Consultation: stereo microscope stand for photomacrography

[Carmen]

Why not 100x100 instead of 50x100, the bigger 100x100 would give you stability in both directions, also better against torsion. . .

Thank you Lothar, the technical advice is greatly appreciated! English is easier for me than German, but google translate serves well for basic german, given short phrases.

I concurr that a 100x100mm profile should be stronger, less prone to vibration, etc... although it may necesitate a heavier base plate. But I still prefer a centered slot to facilitate mounting arca compatible clamps directly to the very center of the column.

I identified this heavy 120x120mm aluminum profile by Item as a possible candidate that seems to comply with the above considerations:




QUESTION 1: Given all of the above, overall ¿what do you think of this 120x120mm profile?

On the issue of fastening the column to the base plate, it occured to me that this profile's 8 holes at the ends may serve as strong anchor points.

QUESTION 2: ¿Are bolts, between the base plate and threaded anchor points in the bottom of the column, the strongest method?

QUESTION 3: Would one reasonably expect normal steel bolts in pre-threaded holes to provide a stronger connection than "self tapping" bolts in un-threaded holes?

QUESTION 4: Given a connection of 8 anchoring bolts to base plate, would re-enforcing gussets on the sides of the column still be adviseable? (i.e L-shaped profiles bolted to the top of base plate and the column?)

[lothman]

Q1: Ok the larger the better

Q2: 8 bolts are strong enough, but also could use angles on the outside but IMO that doesn't look good and at 120x120 there is no need. Your base plate should also be thick enough to support the stiffness of a 120x120 column. I would say at least 4 cm thick.

Q3: I would prefer cutting threads because assembly is easier than with self tapping screws, but both solution will give much more strength then needed (regarding the thread). With steel bolts in alumina I would say the thread depth should 2 to 3-times bolt diameter.
But with self tapping screws this "cavity thing" is recommended, or you take longer screws and drill the holes in the alumina profile about 1 cm depth at a slightly larger diameter than the screw. The deeper start of the thread distributes the force from the tap to a wider area therefore less prone to "wobbling". (A trick often used in high precision machine design).

Q4: no, 120x120 is already a stable oversized solution. Pay attention that the mating surfaces of base plate and column are machined flat and smooth


Since you have a large area in you profile you could think of a piece of steel connected via a cable to your camera arm acting as a counterweight what makes moving the camera easier/safer.

[ChrisR]

It would be interesting to see (laser interferometry?) where the movement actually occurs in a construction like Volker's. It would be easy to go bigger than necessary on one part, which is already less of a problem than some other part. The connection to the camera always seems to be quite feeble in comparison.

I think Volker did very well to get his parts at his price:
Aluminum Profiles and mounting parts for stand(new) 100 Euros

A couple of 90º brackets can cost a lot.

I haven't done an empirical analysis, but isolation from the ground seems to be highest priority, and the set-up must not "ring".
Increasing mass keeps the movements down. Metal structures are rather good at transmitting shocks (think of the toy "cradle" of steel balls).
For that, lead pellets are very good.

[lothman]

The connection to the camera always seems to be quite feeble in compariison.

That's a strong Point. To mount the SLR on the bayonet and additional to bracket it on the tripod thread should help here. By the way I would like to have EFC on my Pentax camera, then lot's of problems would have been solved.

A couple of 90º brackets can cost a lot.

But if you think in terms of interferometry they don't help either. If you fight against microns deflection then the breathing of the connection comes more and more important. Then such things like very smooth (less settling) and flat (good load distribution) surfaces and also the idea with the cavity helps more. The best way to avoid breathing in the connection is to lead the force of the screws to the outer rim of the column creating a pretension. This could be done by a cavity in the column or in the base plate. With brackets it is much more difficult to create a pretension, so I think they won't help much.

[ChrisR]

On brackets, I'd agree - not quite what I meant!
I was thinking of Volker's large amount of aluminium extrusions and parts generally, for so little money.

[Choronzon]

The problem as I see it from a single post design, is that it isn't a closed loop, but merely a cantilevered design. You can make the single post as heavy as you want, but because the camera is going to have to project out some distance from it, there will always be some diving board effect no matter how robust.
There is a reason most production systems have at least a dual post design, which utilize the rectangle. The camera mount can then be positioned close to the post and not sticking far out into space. Remember, in a vertical setup, you will need room to place a specimen with lots of working room. A single post doesn't do that, unless you hang the camera very far away from it.

[Carmen]

. . . There is a reason most production systems have at least a dual post design, which utilize the rectangle. The camera mount can then be positioned close to the post and not sticking far out into space. Remember, in a vertical setup, you will need room to place a specimen with lots of working room. A single post doesn't do that, unless you hang the camera very far away from it.

thank you Choronzon! A dual column sounds brilliant! My initial concern is cost-benefit ratio of a dual column relative to cost-benefit ratio of a single column. How much more cost? how much additional benefit?

[Carmen]

Thank you again Lothar! The technical advice is greatly appreciated!

On the issue of the base plate, my initial investigation seems to suggest that an aluminum plate, adequately machined, may cost a bit more than a 45x270 profile, but it may be worthwhile, given more contact with column, more strength, more attractive, etc...

QUESTION 1: given that the 120x120mm column weighs more than 5kg, which thickness of base plate would you advise? in other words, is there some optimum proportion of the mass of the base plate relative to the mass of the column?

QUESTION 2: What other criteria would you advise regarding the size and mass of the base plate?

QUESTION 3: I'd like to place the back side of the column flush with the back side of the base plate. Will this present any problems?

