Grinding a Threading Tool
B&S 55º Worm Thread
I am repairing a shaft which has a 3.775 major diameter x .875 pitch B&S worm thread. To make the threading tool I used a .750 x .750 H.S.S. tool blank and ground it as follows:
Using a small universal vise purchased several years ago from New England Brass on a whim I first set the "tilt" to one half the included angle of the thread or 27.5º . Then I swiveled the tool 15º. This is necessary to produce side clearance on the tool bit.
After grinding to a mostly sharp point I check the angle of the point using a sine bar and dial indicator.
When swinging the 15º angle the 27.5º angle is affected. It was necessary to make a small adjustment in the vise to get the correct included angle.
This is a birds eye view of how the tool fits into the existing thread.
The new thread section is 4.100 long. I will turn off the damaged thread and weld on the new sleeve. The R.H. photo shows the Higbee thread start. I figured that as long as I was fussing with this thing I might as well do it up fancy.
Here is the almost finished project. The only thing left to do, after a trial fit up to be sure that the thread is correct, is to weld the threaded sleeve to the shaft.
This is the frame work for the Ballista that these guys are building. The thing is about the size of a full scale R.R. car truck To learn more about the NERDS go to http://www.the-nerds.org/ This picture shows the framework laying on its side. http://neme-s.org/Punkin_Chunkin_2003.htm shows a photo of the thing as it is in use.
Far left is the winch that the repaired shaft is for. The other two pictures are of the business end of the winch. The bearings were not available but a trial fit seems to show that the new worm thread is a fine fit to the worm wheel. The winch will be used to pull back the arms of the Ballista.
Errol Groff, 21 July 2005
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