Getting ovals rather than circles

[HowardSmith (Howard)]

Can someone help?

Last year I built a DIY laser diode cutter for balsa using Grbl 1.1 and Lightburn. It took a lot of time, lots of challenges, but I got there and it worked; but not well. My mechanical design was poor, it relied too much on MDF. As a result the gantry was not smooth and the Z axis (bed) kept going unlevel. Spurred on by minor success I embarked on a redesign. Now it uses aluminium extrusion, the gantry just glides and the table is and remains level.

I’ve just got to the exciting part of cutting some balsa, there is a problem and I need your help. Now the gantry is smooth, I changed the DRV8825 to quarter steps on the x axis, to make the machine quieter. It was no surprise that the x steps had to be changed. I got a 2 foot rule, homed the machine bottom left and then marked on the extrusion 610mm. I then put into the console G00 X610 and it fell short by about half. No surprise. Went into machine settings, ‘calibrate access’, put in 610mm for what I requested to move and then recorded the actual movement. It suggested a value which was applied. Went through this routine a couple more times until the actual movement was exactly 610mm. Result.

I then drew a circle in Lightburn of 10mm * 10mm. Set the table height, closed the lid and pressed Start. It buzzed for a few seconds and then stopped.

Rather than a circle, I got an oval. The height of the oval is 10mm and the length is 20mm. X is moving twice as far as intended. Put in G00 X610 and it moves 610mm from the homed position, perfect.

Checked the G Code for the circle job and Lightburn is asking for X and Y to move 10 mm from where it switches on the laser. Just as it should.

Done all the obvious things like rebooting and checking the setting were saved; they were.

I’ve done some reading and seen some folks have had the same issue having switched rotary on. I’ve not touched the rotary, was not even aware of its existence and have confirmed it is disabled.

So close yet so far. I’m sure it will be something simple. Can someone turn on the light for me?

Thanks in advance

Howard

[gilaraujo (Gil Araújo)]

I would check your $$ data

in console try
$RST=*
press enter
$$
press enter
Check your steps per mm $100 $101

Then TRIPLE check your rotary enable featuer… so many times it gets turned on by mistake and then cause troubles

Try also to engrave this pattern once you finish the above
Dot Test with Angle.lbrn (46.3 KB)
Post apicture with results please?

[HowardSmith (Howard)]

Hi Gil

Thanks for you prompt response, I was at the end of my tether having spent too long looking at it.

I took you advice and started again with a reset. It was bound to be something I had done and it boiled down to my misunderstanding as to how G Code works.

The step mode on the X Axis was changed so it was clear the steps per mm would also need to be changed. I marked 610mm on the x axis extrusion from the homing position as my target point. Here’s the mistake. I entered G00 X610, then put how far it moved in ‘Calibrate Axis’, did it a few times until I was bang on.

It’s something to do with absolute moves and offsetes. My bed is 914 on X and 97 on Y. Thought I was telling it to move 610mm to the right. However think I was telling it to move to position 610. I then tried using the move console in Lightburn and told it to move 610mm, it hit the x end stop. Then retuned calibrate axis using the move console and the steps per mm was dramatically different. Tuned it right in and just cut a perfect circle to the desired dimension.

Thanks for reaching out with help.

All the best

Howard

[gilaraujo (Gil Araújo)]

Right you might have been confusing absolute vs incremental
G90 vs G91

G90 and G91 G Codes

(good read if needed)

G90 G0 X610 Is an absolute move to X610 from X0 Y0
G91 G0 X610 is an “move 610 mm from wherei am now”

Overall glad you figured it out!

[HowardSmith (Howard)]

Thanks Gil,

That’s an interesting read.

Just one more problem that I’m investigating and I should be there.

Thanks again.