Gallery, Projects and General > How to's
Another way to make spark plugs
gbritnell:
So as not to hijack Chuck's thread I'll start another on the same subject, making miniature spark plugs.
My plugs differ in that they have a ground strap on them. I have used the surface gap type but have had better luck with this style. The first thing I do is take a piece of steel large enough to accommodate the hex on the plug I'm making.
I then turn it and thread it leaving a small portion on the end who's outside diameter is the root diameter of the thread I'm cutting. The thread could be chased or cut with a die.
The piece is then cut off.
The plug blank is then mounted into a threaded bushing for the next machining steps.
The first step is to center drill the part.
The next step is to drill the smaller diameter down to within about .04 from the end of the plug. This will show up later.
Now I drill out the upper diameter.
I have made a counterboring tool out of drill rod and hardened it. It has 3 cutting flutes on it but could be made with 2 depending on the size of the plug you are making. I then counterbore the first drilled hole down to about .035 from the end of the plug.
Now I turn a small collar on the body of the plug. I make it about .008 wall thickness.
The bushing is transferred to dividing head where I do the remaining cuts on the plug body. First is the hex.
Next I clamp the plug body by the hex and mill away the material from the collar that I left on the bottom of the plug. You can see where the counterbore left the material at the end of the plug. I only side mill the opening. After this is done the ground strap is snipped with a pair of cutters, first the vertical piece and then the horizontal piece. I leave enough on the horizontal piece to file it just to the edge of where my center electrode will be.
The next operation is the insulator. I use a large enough diameter piece of Teflon so that it's rigid while I'm turning it. I first drill the hole for the electrode. I drill it the same size as my electrode wire because the Teflon will spring back a little after drilling making the hole fairly snug. I then drill a short way into the insulator with a tap drill for a 3-48 thread. The hole is then tapped. Depending on the size of your plug these sizes might be different. These plugs are 1/4-32 thread with a 5/16 hex.
When turning the Teflon make the diameters size for size with the hole that you drilled. The Teflon doesn't like to be forced when it's this small. Now insert the insulator into the plug body. If it's a little long it won't hurt, you can take a hobby knife and trim it off. With the insulator pushed into the plug mark where the .008 collar is on the plug body and put the piece of Teflon back into the lathe and put a little angular notch in it just a little below your mark. Now cut the insulator off from the stock. Reinsert it into the plug body and chuck up the plug. Now turn your cutoff tool at about 30 degrees and rotate the chuck BACKWARDS with the tool against the collar slowly rolling it into the notch in the Teflon. Keep checking the insulator. When it won't rotate any more, quit crimping.
Now you can turn a small taper onto the insulator.
The final operation is to make up the center electrode with the threaded cap. In my case I have a 0-80 thread on the outside but you could make any configuration you like. The inside has the 3-48 thread to match the insulator. The cap is also drilled to accept the electrode wire. Once the cap is made the wire is soft soldered in place. If you find that the center hole in the insulator has tightened up from the crimping operation just chase it with the drill to clean it out.
I used to just press the center electrode into the Teflon but when I made some snap on plug wires they would have a tendency to pull the electrodes out so that is why I made them this way. It also has a side benefit that when the electrode wears you can unscrew it, chuck it up and take a light cut off the cap and screw it back in. Another caution: when you screw the cap into the insulator be gentle, it's only Teflon. It will thread in very nicely, it just doesn't like being overtightened . The last two pictures are of the finished plug.
gbritnell:
I should have added, if anyone would like a drawing of this plug let me know and I'll post it for you.
George D. Britnell
dsquire:
George
Very nice work. You make it look so easy. I like the way that you crimp them together, clean, simple and efficient. I'm filing this one away for future use. Thanks George. :ddb: :ddb:
Cheers :beer:
Don
sbwhart:
Great work George well shown.
:clap: :clap: :clap: :clap:
Thanks for sharing
Stew
NickG:
Yes, very nice method and lovely result george.
Nick
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