Build own Rotary - Beginner Questions

Hello everybody!
I want to build my own Rotary unit and I have found a rough guide for it!
But what I still don’t understand is how to calculate the length of the belt and the number of teeth on the gears, for example.
It should be a 3D printed rotary unit, based on one from Thingiverse.

But there are 3 gears installed, apparently the two lower ones with 34 teeth and the upper one in the middle (on the motor) with 17 teeth.
According to the description, it is a 2: 1 reduction.

Can someone explain how this works and I can calculate which pulley has how many teeth and how long the belt has to be?

In the example of thingiverse, the author prescribes a belt with a length of 315mm and a width of 9mm.
I can change the width for example with 3d printed pulleys …
But how can I calculate how long the belt has to be and how does this relate to the number of teeth on the pulleys?
I’ve been using Google all day and I don’t understand it.

I hope you can read this reasonably well, because my English is unfortunately very bad and I had to use Google Translator to help!

Oh yes, here is a picture of the example:

Kind Regards from Austria

Trigonometry is your friend. :slight_smile:

That would get broken down to the length of six sections. You need to know the diameter of each pullley and the wrap angle of the belt for each pulley to calculate the length of belt wrapped around each pulley and then add the length of each straight section of belt between pulleys.

I recently built a project that had 5 pulleys and I modeled it to scale in Inkscape which made it fairly easy to figure out the wrap angle on each pulley and measure the lengths.

i have found some online calculators but even only for two pulleys and not for three like in the picture above.

This is all new for my and i dont understand this :confused:

For a rough idea take a string around the pulleys where the end meets the beginning mark it or cut it there.
Then measure the cut string in mm and you will get a idea i am sure your motor has a adjustment to take up the slack.

Worst case, put a slot below the motor for a smooth idler. Because we all know belts will never be anything but the exact size we want them to be…

specifically, I am concerned with these instructions here

and how i can calculate the right number of teeths for each pulley?

Here is the one I built from pictures on the internet no plans just guessing i think before i got it right it went to rev 6. Its a little heavy but it works. Everything from amazon and mcmastercarr.

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looks great!
but to be honest i still don’t understand how it works with the number of teeth.
besides, i also ponder how exactly it works with the steps

Daniel
What i did was make sure the two pulleys that drive the spindles are the same.
The one on the stepper motor is what ever you choose mine just happens to be smaller just make sure the pulleys have the same tooth spacing like GT2 6mm belt . The software will control the speed which is another whole challenge. You will get there it takes time.

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As long as your two shafts have the same number of teeth, everything else is arbitrary and totally up to you. Once you have it assembled, you still have to do your calibration tests, just like you did for your X and Y when you got your machine. Every cylindrical object sitting on rollers will have a different circumference unless you buy a case of the same tumblers. Only a jaw type rotary will have the same number of steps in a 360 rotation.

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okay thank you!
but i dont have made any calibration tests, when i got my machine :sweat_smile:

Almost every machine needs to be calibrated in X and Y. They are usually close, but not what they should be. It’s not a difficult thing to do, and you check for skew at the same time.

In your software, make a box a little smaller than your max table size, and put a dashed line down the center, now cut that from some paper. I used old Christmas wrapping paper cut from the back side.

Step one: Measure your X and Y cuts and compare them to what they are supposed to be. Enter the drawn vs actual in your controller and retest. It’s not unusual to have to tweak them a second or third time if they are quite a ways out.

Step two: Fold that piece of paper on the dashed line and cross your fingers. If your Y drive axle is set properly, the corners will match. If they don’t, that’s your skew, the amount your gantry is off square from side to side. It’s up to you if you want to go through correcting it.

My gantry is off a little more than 2mm over a 700mm distance.I’m not going to worry about it for two reasons. First, I’m not cutting tight tolerance interlocking pieces or doing large inlays. Second, and more important for me, my Y drive axle is behind my laser tube. I will wait until I have to replace the tube to tweak that 2 plus mm out of the skew.

Oz, I just thought of something. How would an X or Y out of calibration or a skew problem affect the camera usage in LightBurn?

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but until yet i had no problems, so i think everything is in tolerance.
but good to know :wink:

Think is not knowing. Run the check. If you are good out of the box, great.

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