crayford type focuser for telescope

[boatmadman]

My son has an 8" telescope and wants me to make one of these focusing devices - ideas shown here:

http://www.atmsite.org/contrib/Sapp/Focuser/CrayFocus.htm

One thing that isn't shown is the mating faces between the telescope and focuser. Clearly the focuser needs to have a curve machined into it at a 4" radius. Now, getting the workpiece 4" away from the spindle center isn't a problem, BUT, I am looking for advice on the best way to machine the curve.

I wondered if it would be possible using a boring head, but am concerned about flexing at that radius. If a boring head is viable, what size would be recommended, bearing in mind tooling cost is limited.

Thanks for any help.

Ian

[BillTodd]

If you can put up with an ellipse rather than a perfect radius, you should be able to use smaller radius tool (boring bar/fly cutter) at an angle to the work piece. (e.g at 90' the cutter will be flat , at 0' it will cut it's own radius but in between it will cut an ellipse with a major 'radius' close to your 4")

Bill

http://www.practicalmachinist.com/vb/general/milling-radius-tilting-mill-229362/

[dickda1]

Being an old (quite old) telescope maker, I have always wondered about cutting threads for a focuser.  Always heard that a rolled or ground thread was the only one that would be smooth enough to avoid excessive wear.  Please post your experience with the new focuser - I am interested.
-Dick

[kvom]

A flycutter could also machine the radius as could an endmill with the part mounted on a rotab.

[johnf345]

There are two ways of doing this.
Firstly the workpiece is mounted in a vice mount on an angle plate mounted on a rotary table.
The distance from the center of the table to the paart is the radius of the cut.
It is possible to use an endmill in the vertical head if the length of the cut is short.
Better than this is to tilt the head and use a ball ended endmill.

The alternative method is to mount the workpiece in the vice mounted on an angle plate.
The cutting is done with asuitable cutter in a boring head.
Set the diameter of the boring head to the final diameter required.
This can be done by fitting two fences the required distance apart on the milling table and then moving the table
or the adjustment on the boring head till it fits between the two fences.

This sort of part has be made accurately if its going to look the part.

[Old Bill]

I agree that machining the curve would be the cleaner, more professional way to do this, but if you need a simple solution, you could mount two lengths of round stock on the edges of the base that are parallel to the centerline of the tube. You would have to be sure to mount these equidistant from centerline of the focuser tube to prevent canting off center. With some figuring, the base will just touch at the apex of the circle while the round stock provides the 'feet' for mounting. A bit of foam rubber could be used to 'seal' light and dust from coming in from the bottom of the base.