Move, Pulse Unit, and Need Pulse

[Travis Reese]

I've been slowly figuring out the laser machine. I was having a heck of a time calibrating the distance the head actually moves vs what it should move. I have some artwork printed on an 8.5"x11" piece of veneer that I needed to very precisely cut around. Even though I'd align the registration marks in the upper left corner very carefully it was cutting a larger pattern than it should have. I searched and searched both here and around the Internet and determined the setting I needed to change was the Move setting in the Pulse Unit Calculation dialog. See screen shots below. The question was what value to use to achieve a given result? Most of what I found on this was related to the rotary attachments and how to set it for a rotary.


Forgive me if this information is indeed somewhere else in these forums but I've not been able to find it. Below is what I think I figured out. If I'm incorrect on any of this, someone please correct me.


The Need Pulse value is the number of pulses/micropulses the driver must output in order to turn the stepper one full revolution. Everything I read said that the Pulse Unit value is the distance, in mm, that the head would move for a given pulse. There was someone who wrote an alternate manual for LaserCut 5.3 that gave a formula for determining the values to use. Basically everything I've found written says to have the laser cut a square that is 100mmx100mm. Next, measure the square. For each axis measure the respective side of the square. The formula is very basic math which is...


Current Pulse Unit Value/Measured Length = Needed Pulse Unit Value/Input Length. Solve for Needed Pulse Unit Value. Simplified for a 100mm square, a Measured Length of 105mm, and my Current Pulse Unit Value of .00718091 the math would be Needed Pulse Unit Value=100*.00718091/105. That would give me a value of .06666667. Makes sense, correct? If I'm getting a 105mm square and I should be getting a 100mm square and everyone says this Pulse Unit value is the distance that the head moves per step of the motor, then I should get a Needed Pulse Unit Value less than my Current Pulse Unit Value because I want the head to move a shorter distance for each pulse. What was killing me was that this doesn't work. If I decrease the pulse unit value to .06666667 the square actually measures BIGGER than 105mm, not the expected 100mm. This put me back to square one. Am I the only one that's tried this and come up with this result when using this formula? The question I still couldn't answer was what the heck do these numbers actually represent? You can also rejigger the math for the formula above and simply divide 100/105 which is .95238095. That gives you the correction factor to multiply the current pulse unit by resulting in the .06666667 that I came up with.


I believe the Need Pulse value represents the number of pulses needed to rotate the stepper one revolution.


The Pulse Unit value is nothing more than the Move value divided by the Need Pulse value. You can change the Pulse Unit or the Move value and the other will update accordingly. So in my forula above you could have used the Pulse Unit or the Move value interchangably for a given Need Pulse value. So, what does the Move/Pulse Unit actually represent?


On a whim, knowing that increasing the Pulse Unit/Move value actually gave me a smaller measured length I tried using 1/.95238095 as the correction value, in other words, 105. That worked. WHY? Why does increasing the Pulse Unit/Move actually give me a smaller measured length? The big revelation here is that I believe the Move value represents the dimension of some pulley. Makes total sense. If you tell the software you have a bigger pulley then it knows it needs to turn it fewer steps to drive the head a given distance.


Sorry if this is all common knowledge to those who have been here a while but I just couldn't seem to find this information anywhere. Everyone kept saying that the Pulse Unit was the distance the head moved for a given pulse which clearly isn't the case.


Screenshot 2016-01-17 19.07.37.png

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[Dave Sheldrake]

Remember to allow for Kerf or the hole may be the right size but the cutout part will be well undersize and visa-versa

[Rich Harman]

What you need to know is the distance moved per step pulse. In my opinion, you should ignore that calculation where you measure and input a value. It will never be correct.

You need to know three things to arrive at the correct number.

1. The number of steps for a full revolution of the stepper motor. If your stepper drivers are similar to mine you can look at them to see how they are set. Mine are set to 10,000* steps per revolution.
2. The ratio of stepper output to the movement. Mine has a 2:1 belt reduction. The stepper motor rotates twice for the output pulley to rotate once (20,000 steps per revolution).
3. The circumference of the output pulley. The pitch of the drive belt and number of teeth on the pulley will tell you this. I have a 5mm pitch belt and 24 teeth on the pulley. That's a 120mm circumference.

120mm divided by 20,000 steps gives 0.006 mm per step pulse.

* I think that's right - it sounds high but I know my setting is 0.006 - or it used to be before I switched to chain - now the number for my X axis is different, but the process to arrive at the number is correct.

[Travis Reese]

Thanks for the input. The biggest point I was trying to make to help others is that the Move/Pulse Unit value in the software is NOT the distance the gantry will move per pulse. Rich, as you said, the only way to calculate this and do it correctly is to know the values that you mentioned. I think even knowing those values there is probably some slight adjustment to be made. I need to call Rabbit to see if they can tell me without guessing and trying to measure the pulleys. Dave, yes, what I want is to know precisely how far the gantry itself moves then I can account for kerf. For cutting out preprinted artwork getting the distance the gantry actually moves correct is the most important. I can then account for the kerf which I've determined is approximately .004" on very thin material such as paper or .025 thick veneer. I know this will change depending on the material and thickness of the material.

[Rich Harman]

In an open loop belt drive system, we will never get 100% accuracy. If further adjustment is necessary then it will only be applicable to that one specific cut line on that location on the table. If you adjust for a small part, and it comes out perfect, then when you cut a large part it will be off by a significant amount.

My best overall accuracy is a result of entering the settings based upon math as described in my previous post. It's not perfect, and it can't be without ball screws or a linear encoder - but it is better than the way it came from the factory.

[Travis Reese]

Rich, I agree. Never going to get the fractions of a thousandth I'm used to on a CNC milling machine. I realize that I'm going to have to tweak things for individual projects. But without having a good understanding of the settings and why they're set the way they are was driving me a bit crazy. Have to have a foundation to work from.