Telescope Leadscrew Advice

[Dr_GMJNParticipant @dr_gmjn]

All, I’m making a fine adjuster for altitude on my DIY Dobsonian telescope mount. I have just built an equatorial platform for it for tracking. I’ve 3D printed a 200mm long lever arm, and now need a leadscrew. I had originally designed it for an M8 fine thread, for which I was going to buy some threaded rod, and then get a tap to thread the corresponding nut.

For the highest magnification eyepiece I’ll ever be using, I’ve calculated that to move a point from one side of the field of view to the other, it would take about 1/3 of a turn of the adjuster. For lower magnifications, it would be around 3/4 of a turn. I need to get a compromise between getting a fine enough adjustment for small targets like stars and for panning around the Moon’s surface at high magnification, but then when trying to find an object just out of view at low magnifications, not wanting to have to turn the wheel loads of times (or disengage the lever arm clutch knob and move manually).

I think 1mm pitch is about right, but ideally would be around 0.75mm pitch to give a slightly finer adjustment. Since I’ve already got some 8mm thrust bearings and hand wheels with a 6mm bore, I’d like to stick with something around M8.

Are there any finer “standard” threads to use, with good availability of taps/dies or threaded rod? The overall leadscrew length needs to be 150mm.

Also, if I do stick with M8 fine, is it better to buy threaded rod, or thread plain bar with a die? The leadscrew nut is nylon, and I’m hoping that I can get zero backlash by using that in conjunction with a metal thread. The precise tolerance on the thread itself is unimportant, as is the straightness of the thread (within reason). I don’t really want to get into custom thread turning bar and nuts on the lathe – I want to use the telescope rather than spend more days making parts for it, plus I’ve yet to come up with a design for azimuth fine adjustment, which is far more difficult.

Thanks.

[DC31kParticipant @dc31k]

Use a differential thread. One standard M8 x 1.25; the other M8 x 1.0 . Have the nut on the coarse pitch side rotatable for coarse adjustment and then keep that nut stationary and rotate the differential thread for an effective pitch of 0.25mm.

The beauty of a differential thread, especially if you would 3DP a nut, is the actual pitch does not matter, only the _difference_ in pitches – and for 3DP a coarse pitch is easier to print (so M24 x 3 and M22 x 2.5 ISO coarse would make a 0.5mm pitch differential).

[JasonBModerator @jasonb]

I use M8 x 0.75 quite a lot on my models so easy enough to get hold of taps and dies

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