Thursday, February 21, 2008

Beam inserts

Feb 18th – Boston – Medellin dreaming up how to thread the beams.

It’s Monday and the day started very early. To catch the 5:30 am flight to Miami that then pulls into Medellin at about 2, I need to be up by 3:30 and leave home by 4. Today I made it to airport by 4:13, a hair before the large early morning international crowd. This weekend was relatively productive. I wanted to get more work stuff done but ended up doing more home stuff, spent time with Diana which was much needed, catch-up with the mail and friends that where visiting town. I am happy that I managed to get almost all the pieces of equipment that I came here to get - ballscrews, spindle and metering tools. I am still waiting for a communication cable to a new displacement gauge and vibration measurement system. I am hoping to get those during my next visit.

The next things on my mind that I will concentrating on this week;
Electrical diagram for the machine.
Sourcing of enclosure components.
Hook up electronics-controller-motors-screws and get stuff to move.
Build the y,x and x axis.
Talking about the axis, I have this idea to place threaded inserts into the concrete. For inserts, I picked extra long brass inserts (the type used in plastics). These inserts have a thread length-to-bolt diameter of over 2, which will guarantee plenty of grabbing length along for the screw also, since we place the insert from the inside. The insert is in compression against the inside wall of the steel beam. I am also considering testing a combination of an epoxy based cement with or with out regular concrete. The idea is to use the epoxy-based cement along the insets and let regular cement fill in the rest of the space. This will accomplish several things - for one it will reduce cost, since the epoxy base cement is a lot more expensive. It also has better strength and damping properties. Also, the combination of these two masses may have a beneficial affect on the structure, reducing the likelihood of resonance. There is also the possibility to further improve the structure by shifting the neutral axis of the concrete with that of the steel beam. This is accomplished by inserting a filling of different density in an asymmetrical location. In this particular case, I am thinking of using a 1 inch diameter piece off wood. It will also reduce the weight of the structure of a bit. Now I am sure you can get fancy with shape, material and location of this insert but we will keep it simple for now.There are other things to take into consideration. I plan to first pre-drill the beam as straight as possible, with a through hole for the screw, then place the insert inside the beam and thread it through with the screw and hold it in place. Once all the inserts are in place but not locked down, I can double-check their alignment with a straight edge. Actually I will not tight them down completely because this action will certainly move them, causing misalignment. Remember we want them as straight as possible so we can later have more place with the entire rail. Since the through hole on the rail is 7mm, the M6 screws give us half a millimeter place on each side. The more scattered the insert the less clearance there will be. Now, and other idea is to hold the screw in place with something like silicone or hot glue - this will certainly hold them in place but will not be permanent.

I would then process to turn the beam with the insert side down and fill in the first layer of epoxy cement, let that set and proceed with regular cement.

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