The Wimshurst machine project (or my great device as I like to call it) came about as I needed something to make at my metalwork night class that I go to to get access to the lathes and mill since I am not lucky enough to own these myself (yet). The project has a fair bit of machining in it allowing me to keep practicing my skills on the machine tools. I am also able to make a lot of it in my shed by improvising with the tools I do have.
Part two is now available here:
A Wimshurst machine is a device for generating static electricity. Basically you use it to make sparks. Big sparks! Also I just love how they look. They have a very old fashioned scientific look to them and they look fantastic when they are running due to their main feature, two large contra-rotating discs with foil sectors on them. What is also fascinating is the device is totally mechanical. You need no batteries or power supplies. It is hand cranked and will generate electricity seemingly out of thin air!
I decided to make my machine fairly large (I like BIG sparks) so I chose a disc size of 500mm. Initially I had two glass discs made up this size. Once I got them though I quickly realised glass was not a good idea!
1. Glass is very heavy.
2. There is no easy way to attach the hubs to the center of the discs.
3. Glass is somewhat hygroscopic. It will absorb moisture from the air.
The third point is rather important. To successfully generate static electricity you need dry air. Any moisture around will limit the amount of charge you can build up.
So instead of glass I went and got two sheets of clear acrylic 5mm thick and large enough to make 500mm discs from. The glass discs won’t be wasted however as I will need something to sit the machine on when it is finished and I will use the glass to make a small table.
When I was at the plastics place I found a length of 50mm diameter nylon rod lying on the floor. I picked it up and asked how much it would be and the chap said it was an off cut and I could have that for free. That turned out to be the perfect thing to make the hubs from.
I needed to turn my square acrylic sheet into circular discs. First I marked the center of each sheet then I drilled a 6mm hole thorough each one. This hole is where the axle will run through on the finished machine and it allows me to fit the sheet onto my circle cutting jig.
To cut the circles I use a jig I made. Basically it is a flat board with a bolt through the middle of it acting as a pivot point. The sheet is put over this board with the pivot protruding up through the center of it. I then use a plunge router with a cutting bit that I attached to another flat piece of wood. At one end is attached the router. Along the board are various holes. Each hole is a certain radius away from the cutting tool. You select the correct hole, insert that over the pivot and then using the router you can cut an accurate circle to just the diameter you require.
One I had the two discs cut out I was able to begin on the supports that will hold the discs in position. I decided to make these from thick plastic sheet. A nice, cheap source of such sheet is plastic chopping board. It is easy to work with and nice and flat.
To accurately cut the board I again turned to my router with which I could machine the edges of the plastic to be nice and square. I first made a thin wooden template the shape of the supports I wanted to make.
Next I traced around the template onto the chopping board. I then cut around the template outline with a jig saw. This only needs to be done roughly but you want to me close to the edge so when you are routing the edges of the board there isn’t too much plastic to cut away. When routing everything the cutting bit cuts turns into plastic saw dust and you end up with the stuff everywhere so it is a good idea to try to minimise this.
I screwed the template to the cut out chopping board so that the plastic extends past the template by a few mm all around. Next I made another jig for my router. This time I attached the router from underneath the board (I used scrap acrylic sheet as it is nice and smooth) so that the cutting bit protrudes up. I used a bit that has on it’s tip a small bearing. This is intended to be used as a guide when routing along edges. What I did was set the height of the cutting bit so that the bearing would run along the edge of my wooden template and the cutting tool would only cut the plastic underneath it.
You then start the router and simply (but carefully) rung the template around the cutting bit so that the bearing rolls along the side of the template. The plastic will then be cut by the bit to exactly the same shape as the template. I also used the template to accurately position the holes I was going to drill in the supports. The holes are mainly decorative except for the bottom middle hole which will contain the bearings that the crank shaft will run though.
Once the supports were cut out I again used the router and a larger bit to cut the large holes in the supports. As it happens the bit I used is the same diameter as the bearing I am using on the machine. Both the hubs and the crank shaft will contain bearings to ensure smooth running. I use skateboard bearings as they are cheap and easy to get and they will take a 8mm shaft.
With the supports mostly done (I still need to make cross bracing pieces and drill holes to allow fixing to the base of the machine) I started on the hubs. These I machined from the nylon rod at my night class where I have access to a lathe and mill. The hubs are simply short sections of the rod with the ends faced smooth and a large hole 22mm diameter bored ALMOST all the way through. One side of the hub has a 9mm hole that the 8mm shaft can pass through. The larger hole diameter on the other side is such that the bearings can slide down into the hub. Each hub contains two bearings. The first is slide down into the hole, next a compression spring is added and finally the last bearing. Three equally spaces holes were drilled using a rotary head on the mill and the holes were tapped for a 5mm screw. Matching holes were drilled in each of the acrylic discs so the disc can be screwed to the hub. When the disc is screwed to the hub (using countersunk screws so the heads are flush with the surface) the spring is compressed forcing the two bearings apart. One against the inside of the hub and the other against the disc. This allows maximum separation of the bearings on the shaft providing better support.There is also a belt groove machined onto the outside of each hub which will be how the hubs are driven from the crank shaft.
Once the hubs were done I started on the foil sectors. I decided on 20 sectors per disc and they are sized to give me a good length spark as well as a reasonable current. The sectors are made from foil tape. I used tape that comes with a backing attached that you peel off. This allowed me to make a cardboard template of the sector shape and trace around this on the paper backed foil. Then I was able to cut out each sector using scissors.
The foil sectors must be accurately positioned on the discs. Since I am using clear acrylic I simply drew up a paper guide sheet that the clear disc could sit over. Lines and marks on the guide sheet allow me to easily position the sectors. One important thing to note is that you attach the sectors to the correct side of the disc. The sectors are on the same side as the hubs. Perhaps one of the most fun parts of the project is removing the brown backing paper from the acrylic. For some reason I find doing that terribly satisfying!
With all the foil sectors positioned I could then attach the hubs and bearings to each disc. We know have something that is starting to look recognisable as part of the great device!
With the discs finished I turned my attention to the Leyden jars. These are simple capacitors that are used to store the charge generated by the machine and give the sparks it makes more OOMMPPPHHH! I am using square shaped glass jars (that used to contain spaghetti sauce). Each jar has a layer of foil added to the inside and outside of it. The glass jar itself forms the dielectric of the capacitor. Each jar will have a plastic top made from the offcuts of the plastic chopping boards and a round brass door handle on top as a terminal. With the two layers of foil in place I measured the capacitance of the jars and they are both more or less 1nF.
Unfortunately it is now school holidays for the next two weeks meaning no night class and no access to the lathe. I will continue making what I can at home however and I should be able to finish the Leyden jars and start work on the wooden base of the machine and the support stands.
More to follow soon.
Part two is now available here:
http://www.asciimation.co.nz/bb/2008/10/27/building-a-wimshurst-machine-part-2Sorry it took so long!