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Stepper motors with 7.5deg steps?

Posted by heimerdinger 
Stepper motors with 7.5deg steps?
February 18, 2016 07:58PM
Hi RepRap,

First post.

Much to learn, I still have. yes yes.

I've dismantled an old (~2011?) HP laser printer, knowing that I could get lots of stepper motors and useful parts. Turns out they're all 7.5 degree steps. My local 3D printer expert has explained why 1.8 would be ideal (200 steps gives a nice round number for a full rotation and avoids messy calculations/mechanical compensation).

Is it worth doing those calculations to avoid buying new motors when I've got a load of perfectly good ones already? (I think it would be, just wondering if anyone else has done anything similar and could provide pointers)

If this question is too open ended, feel free to harass me for it.

Cheers,
Heimerdinger
Re: Stepper motors with 7.5deg steps?
February 18, 2016 10:28PM
7.5degree is 48 steps per rotation (full step)

This directly translates it to the accuracy of the machine

Presuming a standard belt driven axis using 2mm belt pitch and 8 tooth pulley and 1/16th micro stepping
1.8 steppers, 200 steps/mm gives you a theoretical resolution of 5micron
7.5 steppers, 48 steps/mm gives you a theoretical resolution of 20.82micron

On m5 leadscrews this translates to
1.8 steppers, 4000 steps/mm ie 0.25Micron
7.5 steppers, 960 steps/mm ie 1.04166666Micron

I don't think you will ever notice the difference.. Please try it
Re: Stepper motors with 7.5deg steps?
February 19, 2016 03:02AM
Should be OK for a Cartesian printer, but not a delta because the Z resolution would be too low. More important is whether the rated phase current is in the 1 to 2A range that will work well with standard stepper motor drivers.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Stepper motors with 7.5deg steps?
February 19, 2016 10:00AM
Quote
Dust
7.5degree is 48 steps per rotation (full step)

This directly translates it to the accuracy of the machine

Presuming a standard belt driven axis using 2mm belt pitch and 8 tooth pulley and 1/16th micro stepping
1.8 steppers, 200 steps/mm gives you a theoretical resolution of 5micron
7.5 steppers, 48 steps/mm gives you a theoretical resolution of 20.82micron

On m5 leadscrews this translates to
1.8 steppers, 4000 steps/mm ie 0.25Micron
7.5 steppers, 960 steps/mm ie 1.04166666Micron

I don't think you will ever notice the difference.. Please try it

Right, the 1.04166666666666666.... - on the x/y (and z, technically, but that's not so important) this would leave an infinitesimally small fraction of a micron left over, shifting each layer by that much, correct? I know visually it wouldn't be noticeable between each layer but with a tall enough object / enough layers it would add up and lead to a small deformity every x number of layers. (poorly attempted ASCII illustration below)

.
..
...
....
.....
......
.
..
...
....
.....
......
.
..
...
....
.....
......

that 0.000001 something microns would stack up and after enough layers it would jump back to start, leaving a more noticable difference, and a weird pattern where one is not intended. That is how he explained it to me, I believe.

--

That said I have been lurking various forums/threads and I think I'll give it a shot with the 7.5 steppers, worst case I just buy some 1.8's, best case I have a functional printer without buying new motors! grinning smiley

Thanks for the help.

Further discussion is welcome.
Re: Stepper motors with 7.5deg steps?
February 19, 2016 04:10PM
Quote
heimerdinger
Right, the 1.04166666666666666.... - on the x/y (and z, technically, but that's not so important) this would leave an infinitesimally small fraction of a micron left over, shifting each layer by that much, correct? I know visually it wouldn't be noticeable between each layer but with a tall enough object / enough layers it would add up and lead to a small deformity every x number of layers. (poorly attempted ASCII illustration below)

.
..
...
....
.....
......
.
..
...
....
.....
......
.
..
...
....
.....
......

that 0.000001 something microns would stack up and after enough layers it would jump back to start, leaving a more noticable difference, and a weird pattern where one is not intended. That is how he explained it to me, I believe.

--

That said I have been lurking various forums/threads and I think I'll give it a shot with the 7.5 steppers, worst case I just buy some 1.8's, best case I have a functional printer without buying new motors! grinning smiley

Thanks for the help.

Further discussion is welcome.

With RepRapFirmware (and I suspect with most other firmwares too), there will be no "jump" in X and Y. As with all printers, there will be a "jump" in Z if you choose a layer height that is not an exact number of microsteps (and preferably an exact number of quarter steps too).



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Stepper motors with 7.5deg steps?
March 16, 2016 08:51AM
Hi,
I am planning to use 7.5 degree steppers for my 3d printer.
I am using belt drive to increase resolution.
First z axis test looks quite promising.
My configuration:
Motor pulley - 24 teeth
Threaded rod pulley - 50 teeth
Threaded rod M8 metric
So I have 80 steps per mm resolution even without micro stepping.
VIDEO [youtu.be]
Re: Stepper motors with 7.5deg steps?
May 26, 2016 02:57AM
Hi,

Made some tests with 7.5 degree steppers for X and Y axises.
VIDEO [youtu.be]

Quite good so far smiling smiley


My 3D printer videos [www.youtube.com]
My E-Waste CNC videos [www.youtube.com]
Re: Stepper motors with 7.5deg steps?
June 22, 2016 09:20AM
It works !!! smiling smiley

All motors except extruder are 7,5 deg motors

[youtu.be]

Print quality is not very good, need to calibrate...


My 3D printer videos [www.youtube.com]
My E-Waste CNC videos [www.youtube.com]
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