Pict

This is a photo of my holography bench. I have very little room so everything was very carefully plotted out to take the least amount of floor space possible. It is a little cramped but I find it functional.

The most important thing a bench can do is isolate the optics from the environment. I did a lot of study into a variety of techniques before settling on the system I use now. In my opinion the most important thing is the stiffness of the table. Since my table will need to move because I am a renter, a great deal of thought was put into this factor.

The Laser Educational and Optical Training program (LEOT) that used to be on-line was very instructive in helping me to settle on some design criterion. Their recommendation was a table that would only deflect 1 wavelength with a 100 lb weight. I used their equation for calculating the stiffness of various table designs.

Where:

P = Force exerted by the point load (in lbs)
L = Length of the panel (in ft)
b = Width of the panel (in ft)
H = Thickness of the pane (in ft)
T = Thickness of the skins (in ft)
E = Young's modulus for the skin material (in lb/ft2)
G = Shear modulus for the core (in lb/ft2)

With these units, the deflection will come out in ft.

(The above equation is directly copied from LEOT.)

I made a simple spread sheet and compared a variety of designs:

Steel Caps with a Aluminum Foam Core
Steel Caps with a Blue Foam Core
Steel Caps with an End Grain Balsa Core
Steel Caps with Honeycomb Core
Solid Steel Plate
Solid Aluminum Plate
Solid Concrete Plate
Solid Granite Plate

I then talked to holographers about their tables. They were all very helpful and gave me lots of insight. For cost and stiffness reasons I decided on a concrete table with a steel top so I could use magnetic mounts. I reverse engineered the tables from successful amateurs holographers and found them to have about 8 wavelengths of deflection for a 100lb weight. From my calculations for a 3' x 5' table I got (in summery):

Design

Weight

Deflection in wavelengths
of 658nm light

8" Granite Plate

1500lb

.3

3.5" Concrete

650lb

8.7

2" Steel Plate

1222lb

6.46

8" Blue Foam with 3/6" Steel Caps

250lb

314

8" End Grain Balsa with 3/16" Steel Caps

340lb

18

4" Honeycomb with 3/16" Steel Caps

255lb

18

8" Honeycomb with 3/16" Steel Caps

280lb

6

After researching honeycomb for sometime I decided I was not going to be able to get a supplier to work with me and the gluing was going to be very challenging. Contact me if you want to discuss this option further as I would love to have a light and stiff table.

Construction

After deciding on a concrete table, I had to make one. I had no concrete experience. I had a friend volunteer to assist and we decided to make the table at his house. Since his house was 40 miles from mine it would be a good chance to prove we could move the table.

In talking with Jonathan he told me his greatest regrets were not pouring the whole table in one pour and not taking the time to get a perfectly flat and smooth finish on his table top. With this in mind I went to Darrin's and we started to work.

When you are getting someone's free advice, and they know more than you, I find it is important to follow it to be polite. Darrin wanted to make the concrete ourselves. Knowing that would eliminate a single pour I agreed. I would recommend renting a trailer with the already full concrete in it. Make sure to order the right amount because you have to return the trailer empty and finding a place to dump concrete would not be very easy for some of us.

There are basically three general stages to making concrete, making the form, installing the reinforcing steel, pouring the concrete.

Making the form was very easy. I had decided the table would be exactly one 2x4 thick and I just made a piece of plywood framed with 2x4 sides. I then coated the form with motor oil. I had decided this table was just light enough that 8 people could pick it up if necessary so I installed concrete bolts coming out the sides to attach 8 handles. This involved drilling holes in the sides of my form. I placed the form on the flattest piece of concrete I could find.

Normally a concrete slab is supported by the ground. Since I was supporting this table by it's corners I wanted to make sure the reinforcing steel was properly placed to avoid a crack. Steel is added to concrete so it can never stretch. Concrete is very strong in compression but very weak in tension. Since my design would be supported in the middle for repairs to the legs, and at the ends in use, I decided I would put in two layers of steel rebar.

I made the rebar structure about 2.75" in height and put in a 4x6 grid top and bottom. This was all wired together with baling wire so it would not move when we poured the concrete. I then rested this on a few pieces of gravel so it would sit in the form at the right height. I also installed all of the concrete bolts for handles.

I was now ready to make concrete. Darrin donated some gravel and sand he had on his property and I only had to buy bags of concrete. We mixed with a hoe in a wheel barrow. When we were done mixing we would pour it in the form and do some rough positioning. Then I would make another load. I don't remember how many times I went through that process but it was quite a few. The important thing is to have the right proportions of ingredients. The gravel, sand and concrete are pretty easy to measure but the water is done by experience. If you go to a job site and ask to run a hoe through their concrete you will know the consistency your are after. It is not very hard once you know.

Once the pour was finished, it was time to make the top surface. This is the part that can't be taught. You take a long piece of metal with a handle on it called a float and you smooth the surface out like frosting a cake. This pushes the gravel under the surface and raises the concrete to the top. It also pulls water to the surface. The more you do this the shiner the surface will be when it dries. If you look at outdoor concrete and look at the concrete in your garage the only difference is in how it was floated.

I laid on the ground and looked very carefully for flatness and for twist. This process took about one hour. There is a point when you don't have enough water to continue. The surface starts getting rougher when you work it. I never got to this point. While concrete dries enough to walk on overnight, I did not want to pick it up for a week.

After a week I put on the handles and we set it on a trailer. We were able to pick it up. Getting it home was not as big of a task as I expected but every move was thought out in advance. I filled a van with pallets and placed the table higher than it would end up in the studio. When we arrived, we slid the table down to my cart and rolled onto the legs. We then inflated the inner tubes and were able to remove the cart.

After 3 months of design and planning the concrete work took about 6 hours and the move took about 2 hours including the one hour drive.

 

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Please free to contact me at: colin@designerinlight.com
Copyright 2002 Colin Kaminski