PSN-L Email List Message

Subject: Re: Cover for seismo
From: CapAAVSO@.......
Date: Wed, 11 Jul 2001 08:16:02 EDT

In a message dated 7/10/01 11:53:51 PM GMT Daylight Time, 
dlstevens69@........... writes:

<< Thanks for the building tips. The store where I get the best price on 
plexiglass has many different sizes - anywhere from 2mm and up. Is there any 
advantage to using a thicker pane, such as 1/4" vs 1/8", or could I go as 
thin as say 5mm? >>

Hello Dan,

I should have mentioned that I only covered the pendulum itself and 2 mm was 
the thinnest Home Depot carried and it was plenty thick enough. It is 
important to only cover the pendulum because this gives you access to the 
leveling screws so you can tweak up the period to say ~20 seconds. If you 
have to remove a big box every time you want to tweak up the period it will 
drive you crazy. Furthermore, making the cover out of Plexiglas allows you to 
see the pendulum and center it as you adjust the leveling screws to adjust 
its period 

Here's how to make a box that covers only the pendulum: 

1) cut two sides and one end for a rectangular box that will cover the 
pendulum inside the leveling screws. Glue the three pieces at two corners and 
slide this around the pendulum from the front (the mass end). The box should 
extend back beyond the lower pivot point so it meets the other end of the box 
which is fastened to the back of pendulum's frame. The three sides will not 
be fastened to the back end, just pushed up against it. Now you have four 
sides of the box in place.

2) make the top of the box in two pieces and wide enough so you can glue a 
piece of the 1/4 X 1/4 Balsa wood on either side of both pieces to overlap 
the sides and hold the two top pieces in place like a lid. The back piece of 
the top should extend out from the back end of the box to almost reach the 
pendulum support cable. The other piece of the top should have a slot about 1 
cm wide cut in it to clear the support cable and cover the rest of the top of 
the box.

Now you have a removable rectangular box that adequately protects the 
pendulum from drafts but yet allows access to the leveling screws so you can 
adjust the period
while watching the pendulum to get it centered where you want it. The back 
end of the box is fastened to the back of the pendulum frame, the other three 
sides slide up against the back end from the front (the mass end) and the two 
piece lid sets on top with a slot to clear the support cable. All very nice, 
and you can show it to your friends without disturbing it by removing a big 
box that covers the whole business. 

Have fun and enjoy,

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Larry Cochrane <cochrane@..............>