Stripped Half Nuts

[savarin]

well, not both halves.
I cleaned the lathe the other day and made some adjustments oiled etc.
When running the lead screw from the tail stock to the head stock with the half nuts closed as if threading I noticed the carriage stopped moving about half way along. It looked as if the half nuts had opened a little and let them lose their grip.
I stripped it all down to check what was happening and found one half was ok but the other half had a lot of wear.

The difference in the wear is very apparent here.
I believe these are made with unobtanium as the price is $50+ with a 16 week delivery schedule or 4 weeks if I pay another $60 for expedited air mail. Ordering from the states is no better.
If I hold the worn half nut to the lead screw it seems a good fit so probably will still work if everything is adjusted to zero play and they clamp up tight.
The two gib screws are I assume supposed to move the dovetail section to take up play, they cant because the holes "a" and "b" do not allow sufficient movement to do this.
Should I just bore them out to a larger diameter (easy) or machine then to a slot (harder) to get sufficient adjustment?

Or should I thin down "c" so the dovetail strip can take up the play as its bolted down. Easy but I think difficult to get exact and leaves no adjustment for wear.

There is also I think a bit of a problem with the width of the two parts
With them pushed into the dovetail as shown part one is approx 5+ thou wider (in the arrowed dimension) than part two so even if the dovetails are adjusted to remove play it will only work on one of the half nuts.
The 5+ thou is guess work as I can figure out how to measure it exactly.
Would I be correct in thinking I have to remove this extra material so the are both exactly the same width?
I know the two half nuts are touching here but I didnt check if they touch when the lead screw is between them.
If they do then should I again remove some material so they dont touch when clamped up tight.

Last question (honest, for now)
The dimple for the detant is I believe in the wrong place (no pics but there are two overlapping dimples) because the handle to operate the half nuts can be closed a bit more but then springs back to one of the dimples.
Any ideas how to fill these dimples so I can drill a new one?

 

[Bill Gruby]

I haven't lost you Charles. I just changed the entire apron on mie. I will look at the old apron and try to answer mall your questions.

"Billy G"

 

[Bill Gruby]

Are the half-nuts in the last picture in the engaged position Charles. If so they look to be too tight. The first pic is mine engaged. The second is open.

"Billy G"

 

[ARKnack]

I remember seeing somewhere where a half nut was repaired using tercite. I believe it held up well. Search the internet for repairing a half nut and you may find it.

 

[savarin]

Are the half-nuts in the last picture in the engaged position Charles. If so they look to be too tight. The first pic is mine engaged. The second is open.

"Billy G"

Hi Bill,
No, they are just resting there, I will put the lead screw in and clamp it today to check the gap.

Hi ARKnack, I like the turcite idea and have been procrastinating about using that mod for the cross slide, I will have a look for the half nut version.

 

[davidh]

have you considered the "evanut" made from delrin ? a very interesting solution to your problem and it;s totally a do it yourself project.

 

[savarin]

In thinking about this further I cant see how the delrin would remain in the half nuts after splitting without doing irreversible damage to it with holes or undercuts to hold the plastic.
I think I will attempt to effect a repair as I outlined above and it that doesnt solve the problem then I will cut some keys into the brass and hope the plastic flows into them sufficiently to hold it in place.
Hmm, maybe drilled and pined.
I'm also going to make a larger handle so I can drill the detante dimple further out and in the correct position to hold.

 

[savarin]

I elongated the holes in the gib strips which allowed more adjustment as I hoped.
I added a 0.9mm shim under the operating cam so the half nuts rode on its rear face and the edges of the hole. This went a long way towards stopping the half nuts from tilting.
The fixed gib strip had to be packed on one end so as to bring both gibs into parallel, now the adjustment is equal at both ends and the dovetails slide smoothly with no tipping or twist. Just as they should have from the beginning.
I also adjusted the little screw that stops the half nut cam from turning too far so they opened half as wide again. This will let me place the detante dimples a reasonable distance apart.
Now it all works very well, the action is smooth but firm and the carriage travels from end to end without skipping a beat.
Just have to make a handle now as the vice grips do not look the part.

 

[FOMOGO]

Looks like another successful fix. Just goes to show that if you hammer on something long enough it will eventually submit. Mike

 

[savarin]

Heres the apron showing the new dimple holes.
They are deeper as well and the ball sits in with a distinctive feel.

The original handle was cast in place on the shaft.
This handle is steel, the paint is still tacky which is why it isnt yet on the lathe and I'm waiting for 2x8mm grub screws to hold against the two flats on the shaft.