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Rotary table/dividing head calibration with theodolite

valensdiesel

Plastic
Joined
Apr 3, 2020
Hi all,

Found an interesting method for calibrating a rotating axis. This post is mostly for information in case someone else is searching for this idea, but I would appreciate any advice regarding an equipment maintenance question at the end.

This method basically boils down to "bolt a theodlite onto your rotary table, take a fixed reference point in the distance (church steeple, etc), measure actual table angles relative to fixed point to determine actual rotation vs indicated rotation". I think some discussion was made in a previous thread (Calibrating and Compensating a Rotary Table) of a similar method, but the setup as-described required an autocollimating theodolite. This link to a youtube video shows the method requiring "any old theodolite", subject to your desired accuracy/resolution limits: Dividing Head Accuracy: Unconventional Testing. I thought I would give it a shot on my well-loved Troyke 9" and see how well it has fared over time. In order to do that, I scored a secondhand Hilger and Watts ST-200 which, according to a trade pub I found, is accurate to 1 second of arc and can be interpolated down to ~0.2 seconds. The unit I picked up is in near-mint condition and was university surplus, so I'm not overly concerned that the absolute accuracy has changed drastically (though its of course possible to have it recalibrated), and anyway the R-9 is only supposed to be good to 1 minute of arc.
IMG_1755.jpg
My new-to-me theodolite does have one slight issue, which is that it has hysteresis/sluggish return in the horizontal fine adjustment. The micrometer screw moves in and out smoothly, but when backing it out, the spring-loaded return seems to be encountering some friction which feels like moving through sticky grease. Since the micrometer acts as a positive stop in this arrangement, I believe it is still possible to obtain accurate readings, but it is a bit of a pain having to add manual preload to encourage the return force in the event you overshoot your target. I pulled out the return spring and it is covered in some rather tacky green grease.
IMG_1758.jpg
Unfortunately, as with most Hilger and Watts items, manuals or other publications are somewhat thin on the ground, so there is of course no recommended lubricant for the system. Was this originally grease, or is it oil that has gelled over time? Who knows. I did find a translation of a russian document that actually describes the parameters of watch oil used in a (different) theodolite (Link, PDF is too large to upload), and I think there is probably a Moebius product (I'm not affiliated at all) that is pretty similar: Synta-Frigo-Lube 9030.
So, the question is two-fold:

1. Would you try to clean and reassemble with new lubricant, or live with it as-is? I could try to have it professionally serviced, but I'm not confident that anyone will know anything about servicing an antique theodolite (serial # marks it as 1957-1958 vintage: Theodolite Serial Numbers), and anyway this is more of a project of curiosity rather than necessity, and

2. If you were to clean and re-lube, what lubricant would you choose, and how might you go about cleaning the mechanism? I was thinking a volatile solvent like diethyl ether to sweep the grease out and then after it evaporates, insert watch oil/whatever lube is appropriate. My concern here is will I be able to effectively lubricate the entire mechanism?

Alternatively, if anyone has a manual or other info, or direct experience with this or similar theodolites, then your advice and expertise would be most welcome.

Finally, and this may be of most interest to others in the forum, the eyepieces for my theodolite look very similar to the eyepiece on my Hilger and Watts autocollimator (no surprise there). The AC looks like it has a widefield type eyepiece while the theodolite does not, but I did search around a little bit and found a company that would probably supply the appropriate eyepieces as they specialize in eyepieces for theodolites (Link, not affiliated). I haven't reached out to them because I have good eyepieces for all my equipment, but for everyone else's sake, I'll post the link in case there is need. You might even be able to get one of the OE-style 90-degree eyepieces which are so loved by the AC-using community.

