A friend bought a very nice completed model of 1 1/2" to the ft Alchine traction engine which he was delighted with, the only blot was that the break drum wouldn't work, he found out that the nut that tightens the break drum was not threaded, the bolt was nicely made with a butress thread but for some reason or other the builder nadn't completed the job and made a tap and tapped the nut.
So I offered to do the job for him.
First thing it pays to measure up the job and sketch up how the threads will fit together as you have to think inside out and back to front
.
From this I worked out the sizes for the tap, then by hand ground up the tool for the job this is just like a parting tool but you have to give the leading edge enough rake to clear the thread helix.
This pic also shows the bolt, and nut.
I turned a couple of blank taps up, then set the change gears of my lathe to cut 16 tpi.
My lathe is not that friendly when it comes to thread cutting, even though its got a split but its got lots of back lash so I find the best way is to keep the nut engauged and power the sadle back and two winding the cut off and the end of each pass.
The biggest problem was from the tool wanting to dive under the thin rod as the thread was cut I got round this by slightly setting the tool above centre.
Next I got to try out my new power feed on my mill.
The threaded tap was set up in the spin indexer with the tail stop and supported with a jack to stop it flexing, and three flutes cut.
By hand I filed a small amount of radial releaf, then heated them up to cherry red and quenched them in water, gave them a wire brushing and a quick hone with a stone.
This is what happend to my first attemp of cutting a thread.
For the second attempt I tempered the remaining tap and increased the size of the tapping drill.
This time it worked like a dream, I'm going to bush the square nut with the nut that I've made.
Stew
