Building the Minimag

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madjackghengis:
Hi John, I'm really enjoying this mag build, I am a longtime use of magnetos, but the building of one of model size has not been something I've contemplated before, so this is affirming my notions of what is important, and how to do it most easily and get the best effect. For those who wish information on magnetos in general, I am well versed with such of most types, and would gladly provide general magneto information, so as not to fill this log up with them. All in all, I am quite pleased with how this build is going, and the quality of the components, the engineering which has been done, and of course, the excellent quality of the machinist who is doing the build. I'm glad to see the great care exercized in getting everything right, and simple. :beer: cheers, mad jack

Bogstandard:
Thanks Jack.

Basically, todays offering is finishing off the main lathe work that is required. A few of the finishes look rough as old boots, but in fact they are very fine indeed. I think it is something to do with the lighting.

Carrying on from the last post, the spacer that sits inside the cam needs to made.

The first job was to drill and ream it out to 8mm.

The OD was then turned until the cam sleeve just fitted onto it with a nice sliding fit.

I actually deviated from the plans with this bit.

I lined up the cam sleeve with the end of the spacer and spotted thru the grub screw hole. This gave me a mark so that I could put in a small recess into the spacer.
The reason. When a grub screw is tightened, unless it has a soft end, it will throw up a burr. In this situation, where the cam sleeve has to rotate around the spacer, that burr will jam everything up and most probably cause problems when taking the whole shaft system apart. This groove will allow the burr to be below the running surfaces and so it won't cause a problem.

After this groove was done, the spacer was parted off and brought down to EXACTLY the same length as the larger bearing spacer made previously.

This is what it will look like when fitted inside the main block. The outer races of the bearings will be sitting against the larger spacer that is already stuck inside. So both the inner and outer races have the same length spacers between them.

This is it pulled apart.

The next job was to make up a 5/16" x 32 TPI ME thread half nut (sitting on top of the bearing) from the hex material supplied.

The final job of the day was to make the main 8mm diameter spindle from the ground stock that came in the kit.

First it was skimmed down to plan length, followed by tapping a 5mm thread into one end.

The other end was taken down to size for the projected sprocket or gear shaft and a matching thread for the nut put on, all to plan dimensions.

What the finished article looks like.

Actually there is nothing that was done today that should hold fears for anyone, except maybe having to single point threads if you don't have the correct taps and dies.

Also a little extra care is required to make sure you get dimensions spot on, rather than near enough.

To start to bring this build to a conclusion, the main block has to go back onto my mill and have a few holes drilled, the items shown in this picture made up into the contact breaker setup and be fitted into the block. Then just a dust cover for the rotor making, and it should then be ready for final assembly.

I will have to see how I feel tomorrow to see if it will get done this weekend.

Bogs

John Stevenson:
John,
When you do these post you need to start two.

In this case one called "Building the Minimag"
and one called "Questions about building the Minimag"

This way if everyone behaves the main post is more of a blog with posts running sequentially and the other post stops the detractions.

In which case this post is in the wrong one :doh:

John S.

Bogstandard:
Many thanks for that idea John.

I doubt if ever I will get to build something like this again, with it's unique conditions, unless some other enterprising person wants me to try out their pre production build instructions, and of course, one of their kits.

As it is, I think the members have understood what the restrictions have been, and have really done their best to keep it fairly 'clean', I have no complaints at all.
On other sites, I reckon this post would be up to about 20 pages by now, despite the pleas, as I have done here.

They are a great bunch of chaps on here, with good willpower.

Hopefully in a couple of days, if they still want to, they can let the cat out of the bag.

John

Bogstandard:
I am now into the finishing off stages.
Just a few holes to drill in the main block and make the points system. The holes are coordinate drilled from the dimensions shown on the plans and the points system is built up from the precise instructions as well, so I won't go into super detail unless someone asks about it.

Three holes are drilled in the top face, but I am showing this one especially.

The hole requires two operations, I went down first with a 2.5mm drill to give a recess in the bottom, it can be a fair amount deeper as it is only to give clearance, and then followed it down with a 6mm end mill to the specified depth. This forms the flat bottom hole that is required for correct fixing. See C-o-C below.

I hope this explains what I am on about. A very similar operation to fitting a bursting disc, or core plug as some people call them.

The fourth hole is drilled thru the side to allow access to the cam grub screw, to allow it to be adjusted.
Be very careful to drill the correct side. I nearly did the wrong one, but luckily caught it before making the mistake. It makes adjusting rather difficult if drilled in the other side.

At this stage, the screw for the main shaft was fitted and the rotor Loctited onto the shaft as far as it could go down.

A piece of supplied exact to size ferrule is cut to length and fitted into the centre hole. This hole is in fact reamed, and the ferrule should have been an interferance fit, but it wasn't, just a tight push. So uncle Loctite came to the rescue and fixed it.

At this stage three things had been completed. The first was to fit the ferrule in the centre hole flush with the top. The second was to tap the far hole out to 3mm. In fact, again belt and braces, I fitted a helicoil in there, to me it makes the tightening of the screw a lot more rigid. For the third bit, I had fitted the fixed point as shown in the above sketch.

You will also notice a brown stick thing poking up out of the ferrule. That is in fact a piece of Tufnol type rod, that when cut to the correct length will follow the cam and push up on the contact spring to make the points break, and so fire the stored energy in the coil.

The points system was then made up to instructions from a bit of spring steel strip with two holes drilled thru it (no problems with that, just good tapping lube and a slow speed), and a couple of insulators, plus of course a screw and solder tag.
I will just mention, in my supplied kit, if you are careful with the cutting, there was enough materials supplied to make up a replacement points part, just in case you buggered something up. But no spare tungsten points, so be careful with those.

The operating pin isn't finished just yet, as I still have a couple of jobs to do that requires the minimag to be stripped down. The first is the optional back cover, but the other is one of my own.
If you notice the upstands, I have carried on with the coil mounting screw holes and brought them out to the back.
I have a fetish about cables vibrating and swinging about, maybe fracturing. So I am going to use those two holes to hold some home made cable clamps, in keeping with the overall look.

So other than those, the Minimag is finished. It just needs to be timed to get the maximum spark out of it. That will be done later.

The back view, minus the cover plate.

So in the next few days, I will be making these extra bits, plus an operating stand to put it thru it's paces.

Overall I have really enjoyed making this little out of the ordinary piece of equipment. The materials supplied to me were excellent, the build instructions and plans have been modded in places, and that info has been passed onto Julian in the hope that he will take notice of them and modify accordingly.

So would I recommend this to a total beginner, no. But on the other hand, if you have made an engine for this to fit onto, then I see no problems at all. Just keep things as tight as you can.

Now open to questions and comments.

John

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