I had a problem last week that may be of some interest here. I was modifying an exhibit to make it more break resistant. These are the parts involved.
The handle is held to the end of the aluminum part on the left by the long set screw. A new design required me to make a new aluminum part threaded internally. It wasn't till I had the exhibit apart that I found that the screw was threaded metric 12 mm by 1.75. I didn't have a tap, the exhibit is popular so no time to order one and my lathe a southbend heavy 10 only speaks inches. An article in Live Steam Magazine (December 1978) detailed methods of using additional change gears to approximate special threads. For this case I added the ratio 20/22 to the gear train and set the quick change gears to cut 16 threads per inch.
The ratio was made using 40 and a 44 tooth change gears added to the gear train:
The gear train then cut 16 x 40/44 = 14.5454 threads per inch. This gives a pitch of 0.06875 in or 1.75625 mm. This gives an error of about 1 part in 467.
A test thread was cut on the exterior of the oversize stock to check the pitch.
A comparison to the screw
Closeup view
This should be plenty good enough for holding a handle on.
An internal threading tool was forged from drill rod, filed to shape, hardened and tempered and set up for cutting the thread.
I usually cut internal threads using an upside down tool on the back side of the bore as it involves less setup for me.
A hand crank was used to turn the spindle since the the half nuts had to remain closed for the entire operation and there was little extra room inside of bore to withdraw the tool. The thread dial won't tell you where to reengage the half nuts for each pass when using special ratios like this one.
For short threads, like this one, hand cranking the spindle is fairly quick taking maybe an hour longer than if the metric tap had been available. The exhibit is now back in use.
I hope this has been useful.
Dave
