Drill mill X and Y backlash

[old martParticipant @oldmart]

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.

[old martParticipant @oldmart]

[old martParticipant @oldmart]

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.

[Martin ConnellyParticipant @martinconnelly55370]

2.5mm pitch trapezoidal is not listed in my Machinery's Handbook but the formulae for calculating the depth and other features based on the pitch are shown. Using these should give enough information to form the 2.5mm pitch correctly.

Martin C

[old martParticipant @oldmart]

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.

[old martParticipant @oldmart]

The 2.5mm Trapezoidal inserts arrived the other day and the first 2 nuts are made as well as the X nutblock. Getting the pitch was straightforward, instead of the original 120 idler gear, the Smart and Brown model A has a double 127/125 and driving in that order turns a 10TPI set in the Norton gearbox into 2.5mm exactly. To save money the body is a block of aluminium. One nut will be Loctited in place and the other is screwed in with a 40tpi thread. The flange of the screwed nut will have axial holes in it for adjusting in situ, and there will be a spring loaded clamp to prevent it turning by itself. reversing the motor while threading is easy, I stop before changing direction, it gives time to back off and/or advance the cut depth.

[DiogenesIIParticipant @diogenesii]

Looking good – I think on mine the handles foul the saddle casting before anything runs onto the nuts in X..!

[old martParticipant @oldmart]

I had a good look under the drill mill before deciding to make the nut assemblies for the X and Y. The X still has about 100mm space either end when at the limit of travel, so this doubling of the nut length will not do any harm. The Y axis nut is restricted towards the rear, but there is plenty of room at the front side, so the front nut will be displaced towards the winding handle.

[old martParticipant @oldmart]

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

[old martParticipant @oldmart]

The mill is back together, and I have confirmed that the X and Y travel is unaltered. Both have 0.1mm backlash which is down to the leadscrew thrust bearings axial play, they will need a complete rethink of the original Taiwanese design.

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