We Make America Productive
Misunderstanding how the lead nuts are supposed to work on a Lucas leads to early nut failure.
If this applies to you, you will notice that the threads are almost completely worn out inside the fixed nut, while the threads inside the adjustable nut look almost new.
Lucas nuts are of two types: Fixed and Adjustable. The adjustable nut is the one with the small serrations on the edge of the flange. This nut is 'tightened against' the fixed nut until only a minimum of clearance (typically 0.004 to 0.006) remains between the two nuts.
The fixed nut for Tables and Heads is the one with a bolt thru a hole in the flange. This little bolt results in a great number of failed lead nuts. The bolt - obviously - must prevent the nut from turning; however, the head of the bolt MUST NOT CLAMP DOWN THE NUT FLANGE. There should be enough clearance under the head of the bolt to allow the nut to 'breathe' in and out - just like the adjustable nut. While the amount of clearance under the bolt head is not listed in any Lucas literature that I have ever seen, I have been telling people for many years to allow one pitch of the leadscrew - about ΒΌ Inch.
We will use the Head for our example.
When the Head is going up, the screw has pushed the lower nut upwards until the nut flange hits the mounting tube. The mounting tube then pushes the Head upwards. Meanwhile, the upper nut has 'breathed' upwards on the splines and simply 'goes along for the ride' with minimal wear.
When the Head is going down, the screw has pushed the upper nut downwards on the splines until the nut flange has contacted the top of the nut mounting tube. The mounting tube then pushes the Head downwards. Meanwhile, the lower nut has 'breathed' downwards on the bolt and simply 'goes along for the ride' with minimal wear.
Each nut 'shares' the load and the wear is distributed evenly.
The operation of the Table nuts is the same. The Saddle nuts are properly adjusted 'away from' each other and the 'cap' locks and limits the adjustable nut travel, while the Saddle casting limits the travel of the Fixed nut.
We again use the Head for our example.
With 'brand new' nuts;
When the Head is going up, the screw pushes the lower nut which pushes on the mounting tube and the Head goes up. Meanwhile, the upper nut has 'breathed' upwards on the splines and simply 'goes along for the ride' with minimal wear.
When the Head is going down, the screw pushes the upper nut downwards on the splines until the nut flange contacts the top of the nut mounting tube. The upper nut flange makes contact before the screw threads touch the lower nut since the nuts are new. The mounting tube then pushes the Head downwards.
After the machine wears a minimum of brass out of the nuts;
When the Head is going up, the screw pushes the lower nut which pushes on the mounting tube and the Head goes up. Meanwhile, the upper nut has 'breathed' upwards on the splines and simply 'goes along for the ride' with minimal wear.
When the Head is going down, the screw begins to push the upper nut downwards on the splines; however, before the flange contacts the top of the nut mounting tube, the screw threads hit the 'backside' of the threads on the lower nut. The lower nut now 'pulls' on the bolt - which pulls on the nut mounting tube and the Head goes down. Meanwhile, the upper nut - not having contacted the flange simply 'goes along for the ride' again.
The result is that the lower nut does all the work and takes all the wear. The upper nut isn't doing anything and gets only minimum wear.
Please note that some older Lucas drawings do NOT show this properly, however, the simple mechanics of the design require that the lower must not be 'clamped' - regardless of what is shown on the drawing. The Table works the same way. Although there is no bolt used on the Saddle, the basic idea is the same - one nut is used for one direction and the other nut is used for the other direction. 'Locking down' the fixed nut on the Saddle will produce the same result - greatly accelerated nut wear.