New knee nut for Tom Senior

Today, I decided to go for an even simpler option, just a clamp on the side of the nut. I am getting 0.004" backlash in the most worn centre of the leadscrew, and nothing either end of travel for the Y axis.

Edited By old mart on 30/07/2022 17:23:14

Edited By old mart on 30/07/2022 17:31:47

While the Tom Senior is in bits, I decided to make the Yaxis nut backlash adjustable without didmantling the machine. The pictures show my way of doing it and Mike and myself did some turning, hacksawing and milling to make a tool that can reach under the Y axis.

The bed, and head are now fitted, and I got the head trammed in only 30 minutes, despite the design made especially to make tramming difficult. Now only the motor remains unfitted.

Nearly, but not quite right, the knee being extended by 25mm to increase the Z height slightly has not exactly gone to plan. The knee stops at plus 25mm from the bottom. Investigation found that the top of the nut was running out of leadscrew thread. Not slowly binding, but stopping dead. It is easy to check that it is indeed the nut running out of thread because if the knee is raised to near the top of its travel,and locked, the nut can be screwed up until it stops and the nut height recorded. Then the nut is returned to the bottom of the leadscrew and the distance that the bed can be lowered is the same.

The nut flange has been thinned, and the height of the cast iron base that the nut sits on shortened. The flanges on the nut base are wide enough to foul the narrow underside of the knee, so they have been turned smaller and the bolt seats have been chamfered to use countersunk head 3/8 W bolts (in the post ).

It is possible, just, to remove the nut and nut base without taking everything off of the knee. I rotated the head 90 degrees clockwise and wound the bed right up to it. The nut and base can be blocked up when lifting the last bit and then the knee locked and chocked. The nut on its own can be screwed up the leadscrew and there is room for the nut base to be removed followed by unscrewing the nut. I have added a picture of the modified nut assembly to compare with the old unmodified one earlier in this thread. The screws remain to be sorted at present.