Alan and I took the bed and Y axis of the museum's drill mill to bits yesterday. The leadscrews are in good condition, but the design of the nuts leaves a lot to be desired. Until now I have not been able to contemplate redesigning the nuts as none of the UK suppliers that I know of had laydown inserts in 2.5mm trapezoidal in their stocklists. Last week, I had another try on ebay and found external and internal 2.5mm pitch in stock from China. A box of ten er16 internal cost £19 so I have ordered some. The reason for taking the mill to bits was to assess the possibilities of fitting an antibacklash design. I needed to know the nut dimensions and find out if there was room to accomodate bigger nuts. The Y axis will be easy as there is a lot of space to the front of the nut and unused leadscrew threads on the winding handle side. The X will have to be assessed in the new year to find out whether a longer nut might restrict the travel. I made rough drawings of possible nuts and found that a block of sae660 gunmetal would be very expensive, so any nuts made will be from barstock and have bodies made from mild steel.

I will keep you posted when things start to happen.

I have found that 2.5mm pitch trapezoidal is not included on the charts, but helpfully, the calculations are very similar to ACME. The nominal thread depth is half the pitch. Better still, having changed all the change gears to MOD1 for lack of original gears for the Smart & Brown model A, I started off getting a 125/127 and it is all that is needed for producing the 2.5mm pitch. If 1/10" pitch is set and the 125 is driven and the 127 is output, the result pitch will be 0.0984251", equivalent to 2.4999975mm, close enough for government work.

Another bad design is the fixing of the Xaxis nut. When you remove the bed, an easy job, you have the choice of removing the whole Y axis casting together with the X axis leadscrew or unscrewing the leadscrew first from the nut, about 180 turns. This is because the nut is attached from underneath the Y axis casting. That will be remedied.

The whole thread production will be straightforward using singlepoint on the lathe. Because the thread is fine pitch, I can use an IR16 insert on a 16mm bar which gives plenty of clearance, unlike the 5ACME with an IR22 insert which required a custom made bar. The thread diameter is also unusual, being about 22.8mm, try finding a tap for that.

We have almost finished the re assembling of the mill. Here are some pictures of the old nuts next to the new ones.