Looking for autofocuser bracket for Crayford style focuser and NEMA17.

[ofranzen]

Hi all. I'm building the wonderfully over engineered myFocuserPro2 by Robert Brown and am looking for an STL for a NEMA17 stepper motor bracket to fit my MN190 with the dual speed crayford focuser. I want a bracket that is not "direct drive" but aligned so a belt can run from a smaller pinion on the stepper motor to the coarse focuser knob, or a replacement pulley.

Thanks in advance.

/Ola

Edited November 26, 2023 by ofranzen

[ofranzen]

I'm lifting this again and took some photos of the focuser. Any suggestions are highly appreciated. 

[symmetal]

Hi ofranzen,

Is this the kind of bracket you're looking for with your belt drive? It can use the two focuser mounting holes if you replace them with longer screws, and the bracket plate just sits flush with the face the screws go through. If the plate is thick enough it shouldn't flex too much. The bracket vertical end plate with the Nema 17 mounting holes will add some stiffness anyway.

Can you give the spacing between the two focuser screws and what size hole will be needed in the plate for them, M4 or M5? The length of the focuser mounting plate would be useful too, so the bracket sits flush with the ends.

Also the distance available between the focuser cylinder and the stem of the finder bracket assuming it's just a flat plate parallel to the focuser screws mounting face. I may need to make cutouts to clear the finder bracket if the Nema 17 is too wide to fit the available plate width as it's shown. I have Nema 17 steppers so have their dimensions already.

I use Blender to create my 3D printer STL files.

 

Alan

[ofranzen]

Much appreciated. Are these the measurements you meant? 

[symmetal]

Yes, those measurements are very useful. Also, the maximum possible width of the base plate as shown in'red' would be handy too, ie. where it would touch the finder scope 'riser' when the plate is screwed in place. If this isn't wide enough to fit the stepper mounting I'll need to make a cutout in the plate to fit around the finder scope.

I'll post a quick image and stl file later to see if it's basically ok and can then modify it later as required. 🙂

Alan 

[ofranzen]

I would say 50 mm is the maximum width between the guide scope riser and the focuser barrel.

Again, many thanks.

[symmetal]

Oh. I need 60mm to fit the Nema 17 as shown. I'll make a 20mm cutout which should give plenty of room. I'll post version 1 shortly.

I could make the Nema 17 flush with the left hand edge of the base plate but that would mean the Nema would obscure the base mounting hole and would need the Nema fitting after the base plate is mounted on the focuser. I'd also need to raise the Nema up a few mm to clear the screw head. This means the base plate would be 48mm wide. I can post one of these after if you wish.

Alan

[symmetal]

Here it is 🙂 

I've used 3mm thick panels. Here are two stl files, one without and one with the cutout in the base.

Bracket v1.stl

Bracket with cutout v2.stl

Any mods like curved edges, chamfers, let me know. 🙂

Alan

Edit: I use Cura for slicing and would select 25 or 33% Tri-Hexagon infill. The default infill is not mechanically very strong. Printed with the orientation as shown I'd likely select supports to give a cleaner cutout circle. You could rotate it 90 degrees and print it with the nema face down to give the best circle cutout but the base itself would not be as smooth and also be less rigid.

Edited November 27, 2023 by symmetal

[ofranzen]

This is very kind. I am printing the cutout version today and will see how it fits tomorrow. I'll keep you posted and once again thanks.

[ofranzen]

Apologies for the delay. I have had printer issues due to cold weather and the workshop not keeping full room temperature, with under extrusion as an effect. I've finally managed to print the part and it won't fit as intended. I overlooked that even if the base of the bracket clears the base for the guide scope riser, the top won't as the diameter of the guide scope is of course wider than the riser base. I will try to flip the part 180 degrees and fit it on the other side of the focuser. That will get rid of any clearance issues and the belt should still fit on the course focuser wheel as intended.

@symmetalThank you again for your help. It was my fault for not considering all constraints and I will post here how the part fits on the other side of the focuser. It was getting a bit cramped on that side anyway

[symmetal]

I wondered how you were getting on. One of those 3D printer flexible zip-up thermal enclosures may help with your temperature issues. I didn't think about the height of the finder scope either. 🙂

Fitting the bracket on the other side would be easier, but from your initial photos it looked like the manual focus assembly might get in the way, depending on the clearance between it and the fixing plate the 3D printed bracket attaches to. I thought that a thicker base plate, like 4 or 5mm, may also help with any flexure but that will likely need a cutout to clear the manual focuser. Let me know if you want a thicker base plate and whether a cutout is required between the mounting screws.

How are you going to set the belt tension from the stepper motor? Do you need some adjustment in stepper distance from the focus knob which may require a separate stepper mounting plate with a slotted screw hole cutout to then fix to the main base plate.

Alan

Edited December 9, 2023 by symmetal

[ofranzen]

I need to rethink a bit how I want to solve this. Either I go for a side mounted solution with a belt or I go for in-line as seems to be the most common. Belt tension was actually planned to be solved simply by using a rubber band. 

[symmetal]

A rubber band likely won't give a repeatable focuser position unless it's possibly very tight as the first few stepper movements will just increase band tension on one side until there is enough force to rotate the focuser. Then when you move in the other direction even more steps are needed as the tension on one side is reduced and the other side is increased. It's non consistant backlash in effect. A toothed belt would be better but needs to match the grooves on the focus knob, or have a toothed pulley wheel which fits the focuser shaft.

In line drive would be mechanically simpler but finding a way to mount it is tricky. A base plate that attaches to the two front focuser mounting screws and extends forward to hold the stepper might work but accurate dimensions and the fixing screw angles need to be determined, for it to fit.

Alan

[ofranzen]

I think this way could work well with a similar bracket but for a NEMA17. I think there are available ones already but I didn't want to mount it this way because I then lose the possibility to add gearing through different sized pullies. But, any backlash issues goes away and I think that the stepper motors with micro stepping will be good enough. I've read in several places that you should not mount the focuser on the fine adjustment shaft as is shown in this video though. Worst case I need to buy a geared motor instead. Thanks again for your help.

 

[symmetal]

Microstepping requires the stepper motor to be continuously powered even when it's stationary so I try to avoid that. I've always tended to use Lakeside stepper motors, with my own arduino based controllers, as they are heavily geared down and so have a lot of torque as well as not needing microstepping.

Using the fine focus knob with the Nema 17 would help if the main focus knob has too coarse steps without microstepping. I think the opposition to using the fine focus knob is that it introduces more backlash, but that's not an issue really, as geared backlash is consistant, and if your final focus direction is always the same direction it doesn't matter, if you set sufficient backlash steps in the driver software.

Alan

[ofranzen]

Very good info. Thank you.

I thought that the fine focus shaft should be avoided because it is more fragile than the main shaft. That would make sense with a side mounted belt setup with tension on the belt but with an in-line mounting I never really understood the issue.

So Back to the drawing board I think. Maybe I get one of those smaller but geared motors instead. I really doesn't need much power and almost any size should have torque enough when geared.

[symmetal]

Particularly with using  the fine focus shaft it's important that the helical 'spring' coupler is well aligned to avoid a constant sideways force on it. I noticed on the video you posted not much effort was put into aligning the motor shaft with the focus shaft as there is a noticeable bend in the coupler.

Good luck with what you decide to do. 🙂

Alan

[ofranzen]

Yes I saw that too and think that will be an issue in the long run. It should be fairly easy to make the bracket/mount adjustable enough to take any base tension out.