Re: vibrating table


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Posted by bpfink on July 20, 1999 at 08:09:56:

In Reply to: vibrating table posted by M. Markham on May 08, 1999 at 11:40:32:

This is in 2 parts covering 2 areas.

#1 There is a very fine system on the market for larger molds or even building foundation amounts of concrete.

New at about $350 US. I got mine a few years ago from Northern Tool and Equipment (prviously Northern Hydraulics) (phone 1 800 533-5545)

I assume this is in a volume over what you want but the principle can work the same way. It is simply a long 'vibrator' shaft or a flexible shaft that has an off center end encased in the end tube and it sends a frequency shock into the concrete when submerged so the fines (differing sized components making up the mix) start to very slightly move and consequently release the entrapped air bubbles in the process.

With concrete there is a danger of OVER vibrating as the mix can start to over separate and the heavier fines sink to the bottom. It only takes a few short seconds in several different locations of the mix to do it right.

With plaster investment I find it is NOT that effective to use this bigger model as the mix has fines nearly of the same size and does not transfer the shock as well, AND the waxes have a tendency to also break apart if there are any weaker joints (like between sprues and vents to the body of the piece.

SO I don't recommend this model BUT

I mention this for two reasons as this can also be done with a smaller more delicate flexible shaft made from one of your old hanging drill shafts or cars lying in the back yard.

Take the speedometer cable out and tie one end to a small motor and then on the other end weld a small off center blob about the size of a small marble. Put a metal cover over it (a small 1/2 inch pipe with the end shut) so the whole marble can still turn and be the vibrator.

#2 If you are talking about bigger investments than about 25# I would definetly go for the table system.

The best one I ever had was a simple upside down plywood box nailed, screwed and glued real well with 4 legs made from the springs of car tappet valves.

(about 3 inches high , strong, and about 1 1/2 inch in diameter)

(and No, not from my back yard)

The plywood box housed an old 1/3 HP washing machine motor with the steel metal pulley edge cut off on one side of it so it was the vibrating mechanism.

The investments were simply cast on the top of the upside down box floor and vibrated with short blasts from the switch located on the end of the cord (in case of a plaster spill you don't want the switch to be mounted on the box itself).

It worked very well and would safely and with good control handle investments up to about 300 #.

These days I don't vibrate anymore as they are usually much too big and both the gravity and pre surface application methods have started to handle that better.

These involve putting a sprayed or flecked pre mix onto the waxes with a greater amount of retarder in that covering so the later investment pours have a chance to combine and cover before the surface mix really hardens. It is also because the vibrating is too rough on the whole investment when it is getting to be in the 400# to 2000# ranges and the plastic sheet container has no bottom so the whole could blow it's way out if shaken too much.

(I like my investments just poured and not shaken as it also tends to bruise the gin).

As for the mention here of 'no bottom'...

I pour the bottom seal with the same investment (and no retarder) before starting the whole process, let it harden and then start the segmented pours. This way I can control the layers and do about 200# (or 20 gallons) per mix pour. It is still soft as the next pour goes in but solidifies shortly thereafter. The next pour does the same and there may be from 3 to 10 pours or more to make up the whole investment.

This, by the way, is the same as how they cast those great grain silos you see in the midwest. 20 to 40 stories high and cast with a single upward sliding form that is resting on the very cement they had just cast a few days before. In their case it is a slow and continuous pour taking apparently weeks to get to the top. It is also the reason it sometimes fails and the scaffold peels off the still insufficiently uncured cement and can kill a whole shift of workers falling, falling, falling. Hate to even think about it.

So do be careful, even though this may be just a little plaster jewelry or sculpture investment... it's a jungle in here.


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