small mill for machining steel. Max spend £500 including computer

[Andrew Wilding]

Phil,
Thanks for you experiments. I realise that ball screws are more efficient. I have chosen larger steppers than I would have with ball screws. The teflon coated slides also help increase the overall efficiency of the system a bit.

As you will see when I post the detail of how I did the knee, it is counter balanced. This obviously improves the force balance on the system but the knee is still a fairly heavy part to accelerate. I tried to take the easy route out with the design and replace the threaded rod with the screw stock but I was not happy with it and had to go back to the drawing board! more on that later.

Jonathan,
Thanks for the post. I was at Notts 6 or 7 years ago and studied mech eng (ahh the smell of the Coates building!). I now work for a gas turbine manufacturer but still live in Nottinghamshire. I did do an autocad sketch of the nut but it was only to see if it looked right. I will try to post this or possibly just a better picture, although there was not much science behind it. The slave nut is castellated to allow the spring load to be adjusted and so there was a bit of experimentation involved. The two halves mate with a 60 deg chamfer so an increased locking torque is experienced between the two parts when compared with just plain faces for a fixed spring rate. This should increase the thrust required to separate the two halves of the nut with the same spring rate. It seems to work ok. The lash in each axis is about a thou but I an now adjusting everything to see if that can be reduced. Without the slave nuts the lash is 6 or 7 thou (I deliberately made this a loose fit so that the only friction is on the useful faces)

[M250cnc]

Andrew not easy to counterbalance the knee, but good to see you have it worked out.

Good luck with the rest of the build which i will keep an eye on.

Phil