There are two major sizes of drive screws on my machines. As my machines are in metric scale, sherline cut the standard M6 1mm pitch thread on the drive screws and nuts, but they are using the 0.25 inch (6.35mm)and 0.375 inch (9.525mm)diameter stock to make the screws. This result in unique specifications of threads.
At the very beginning, I plan to make a brass anti backlash nut with center expansion spacer; the spacer was driven by a torsion spring. But the overall size of the nut was so small that I was hardly to cut the parts out. (I will need to use 0.5mm spring wire to make the torsion spring in order to keep the overall size under the limit.)
During making the nuts, I’ve tried different methods to cut the internal and external threads. I’ve spent 2 weeks to grind the LH and RH internal threading tool. Although I can cut the thread as I want, the process was very slow, especially there is no power assist when using the threading attachment on my lathe.
Thus, I decided to make my own tap. I cut the taps from sliver steel, and try to heat treat them on a household gas strove. The heat treating result was not very good, the tap wear faster than my expectation.
In my design, I want the nuts can be:
1, Easy to cut/make
2, Flange mounting
3, Little / no modification on the machines
4, Last reasonably longer
5, Self compensation for the wearing
Brass was a bit hard for my homemade tap, so, I am going to use a soft and wear resistant material, that is delrin.
I also decide to use an expansion spring to replace the expansion spacer. Although the spring will push the nuts at all time, introducing loading on the nuts and screw. The thread pitch is 1mm, that’s mean if the spring push out 0.5mm for wear compensation, the nuts are going to be replaced. And 0.5mm pre-compression will not introduce large amount of loading. Moreover, my machines are light duty machines, the resistant introduced by work tables and dovetail slides are only several kgs.
I am going to add this nut on the cross slide of my lathe first, as the cross slide don’t has any anti backlash devices. I can also use this axis to test my anti backlash nut.
Hopefully the delrin nuts can last for at least 500 hours.
