Used Autocollimator Risks? How Often to Calibrate?
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    Default Used Autocollimator Risks? How Often to Calibrate?

    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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    Quote Originally Posted by TMS8C8 View Post
    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?
    .
    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)
    .
    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
    .
    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
    .
    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
    .
    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
    .
    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
    .
    adjusting reticle or crosshair screws need rechecking later as sometimes it will creep or change after many hours, hard to describe

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    Quote Originally Posted by TMS8C8 View Post
    But I'm always a bit cautious about buying used metrology gear, especially if I don't have a trusted "standard" to test it.
    That's what you should be prepared to do. HAVE a standard.

    IOW, make that a part of your search when scouting for used-but-good.

    A special mirror called an optical 'wedge' serves for my Davidson D600 "differential" autocollimator stable.

    It has a known difference between its two surfaces.

    Look first for maker info and manuals, books and articles and online how-to's, etc. THEN pick what goods you can get a decent collection of so you have the standard and wotever else it expects to work with - not just the one item.

    If I had it all to do over again? I'd skip the "visible light" goods and go directly to laser. Laser tech has moved further and faster then also gone less-costly, which helps find new markets at far higher unit volume.

    Ergo, "obsoleted" goods come into the marked in greater choice and volume, and can still be more than good enough for a patient user... because...

    "Some" of the new goods are displacing old goods off the back of saving EXPENSIVE expert labour by requiring less training and being faster to use.

    Not all are actually improving resolution or accuracy.

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    if you every used a ultra high precision spirit level like 0.02mm per meter sensitivity you would quick see how a little dirt, dust and even the heat of your hand can effect things.
    .
    once using a precision laser level with a room heater running near by cause outside door was open and building was below freezing. just turning heater off and rechecking laser level readings on large machine sections readings changed 1/4" per 20 feet easily. mirage effects or wavy air can easily distort optical readings. just saying temperature changes from air and touching stuff can easily change measured readings 10 to 1000 times more than calibration tolerance
    .
    at 100 foot distances a 10 arc second wide reticle crosshair looking through instrument the crosshair is about .060" wide, if instrument has a 4 arc second wide reticle its .024" wide at 100 feet and much harder to see the crosshair cause its so thin...... just saying at 100 foot just refocusing you can easily get a .010" change in readings, or close door so air temp more even and readings easily change over 0.100" per 100 feet...... thats what i mean by reasonable repeatable tolerance
    .
    usually its a joke when somebody says instrument is calibrated to 1/10 a crosshair width, they will say thats the average of 10 readings not mentioning readings had a spread of a full crosshair width (or more), its like saying you can measure to .001" with a hand held tape measure and obviously at 100 feet just pulling on tape measure you can stretch tape measure easily over .010" just pulling on it
    .
    its why most optical instruments are 20 to 40x magnification. obviously if using a instrument of higher magnification you would see air shimmer or varying readings often while you are looking through the scope. i have also seen outside building steel columns change elevation .030" over a 6 hour period from heat of the sun warming up the outside walls of a building. very slow moving targets often you dont realize they are moving targets
    .
    i have used laser levels where 200lb man walking on steel sole plates designed to hold 20 ton machine and floor sole plates would go up and down .0001" or more just person walking or moving. concrete floor can easily got down over .001" or .010" if 200lb man or 5000lb fork truck is moving around on the floor. many machines need releveling cause assembling tons more parts on to machine it sinks or settles lower in spots than the rest, like being on a trampoline
    .
    i have seen straight steel rails go wavy 10x past tolerance just from sunlight shining through a skylight window. just saying calibration repeatable tolerance and repeatable next day instrument use tolerance can be a lot


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