Reversed Crayford 'CRC'

[Chriske]

Not my idea, but I completely redesigned this 'Reversed Crayford'
Flat and bearings are at the same side of the focuser. So the unit is very small.
That flat(purple) is nothing more than a piece of aluminium screwed onto the side of the eyepiece barrel.
With a normal Crayford the focusing rod pushes the eyepiece barrel against the 4 ball bearings at the opposite side of the focuser.
In my case the focusing rod pulls the focusing barrel toward the observer, against these ball bearings. I choose to print the eyepiece barrel instead of making it out of aluminium(as I always did in the past).
I printed that barrel with a very small layerheight so it has a very smooth perimeter. To make very good contact between that barrel and the ball bearings I mounted the focusing rod springloaded.
Purpose is to allow others to copy(print) this design without the need of that aluminium eyepiece barrel. That printed barrel is rather tick so there little chance it will deform.

 

 

 

[Gina]

Clever ?

I see the third drawing has gone a bit wrong where the focus shaft touches the flat plate (between the sides of the casing).

[Chriske]

Not sure what you're trying to say about that third drawing Gina...

[Rusted]

What retains the white tube from flying off into space?

Is the 'purple' piece a dovetail?

[Gina]

59 minutes ago, Chriske said:

Not sure what you're trying to say about that third drawing Gina...

Oops, I beg your pardon Chris - it was an optical illusion, looks fine now.

[Chriske]

12 minutes ago, Rusted said:

What retains the white tube from flying off into space?

Is the 'purple' piece a dovetail?

Well, against the white tube is bolted a piece of aluminum(purple).
The (springloaded) focusing-rod pushes against that square aluminium rod. Doing so the eyepiece tube is pushed against the 4 ball bearings.
It is in fact exactly the same as a 'regular' Crayford, except that the focusing 'rod's action is in the opposite direction.
Almost done printing. I'll will assemble it later on.

[Chriske]

There are four ball bearings guiding the eyepiece-barrel.
In the cutaway drawing you see in the background 1 of the two other bearings to supports the focusing rod.
That (brass-coloured) focusing rod(6mm diameter) is pushed against that purple aluminium square rod. If you look closely you'll notice in the lower drawing that ther's no contact between focusing rod and eyepiece-barrel.
Maybe this cutaway says it all...?

The eyepiece-barrel in this drawing is in its most lowest position

 

[Astrobits]

I made some similar to these about 10 years ago and still have one on my scope. I did not include the spring arrangement that you have included and that would be better than mine. I found that the flat bar parallelism is critical otherwise it is too tight at one end and too  loose at the other. I also used a 3mm dia rod from the focussing knobs and believe that it is too thin and flexible. Again your 6mm will be better.

Here's a pic on one that I made:

Nigel

[Chriske]

That's what I thought Nigel, about not been parallel. What's more in my case I couldn't go any other way because of that printed eyepiece barrel. If for some reason that barrel is not perfectly straight that spring-loaded-compensation is an absolute necessity.
Indeed a 3mm focusing rod is far to thin imo.

Yours looks very nice, good job...!!..

[Gina]

The Crayford drive shaft on my MN190 that I refurbished is 5mm but I cant remember much else about it.  I remember I replaced the ball bearings and they were on the opposite side as usual.

[Chriske]

Looking good, but one issue has to be dealt with.

Turning the focusing know, it is as there are some pre-programmed locations the focuser  is jumping toward. Very strange.
The layer-height of the printed eyepiece barrel is very small(0.2mm), so that is not the problem at all. I think 'over' the layer-thickness-pattern there is a second pattern that is causing this 'jumpy' movement.
Ball bearings are not on the suspicions list because I use these ball bearings all the time in other focusers and in my 3D-printers. So that leaves only the focusing rod as second reason why this is happening. Just to bring in some colour, this is the first time I used a brass focusing rod 6mm diameter. In the past I always used stubb-metal. Stubb is very hard but most of all very accurately grinded. It is grinded within very narrow specifications. Don't know if brass rods are treated with the same narrow tolerances.

Time to investigate...
And also to restart working on postponed (3D-printer)issues...

[Gina]

Some brass rods I have used are certainly not accurately machined - I think they are simply hot-drawn.  Same applies to aluminium.  I think that is most likely your problem, Chris.

[Chriske]

Found it...!
Focusing rod got out of the suspicions list.
Just to be sure it had nothing to do with the eyepiece barrel I temporary installed a very thin brass shim(0.2mm - 0.008") between barrel an ball bearings.. et voilà... perfect and very smooth action...!! So my first thought about that secondary pattern in the barrel's perimeter was correct.
While testing with that brass shim it got imprints in the brass shims caused by the pressure of the bearings. After a minute or so the shims got deformed and that secondary barrels pattern got embedded in the shims. So that jumpy focusing got started again.
Need to redesign that barrel. I will add some thicker aluminium strips at the ball bearing's path.

Got me thinking about the quality of my printed parts. I did some measuring of that barrel and there was a slight difference in diameter at different heights. Although the perimeters of my printed parts are very good, in this case there was a difference of 0.045mm(0.0018") measured at different heights.
Lesson learned..!
So back to the drawingboard now.

 

 

[Gina]

Ah yes, you may remember I've had that problem with 3D printers, Chris.  Difficult to find the cause.  It's difficult to get very accurate results from 3D printers as you know.