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Y axis feed nut repair

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catsparadise

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#1
A Happy New Year to All,

I have an Elliott Model 00 Mill that I am in the process of stripping down, cleaning and reassembling. This mill is probably older than I am (from its serial number 5720... I'm guessing it might be a 1957 vintage) and despite the expected coat of baked on grease & grime is in pretty good shape. The ways are pristine given its age and the X axis (longitudinal) feed screw and nut are in excellent shape. The Y axis on the other hand isn't so good. The feed screw, 3/4" x 8tpi ACME looks unworn. The cast iron nut it rides in has threads that look like a mixture of a 60 deg. thread that has been double threaded:

Y axis feed nut.jpg

My thought on how to rectify this situation given the amount of slop (its far too loose to be called backlash!) is to single point a cylindrical nut from a piece of bronze rod (I have an offcut of 1" OD, 1/2" ID tube), then bore out the cast iron nut and loctite the new thread in. There isn't a huge amount of surplus material around the edge to support the new nut and aligning the above piece in the 4-jaw to bore it true will be a challenge - note the spring pins which are supposed to line it up, although the hole is tapered so those pins may not be in the right place.

So, my question is - am I going about this the right way, or does anyone have a better way of fixing it? A replacement part is not going to be available, of course.

Oh, and to satisfy curiosity, this is the mill, as delivered, before I started stripping it down:

Mill - original condition.jpg

Thanks, Rob
 

Bob Korves

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#2
I don't think I would trust the Loctite by itself to contain the thrust load. I am not sure how much load the perhaps oily cast iron, Loctite, and bronze fit up would take. I think I would prefer a sleeve with a flange on one end and a nut on the other to contain it axially, and perhaps a means of adjusting the thread fit with wear as well. If necessary, shorten the current part as needed to retain full travel of the axis.
 

4GSR

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#3
Bob has a excellent suggestion to try. Yeah, that's going to be a close retrofit trying not to make the cast iron paper thin. Don't put any interference in the fit or it may crack the cast iron into. Don't want to do that. Give it a half to one thousandth clearance for the fit and say a 16 or 18 T.P.I. pitch on the thread. Don't worry about making the bored ID of the cast iron or the new nut to a fractional size, unless you get lucky.
 

tfleming

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#4
I agree with Bob on the thrust capabilities of Loctite. Personally, I would pin it in after machining it to a light press fit. Heat the cast, cool the bronze, press it in and drill and pin it.. When done, if you have a 4 jaw chuck, you could clean up the threads a bit. The other option, would be to chamfer the leading edges and braze it in. I would not trust this application to just Loctite alone.
 

The Liberal Arts Garage

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#5
Has anyone mentioned how much we learn from multiple how- to 's even if
we don't need them right now- a few words to explain the method is always
Helpful ! ........BLJHB
 

tfleming

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#6
Add on comment: Looking at the assembly closer, there should be plenty of "meat" to create an insert. Remember, the bronze insert OD only has to have maybe 0.010-0.020 beyond the root of the thread. The bronze can be very thin, as it will be captive in the cast iron. The cast only has to be bored to the root of the original thread + whatever sidewall you make on the insert. Again, you can pin it or braze it (or even better, silver solder). In either case, there should be plenty of cast left to accept it. Even if you make it a slip fit and braze/solder it, she won't move. Remember, if you are going to silver solder it, you will have to have a wee bit of clearance (at least 0.003) for the solder to flow. Silver solder flows beautifully in situations like this..........if you wear the repair out over time, you just make a new one and solder it in.

Lastly, you can cut your threads before you machine the OD of the insert. Leave the insert length 2"-3" proud to hold in the chuck. Cut your threads. Then, using a small pipe center on the tailstock, turn your insert to the desired OD. Cut the extra length off once you have the threads and the OD where you want them.

IMHO, I think there is plenty of material in the cast to do this. If you are not running production work on this, and keep the new one lubed and clean, it should last a long time.
 

