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How to make a Helical Crayford Focuser

How to make a Helical Crayford Focuser

This is a simple article I wrote a while ago and rather than re-edit it completely I just added an update

Somewhere on the web I found something called a Helical Crayford.
It looked so easy to build and as I wanted a very low profile focuser I decided to build one although these pictures show a long eyepiece tube for clarity.
The best part is that with a selection of different length tubes you can always get an eyepiece to focus irrespective of its foibles.
I was amazed how easy it was and required very few tools. I used a cheap imported drill press, hacksaw and file so it should be possible even for those with little or no fabrication skills.


The tolerances are looser and easier to set up than for a conventional crayford and far less work is required but it still produces a precision focusser.
I scrounged a bunch of old broken 3.5" floppy drives and dismantled them to get the small bearings. They are not always the same size so you often have to keep looking for similar sizes.

 

f3L

Update – the floppy drive bearings went rusty from dew so I now have these for my new binocular helical crayfords
http://www.vxb.com/page/bearings/PROD/kit680

I started off by finding some thick walled aluminium tube with a 1.25" bore for the focuser tube.
This gave a dimension for the hole in the 6mm aluminium plate.
I found a hole saw/cutter that would be the closest fit to this tube with some clearance available.

The marking out was the most difficult part of the project.
First a centre pop mark, then scribe a circle the size of the external dia of the tube, then another circle the outside dia of the hole cutter.
Divide the circles into 6 equal segments and scribe all 6 lines across the diameter and extend them to the edge of the plate.
The bearings need to be separated by 120 degrees with the pressure screw a further 120 degrees so choose the points that divide the circle into thirds and Lay the bearings on two of the lines so the outer edge of the bearing protrudes into the hole that the eyepiece tube will slide through.
Now scribe a circle that will fall through the middle of the bearing shaft, this is to ensure that the two bearings are equally placed around the eyepiece tube.

layout

Drill the large hole for the eyepiece tube first using the hole saw whilst the table is still horizontal as there is less chance of tearing into the bearing holes this way. (don't ask me how I know)
Set the table of the drilling machine to anything from 2 - 5 degrees off horizontal.
The higher the number of degrees the greater the movement when turning. I set mine at 2.5 degrees and for me that feels fine. Go down to say 1 degree for an ultra fine feed, The coarse feed is just pull or push with the twist to fine focus.
Now sight up one of the lines that a bearing is on with the centre of the drill and its support shaft.
The idea is to get the center of the bearing shaft exactly on line with the center of the eyepiece tube axis then drill the two holes suitable for the bolts that hold the bearings.
I used stainless 4mm cap screws with nylock nuts.
Depending upon how thick you can get some spacers or washers to raise the lower edge of the bearing from the flat plate
you may have to drill a shallow clearance hole for that edge of the bearing to sit into, I did it by grinding an old drill flat and forming a cutting edge so it just took a skim from around the hole deep enough for the tilt of the bearing.
Drill and tap the hole for the Teflon pressure pad.
I then cut a recess on the inside of the plate so I could use a "T" shaped piece of Teflon to give a wider surface area although I do not think its necessary.
Assemble the bearings on their shafts and through the plate using spacers so the outer ring of the bearing does not contact the plate or heads of the nuts and bolts.
Add the tube and apply pressure to the Teflon pad, twist the tube, check for sufficient clearance and if required file some off the inside of the hole.

And here is the finished focuser with an extra top ring for grip.
With the polish I think it looks pretty good and more to the point it works very well and smooth.
The wooden mounting block was made by taping a sheet of sandpaper to the OTA and sanding in the curve.

I think another improvement could be sandwiching a layer of felt between a thin sheet of aluminium on the top of the plate with clearance for the bearings but a tight fit for the eyepiece tube to keep dirt and dust out of the scope.
Good luck.

finishedL