Thanks for any replies, and hopefully at least you find the youtube video interesting!
 

neanderthal mach

Cast Iron
Joined
Dec 18, 2008
Location
princeton b.c.
I can't offer much that that might help, but fwiw. I thought that YouTube video showing the use of that surveying theodolite was extremely clever when I watched it shortly after it was first uploaded. However I asked a question that afaik was never properly answered in the comments by the video poster. I've done enough shooting with high power rifle scopes to know a distant target can actually be not exactly where your optically aiming due to mirage effects. And the more distant the target the more those effects are going to matter especially when doing accuracy checks like this. Better might be doing the same checks at night with a distant light as the target to lessen the mirage caused by temperature gradients in the air. But even then it's still possible to have inaccuracy's due to changes in cooling or continued heat dissipation, open water, road surfaces etc. So it's something to at least be aware of. Over short distance such as optically checking a surface plate, then those effects are probably so minor they can be ignored. Over the distances he was showing in that video, they are going to change or affect your measurements. With a bolt action rifle I'll usually choose to use a distant light at night and center it in the barrel bore, then zero the scope cross hair to the same light. That method is accurate enough to get me within 1-2 minute of angle of zero at 100 yrds. No where even close enough when trying to check to seconds obviously. But a high accuracy theodolite vastly improves my amateur attempts IF you can compensate or remove that mirage problem.

As far as that grease? A whole lot of instruments use what's called damping or dampening grease. A Google search will pull up some details about it. Strangely it's use is seldom mentioned or even hinted at by the manufacturers. It's quite common in rifle scopes, some camera lenses, binoculars etc. as it gives a feel that the parts assembly is much tighter and higher precision than they are and helps to ensure any adjustments are and stay at what your adjusting them to. Those turrets on rifle scopes as a good example. I obviously can't say for sure that's what H&W used on your theodolite, but it's one definite possibility.

Your Troyke table would be built to and trustworthy for it's + - accuracy deviations when it was new. I have a great deal of suspicion about both my Vertex rotary table and dividing head and there "certificates" of accuracy given there price and what Moore Tools have to say about there efforts just manufacturing much simpler but very high precision feed screws and nuts. Hard to double check items most aren't set up to properly verify those accuracy claims almost invites cheating by any less than honest manufacturers. I know just enough about this to realize I don't know nearly enough and that there's quite a few members here who know a whole lot more than I do. What I've yet to run across is and since there were so many manufactures of rotary tables and dividing heads over the last maybe 100 years. There must be a fairly simple and quick method they all used to double check and verify there accuracy claims. That's so far an unanswered question I've had for more than a few years.
 

valensdiesel

Plastic
Joined
Apr 3, 2020
I can't offer much that that might help, but fwiw. I thought that YouTube video showing the use of that surveying theodolite was extremely clever when I watched it shortly after it was first uploaded. However I asked a question that afaik was never properly answered in the comments by the video poster. I've done enough shooting with high power rifle scopes to know a distant target can actually be not exactly where your optically aiming due to mirage effects. And the more distant the target the more those effects are going to matter especially when doing accuracy checks like this. Better might be doing the same checks at night with a distant light as the target to lessen the mirage caused by temperature gradients in the air. But even then it's still possible to have inaccuracy's due to changes in cooling or continued heat dissipation, open water, road surfaces etc. So it's something to at least be aware of. Over short distance such as optically checking a surface plate, then those effects are probably so minor they can be ignored. Over the distances he was showing in that video, they are going to change or affect your measurements. With a bolt action rifle I'll usually choose to use a distant light at night and center it in the barrel bore, then zero the scope cross hair to the same light. That method is accurate enough to get me within 1-2 minute of angle of zero at 100 yrds. No where even close enough when trying to check to seconds obviously. But a high accuracy theodolite vastly improves my amateur attempts IF you can compensate or remove that mirage problem.

As far as that grease? A whole lot of instruments use what's called damping or dampening grease. A Google search will pull up some details about it. Strangely it's use is seldom mentioned or even hinted at by the manufacturers. It's quite common in rifle scopes, some camera lenses, binoculars etc. as it gives a feel that the parts assembly is much tighter and higher precision than they are and helps to ensure any adjustments are and stay at what your adjusting them to. Those turrets on rifle scopes as a good example. I obviously can't say for sure that's what H&W used on your theodolite, but it's one definite possibility.

