Linear motion components for DIY focuser?

[vlaiv]

I started fiddling around with building 3d printed scopes, or rather using 3d printing to assemble telescopes.

One of the biggest hurdles that I foresee for time being is - focuser.

Commercial units are either very poor performing and cheap, or too expensive for cheap DIY scopes.

I made little 3d printed non rotating helical focuser, and to be honest, I'm rather impressed that it works at all, but one of its major drawbacks is printed draw tube. Draw tube is printed in "standing" orientation due to threading involved - which puts layer lines on sliding surface. That obviously makes sliding motion seriously rough.

All that made me thing think - what about using linear rods or linear rails as sliding support for DIY focuser?

Linear rods can be sourced very cheaply and PTFE bushings + PTFE dry / spray on lubricant for rod itself - should create very smooth motion of the draw tube.

What are your thoughts and does anyone have experience with building such focusers?

 

Edited December 3, 2022 by vlaiv
typo

[Peter Drew]

Not focusers, but I have made similar action components for other purposes.  It's one of those designs that look good on paper but which can be quite difficult to implement.  Unless there is a decent length of axial support, multi component bearings have a tendency to "crab" if the motive force is not on a central axis.     🤔

 

[markse68]

@Chriske has made loads of interesting 3d printed focusers- here’s a thread for inspiration if you missed it

 

Mark

[marcopolo]

Did you see mine? All assembly details, BOM, files and CAD available. I looked ages online couldn't find anything descent, so designed and built this. There have been variants of it made since even motorised so folk are quite happy with it.

https://marcosatm.com/2020/06/04/2in-crayford-focuser/

 

 

[vlaiv]

2 hours ago, marcopolo said:

Did you see mine? All assembly details, BOM, files and CAD available. I looked ages online couldn't find anything descent, so designed and built this. There have been variants of it made since even motorised so folk are quite happy with it.

https://marcosatm.com/2020/06/04/2in-crayford-focuser/

 

 

I've seen that, and I must say - its one of the nicest looking designs that I've came across.

For me, probably most difficult thing to source would be 3x8x3 bearings. They seem perpetually out of stock locally.

Anyways, I was thinking more in line with refractor focuser and rack and pinion or lead screw type.

 

[saac]

@vlaiv a few years back I built an interferometer for use on a mirror grinding project.  I based the design on a standard  xy optical stage. The stage, aluminium plate, rode over ptfe blocks which acted as linear bearings. Control over positioning was affected by use of a micrometer stem which was secured to the stage such that it could be pushed forward against the tension of a retarding spring. The action was really smooth and allowed the precision of movement needed to set up the interferograms. Micrometre stems can be picked up relatively inexpensively now. I wonder if something similar could be useful in a focuser. 

Jim 

[vlaiv]

I just got a meter long linear bearing rod that I cut into pieces (18cm x 2 for now). SKF PTFE bushings also arrived. Both are 6mm and fit perfectly.

In fact - fit is too good. Linear rod glides like nothing is there

I'll probably need to design some sort of tension breaks into system to allow focuser to hold some weight without slipping.

First step will be to design prototype rig for sliding and 3d print it. Some interference fit testing first for both bushings and rods into 3d printed frame.

I already printed some herringbone rack and pinion. Due to 3d printing precision - I can only print components that will move something like 40mm per turn (1mm module, with pinion having 11-13 teeth) - and that is too coarse. Will need to include 3:1 or 4:1 reduction stage into design.

That will come in prototype stage 2 - sliding rig + rack and pinion part. When I get that right - rest is easy, I just need to add 55mm aluminum tubing (easy to source, very cheap) and print casing for the lot.

[marcopolo]

Thanks Vlaiv. Appreciated! I’m a mechanical engineering so no excuses for poor work!!

Edited December 9, 2022 by marcopolo

[vlaiv]

Just had an idea:

How about 2GT timing belt instead of rack and pinion?

It would certainly need reduction with 16T pulley as that gives 32mm per revolution, but then again, same can be done - another 16T paired with say 32T or 40T pulley?

[skybadger]

I always had a hankering to design an essentially rack and pinion system that used three racks around a cylinder and the racks are operated by worms, one for each, and the worms are synchronised by a belt or large ring gear. The aim is to get positive lock and very fine adjustment without rotation of the cylinder over a decent strike range. 

[vlaiv]

1 minute ago, skybadger said:

I always had a hankering to design an essentially rack and pinion system that used three racks around a cylinder and the racks are operated by worms, one for each, and the worms are synchronised by a belt or large ring gear. The aim is to get positive lock and very fine adjustment without rotation of the cylinder over a decent strike range. 