QUESTION 4: Any thoughts on cost-to-benefit ratio of a dual colunm stand, compared to cost-to-benefit ratio of a single column stand? See Choronzon's post and photo.

Your idea of placing the counterweight within the column is brilliant! I had a similar idea of a counterweight, but hanging down the back of the column.

P.S. I agree that the angles on the outside are not attractive. Black color is preferred to minimize stray light, and IMO is attractive.

[ChrisR]

Well I'm not convinced. Happy to become so, but it doesn't seem likely at the moment .

First you isolate your rig from the ground (cycle inner tubes, Sorbothane, whatever), so only very low frequencies can be transmitted.
You make sure your rig is stiff enough that it can't ring or bounce at any frequency it could get from the ground.
Then you let all the rig-mounted vibration sources die away. You have no diver.

Then it makes no difference whether you have a cantilevered design or not, nothing can move on your rig relative to another part of the rig.


If you were trying to "hold firm" something like a noisy shutter, it would be a different matter.

[Choronzon]

You make sure your rig is stiff enough that it can't ring or bounce at any frequency it could get from the ground.

And a single column filled with lead shot is better at this than a triangulated one?

[ChrisR]

And a single column filled with lead shot is better at this than a triangulated one?

It depends how low you've pushed the "natural" frequency of the table supports.
If you've pushed it low enough, then as long as the rig is stiff enough then making it stiffer won't make any difference.
The natural frequency of the rig should be "significantly" higher than that of the isolators it rests on to ensure that energy coupling does not take place.
Adding mass does reduce any movement due to energy transfer, and lead dampens any vibrations.

Adding rig stiffness does make it easier to get the natural frequencies apart, but once that's achieved it makes no further difference. You're on the "flat part" of the curve, though I'd struggle to find the mathematics behind it all right now, I admit.

(If you want "triangulated" - that's a tripod with reversed column !)

[Choronzon]

The rig can never be as stiff when it's supported a longer distance from the post as opposed to directly bolted to it with little or no extension.

[lothman]

On the issue of the base plate, my initial investigation seems to suggest that an aluminum plate, adequately machined, may cost a bit more than a 45x270 profile, but it may be worthwhile, given more contact with column, more strength, more attractive, etc...

yes, you can get alumina plate machined on both sides, temperature treated so that they stay flat. Also positioning bolts in solid material is easier than in the profile. The additional mass helps against vibrations.

QUESTION 1: given that the 120x120mm column weighs more than 5kg, which thickness of base plate would you advise? in other words, is there some optimum proportion of the mass of the base plate relative to the mass of the column?

QUESTION 2: What other criteria would you advise regarding the size and mass of the base plate?

That's again not only a question of mass also but of stiffness. Your 120x120 column is rather stiff and if it would be bolted to a let's say 5mm base plate, then the baseplate would flex and the column will do the same together with the base plate. So what makes something stiff? E-Modulus and thickness (second moment of inertia). A high and ripped cast rack could have less weight but being stiffer than a thinner but solid plate.

So thicker and heavier is always better. In order to squeeze out the abilities of a 120mm column I would day the baseplate should be at least 50mm thick and 200mm wide. Just a guess, I did not calculate. IMO it makes sense that the second moment of inertia of the base plate and the of the column are in the same range.

QUESTION 3: I'd like to place the back side of the column flush with the back side of the base plate. Will this present any problems?

No such a connection will weaken your setup. On the face end you can support all your profile with the 8 screws, mounting on the backside gives you only one half of the profile of the column to carry load from the base plate to the column.
It recommend to use this lovely distributed 8 screws on the face end put the column vertical on the base plate. Or you use no base plate at all and go for a horizontal setup placing camera and object table on the same column/rail.

QUESTION 4: Any thoughts on cost-to-benefit ratio of a dual colunm stand, compared to cost-to-benefit ratio of a single column stand? See Choronzon's post and photo.

Alumina profile is rather cheap so material cost would not bother me at all if I would spend all the effort to build a custom stand. And I agree to 100% to Choronzon's suggestion that a two post design will always be stiffer then a single post design. The benefit for stability is great with a two post design but on the other hand the access to the working area is limited. So it's your choice ;-)

[lothman]

Then it makes no difference whether you have a cantilevered design or not, nothing can move on your rig relative to another part of the rig.

I doubt, a cantilever design will swing more like a bell than a tripod design would do, so every external/internal vibration will have a greater effect.

The question is how stiff do we need to be? What is the weakest point?

[ChrisR]

Very obviously, a support from more than one direction can be stiffer.
As above, Second Moment of Area is well understood.
I'm trying to convey that stiffness is not the primary aim. Lack of movement is the aim.
The Most Stiff arrangement, bolted to the ground where a railway passes, would be the worst you could have.

Stiffness may be the most familiar property and intuitively compelling to work towards, but mass, damping, and separation from movement sources overwhelm it.

a cantilever design will swing more like a bell than a tripod design would do, so every external/internal vibration will have a greater effect.

Your bell doesn't ring at all unless you pass energy to it.

so every external/internal vibration will have a greater effect.

Remove the internal vibrations. Time is your friend. EFSC helps a lot!
You can effectively remove all of those relatively easily.

External sources:

I doubt

Rather too many years ago, I was taken though the maths of energy transfer between systems with "significantly" separated Natural Frequencies. I remember the results were illuminating, though I couldn't reproduce the workings now.
I urge you to look for it. There's some explanation of part of it here.

A twopod or tripod design may be what you prefer, but note that photos are shown on this site at the limits of resolution, without using them.