Slow circles and arc cutting with rotary

When cutting arcs or circles using the rotary. Laerweb produces G-code that slows the speed to a crawl proportionally depending on the amount of A axis movement vs X axis. I’ll post the gcode at the end. It does not exhibit this behavior when cutting rectangles or straight lines. Basically it ends up over melting the ends of my holes I am trying to cut.

I am on a custom machine running GRBL-LPC

G21 ; Set units to mm

G90 ; Absolute positioning


; Operation: 0

; Type: Laser Cut

; Paths: 1

; Passes: 1

; Cut rate: 1500 mm/min


; Pass 0

; Pass 0 Path 0

G0 X17.00 A0.11



G1 X17.99 A0.45 F1483.44

G1 X18.96 A1.01 F1454.56

G1 X19.90 A1.77 F1414.56

G1 X20.81 A2.71 F1367.52

G1 X21.69 A3.82 F1314.12

G1 X22.53 A5.08 F1255.08

G1 X23.32 A6.47 F1189.80

G1 X24.07 A7.98 F1126.92

G1 X24.78 A9.60 F1063.01

G1 X25.46 A11.30 F1013.63

G1 X26.09 A13.08 F944.94

G1 X26.68 A14.92 F888.26

G1 X27.23 A16.83 F826.22

G1 X27.75 A18.79 F779.30

G1 X28.24 A20.80 F731.61

G1 X28.69 A22.85 F674.13

G1 X29.11 A24.93 F629.97

G1 X29.49 A27.04 F572.29

G1 X29.85 A29.19 F537.41

G1 X30.18 A31.35 F495.70

G1 X30.48 A33.54 F449.30

G1 X30.74 A35.75 F390.50

G1 X30.99 A37.98 F373.27

G1 X31.20 A40.22 F315.10

G1 X31.39 A42.47 F285.01

G1 X31.55 A44.73 F240.24

G1 X31.69 A47.00 F209.93

G1 X31.80 A49.28 F164.85

G1 X31.89 A51.57 F134.56

G1 X31.95 A53.86 F89.91

G1 X31.99 A56.15 F60.00

G1 X32.00 A58.44 F15.01

G1 X31.99 A60.73 F15.01

G1 X31.95 A63.03 F59.74

G1 X31.89 A65.32 F89.91

G1 X31.80 A67.60 F135.15

G1 X31.69 A69.88 F164.85

G1 X31.55 A72.15 F209.93

G1 X31.39 A74.41 F240.24

G1 X31.20 A76.67 F283.79

G1 X30.99 A78.91 F315.10

G1 X30.74 A81.13 F374.85

G1 X30.48 A83.34 F390.50

G1 X30.18 A85.53 F449.30

G1 X29.85 A87.70 F493.67

G1 X29.49 A89.84 F539.60

G1 X29.11 A91.95 F572.29

G1 X28.69 A94.04 F627.49

G1 X28.24 A96.09 F674.13

G1 X27.75 A98.09 F734.40

G1 X27.23 A100.05 F779.30

G1 X26.68 A101.96 F826.22

G1 X26.09 A103.81 F885.13

G1 X25.46 A105.59 F944.94

G1 X24.78 A107.29 F1013.63

G1 X24.07 A108.90 F1066.28

G1 X23.32 A110.41 F1126.92

G1 X22.53 A111.80 F1189.80

G1 X21.69 A113.06 F1255.08

G1 X20.81 A114.17 F1314.12

G1 X19.90 A115.12 F1365.05

G1 X18.96 A115.88 F1414.56

G1 X17.99 A116.43 F1456.10

G1 X17.00 A116.77 F1483.44

G1 X16.00 A116.88 F1498.28

G1 X15.00 A116.77 F1498.28

G1 X14.01 A116.43 F1483.44

G1 X13.04 A115.88 F1456.10

G1 X12.10 A115.12 F1414.56

G1 X11.19 A114.17 F1365.05

G1 X10.31 A113.06 F1314.12

G1 X9.47 A111.80 F1255.08

G1 X8.68 A110.41 F1189.80

G1 X7.93 A108.90 F1126.92

G1 X7.22 A107.29 F1066.28

G1 X6.54 A105.59 F1013.63

G1 X5.91 A103.81 F944.94

G1 X5.32 A101.96 F885.13

G1 X4.77 A100.05 F826.22

G1 X4.25 A98.09 F779.30

G1 X3.76 A96.09 F734.40

G1 X3.31 A94.04 F674.13

G1 X2.89 A91.95 F627.49

G1 X2.51 A89.84 F572.29

G1 X2.15 A87.70 F539.60

G1 X1.82 A85.53 F493.67

G1 X1.52 A83.34 F449.30

G1 X1.26 A81.13 F390.50

G1 X1.01 A78.91 F374.85

G1 X0.80 A76.67 F315.10

G1 X0.61 A74.41 F283.79

G1 X0.45 A72.15 F240.24

G1 X0.31 A69.88 F209.93

G1 X0.20 A67.60 F164.85

G1 X0.11 A65.32 F135.15

G1 X0.05 A63.03 F89.91

G1 X0.01 A60.73 F59.74

G1 X0.00 A58.44 F15.01

G1 X0.01 A56.15 F15.01

G1 X0.05 A53.86 F60.00

G1 X0.11 A51.57 F89.91

G1 X0.20 A49.28 F134.56

G1 X0.31 A47.00 F164.85

G1 X0.45 A44.73 F209.93

G1 X0.61 A42.47 F240.24

G1 X0.80 A40.22 F285.01

G1 X1.01 A37.98 F315.10

G1 X1.26 A35.75 F373.27

G1 X1.52 A33.54 F390.50

G1 X1.82 A31.35 F449.30

G1 X2.15 A29.19 F495.70

G1 X2.51 A27.04 F537.41

G1 X2.89 A24.93 F572.29

G1 X3.31 A22.85 F629.97

G1 X3.76 A20.80 F674.13

G1 X4.25 A18.79 F731.61

G1 X4.77 A16.83 F779.30

G1 X5.32 A14.92 F826.22

G1 X5.91 A13.08 F888.26

G1 X6.54 A11.30 F944.94

G1 X7.22 A9.60 F1013.63

G1 X7.93 A7.98 F1063.01

G1 X8.68 A6.47 F1126.92

G1 X9.47 A5.08 F1189.80

G1 X10.31 A3.82 F1255.08

G1 X11.19 A2.71 F1314.12

G1 X12.10 A1.77 F1367.52

G1 X13.04 A1.01 F1414.56

G1 X14.01 A0.45 F1454.56

G1 X15.00 A0.11 F1483.44

G1 X16.00 A0.00 F1498.28

G1 X17.00 A0.11 F1498.28


M5 ; Switch tool offEnd

@GrblGru is this rotary-axis-on-inner-surface application a design you have a model for in grblgru? I recall from breaking out your release notes that you added surface projection but don’t know whether it would apply to inner surfaces.

This example is showing the inner surface cut. It also happens when doing our standard outside surface rotary cuts. It should not matter since neither GRBL nor Laserweb is aware which side of the tube they lasering.

