The two middle screws hold the nut, the two outer screws just hold the protective leadscrew covers.
You can measure the distance from the outer edge to the leadscrew down the outer hole, and to the ‘obstruction’ down the middle screw hole. Compare that to the right hand edge and the hole for the leadscrew to confirm that what you are seeing is the body of the nut and not some additional screw/pad.
The original design just has a round ‘nut’ pushed down and along the leadscrew and two bolts driven in to lock it. There might be small dimples in the nut for the bolts to seat in. If something locked up it is likely the nut was moved round or along a bit creating big burrs that are now graunched into the saddle body. I don’t think drilling down the bolt hole will help and may raise further burrs.

It is going to be brutal but I think you need to put the leadscrew protective cover (or some other tube if you have it) over the leadscrew down to the nut so that you can hit it and hence the nut to push it out of the hole rather than applying force to the nut via the leadscrew.

Well down. Don’t be in too much of a hurry. Let the releasing fluid do its work and keep fiddling with it to try and get a little movement. I would hold the lead screw vertically with one hand and tap the nut with a nylon hammer whilst successively rotating the lead screw so you give it a lot of gentle taps all the way round. Then see if you can get a bit of movement. If you can don’t just try and wind the nut straight off work it backwards and forward.

Pure guess work but if the Leadscrew is 16 mm diameter, it might be 1 or 1.5 mm pitch

If you lay the Leadscrew in the bench, hitting it with mallet , whilst rotating the assembly might free things off to give a little movement. The blows will produce a temporary distortion which might free things off a little. But avoid excessive force for the blows, or attempts at rotation, for fear of doing more damage.

It may be that the nut will then move more easily on one direction than the other, away from what is causing the problem, (swarf?)

Once you have it apart the cause of the problem should be visible, and hopefully repairable.

Howard

Hi, my MD65 leadscrew is 1mm pitch, the hand wheel does have 0.025mm increments, but there are ten sections on the handwheel with four divisions in each, so, 4 x 0.025 = 0.1 x 10 = 1mm.

Personally, I would lay the nut on a piece of stout timber and gently tap it with a nylon mallet a few times, while rotating it, and I would use proper penetrating oil to help free it off, and turn it the opposite way to the way it first jammed up.

Regards Nick.

Agree with the Plus-Gas, better releaser than WD 40.

I couldn’t see mention of the material of the nut? If it’s bronze, probably going to be OK. But if it’s a soft material, and aluminium / mazak based material there is a risk that a piece on ingested swarf has caused a ‘pick up’, basically raised a ball of the softer material and jammed it up solid. Turning it further just increases the grab and it’s a bad doo.

If it’s bronze based. Heat is the answer. Lots of it and FAST. I.e. heat the nut fast so that the lead screw stays as cold as possible  ( even pre cool the lead screw with cold water / ice ), this gives the greatest degree of differential expansion and has the best loosening effect. Then gip the nut with a Stilson or mole and twist… Do the whole job as fast as possible to minimise heat transfer to the lead screw. You can also put tube over the exposed parts of the led screw to act as a heat shield. Basically you want the nut as hot as possible and the screw as cold as possible….

Also when holding the leadscrew in the vice and grabbing the nut with aggressive jawed wrenches, protect the parts with some thinner brass sheet bent over the part so that it eliminates damage an you don’t have to faff manually holding the protectors in place whilst it’s scalding hot. Make a thin brass ‘U’ shape, cook the nut, drop the protector on to it, then grab and twist…..

If it’s alloy based. I would try to find a spare / replacement nut. Then try the heat, and if not, cut the nut off, dress the leadscrew if there is any damage and fit the new nut.

A badly picked up nut on a thread can be terminal….. But anything can be fixed, just a bit more work required….

All the best and hopefully the Plus-Gas and Heat will do the trick.

yes, agree it’s 16×1, the form is standard metric 60 pointy.

The nut is a bronze cylinder and Heritage Lathes will sell you a brand spanker for £45 I think.

Sincere apologies, you are quite correct and I made an incorrect assertion twice – hangs head in shame..