Shenhui Accuracy Issues

[Khalid Nazim]

I have a new accuracy issue and I don't know how to solve it.

If I cut a square, the dimensions on x-axis are different then the dimensions on the y axis. Therefore, I don't have a perfect square. I have tried adjusting it using the step-length correction in the vendor setup and its somewhat ok. I can probably live with the difference.

However, I noticed a major issue. If I move the laser head down on the y-axis and cut the same square, its about 3 mm larger on y axis! How do I adjust that?

Regards
KN

[Rich Harman]

Sounds like stretched belts, but a 3mm difference is huge especially on a smaller machine. Check that the pulleys are not loose on the torque tube and also check that the torque tube is not loose anywhere.

The step-length correction is a kludge in my opinion. Step length really only has to do with steps per revolution of the motor and the geometry of the pulleys. For my machine it is 6.0000. If any other number is required to make a correct size then the real problem is elsewhere.

[Khalid Nazim]

Rich,
Thanks for your response. Couple of questions:

How did you calculate the step length for your machine to be 6.000?
What/where is the torque tube on the Shenhui?

Regards
Khalid

[Rich Harman]

First you need to know how many step pulses make one revolution of the motor. You can check the stepper driver to see how the switches are set. Mine are set to 5000 steps per revolution.

On the motor is a pulley that has 20 teeth. A belt connects the motor to a 40 tooth pulley. So now it takes two revolutions of the motor to turn the larger pulley one revolution. That 40 tooth pulley is attached to a long tube (torque tube) that spans the machine from left to right. On each end of that tube are 20 tooth pulleys that drive belts, which connect to each end of the gantry (X axis).

Two turns of the motor (10,000 steps) equals one turn of the torque tube. One turn of the torque tube is one turn of the 20 tooth pulleys which moves the belt 60mm. The belt pitch is 3mm.

So 60 mm of movement divided by 10,000 steps equals .006mm or 6.000 um.

[Khalid Nazim]

Rich,

Thanks again for your detailed explanation. I will check the pulleys and the torque tubes to see if everything is properly secured. Also, I have to now calculate the step length of my machine :)

Regards
Khalid

[Daniel Wolanski]

Another way I did it was to get a certified 1000 mm scale and vector cut a line in wood (not all the way through) in the X and Y dimension. Then tweak you steps in the software until you get it to match 1000 mm on the screen and 1000 mm on the board. You want to do this with the largest possible scale. I also looked at the line comparison in a 10X lupe to make sure they lined up. Now, anything you make that is say 100 mm x 100 mm with have 10X the accuracy of any errors you may have been off by with the 1000 mm scale. So, if you were 1 mm off @ 1000 mm you would only be .1 mm off at 100 mm (theoretically). You are basically approaching the resolution of steppers/ belts combo at that point and have taken the software step issue out of the equation. If it continues to give parts that are not accurate, it is not the step setting.