Your Troyke table would be built to and trustworthy for it's + - accuracy deviations when it was new. I have a great deal of suspicion about both my Vertex rotary table and dividing head and there "certificates" of accuracy given there price and what Moore Tools have to say about there efforts just manufacturing much simpler but very high precision feed screws and nuts. Hard to double check items most aren't set up to properly verify those accuracy claims almost invites cheating by any less than honest manufacturers. I know just enough about this to realize I don't know nearly enough and that there's quite a few members here who know a whole lot more than I do. What I've yet to run across is and since there were so many manufactures of rotary tables and dividing heads over the last maybe 100 years. There must be a fairly simple and quick method they all used to double check and verify there accuracy claims. That's so far an unanswered question I've had for more than a few years.

Very interesting points. I hadn't considered the effect of refractive index but now that you mention it I can see it being a significant issue. I did and still do think that achieving single-second accuracy with this method is hopeless (unless maybe I could set it up in a linear accelerator beamline where there are a couple miles of climate-controlled straight shot, slim chance of that), which is probably why the typical technique uses the tried-and-true polygon mirror method. However, as johansen points out, such things are not cheap, and for a rotary table not even close to single-second spec'd accuracy, definitely overkill. What really interested me about this method vs. the polygon mirror is that you can take as many measurements around the circle as you like without disturbing the setup, so with patience you could get a significantly larger n for estimating your confidence interval.

I'll do some more reading on damping grease, that seems like a reasonable explanation for what I observe, though a cursory look through the nye lubricants website shows that some such greases are formulated with silica fill. This would seem to me to be the worst thing you could put in a precision mechanism. Do you happen to know how your rifle scope grease is formulated? I suppose that since the actual measurement made by the theodolite doesn't depend on the accuracy of the fine adjust screw but rather on the optical micrometer's intrinsic accuracy at manufacture that this might be okay, but I will confess to having reservations around using anything that looks like lapping paste as a lubricant.
 

John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
Valensdiesel --

If I'm remembering correctly, the Watts second-order theodolite has separate micrometers for the horizontal and vertical circles, not the more common single micrometer that is used for both circles. The Watts is an excellent instrument (second only to the Kern DKM-2 and DKM -2A in my estimation).

I would strongly suggest you clean and lubricate the tangent screws and their return springs, plungers, AND the inside of the plunger guide cylinders. Mineral spirits is a good cleaning solvent for oil or grease that's in good condition, but something stronger is warranted if the existing lube is oxidized.

Spray-can brake or electric-motor cleaner from an auto parts store is good, but don't spray it directly onto your T-lite. Instead, spray some into a small jar, use a cotton swab or small paintbrush to your instrument.

It's now been 40 years since I last used a Watts theodolite, and I don't remember how to read the circles and micrometers, so I can't help there. I do remember that the vertical circle numbering is formatted for "vertical angles", which, in the telescope-direct position, has a nominal value of 000 degree 00 arcminute 00 arcsecond when the telescope line-of-sight (LOS) is horizontal. The vertical circle value increases through a full rotation of the telescope LOS; at 090 00 00 the telescope LOS points up to the zenith, 180 00 00 when the LOS is horizontal but pointing directly opposite the zero direction, 270 00 00 when the LOS points down to the nadir, and 359 59 59 when the LOS points 1 arcsecond down from 000 00 00.

This vertical circle numbering system is relatively "oddball", in that most of the competitive second-order, glass-circle instruments use "zenith angle" numbering on their vertical circles. (I'll note here that Wild's T-3 and T-3A first-order theodolites also use vertical-angle notation, with REALLY ODDBALL system of graduation-and-reading).

Back to your instrument . . . The U S military specification for second-order theodolites required that the instruments be lubricated with grease meeting the MIL-G-23827, which is a low-volatility grease primarily used in aircraft instruments and gears. It can be used inside the telescope and in close proximity optical elements.

The MIL-G-23827 specification has been replaced by MIL-PRF-23827. I'm still using Aeroshell 7, which complies with the older spec, but I don't think it complies with the PRF spec.

