Question about sand table and magnetic bases

[Arturo]

Hello everyone...

I have a question regarding the use of sand table... I have some magnetic bases that I would like to use... I was wondering if using an mdf+steel board on top of the sand as a tabletop makes any sense or this simply defeats the whole purpose of the sandtable because the sandtable requires the components to be inserted in the sand directly (as in Unterseher's book).

Thanks

[BobH]

I'd personally recommend a steel top table over sand every day of the week. There's nothing that can be done in sand that can't be done just as easily on steel, unless the sand itself is the subject as with a whole bunch of Rudie Berkhout's work. But to answer your question, I'd not recommend putting the top you described in a sand box unless it was anchored to the bottom.
That way, it won't bend and shift with the moving loads on it. Cans full of sand (to hold components) on a steel table would be better than steel bases on a sand table in my opinion.

[erose]

Correct me if I am wrong but the real purpose of the sand was to create a huge mass and thus dampen vibration. The ability to move/position optics was just a bonus. Also, as a designer & exhibit builder I use lots of MDF. I find it very prone to changes in humidity and temperature. I would consider it a very UNstable material.

FYI: Rudie's table in NYC was both sand and plain concrete slab. It was a big L-shaped thing with sand at the end where the art was made. Most of the optics were on steel supports coming off the concrete sections.

[Dinesh]

I'm afraid I don't know what an MDF is, but I did have a 6' x 8' x 15" sand table in the early 90's. I covered it with two metal sheets of about 1" thickness (I had to have two because the local metal shop did not have a 6 x 8 metal sheet).The ease with which I could use mag mounts made it easy to work on the table and change geometries relatively quickly. My problem was that putting heavy optics at one end of the table caused the metal sheet to lift at the other end, as well as warp and twist. Bob's suggestion of anchoring it would have helped, but I didn't think of it at the time. Also, the metal sheet didn't go all the way to the edge of the table, so sand seeped around the edges and covered the surface. Sand underneath a mag mount will destabilise the mag mount and cause motion so I had to wipe down the surface every morning. Sand also got into the optics, so I covered all the small optics with ziplocks (plastic sandwich bags) and the large optics with trash can liners every evening.

By the way, the whole table was a brieze (breeze?) blocks, with two sheets of plywood on them, on which were placed 9 inner tubes and on top of that was the sand box with a couple of tons of sand. However, when one tyre deflated, I realise that I'd put the nipple to inflate facing inside - and was thus inaccesible! I had to shovel a couple of tons of sand out of the box, re-inflate the tyre, then shovel all the sand back!

But, I digress. The point is that you need to take into account the response frequency of the sheet to various effects (trucks running engines nearby etc) as well as frequency modes, against your exposure. If the response is approx 1 - 10Hz, then the metal sheet goes through one entire vibration in 1 to 1/10th second. Any exposure of about 1 - 10 seconds probably won't come out. I believe that a truck with its engine idling has a frequency of about a Hz or so.

[Arturo]

Thanks all for you comments and information It's been really illustrative... I let you know how it all turns out. My biggest problem is that I live in a flat downtown Madrid, meaning (i) some kind of isolation is needed, since there is too much going around (and underground ) at every time, and (ii) I have limit the mass I can load in my table... I'd like to make the holograms in my apartment, not in the one right below me

Thanks a lot.

P.S.: Dinesh, MDF is a type of engineered wood, see http://en.wikipedia.org/wiki/Medium-density_fibreboard. BTW, I DO miss some of your maths!

[Jeffrey Weil]

Hello Everyone,

I made a bunch of crappy tables at first but here is how I made my first good one.

A few breeze blocks were the legs. No carpet or anything, just the blocks, on their side. On top of that I built a well out of plywood and poured a steel reinforced slab 2 or 3 inches thick. When that was dry I put down some inner tubes and built another well on top of those (remember to put a piece of a 2x4 inside the donut hole of the tube, that way if it goes flat you can still get a jack in there to lift up the table to replace the tube).

After that 2 or 3 inch slab was dry I put about 50 blocks on their side mortared together. On top of that I poured a 1 inch non reinforced slab to further bond the blocks together. Once that was dry I glued a piece of steel to the top using the construction adhesive "liquid nails". Fully glued, edge to edge.

I also built a short stool like thing that could take the weight of the table. It allowed me to place it on one side or the other and use a simple car jack to lift up the table to replace tubes.

That was a pretty good table, I got a whole bunch of Chinese mag mounts for about 15 bucks each and I was ready to go. The only downside was the top was plain steel so it had to be kept clean to keep from rusting.

I used to joke that whoever bought my house from me was going to have a pretty hard time removing it.....a few years later I decided to replace it with a "proper" table and I had to remove it with a hammer and chisel. Believe me, that was a solid design!

When I got the newport table I lifted it up from my yard where it had been delivered, rotated it so it was on edge, jumped it over a 6 inch step, moved it through a few doorways in my house, rotated it again and lifted it high so legs could be put underneath and lowered onto the legs. I did that by myself with a wooden crane I built out of 2x6's and winches. But that's another story.

Jeffrey Weil
NorthBeach Holography Inc.

[Jeffrey Weil]

Hello Arturo,

I missed the part where you want to keep the mass down. My table wouldn't work well for you as it weighs a lot. I left the post up for others.

Jeff Weil

[erose]

I made single beam reflections and a variety of diffraction gratings in my 5th floor walk up in NYC. I was on 10th Avenue and big trucks would roll by, especially at night. I had a 10mw HeNe and was using Agfa plates cut small. I did it by having everything bolted tightly to a 5 foot long steel Beam. But I still had to keep the exposures short. The steel beam was an "H" beam and very rigid. optics (Laser & SF), plate holder and objects were clamped to the beam plus lots of hot melt. Do folks still use hot melt glue? I have holograms that when looked at from odd angles have little hot melt webs running thru them.

[Arturo]

erose, thanks, that is encouraging, yes!!!!!

[holomaker]

In my early days i made a table from one 20" bicycle innertube and marble slab 24"x24"(less than 75 lbs.), with the laser was located across the room with the spatial filter, i still have the slab today and use it on my large table when i need an elevated surface ..........