Hi all - in my free time, I've been getting into machine rebuilding. None of my paying jobs are likely to require an autocollimator but I'm starting to think it would be a nifty toy to aid in machine rebuilding and checking some of my longer straightedges / granite plates. But I certainly can't justify buying new, especially when I can have a calibration company out to check and lap my granite plates for way less than the cost of a new autocollimator.
But I'm always a bit cautious about buying used metrology gear, especially if I don't have a trusted "standard" to test it. For the folks who have a lot of experience with these things, what have you noticed? Are your autocollimators on a regular calibration schedule? How do you insure it's working properly? What are the possible sources of error? Seems to me like the optical train could get knocked out of alignment due to damage (e.g. dropping!) but aside from that, the wear would be in the adjusting screw, resulting in slight errors on the measured deviation.
No matter how cheap it is, it doesn't do me much good if I can't trust it. On the other hand, maybe there's not much to go wrong. What are your thoughts / opinions regarding buying one used?
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Optical level collimator I would check with a peg test before use. you center between 2 targets and measure elevation difference, then move closer to one target ( farther from other target) and see if elevation difference changed. reticle screws sometimes needing fine adjust especially after a large temperature change is not unusual. (below freezing and back to room temp)
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autocollimator (and most optical instruments) there is straightness of focus test using near, middle distance and far target. you measure elevation difference in the 3 targets and then rotate autocollimator 180 and see if 3 targets measure different elevation differences. it is extremely rare to need to adjust straightness of focus. usually only done at factory when instrument first made
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many optical instruments use dampening grease to make a slight resistance to focus and eliminate loose rattling of parts. if instrument points
down you dont want gravity causing lenses to move or focus to change by itself. if dampening grease is dried out it can be too stiff to adjust focus where force required to change focus might move instrument. this is rare but cleaning and reapplying dampening grease on many decade old instruments that are too stiff to adjust is not unusual. NYE makes a dampening grease kit of various stiffness so you can pick and choose what feels best
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some instruments that were over tightened might have some warpage or damage. for example a optical level with a tapered solid bronze bearing can have bore out of round from leveling screws being way over tightened. lapping tapered bearing requires disassemble and is time consuming. newer instrument often use a precision ball bearing instead. some parts are replaced rather than attempt repair
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i have seen someone unscrew a 2 speed fine adjust azimuth screw all the way out and the loose floating pin fall into the instrument interior rattling around. I had to disassemble and reassemble instrument and do a full calibration on it (takes many hours). new person was curious to see how it worked and unscrewed the 2 speed fine adjust screw. hard to describe but annoying to have to "repair" instrument cause somebody was curious to see how it works
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how often to check calibration depends of tolerances of work being done. some calibration takes 10 minutes and some can take all day. just saying sometimes calibration can take longer than expected to do. usually need to decide on a reasonable tolerance and see if instrument is in a reasonable tolerance....... somebody saying instrument is calibrated to 1/10 the reticle crosshair width is usually not going to mention if it will repeat the same the next day...... just saying often you can refocus and get more than that difference in readings just refocusing
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adjusting reticle or crosshair screws need rechecking later as sometimes it will creep or change after many hours, hard to describe