Labophot 2 focus slippage

ctron · Post by **ctron** » Thu Feb 25, 2016 10:15 am

Well, wouldn't you know, last night's "first light" since replacement of fine focus gear didn't go too well. It actually did go "ok", but I noticed that sometimes the focus either wouldn't advance for a while or it would jump ahead. In the final stack, this showed as a distinct area of blur since it jumped over that. So, time for more troubleshooting. It started doing this again today, but when I look at the gears carefully after removing the focus knob covers, I can't see that there's any hesitation... seems more likely in the stage? area. I also notice it more when I change directions of course focus, takes a bit longer than it should to "catch up" with the reversed direction. Sounds like slippage somewhere. Anything to look for welcome. Just to note that when I replaced the fine gear, I didn't glue or set screw it to the rod as I felt the friction was enough to keep it in place (I actually had to use the edge of the vice to move it up the rod, so I really don't think that's the problem). Thanks.

ChrisR · Post by **ChrisR** » Thu Feb 25, 2016 10:44 am

Could gravity be involved?

ctron · Post by **ctron** » Thu Feb 25, 2016 10:55 am

In what way, Chris? Right now, the microscope is stage is tipped 90 degrees on its back, so the stage moves horizontally. It shows this behavior whether a light load is on it or no load. Tried adjusting torque but no difference.

Pau · Post by **Pau** » Thu Feb 25, 2016 11:04 am

Do you use the focus block mounted horizontally? If so, be aware that they are designed to work vertically, i.e. loaded with mass whose weight is part of the design. To use it horizontally some force provided by a rubber band or spring could be convenient.

Some backlash when reversing is to be expected in any mechanical system.

ctron · Post by **ctron** » Thu Feb 25, 2016 11:09 am

Pau wrote:Do you use the focus block mounted horizontally? If so, be aware that they are designed to work vertically, i.e. loaded with mass whose weight is part of the design. To use it horizontally some force provided by a rubber band or spring could be convenient.

Some backlash when reversing is to be expected in any mechanical system.

Yes, it's horizontal and that explanation makes perfect sense. Will add a spring/ rubber band and give it a go. Thanks!

Chris S. · Post by **Chris S.** » Thu Feb 25, 2016 1:19 pm

As ChrisR and Pau have said, this is exactly how a focus block behaves when not "loaded." The spring or rubber band doesn't have to be very powerful, though--surprisingly little force is needed to keep the gears meshed.

Here is how I mounted a spring on one of my blocks. Surgical tubing also works very well.

--Chris S

ctron · Post by **ctron** » Thu Feb 25, 2016 1:45 pm

Thanks, guys! The problem was definitely lack of tension since not using the stage vertically. Latest stack is completing and looks quite impressive as it finishes. For now, I'll just use the rubber band but may secure it in the future like you have it Chris (thanks for sharing).

zzffnn · Post by **zzffnn** » Fri Feb 26, 2016 7:29 am

You may have known and tried this:
Labophot 2's focus knob has tightness control built in. Gently turn the inner ring of the coarse focus knob and you can tighten up or loosen its focus movement.

ctron · Post by **ctron** » Fri Feb 26, 2016 7:41 am

zzffnn wrote:You may have known and tried this:
Labophot 2's focus knob has tightness control built in. Gently turn the inner ring of the coarse focus knob and you can tighten up or loosen its focus movement.

I think you're talking about the "torque" adjustment; yes, I tried that but the jumpiness was still there until using the rubber band as suggested, thanks though.