My Aeroshell 7 came from a "Fixed Base Operator" (FBO) repair station at a local airport. About 1/4 cup of the grease set me back a sixpack of Dr. Pepper. Ask nicely, and take in a clean jar.

John
 

neanderthal mach

Cast Iron
Joined
Dec 18, 2008
Location
princeton b.c.
Just about all my rifle scopes are Leupold and there not giving out any secrets about the grease brand/type used that I've been able to find so far Valendiesl. But anything with silica in it I really can't see being used anywhere around fairly expensive optics. I have had some good luck calling a few of the bigger names in the lubrication business for current lubes to substitute for lubrication products no longer produced. They literally have application engineers on staff with cross reference charts to answer questions exactly like this. Mobil, Shell Oil, Chevron might be worth checking to see what they recommend. Shell was/is pretty huge in the UK where your H&W theodolite was made. So they just might be the best one to check with first.
 

John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
I have now realized that the theodolite characteristics described in my earlier essay apply to the Watts Microptic 2, and I don't have any hands-on experience with an ST-200. I wouldn't be astonished to learn that these different models are very similar, but I can't guarantee such similarity.
 

valensdiesel

Plastic
Joined
Apr 3, 2020
I have now realized that the theodolite characteristics described in my earlier essay apply to the Watts Microptic 2, and I don't have any hands-on experience with an ST-200. I wouldn't be astonished to learn that these different models are very similar, but I can't guarantee such similarity.

You described my instrument perfectly, and in fact, the ST-200 IS the Microptic No. 2. I know that there were several successive model numbers because in my travels I have seen a different Microptic No. 2 with the model number ST-200-3. No idea what the differences are, though.
IMG_1773.jpg
Thanks for your reply. That is a wealth of information and I will see about heading over to the Martin FBO and rounding up some Aeroshell 7. I'll bring gloves and a packaged carry-out spoon in addition to the thank-you gift ;)
 

valensdiesel

Plastic
Joined
Apr 3, 2020
I actually have one of those but am not sure how to best use it. Do you happen to know of any instructions online somewhere, I'd be interested to get the link.

This is my best search so far: use of optical polygon for calibration - Google Search

but maybe there's better terms to look for?

Vermont Photonics has a guide you can download of general methods for autocollimator calibrations. The last one down the line is for rotary tables that describes the setup: Autocollimator Applications

That method assumes you have a perfect polygon mirror, but actually any will do, provided you know the actual angular deviations so you can account for that error. There are lower grade versions all over eBay from laser scanning heads, and there are various methods of calibrating them requiring one (e.g. NIST link, Beamsplitter method) or two autocollimators (e.g. Angle Standard Calibrations). If you already have a "real" polygon though, then you just need an autocollimator and some patience and you're off and running. If you want to be really thorough, you can clock your polygon multiple times around the dividing angle of the polygon to check for cyclical errors that you might miss with only a single setup, which was what I was alluding to above with the theodolite method allowing larger "n".

Hope that helps, or at least gets you started down the rabbit hole.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Thanks, that's helpful. I'm pretty sure my polygon is "real", I rescued it from being trashed when the space research lab I worked in was cleaning house during a "scope of work" change.

I haven't even looked for it in a while, but it's got to be around somewhere. Can't remember if it had a calibration tag, it'll be (well) out of date...
 

John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
Another afterthought: Light lube oil meeting U S specification MIL-L-6085 or its replacement, MIL-PRF-6085, is a low-volatility oil intended primarily for aircraft instruments. It would be worth considering for you theodolite's tangent screw systems.

Radiator Specialty used to sell MIL'L-6085 oil under their Solder Seal or Liquid Wrench brand (I forget which) in 4 ounce dropper-spout cans available in hardware and auto-supply stores. I believe that the 6085 oil was also sold, under a number of different brands, in "Zoom Spout" plastic bottles.

I do have a hazy memory that the 6085 oil would soften some types of plastic and paint.
 

John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
The "dividing head calibration" video mentions a general consistency of the differences of the theodolite direct and reverse horizontal circle values for the various dividing head positions. The presenter -- Alec or Alex, not sure which -- speculates that such consistency is probably due to error in the theodolite's circle-reading optical micrometer.

