3d Printed non rotating helical refractor focuser - version 1.

[vlaiv]

I completed prototype of first in series of 3 of non rotating helical type focusers.

Actually, I'm not sure if all three could be called helical, only this one really is, but at least two will be operated the same way one operates helical focuser, neither will rotate and all will operate on basis of helix

This is actually upgrade on my previous design that I built for little hand held scope that I 3d printed around 40mm binocular objective.

Here are the specs:

It is 1.5" focuser (38mm clear aperture) with T2 thread at eyepiece end and M72x1 at telescope side. It has 40mm of draw tube travel and precision of 8mm per turn.

This version weighs in at 290g, but that will probably depend on choice of components and printer settings (infill mostly).

It took about 25h of printing time to print everything. I used Creality HP PLA. Most parts were printed with 0.2mm layer height except for thread parts which was done with 0.12mm. Infill was set to 55% except for drawtube where I went with 15% to save the print time (and rigidity is enforced by linear rods).

Item consists out of 5 printed parts, and about half a meter of 6mm linear guide rod, or rather 570mm to be precise - it needs to be cut to 2x150mm and 3x90mm. 4x6mm PTFE lined bushings were used as well. No glue or fasteners were used in this version - everything was interference fit.

Total cost is less than 20e

Here are some images and video of action:

 

Video has audio part removed as it is low quality - but focuser is not silent. It does produce sound of plastic rubbing on plastic. Helix part is 3d printed and is not as smooth as I was hoping, although overall performance is not bad by any means. I even applied some lighium grease (although I feel Synta-glue would be better option for this design ).

Here is internal construction and individual parts:

Two body parts are held together with 3 pieces of linear rods (those 90mm long ones):

Focusing ring sits in between above two parts (it is fairly simple design):

it has 4 starts trapezoidal thread (30 degrees) with 8mm pitch each

https://en.wikipedia.org/wiki/Trapezoidal_thread_form

Draw tube looks like this:

With two linear rods supporting it (and going thru bushings embedded in housing).

It is mostly single piece except for that part on top that holds ends of linear rods:

As this is first prototype, I've already identified couple of points that need improvement.

1. Too shallow holes for linear rods in draw tube.

Both top and bottom parts that hold linear rods are 10mm wide. Bottom one has 6mm holes all the way, while top one has only 7mm holes that are capped on one end for aesthetic purposes - that side is visible as it is facing the eyepiece, other is inside tube. It would be better for those parts to be at least 20mm wide. That will add to rigidity and interference fit will be stronger.

2. Tree pieces of linear rod holding body together are rather nice idea - but in order to assemble the thing - interference fit can't be very tight (even as is, I needed to use sort of plastic / rubber mallet to force things together). Any play in focusing ring depends on spacing between top and bottom body parts and if they are held by friction alone on 3 smooth rods - they can budge by tiny amount creating enough room to create play in focusing ring.

It does require some force to do so - but I'd prefer if those two parts were not so easily moved apart.

Solution would be to use m6 threaded rod with nuts instead of linear rod. Holes would need to be smaller as any play will throw things out of alignment so that part must be tight fit for everything to slide and move smoothly. Also - one side should have double nut as front and back nut should be tightened just right. Front nut would sit in 3d printed part in hex hole so it would not rotate on its own (and can even be glued in place while still enabling disassembly) - but back nut needs to be tightened against another nut so it does not come loose.

Btw, I think this is very nice idea for small refractor build - like 60-80mm range, or even as an upgrade on stock focuser on something like ST80.

If anyone would like to try building one (hint @Chriske ?) - I'd be happy to post any files required (I have everything as a single FreeCAD project, but can export individual components). I'll probably upload it to one of 3d print model sharing services in near future as well.

 

 

[Chriske]

I need to make some time to print that focuser....sigh...!
This very moment I'm busy building a Delmarva Schief. A few others of my ATM team join me in that quest.
At the same time I'm building, together with another team, a 'full scale' model of Wall-e.
And I'm also working on a 1:33 scale model of SLS Block1B Crew. Just finished Printing all parts of the Solid fuel boosters ready to assemble. When aal is done I also build SLS's LUT. The complete assembly will be almost 4.5m high.

But hey, I'll reserve one of my printers to print your designs. So go ahead and post that latest Helical(STP)

[Chriske]

Finally found some free time to start testing this Helix.
Sorry about the delay. I was very busy these last months.
Before starting I have a question ; you added a  13mm long thread into the lower part of the casing. It is not needed at all imo, correct..?

 

[vlaiv]

1 hour ago, Chriske said:

you added a  13mm long thread into the lower part of the casing. It is not needed at all imo, correct..?

Quite right.

Here is original post:

That design is part of small telescope made out of binocular lens, so it has some features related to that telescope integrated.

Above design is stand alone focuser and if you wish, I'll provide files for that as well.

[Chriske]

Can't print this thread, to small for my printers, pity...  Even with a 0.3 nozzle. I'll try one more time lowering Extrusion Multiplier.

[vlaiv]

1 hour ago, Chriske said:

Can't print this thread, to small for my printers, pity...  Even with a 0.3 nozzle. I'll try one more time lowering Extrusion Multiplier.

That rather strange. I printed it on Ender 3v2 with 0.4mm nozzle and 0.12mm layer height.

I managed to print down to 0.6mm pitch thread (that sort of works) with this setup so far.

[Chriske]

The idea making a Helix this way is very nifty indeed but imo a pitch this small is not practical at all while focusing.
Lets assume working with a 1mm pitch. Turning the barrel say 1/8 of a revolution, you'll end up with a axial shift of only 0.125mm. Using a camera with a pitch this small would be perfect I agree.

Using this focuser for visual observing, or even with little bit a coarser pitch, again, it is not practical, it is to slow.
In the past I made rotating Helix focusers with a 3mm pitch and it worked very well.

I'll try and redraw the two threaded parts with a coarser thread.

[vlaiv]

17 minutes ago, Chriske said:

Lets assume working with a 1mm pitch. Turning the barrel say 1/8 of a revolution, you'll end up with a axial shift of only 0.125mm. Using a camera with a pitch this small would be perfect I agree.

You are quite right. Even at F/4 (this is for binocular lens telescope) - it is still to fine adjustment.

It could have easily worked with x3-x4 coarser focusing mechanism.

[Chriske]

looking a second time and more closely to this print my printer did the job very well.
Using a smaller layer height it would even be better. Say 0.1 instead of 1.5 as I used now.

Anyway I'm not going to. As I told before I'll use the same layer but a larger pitch instead.

Measured value of this pitch I think it's about 0.8mm... correct...?

Edited July 28, 2023 by Chriske

[vlaiv]

9 minutes ago, Chriske said:

Measured value of this pitch I think it's about 0.8mm... correct...?

It's 1mm

[Chriske]

Need to adjust my callipers...

...or my eyesight...😃

🥸

[Chriske]

Busy redrawing using a 3mm pitch...