Looking for tips on getting cuts scaled correctly

[Baronjutter]

Hi guys, my makerspace recently put together a new laser using the X7 DSP controller. The laser is up and running and cutting but after putting in quite a few hours struggling with the software/config we're still hung up on some issues and I'd appreciate any sort of insight or advice.

SCALE
Last night I spent 2 hours trying to get the thing just to scale correctly. I'd cut a 300mm line and measure it, 301mm. Adjust the X and Y settings in "manufacturer settings" up a bit, now it's 299.5. Ok adjust it back down. 299.9. Oh, almost there, just give it a tiny bit more, 300!! Great! I found the correct setting to get the laser printing at 1:1. Saved all the settings, turned off the laser, came back to print a 20x20mm square and it came out 23x23mm. Wait that can't be right, I spent so long getting it to scale!! Go back into "manufacturer settings" "read" to get the values, and the values are not what I had last set them to. But I wrote them down so I should be good right? Wrong, putting in the same values did not make my 20x20mm box correctly scaled, ended up needing totally different values. Am I doing something wrong? Is there a trick to configuring the laser to print exactly 1:1 with no scale issues??

MOMENTUM
On our old laser the cutting head moved very quickly and could instantly change directions and speed. This laser moves like a freight train, every motion the laser makes involves it ramping up and down speed between each movement, this results not just in slow cutting but extra power being applied where the laser is traveling slowly. Is there any way to make it start/stop more on a dime?

POSITIONING SPEED
Similar to the momentum problem, the laser can move fairly quickly when cutting, but when re-positioning between cuts it moves verrrrry slow. So if it's doing a series of lines it will quickly cut the line then very slowly back-track and move to cut the next line. Also when you do a "run box" it moves excruciatingly slow as well.

Any help appreciated!!

[electronrancher]

SCALE:

You should be able to determine it mathematically - if it's not fitting that, something unusual is going on. For a 1.8deg stepper, the steps per revolution is 200. For a 0.9deg, it's 400. You probably have 1/8 or 1/16 microstepping turned on in your driver, resulting in something like 1600 or 3200 steps per revolution (1600 is from 200 steps per rev * 8 microstep, for example). Next you must look at your belt gear, it's probably MXL (2.03mm per tooth) but may be GT2 (2mm per tooth) on a homemade rig made of US components. The tooth count for this gear is most likely 20 tooth. Might as well count it, dot one tooth with a marker and turn slowly when counting teeth until you reach the dot again.

Then, take your mechanical params to determine your step length. An example of 1.8deg motor and 1/8 microstep is 3200 steps for 20 tooth * 2.03mm belt drive length, or 40.8mm/3200 steps, or 12.6875um per step. Calculate this number for your rig, put it in, and don't change it. If you find your cuts off by a quarter mm or something, you either calculated wrong or your machine has backlash that is causing an error in your position as the motor goes back and forth.

If you don't math, no problem - use the steps per mm calculator here: http://prusaprinters.org/calculator/

Momentum:
The slow ramping is most likely due to the acceleration setting in Options->System Options-> Cut_Accel or Engraving_accel. A bigger number means the carriage has less "overtravel", it scales with speed. 5mm overrunl at 200mm/s engraving will become 10mm overrun at 400mm/s engraving because the controller is changing the speed of the head at a constant rate.

Positioning Speed:
Options->System Options-> Key_Move_Speed, Run_Box_Speed, etc.

Hope it helps!

[Baronjutter]

Thanks! I think some of that went over my head but it's a good start. I didn't build the machine but I'll ask about the gears. Just to confirm though, eveything I need to do to get the controller moving the laser at 1:1 is in this screen?


I should just have to adjust the X and Y right?
Also after 2 hours of trial and error I came upon a number almost exactly double what you used in your example, does this mean I'm close?

[electronrancher]

Yes! To both of your questions!

If you have arrived at a number that is double my example, it's quite likely that it is the correct number and your driver is simply set to a different microstep. Congratulations!