure "IJRelative" is enabled (GRBL prefers relative arc centers).

2. Tweak GRBL Settings
Send these commands in UGS (or via terminal):

??
$32=1 (Enable exact arc path mode)
$4=1 (Enable arc tolerance checking, 1=strict, 0=loose)
If the issue persists, try $4=0 to loosen arc checks (may reduce precision slightly).

3. Use UGS "Arc Welder" Feature
In UGS:

Go to "Visualizer" > "Arc Welder".

Enable "Convert arcs to lines" with a small tolerance (e.g., 0.01mm).

This dynamically replaces arcs with line segments before sending to GRBL.

4. Update Firmware
If your GRBL version is <1.1, upgrade to GRBL 1.1f (better arc handling).

For newer machines, try GRBL-HAL or FluidNC (more forgiving with arcs).

5. Quick Workaround
In CamBam, manually replace problematic arcs with:

"Edit > Convert > Arcs to Lines" (for drilling holes, use line segments).

Or use "G1" linear moves instead of G2/G3 where possible.

Why Candle Works but UGS Doesn't:
Candle may silently:

Pre-process arcs (like Arc Welder).

Use looser GRBL tolerances by default.

Ignore minor endpoint mismatches.

Final Tip:
If you prefer UGS, stick with "Arc Welder" or CamBam’s "ArcAsLines" for now. For critical jobs, test arcs in the UGS visualizer before running them on the machine.