I suggest that the observed consistency of the reverse-minus-direct values is more probably caused by the telescope line-of-sight not being precisely perpendicular to the theodolite telescope trunnion axis. Such non-perpendicularity is commonly termed "horizontal collimation error".

It's worth noting that horizontal circle measurements are quantified based on the projection of the telescope line of sight onto the plane of the horizontal circle, and therefore increase in magnitude as the line of sight is raised or lowered away from that plane.
 

Conrad Hoffman

Titanium
Joined
May 10, 2009
Location
Canandaigua, NY, USA
Don't worry about silica as a grease filler. It's fumed silica and so fine that you couldn't abrade anything with it in your lifetime. I think it's the filler in Superlube grease and pretty sure it's used in Nye damping greases. Light damping greases are good for some things but probably not precision adjustments. Stick with very light grease or oils.

Hydrophilic Fumed Silica | Cabot Corporation
Our Products - Evonik Industries
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
I have some old Dow Corning "high vacuum" silicone grease that's got fumed silica thickener in it. Just for you guys I did a test with a polished block of aluminum and a bit of paper towel with a dab of the silicone grease on it. Survey says: very slightly abrasive, not a whole lot more than the paper towel itself. Barely a whisper of gray on the white paper towel with about 5 solid minutes rubbing as hard as I could press down. This is on aluminum, so for a hardened (or even not) steel screw it would probably take 50 years of constant use to even polish it. Potential abrasion due to contaminants or dirt/dust ingress would probably be far more significant. I've seen reference to fumed silica particles being in the range of 5-20 NANOmeters, (.005 - .020 μ) so it's extremely fine. It's also a very small portion of the grease - maybe 10-15% at the max.

For reference, a dab of something like Mother's aluminum polish would turn that paper towel pitch black in about 5 seconds and produce a high polish...

k4xfP2v.jpg
 
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valensdiesel

Plastic
Joined
Apr 3, 2020
Very interesting. Thanks for the insight on the lubricants! I've got some aviation instrument oil and aeroshell 7 in hand so I will clean up the theodolite and give those a go and report back regarding their performance.

I'm doing some more thinking about this project as it relates to the above concern re: variable refraction over long distances. I've come up with a "poor-man's autocollimating theodolite" technique based on some of the other posts in the old thread, which I will share the details of once I finish fabbing it up and testing it. I'm like 98% certain that it's going to work quite well, probably better than the "stare into the distance" method I was originally planning after watching the video.
 

John Garner

Titanium
Joined
Sep 1, 2004
Location
south SF Bay area, California
Valensdiesel --

Don't get too hung up on the AUTO aspect of autocollimation (which essentially means "self-collimation"). Instead, concentrate on the COLLIMATION part.

Collimating two reticle-equipped telescopes is performed by positioning them so that their line-of-sight (LOS) are very nearly anti-parallel (pointing in opposite directions). The LOSs do not need to be colinear, just pointing directly into each other.

I'm pretty busy the rest of this week, but would be happy to spend some time discussing this with you. If you're agreeable, contact me by Practical Machinist private message.

John
 
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valensdiesel

Plastic
Joined
Apr 3, 2020
For those of you playing along at home, I was able to resolve the issue with the horizontal adjusting screw, but it turned out to not be sticking in the return mechanism, but rather sticking in the bearing that the theodolite rides on. I took John's advice and used a cotton swab with some solvent (VM&P Naphtha) and allowed capillary action to draw it down into the pivot bearing area while holding it on its side. I then worked the thing a bit around to work the solvent in, rinse and repeat. Did that a bunch until it seemed like it was spinning much easier, then I re-lubed with the same method. Drip a little lube in using that natural wicking action, spin her around a few times, repeat. I used Brayco 885 (aviation instrument oil), there was no way I was getting Aeroshell 7 in there, and even if I could, I judge that that is not the action where dampling is desirable. After all, that seemed to be what was causing the problem.

More to come soon...
 








 
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