# LPI Ratio Question

I’m sure it was addressed here somehow, but is there an optimal LPI ratio base on motor steps? Are the maximum motor steps on the Y axis divisible by the proof grade LPI defaults of 270, 450, etc. Or are they rounded arbitrary numbers that just ended up looking the best?

The GF uses belts and pulleys with a pitch of 2mm and the pulleys have 30 teeth. This makes one revolution 60mm. The motors are 400 step motors driven by microstepping drivers set to times 8 mode. That gives 3200 steps per revolution, which makes each step 0.01875mm or 0.000738188976378".

That gives a maximum LPI of 1354.66666. All the other LPI numbers have to be integer divisions of that but for some reason the GFI shows slightly different numbers. For example 655 is actually 677.3333333, 450 is 451.55555, etc.

11 Likes

I just read a response from @dan about this exact question. I don’t have the quote or link to his post, but he said that the LPI were derived from stepper motor steps, but it was a long time ago and his memory on the subject was a bit fuzzy.

I would assume that the LPI would be based on an integer value of motor steps, otherwise we would get into problems like playing 24p content on a 60hz TV. Judder…

As such, we can probably back into figuring out the linear travel of each step by looking at the LPIs that were chosen, but I don’t want to fill my head with yet another esoteric hardware thing

I try to match my rasters to the same lpi, as well. Not sure if I get better results or not, but there it is.

Wow, are you sure the gear on the stepper is 30 tooth? I haven’t looked inside my 'forge yet, but that seems rather large. My 3d printer is a 16 tooth pulley. I would assume they would want something with fewer teeth (more resolution), but that also means it would have to rotate faster to get the current full speed (and get hotter due to higher speed)

I’m sure they weighed options and came to the best for the product. It’s just interesting.

No Idea why they picket 30 tooth, I would have used 20, which gives rounder numbers in metric. If I had wanted to hit round LPI numbers I would have used imperial pulleys.

Stepper motors don’t run hotter when they go faster, if anything they run cooler because the current falls. I think the speed is limited by using 10kHz motion files and that is probably also why they don’t use the full x32 mode the drivers support.

So maybe the LPI in the interface is just an approximation and it rounds to an optiumum setting? Just seems like there would be instance where we would see banding or moiré patterns if we had exact control of the LPI.

The machine uses exact step multiples, it is just the GFUI that rounds it to make it look pretty. So if you want to match the LPI with your image to avoid the resampling artefacts it is difficult because you would need to use the real numbers, which are not even integers when expressed as LPI.

Not that you could see a difference, but if I change something from 300LPI to 302LPI, chances are that I didn’t change anything at all?
Which is probably for the best but chafes at my desire for order in chaos

1 Like

Do you mean in your source image? That depends on the program generating it.

Hopefully the GF use a good resampling method so that you don’t get aliasing. I think it uses a Lagrange filter.

No I meant in the cut and power settings in the GF interface… but now I see that you can’t enter a value but only pick from a drop down, dope… my bad

My assumptions were higher speed means more current going to the motor. More current means more energy, meaning hotter.

Often times reality goes contrary to what I see in my head…

I was designing a test pattern file that addressed the LPI but didn’t actually look to see if I could enter any variable into the LPI… so I guess that answers that

That is the case with a DC motor but with a stepper motor you just step it faster. The driver attempts to provide a constant current but at high speed the inductance and back EMF thwart it and the current fails.