# how to calculate speed?

Posted by The Borg
 how to calculate speed? July 14, 2016 07:08PM Registered: 5 years ago Posts: 9
im trying to find out how fast a lead screw will run compared to belt drive. i seen the calculators but all they show me are 'steps' how do i find my mm/s?
 Re: how to calculate speed? July 14, 2016 07:25PM Registered: 7 years ago Posts: 1,401
Your steps per mm and the feedrate your stepper can do will define the speed. But usually a lead screw will be slower than a belt unless the pitch is high enough to compensate for.

To give you an example: a stepper motor with a 20 tooth gear and a gt2 belt will move 40mm for a full revolution of the stepper (5 revolutions to cover the standard 200mm built plate), to do the same you need a 40mm pitch lead screw, even if someone makes such a thing you will be end up loosing accuracy on small movements.

Edited 1 time(s). Last edit at 07/14/2016 07:33PM by ggherbaz.
 Re: how to calculate speed? July 16, 2016 07:31AM Registered: 6 years ago Posts: 3,525
Lead screws are nice for z, although I contend that belts do the job just as well and are cheap and easy to use. For x and y leadscrews suit a cnc where high thrust is needed but it isn't for 3d printing.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions
 Re: how to calculate speed? July 16, 2016 10:15PM Registered: 5 years ago Posts: 9
sorry but im still a little confused. i see numbers like 50 to 200mm/s listed. typically i see something arond 75-150 being the average people run at. but what i don't understand is the math to get those numbers to compare a belt and screw. it seems typically people are using 2 pitch 4 start screws these days which i guess means 8mm per revolution. and motors are 200 to 400 steps depending on model + any microstepping used.

but these are steps and distance, where does time come in when its not listed anywhere? i have no idea how long it takes a stepper to turn. why is there no calculator to come up with this 150mm/s type format that everyone uses?
 Re: how to calculate speed? July 17, 2016 02:06AM Registered: 7 years ago Posts: 1,401
It's not simple, several factors are involve like acceleration, torque, weight, voltage used, etc.

 Re: how to calculate speed? July 17, 2016 02:36AM Registered: 6 years ago Posts: 3,525
The problem with measuring print speed is that travel non-printing moves can go as fast as possible but still have to accurate positionally, external perimeters have to print much slower or the object will look rough, perimeters can go at medium speed, infill can print fast. So printing speed is an approximation of the mean of these values. Average for Fdm printing is 50mm/s, if you can print an object well faster than this you're doing okay. You want to be able to move x and y at 100mm/s travel speed, or it will seem slow. You don't need to move z faster than 1 mm/s really but when homing it's nice to be able to get 10mm/s so you aren't waiting around for it to home or if using it, autolevel. Of course if you double these and still get quality objects then your doing well.

Stepper motors are often being run from slow controllers like ramps/mega so maximum number of steps that can be generated per second is quite important it's around 40,000. Faster controllers are not so limited. Its no problem to have a fine leadscrew if your controller can send enough steps at 4000 steps per mm without slowing down. So if you want your axis to move at 100mm/s and it needs 4000 steps per mm that's 400,000 steps/sec which the controller can't do, so people opt for 8mm leadscrews which require far fewer steps to generate usable accurate motion.

Generally speaking it's belts for x and y and leadscrews for z, if you have a fast controller you can use smaller pulleys and shorter leads in your screws for improved resolution.

If you want really fast check out this guys dc motor controllers [hackaday.io] but whether you can lay down plastic at these speed is very doubtful.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions
 Re: how to calculate speed? July 17, 2016 03:13AM Registered: 10 years ago Posts: 2,470
Quote
The Borg
sorry but im still a little confused. i see numbers like 50 to 200mm/s listed. typically i see something arond 75-150 being the average people run at. but what i don't understand is the math to get those numbers to compare a belt and screw. it seems typically people are using 2 pitch 4 start screws these days which i guess means 8mm per revolution. and motors are 200 to 400 steps depending on model + any microstepping used.

but these are steps and distance, where does time come in when its not listed anywhere? i have no idea how long it takes a stepper to turn. why is there no calculator to come up with this 150mm/s type format that everyone uses?
Maybe it is something more fundamental.
Belt pitch, thread pitch etc. only give you the distance per motor revolution, or ultimately distance / motor step. How many steps are taken and in what time are something controlled by the GCode and the firmware. There is a maximum acceleration any mechanical setup can take, this determines how much time it takes to accelerate to the set speed. Often this can be different for each axis. It depends mostly on how much mass needs to be accelerated.
The maximum speed is determined in theory by how many stepps the electronics can calculate per second. This is actually not very much for ATMega based boards, since the CPU is pretty slow. ARM based CPUs have a lot more power and in theory can drive motors much faster. In reality both can drive RepRap hardware at the very edge of what it can stand up to. Keep in mind that speed is nothing without control, if the precision of positioning is not achieved all the speed or acceleration in the world doesn't help anything.
To answer your original question. Both can be equaly fast, but a belt system has less mass that needs to be accelerated so acceleration can be higher and thus the top speed is reached earlier. Since most of the time the top speed isn't kept for long due to the short distances the higher acceleration often determines the time it takes to print something.
So what you are looking for is not only what is the maximum speed, but als for the maximum acceleration and ultimately the quality of the print.
It is very easy to make something fast (just put enough energy into it), but it is a lot harder to make it work reliable and precise.

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 Re: how to calculate speed? July 17, 2016 11:11AM Registered: 11 years ago Posts: 7,616
This list should give a good idea about the achievable step rates: [reprap.org]. Formulas to calculate it for other constrains are also given.

 Generation 7 Electronics Teacup Firmware RepRap DIY
 Re: how to calculate speed? July 20, 2016 06:57AM Registered: 5 years ago Posts: 2
May i think one software will be there just search those things, because i don't have much idea about the calculation. I hate maths
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