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Alpollo

Is a geared Nema 17 required for "myfocuserpro"?

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I think I have everything here to build a "myfocuser", except my nema 17 motor does not have gear reduction.

Do you think the gearbox is necessary? I haven't built the focuser yet, so I don't have a defined speed ratio.

I suppose I could try and do  some math, it's just I noticed lots of pictures of stepper-focusers without gearboxes, so I thought I would ask.

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I've built some without the geared stepper, so depending on the driver module you are using and if the motor has a high torque, you can just use micro-stepping to achieve the same thing...

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I have two focusers one with a gear and one without and they both work fine.

Personally I prefer the geared stepper though as I have 32000 steps to play with on the range of travel of the focuser.

With the non geared stepper I have 200 steps x 8 (microstepping steps) = 1600 per revolution if I remember correctly

I use about 5000 steps for the range of travel of the focuser.

I use the non geared on an ED80 and the geared on a C11.

The most critical thing with the focuser is to ensure everything is tight and your coupling is solid. Any slack in the system

will add to the backlash and the inacurracy in the focusing.

I have not had any problems focusing the ED80 but I wouldn´t stick the non geared stepper on the C11 as the focusing

is more important at the higher focal length for me.

 

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I have built three Myfocuser auto focusers to date. Nema17 geared and non geared and as Neil as pointed out the geared Nema17 stepper motors give you more steps to play with. The Nema17 27:1 ratio giving the most steps, it is also substantially heavier. One issue you have to be aware of in using the  Nema17 27:1 is the torque. It has the power to rip the teeth off of a R&P focuser if you allow it to "bottom out" If I were to build any more auto focusers I would use the Nema17 5:1 gearbox stepper motor.

Steve

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Thanks guys.

One thing that I do like about a gear box is it would have good holding power even with microstepping, the coils might even be able to be de-energised when not in use.

I have a Pololu DRV8834 which with the 1/32 microstepping gives me 6400 steps per rev, albeit the partial steps are weaker. With a small enough pinion rod in the Crayford, perhaps even some 2:1 belt reduction, it sounds like it might be sufficient for my 900mm focal length.

Tough call but I think I am going to try it without the gears, this motor and driver board are just sitting here begging me to be put into the focuser. It's slightly smaller without the gears and I won't have to wait a month for it. You'll see what I mean eventually, it's not a conventional focuser, but a focusing sled platform that the DSLR is attached to, so I could probably do without the extra length of a gearbox in this case.

Awesome, I will carry on.

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I am about halfway through reading the "myfocuser" PDF, it is pretty good documentation. It has the math in there, in the morning I'll punch some numbers to see if I can get the required 10 steps in the critical focus zone.

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Remember you have got 1/2 stepping and beyond dependant on which driver board you use. Although it does reduce the torque on the motor. but I wouldn't worry too much about this as once the Nema17 is energised it doesn't move. Unless to tell it to :)

Steve

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Hey Sloz.

 I have the Pololu DRV8834 which has 1/32 microstepping as an option. I would probably like to have at least 2 full steps intersect the 10 step critical zone of focus, so I suppose that may limit me to utilising 1/4 steps.

If I have to rely on the full 1/32 microstepping alone, I will probably just get the motor you suggested earlier with a gearbox or try belt reduction, whichever fits better.

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I have the belt reduction on two of my scopes. You need to ensure your auto focus programme has support for backlash compensation. If you want to go down this route.

Steve

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Noted.

I intend to use SGPro, it appears to have backlash compensation of sorts based on a quick glance.

Thanks for bringing that up, I hadn't thought that far ahead yet. I can't say I wont have backlash, because the camera will be flipped about at weird angles, so I don't think I can rely on gravity to keep the slop out. I guess I will have to make the motion snug instead of loose and account for backlash.

 

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If you go down the gear & belt route you will have some form of backlash. Mine are tightened up enough to be tight without putting any strain on the focuser or the motor & still have around 15 steps of backlash. I set mine backlash in SGPro to 50 steps and that does the trick.

Steve

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That sounds like a plan.

I realized I have to decide on a pinion diameter first before I can do the math so I'll have to continue drawing here a for a little bit.

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