Introduction
Counterweights for an equatorial mount can be quite expensive to buy, especially by mail order! Fortunately my friend Roger Brownless recycles lead, mostly into diving weights, and offered to make me a counterweight if I supplied him with a suitable mould. He suggested a large tin can.
Making the mould
A quick scavenge through the waste bins outside the canteen at work yielded a good size can that had contained tomatoes. The can is about 155mm diameter, just a few millimetres more than the EQ-6 counterweights. The other raw materials are a length of 22mm copper pipe and part of a wire coathanger.
The first step was to cut the tin can down to a suitable height. This height will determine the weight, so I chose to go just a little over the height of the standard EQ-6 counterweights. I didn’t want anything too unmanageable. I removed the can’s paper wrapper and marked the height with a thick indelible marker.
I cut down the can with tin snips. The easiest way to do this was to make several vertical cuts, then use short sideways cuts to remove sections. The final trimming was done using the corrugations of the can as a guide. I left a margin to allow the edges to be folded over as described below. Note that the cut edges of the can are razor sharp – you will cut yourself, so have some sticking plasters ready.
Next I bent the coathanger wire to a slightly larger radius than the can and cut it to length so it would just fit inside the can.
Using pliers I folded the edge of the can down and bent it over the wire. I then tried to tidy up the edges with careful use of a hammer. The result is not perfect, but is a lot safer than the cut edge of the tin can.
Finally I cut a piece of 22mm pipe and attached it to the base of the tin can. 22mm pipe is a loose fit on the EQ-6 counterweight shaft – for other mounts a different size pipe will be required. The length of the pipe is slightly more than the height of my cut down tin can. A small amount of excess length should fit into the recess in a standard counterweight without a problem.
I attached the pipe with a nut and bolt and two large washers. These will be removed after pouring the lead and the small hole enlarged to the pipe diameter. The nut and bolt were done up very tight – I didn’t want molten lead seeping into the pipe.
Pouring the lead
Pouring the whole counterweight in one go would not be practical, so Roger used two burners: one to melt the lead in a ladle, and one to keep the lead in the mould molten. A steel plate was used to spread the heat under the mould.
The weight required nearly three ladles of molten lead. The picture shows the first ladle going in. After filling the mould to the brim, the gas rings were turned off and everything left to cool.
Here is the finished weight, after cleaning it up and opening out the hole in the base. It weighs around 19 to 20 lbs, according to my bathroom scales (which are not very accurate).
A brief test fitting of the weight on the mount showed that it was quite a loose fit, which might cause a problem if the weight shifts a little in use. The inside diameter of the 22mm copper pipe is about 20mm, while the EQ-6 counterweight shaft is 18mm diameter. I cut a rectangle of 1mm thick vinyl flooring to just fit inside the tube, and inserted it with its smooth side inwards. The weight now slides smoothly along the EQ-6 shaft with no apparent wobble.
I haven’t fitted any sort of clamp to the home made counterweight, as I always use it sandwiched between the two original weights.