# Mystery mill - telescopic knee screw



## British Steel (Nov 11, 2017)

I've today lifted the knee on my Mill of Mystery, an Italian 1960s Testa 2U (I've only heard of one other in captivity), as I'd stupidly run it up to the point where the screw and nut disengaged, damaging the top thread of the bronze nut - all came apart pretty easily, and when I lifted the nut housing out of the base I discovered about 12" of bronze male thread and that someone has been there before me... (Why can nobody trouble themselves to find a screwdriver that fits the slots??)





 I assume that this (which is locked up solid) should also rotate, when the end of the (steel) elevating screw is reached, to give more elevation and that this is what has caused the screw to run out of the bronze nut...

 End of the (inner, power-driven) elevating screw looks like there's room for a stop to butt against the bottom of the bronze screw's inner thread





 And the damage done - looks like this has happened before and someone before me has "cleaned up" the top turn of the thread, I'll do the same as there's still nearly 2" of 5 TPI bronze Acme left in there





 Both threads are the same 5 TPI, so there'll still be the same travel whether or not the nut rotates in its housing, but allowing the nut to screw in and out of the base should (I think) allow the full range of movement - anyone care to shoot me down in flames? I'll have to apply some BF&I to the nut to get it rotating in the housing, but it *should* simplify reassembly as I'll be able to screw the nut up to meet the elevating screw, attaching the stop to the bottom of the screw may be interesting though...

 Thanks in advance for any suggestions, cautions, encouragement!

 Dave H. (the other one)​


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## f350ca (Nov 11, 2017)

Sounds like your on the right track Dave, The smaller screw should have a stop at the bottom as you thought. When it contacts the bottom of the inner nut the outside screw starts rotating to double the travel. My Peerless shaper uses the same system on the table lift, gives twice the lift in half the distance.
This is a photo of the lift mechanism before I cleaned up the old girl,


Mine has two steel threads with brass nuts.

Greg


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## Silverbullet (Nov 11, 2017)

Sounds like a stops missing on inside thread . I think Allen drill presses use the same set up on  large models . You may need to pull both out to repair .


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## British Steel (Nov 11, 2017)

Thanks Greg, I'll get to it and add a couple of improvements along the way - at the moment I'm struggling with getting the bronze nut free in the steel housing...

Dave H. (the other one)


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## British Steel (Nov 11, 2017)

Yup SB, what I thought was a castellated nut on the end of the screw turned out to be a pair of locating key slots and the remains of an M10 capscrew... 

Plan now is the following stack on the end of the screw:
 M10 x 25mm Capscrew through:
 35mm dia Sturdy Washer;
 20mm ID by 35mm OD thrust bearing;
 20mm diameter by 12? mm spacer with lugs matching the key slots in the screw;
 finally screwing into the thread in the elevating screw itself.

 On the principle that when the screw's extended and reaches its limit on the ID of the bronze nut/screw thing, the thrust bearing will prevent friction locking the two together - which I assume is what Boroughly Thuggered it in the first palce?

It looks like it should be possible to assemble everything without lifting the mill off the floor (I hope so, it's about 3500 pounds of machine...) by paying attention to the order I put all the bits back together...

Dave . (the other one)


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## British Steel (Nov 11, 2017)

Some progress - ground out about 60 degrees of the first turn of the bronze nut's thread (where would we be without our Dremelloids?), wound onto the elevating screw nicely, a bit more BFI and the nut started turning in the housing - refitted it apart from the thrust bearing (found the exact one in the Misc. Bearings drawer, I think I rescued it from the WBS a few months ago!), just need a few of the Right Washers, a M10 x 25 SHCS and a drop of Loctite and I can button it back up - at last!

Dave H. (the other one)


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## Silverbullet (Nov 11, 2017)

Makes YA feel good doing a job like that, imagine if you had to pay someone to come do it. You could buy another used machine.


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