Sinar Goniometer Stage, Replicating? Also, Delrin applicability question?greenspun.com : LUSENET : Large format photography : One Thread
Hi there folks. I'm currently machining what I call a "MiniNar" for my personal use, which is very similar to a Sinar P2/X in terms of the way the movements are put together, but on a smaller scale with 2/3 the movements of the real camera. The material used is 6061-T6 aluminum with 60 degree dovetails cut via m42 tooling. Simple brass racks and delrin pinion gears are used, 32 pitch. The movements are "self-arresting" by use of gibs captured by end-screws. The final product will be hard anodized with NITUFF to 0.005" through NiMet Industries in South Bend.
My question is involving the tilt mechanism on the sinar. I know that Sinar uses what is commonly known as a Goniometer Stage for their tilt mechanism. Machining fairly precise linear dovetail slides for me is an "easy" affair - their manufacture is straightforward. However, I'm really at a loss as to how to manufacture a Goniometer Stage efficiently.
The method that I'm currently planning to use is bar-stock with screw-mounts drilled into them. The bar-stock will then be attached to the headstock of a lathe, and some custom made dovetail forming tools will be use to form first the radius of curvature of the top edge, then the female dovetails. The same method will then be used to cut the same radius of curvature on the top of the stage with the corresponding male dovetail cuts.
Has any of you ever made a goniometer stage? Any experienced machinists out there? Will my method work? I've worked it through with some other machinists and they seem to agree that this is the only way to do it without having strange and dangerous tooling whipping around on a mill. I'm always looking for hints and/or tips.
Also, does anyone know how the drive mechanisms work on a goniometer stage? Is it done via rack and pinion, or by lead/ball screws?
Also, another alternative for me is to use Delrin Homopolymer to ease in the creation of the goniometer stage. It's significantly easier to machine, so I wouldn't have to make lots of new custom tooling to make my cuts (6061-T6 smears like nobody's business. If anyone has an economical source for 6013 or 7075 barstock, I'd love to hear about it). The questions I have is how well does it respond to wear applications against itself?
-- edward kang (firstname.lastname@example.org), May 27, 2001
I applaud your ambition. I assume the self-arresting gibs are of the law-abiding variety. I had a similar set of circumstances befuddling me, so I took up photography instead. What kind of film/developer are you going to use with this contraption? :) Good luck!
-- david o'connor (email@example.com), May 27, 2001.
The female radiused dovetails will be a nightmare to calculate the tool size for, won't they? The rotation of the tool or job to give the goni radius will enlarge the dovetail channel to a size greter than the tool.
A goniometer stage simply gives a geared rotation about the axis of its radius of curvature doesn't it? I don't see any advantage to swinging the lens about a fixed but arbitrary point in space. Why not make life simple and just move the pivot point to the centre of the lensboard? You could then use a planetary gear to reduce the tilting motion.
-- Pete Andrews (firstname.lastname@example.org), May 29, 2001.
> The female radiused dovetails will be a nightmare to calculate the > tool size for, won't they? The rotation of the tool or job to give > the goni radius will enlarge the dovetail channel to a size greater > than the tool.
If one were to machine the dovetails on a vertical mill by mounting the billet on an indexer, then yes, it would have many problems.
However, I proposed machining the dovetails with a cutter parallel to the axis of rotation on a lathe. Therefore, the dovetails are cut from the _side_ and not the top. There will be no enlarging of the dovetail radius.
> I don't see any advantage to swinging the lens about a fixed but > arbitrary point in space. Why not make life simple and just move the > pivot point to the centre of the lensboard? You could then use a\ > planetary gear to reduce the tilting motion.
Well, that's a good point. But I guess my original purpose is to make a "scaled down" version of a sinar.
Maybe I should just use a bogen pan-tilt head mounted upside down instead of any of this other stuff! :)
-- edward kang (email@example.com), May 30, 2001.
Edward, If you have a lot of time on your hands, why not? Consider making the larger female stage as an assembly rather than one piece, with a plate on each side that has a radiused groove (inside) that matches the goooves in the smaller one piece stationary block (outside). If I were doing it I would mount the segments on a faceplate and plunge a thread cutting tool to an appropriate depth so that a bent .187 nylon rod will push in, say, .080. You are creating grooves on the face, not on the O.D. Repeat for each segment without changing the position of the tool and all the pieces will have the same radius. The picture is this: two opposing grooves on each side of the stage in which you cram a .187 nylon rod (or Delrin or Teflon). When you calculate the fit, allow for some shims under one of the side plates on the larger assembled stage. This will allow you to take up the slack as it wears. And you will also have to stabilize the rod so that it doesn't slide out. It's all slop; no precision required.
If you actually choose to do this, I would be very interested in hearing about the results.
-- Bruce Wehman (firstname.lastname@example.org), June 09, 2001.
I'm currently investigating a three-piece solution like the one you mention, except I've currently got the offer for some free CNC work so I think my workload will be significantly reduced! Thanks for the tips. I'll let you know if it works out!
I wonder if CNC cut curves will be precise enough...
-- edward kang (email@example.com), June 14, 2001.
I asked our workshop people how they go about making goiniometer stages and they said they don't, they buy them. Admittedly this is making instrumentation for state of the art angle-resolved spectroscopy, but they're not afraid of tricky work in general.
That said, when I described the requirements for a view camera standard, my workshop wizard reckoned it was the sort of thing our CNC mill can do in its sleep. Just mount a lump of metal and cut away all the parts that don't look like a goiniometer. The only hard part is getting the design right, and all the holes drilled and tapped *before* you hard-anodise it :-)
If you want to buy, or take a hybrid approach, these people do good work at low prices: www.standa.lt
-- Struan Gray (firstname.lastname@example.org), June 15, 2001.