I was thinking of grblgru’s modeling functionality, where it lets you have a model of your device and stock and watch it work a job simulation. It’s an interesting concept. You wouldn’t need an inside stock feature to use it and you could certainly compare it if you are running windows (it is windows-specific) but also @GrblGru has been interested in adding device types to his set of models anyway.

(I’m a Linux person, so my understanding of GrblGru has remained basically theoretical. :slight_smile: )

I’m sorry I am missing the connection between GRBLGRU and the issue with the gcode output making super slow arcs.

I’m suggesting to @GrblGru that he might be interested in modeling this setup if he hasn’t already. This won’t fix the problem with laserweb, but then when I look at the laserweb repository I’m not seeing any commits this year.

I can move the question to the GrblGru category and unclutter your request for help…

I do not know the construction of the machine, unfortunately I cannot watch your video and I do not know your workpiece. :frowning:
But I agree with you that my program can’t help you with that. The problem are the calculated feeds for each move command.

I can only tell you how I calculate the GCode. Here is an example: A 30 mm circle is projected on a cylinder of 50 mm. As you can see in the little video there is only one line with feed. Maybe you can change your GCode accordingly.


1 Like

Right, that is how I would think Laserweb should process the circles or arcs. Interesting thing is that Lightburn also exhibits the same behavior… It maybe something I am missing on my end.

Then I’m sure it will make some sense. I can still imagine that it might have something to do with laser mode. Constant or dynamic ?

@Jonas_Concepcion — Google appears to have broken links being embedded in other sites; they work for a few hours and then expire, as far as I can tell.

Oh, It looks like there has been a candidate fix since the last time I looked, so I can try a site update and hope that makes the video playable again!

…I did a site update, and now it shows a static image and links out to play within google photos; it looks like google broke embedding, probably intentionally.

@GrblGru you should be able to see the video now. The laser is cutting from the inside.

@Jonas_Concepcion I tried to generate rotary g-code with LW and got the same crazy stuff. It doesn’t make sense that the feed is changed when moving over the surcumference of a tube. This would only be needed if you would change the working diameter (sideways on the rotary).

@cprezzi is there anyone still active on laserweb who works on gcode generation?

I don’t know if any of the core devs (beside me) still reads the issues, but I would suggest to open a issue anyways, so other users can find the problem and probably step in to correct it.
I will look into it when I get some spare time…


could this be part of the problem? mixing distance and angular units?

Thanks! I did post a similar question in the Lightburn forum since the post processor from that software spat out a similar result with the oval shape. Oz gave a similar explanation to what is discussed in the video but I got lost in the math. I’ll give that software a try tomorrow.

@Jonas_Concepcion did you ever figure this out? Does Lightburn do it any differently now that there is a Rotary Setup option to enable under the Tools menu?

No solution from laserweb or Lightburn directly. But basically I just replace all the Fxxxx commands in the gcode manually to the highest number that LB or LW generates. Notepad ++ can do this with a find and replace pretty simply. Works fine for me.

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Interesting. I just added a rotary to a small Ortur laser engraver( Laser Master ) using the Y axis and had to slow down the max Y axis motion in the machine setting(firmware) from 9000 mm/s to 900 mm/s. It moved too fast. After seeing your post I started testing a 45mm diameter circle and it all seemed ok. Most likely it’s an “A”/rotary axis thing and not the standard XYZ linear axis’s. My 3040 has an A axis so I’ll have to keep notes on this when I get to using that.

The Ortur uses a controller with only X and Y outputs so I have to swap the rotary to the Y axis. On my K40 with the MKS sBase v1.3 controller I will either be using the Y axis for the rotary too so shouldn’t see this effect there either.

The Ortur runs GRBL and I’m using Lightburn v0.9.19

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