Sinar Goniometer Stage, Replicating? Also, Delrin applicability question?

[edward_kang]

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.

 

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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.

 

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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.

 

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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.

 

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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?

 

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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?

 

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-ed

[david_oconnor1]

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!

[pete_andrews]

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.<br>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.

[edward_kang]

> 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.

 

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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.

 

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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.

 

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> 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.

 

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Well, that's a good point. But I guess my original purpose is to make

a "scaled down" version of a sinar.

 

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Maybe I should just use a bogen pan-tilt head mounted upside down

instead of any of this other stuff! :)

[bruce_wehman3]

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.

 

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If you actually choose to do this, I would be very interested in

hearing about the results.

 

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Regards,

Bruce

[edward_kang]

Bruce,

 

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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!

 

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I wonder if CNC cut curves will be precise enough...

[struan_gray]

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.

 

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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 :-)

 

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If you want to buy, or take a hybrid approach, these people do

good work at low prices: www.standa.lt