If I get that right - it is very similar to the non rotating helical design that I printed:

Draw tube:

Focuser housing:

(thread that is visible at the bottom serves to screw in lens cell - it is not the part of focuser)

Focuser knob (you'll have either belt or large ring instead of this):

And finally - top part of the housing:

 

[skybadger]

Yep I guess that about does the trick. 

[vlaiv]

Just a small update.

Linear rods work exceptionally well. In fact, as predicted - they work too well

Motion is very smooth, silent, there is zero play in any direction and setup is rather stiff:

 

Problem is that motion is too smooth - there is no enough friction to hold assembly in position under its own weight - let alone when adding diagonal and eyepiece:

 

Does anyone have idea what would be the best way to implement friction brake of sorts to stop this?

[markse68]

Hi Vlaiv, wont the drive whatever that turns out to be hold the position?

BTW what video settings do you use to upload embedded video like this? I couldn't get it to work at all

Mark

 

[vlaiv]

49 minutes ago, markse68 said:

Hi Vlaiv, wont the drive whatever that turns out to be hold the position?

BTW what video settings do you use to upload embedded video like this? I couldn't get it to work at all

Mark

 

Well, maybe motorized focuser would hold it, but I'm doing manual version and I don't think that rack and pinion will be able to hold it in place, at least not "loaded" with 2" diagonal and an eyepiece.

I might be wrong though. My next step is to design and add rack and pinion. I already printed few "test samples":

First one is 9 teeth and it did not come out quite well - there was some strange warping in the corner of the rack and teeth on pinion are not that great.

Other two are much better: middle one is cycloidial profile, 11 teeth for pinion (1mm module) and 24 degrees angle on herringbone pattern.

Top one is involute profile, 13 teeth for pinion (again 1mm module) and 30 degrees angle.

In any case, there will be additional reduction stage of about 3:1 - so that might help. For the time being, I'll be using 608 bearings for the test and 5mm threaded rod as shaft for these (I'll design and print threads so all bits can be threaded onto the rod and held in position with m5 nuts).

Btw video is encoded as mp4 video, and I just drag&drop it into post like I do with images.

[Mandy D]

13 hours ago, vlaiv said:

Well, maybe motorized focuser would hold it, but I'm doing manual version and I don't think that rack and pinion will be able to hold it in place, at least not "loaded" with 2" diagonal and an eyepiece.

I might be wrong though. My next step is to design and add rack and pinion. I already printed few "test samples":

First one is 9 teeth and it did not come out quite well - there was some strange warping in the corner of the rack and teeth on pinion are not that great.

Other two are much better: middle one is cycloidial profile, 11 teeth for pinion (1mm module) and 24 degrees angle on herringbone pattern.

Top one is involute profile, 13 teeth for pinion (again 1mm module) and 30 degrees angle.

In any case, there will be additional reduction stage of about 3:1 - so that might help. For the time being, I'll be using 608 bearings for the test and 5mm threaded rod as shaft for these (I'll design and print threads so all bits can be threaded onto the rod and held in position with m5 nuts).

Btw video is encoded as mp4 video, and I just drag&drop it into post like I do with images.

No Citroens were harmed in the manufacture of those racks!

[Chriske]

17 hours ago, markse68 said:

Hi Vlaiv, wont the drive whatever that turns out to be hold the position?

Indeed, and why not just use threaded rod to do the motion.
Of coarse you need to redesign the whole setup.

[vlaiv]

31 minutes ago, Chriske said:

Indeed, and why not just use threaded rod to do the motion.
Of coarse you need to redesign the whole setup.

That might be an option - but I'd still need gear set - only not as reduction, but rather overdrive.

I don't like the idea of having three things stick out with draw tube . I barely accepted these two. But in all seriousness - would threaded rod need lubrication?

I like the idea that these linear rods don't need it if I'm using PTFE bushings.

There is option for threaded rod to be stationary inside casing, and nut to be attached to draw tube - but then we get limited travel - draw tube can only move the same distance nut is able to move inside casing.

With above approach - we can have more travel.

I did look up usual travel of refractor focusers, and most are limited to say 90mm (that is actually on longer side of things - most are 60-70mm), so yes, that might be an option.

Now that I'm thinking about it - I sort of like the idea. Might design one of those next. There is a way to extend draw tube travel by "hiding" part of the mechanism inside telescope body - same way draw tube is inserted in telescope body deeper than the focuser itself when focuser is fully racked in. No reason not to have threaded rod extend on that side.

[vlaiv]

Here are a few "teaser" images of new contraption.

This is more proof of concept and to get the feel of how smooth 3d printed gears will be:

I've yet to add bottom part of housing that will hold bearings for shafts and of course couple of knobs to be able to operate it.