4GSR

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#7
You want the wall thickness for the nut to be more than 0.020" thick. At least 0.035-0.045" thick. Making it 0.010-0.020" thick is not going to make it very stable, even if you sweat it in place. I still prefer Bob's suggestion, especially if you don't have a way to heat both the cast iron to get the silver solder to flow before you melt the bronze or make it cheezy and move on you with 0.010" thick wall.
 

tfleming

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#8
Ken, I can't argue with your point. I have done similar repairs (that I outlined) in the past, but they were bushings. I have had great success with silver solder, but if you don't have the torches or a bit of experience soldering cast iron, then I would absolutely tend to agree with you. I have silver soldered many dissimilar metals, with cast iron being many of them. Once it flows (if done with the correct amount of heat), the insert and the cast iron become "as one". If you have never worked with cast and silver solder before, this may not be the best approach.......I absolutely will concede that one! Great discussion though.

what kind of bronze stock is it you have? Sintered "oil lite", silicone bronze, aluminum bronze? all have different tensile strengths and melting points.
 
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f350ca

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#9
Depending on how much wall thickness you have after boring out the threads this is a slick way to get an adjustable nut. Had to make a new cast iron housing for the compound on my lathe after a crash. Just duplicated what was there. One half the brass nut is pressed then pinned in place with tapered pins. The other half has spring loaded pins that drop into the slots on the outside of the nut. They're spaced to give 1/2 the spacing of the slots, i.e. one drops in the other is held back. Just push the pin in and turn the nut till you have the back lash you want.
IMG_1558.jpg

IMG_1559.jpg

Greg
 

middle.road

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#10
Has anyone mentioned how much we learn from multiple how- to 's even if
we don't need them right now- a few words to explain the method is always
Helpful ! ........BLJHB
now, if one can just remember when and where seen when it does come time to need it. :grin:
 

catsparadise

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#11
Thought I'd follow up on this repair as folks were kind enough to give me their opinions. As I've been cleaning / painting / re-wiring the whole mill it's taken me a while to get to the point where the knee, saddle and table are assembled on the column again.

I bored out the threads out of the cast iron nut/bracket. The photo below shows the technique I used to line it up on the lathe using the leadscrew. I set it up so there was equal slop in both axes, then bored it out:

20180109_180608_LLS.jpg

The bronze tube destined to become the new threads is on top of the headstock. I ground a piece of hss to make an acme threading tool, then realized my boring bar was too big to give clearance to cut the thread so had to make the boring bar too. The thread is 0.75 x 8 tpi. Couldn't find a tap to cut the thread and even if I had I wouldn't have wanted to spend that much money for one thread.

20180113_154204_LLS.jpg

I cut a trial one in a piece of aluminium then cut the bronze. The tool tip was narrower than the required width of the thread so I cut the thread to depth then used the compound to widen the thread until the leadscrew was a nice fit.

At this stage I wasn't sure how I was going to secure it into the bored out hole. I talked it over with some colleagues at work and was persuaded that loctite was the answer. If loctite fails, I can soft solder or silver braze it in. I only have a propane/air torch and would have difficulty getting the cast iron bracket up to temperature - I think.

The bronze nut was trial fitted without glue when I reassembled the knee and saddle. The leadscrew seemed to line up nicely so I glued it in using loctite 603 (high strength retaining compound). Once I'd given it a while to cure I tried the Y axis action. It feels really smooth with very little backlash so I'm really chuffed at how it's turned out.

20180310_165514.jpg

If it fails in the future, I'll post again and admit glue was the wrong choice!

Rob
 

tfleming

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#12
Rob, good start. You can also pin it if it decides to move on you. If you decide to go the silver solder route or plumbing solder route, you can pre-heat the cast iron in a 300 degree oven. Then it won't heat sync so badly. Also, leave the oven on. When you are finished soldering, place the part back into the oven and then shut it off. Leave it cool slowly in the oven for at least an hour. Great job so far.
 
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