Then it is 3d printing time

[vlaiv]

@Chriske

I've considered the idea of using T8x8 P2 lead screw as driving element, and I have a bit of spacing issue.

Say that I want to design 2" refractor focuser. Draw tube will be OD 55mm (that is what I can easily source, with 2mm walls, so ID is 51 - enough for 2" eyepiece / 50.8mm to fit in) and I'm hoping to use it on 90mm OD tube, with again 2mm walls - which leaves 86mm of space to fit focuser into OTA.

If focuser is 86mm wide and draw tube is 55m wide - we have something like 15.5mm to work with. That is plenty for 8mm lead screw and 3d printed integrated nut, but I don't know how to transfer knob motion onto the shaft.

Bevel gears should do the trick - but I can't fit anything sensible in those 15.5mm, or rather lead screw will be like 3mm-4mm away from draw tube - so any bevel gear that I can fit there must have diameter of 16mm max. That is too small.

Alternative is to use GT2 timing belt to move bevel gear away from draw tube? Adds complexity and I'm not sure how to connect 8mm lead screw with small gt2 pulley without needing a lathe to drill m5 bore into lead screw.

Any ideas?

[Chriske]

Not sure I understand the concept you're building here but you do have more space then you think in that OTA.
A 2" eyepiece has a 50.8 mm OD. But most of these big eyepieces have a field-lens of only 40 to 45mm diameter. So that give you more space to work with in that optical tube.

Yet again I'm not sure what your building, sorry about the ignorance...😟

[Chriske]

On 09/12/2022 at 14:02, vlaiv said:

If I get that right - it is very similar to the non rotating helical design that I printed:

Draw tube:

Focuser housing:

(thread that is visible at the bottom serves to screw in lens cell - it is not the part of focuser)

Focuser knob (you'll have either belt or large ring instead of this):

And finally - top part of the housing:

 

Is it possible to send me these files so I can open and assemble that focuser in my CAD software.

[vlaiv]

1 hour ago, Chriske said:

Is it possible to send me these files so I can open and assemble that focuser in my CAD software.

Sure, here it is:

Telescope body.FCStd

It contains several bodies - which should be thought of as parts. I did not bother to properly arrange object hierarchy.

Only thing that is missing is 1.25" eyepiece adapter at the end - but it screws in via T2 thread.

M43 thread on the start connects to binocular lens cell.

 

[vlaiv]

1 hour ago, Chriske said:

Not sure I understand the concept you're building here but you do have more space then you think in that OTA.
A 2" eyepiece has a 50.8 mm OD. But most of these big eyepieces have a field-lens of only 40 to 45mm diameter. So that give you more space to work with in that optical tube.

Yet again I'm not sure what your building, sorry about the ignorance...😟

Ok, here is a brief sketch showing how much room there is:

86mm above is ID of OTA and focuser needs to "slide" in it (It does not need to do that, but it's nice to have focuser as wide as telescope body and not wider).

55mm OD is draw tube. It's made out of aluminum and has 2mm thick wall. It will have 2" 3d printed adapter at the front.

8mm white circle is T8x8 P2 lead screw used for driving the tube back and forth. It is stationary (does not move with tube) and tube has "nut" attached to it and lead screw is threaded thru that screw.

Lead screw needs to sit between two tubes and can't touch either of them because there needs to be some space for:

a) nut

b) telescope housing that goes into OTA tube and holds nuts for OTA attachment screws

So it is pretty much set in position where it is.

I need to transfer rotation of focusing knob - which is in X direction in above drawing to lead screw (Z direction in drawing) - so I need some sort of bevel gears (or something else) - like those marked in blue.

Issue is - lead screw to draw tube distance is set and small and draw tube needs to move freely with respect to lead screw (pass over it back and forth along its length), and I don't think there is enough room to put any sort of sensibly sized bevel gear in there (one that can be 3d printed).

Clearance marked with red arrow is not issue - bevel gears will sit outside of the OTA tube, but clearance marked with green is the issue - as draw tube will pass in that space where bevel gear needs to be.

One possible solution that I came up with is this:

But it has implementation difficulties and those are:

- sourcing so small closed loop timing belt (alternative is nice way to make one from open to suit, but I don't know nice and easy method of doing that)

- attach timing belt pulley to lead screw

- increased number of off the shelf components - which increases total cost of the unit (and I was aiming for cheap solution - this is going to be paired with 80mm achromatic lens from ali express, I'm aiming to provide nice working telescope at lower cost for those that have access to 3d printing).

[Chriske]

The way I see it about that spacing between drawtube and telescope tube.

Let's assume the largest fieldlens of your eyepieces is 40mm. Then you have lots of space between the two tubes.

Therefore the drawtube needs to be like this.