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Grafting New Cross Slide Screw

BobS

Plastic
Joined
Sep 30, 2006
Location
UK
I have been looking at various discussions about grafting on a new cross slide screw. Everyone seems to bore out the remaining stub and turn down the end of the new threaded rod to fit with various levels of interference and sometimes Loctite. Has anyone ever tried threading the components and just screwing them together, possibly with Loctite?
Also how do people hold the threaded rod when turning the end down to minimise the likelihood of damaging the thread?
 
In a collet typically. I think the size of the overall assembly argues against threading the two pieces - also a threaded joint is probably less aligned than a bored/turned fit.
I could imagine silver soldering the parts, but did recently modify a drawbar with a new (different size) threaded end, used locktite 620 - the green stuff. So far holding up well.
 
At a guess, the new threaded rod will have excess length that needs to be cut off. So that could be a factor, in where and which side to cut off.

Also look at the total travel of cross slide, and the position of cross feed nut in relation to threaded portion of acme thread. Note the dial side threads will most likely have a couple/few inches that are never used. Just food for thought. Could be you're not using the entire length also, but you need to be able to initially screw nut on.

If it were me, I'd take the old cross feed screw. Get a hack saw. Cut the old threaded section off, with the exception that I'm leaving 3/4" of the old threads still on. Now clamp the remainder of cf screw in the chuck of a lathe by the smooth dial side portion of shaft. Then turn down the 3/4" length of remaining threads to something appropriate, maybe .002" or so over 3/8", call it .3775". Now bore the thread rod with a 3/8" drill bit to maybe 1" depth. Press the two together and use loctite or bearing retainer like Jim mentioned.

That would probably hold as is, if the press fit is nice. Worst case, cross drill and hammer a pin through both.

If you don't have collets, might be able to stick brass shim stock between jaws and acme thread.

If you didn't try a new cross feed nut yet, that can do a lot to remove some slop all by itself.

Another fyi, crossfeed is usually left hand threads.
 
Now there is food for thought. The suggestion by Texasgeartrain is the opposite of what I was going to do. I was going to completely cut off the old screw, drill the remaining stub and then turn down the new screw so it can be pressed or screwed in. This would mean that the graft could be a bigger diameter and hopefully stronger.
Incidentally I am going to fit a 1/2" acme screw in place of the existing 7/16" as 1/2" ones are easily available in the UK.
 
Clearance is a bit tight there, make sure the larger diameter screw, and the larger ('taller") nut will clear the trough on the cariage.
 
when I made a new extended tailstock screw I made the bearing/ handle end first, drilled/ reamed for some size or other (1/4", 5/16"?) then turned down a piece of LH acme rod to that size. Glued and pinned the two together. It was a pretty straightforward piece of turning.
 
I've found that cutting off the old acme screw at the rear face of the pinion gear or on a "C" type, the gear blank, then drill the gear blank in a collet or chuck undersize. Bore to 0.010 undersize, then ream to exact size. About an inch deep. This keeps everything concentric to your allowed setup, that means check your collet or chuck at dial in first. Then chuck your Acme replacement rod, center drill and turn down a 1.375 length to 0.001-0.0015 undersize of the reamed hole in the gear. This is important, as it gives clearance for the Loctite 680 film thickness.
Once you have this sliding fit, cut an air groove down this length, about 0.020-0.030 deep. Debur the edges of the cut. re-setup/dial in, the gear/handle blank in the collet and test fit the acme blank with the tailstock center in, do not glue yet. Setup a dial indicator on the acme threads just in front of the gear blank, now rotate the screw assembly including the acme screw. You are checking the run-out of the acme blank to the reamed gear hole. Slowly rotate the acme blank as you read the dial for the lowest run-out between the two parts. Once you have found this, mark the alignment points on both parts at the high point. Remove the acme screw and apply the Loctite 680, twist the acme blank as you re-install in in the gear blank, this will let the air out thru the slot. align your marks and position the aligned marks on the down side. The Loctite expands as it cures, and it will pool slightly on the lower side. This lifts the screw slightly and will help to bring the final center concentric a little closer. If you have less that 0.001 runout, turn on the lathe and run at very low speed for an hour or so, depends on the temp/heat, just enough to keep the Loctite even. If you have more that that, just let it sit with the mark down.
I've found that 0.001 is about the limit it will push up. Let it cure 24hrs before removing from between centers. After 48hrs you can re-chuck and check runout on the far right end of the screw, straighten to best run-out. The extra 3/8 inch on the screw stub length is to match the factory thread relief and get you acme blank length correct. there is no need to pin the glued screw if you use 680 Loctite, after about a week it takes 900-1100 degrees heat to break it's bond, and its rated at close to 3000 lbs shear. To make the air gap cut, take a sharp point turning tool and use it like a shaper blade in the lathe on center, works great. Very Important: After about 30 minutes of work, and you are watching it cure, have a "congratulatory achievement" beer!!
 
That is a very useful and detailed set of instructions, thank you very much for taking the time to write them.
One further question, I am based in the UK and 7/16" acme rod is unobtainable but I can get 1/2". I think I could use this if I tap out the nut to the larger size. I think there is just enough material in the nut to do this but the wall thickness will be quite thin. Have you ever tried doing this?
 
That is a very useful and detailed set of instructions, thank you very much for taking the time to write them.
One further question, I am based in the UK and 7/16" acme rod is unobtainable but I can get 1/2". I think I could use this if I tap out the nut to the larger size. I think there is just enough material in the nut to do this but the wall thickness will be quite thin. Have you ever tried doing this?
Thank you, I appreciate that.
Yes, the 1/2" will work as you stated. Keep in mind that a normal 2G tap is going to be oversize, enough that it will be a loose fit. It would be better to single point the nut for a nice fit to the screw.
South Bend's print on the screw was a standard 2G print with a 0.010 square gutter in the minor dia. The rougher bit had a square tip for this. the measurements over wires were held fairly tight to control backlash. The auto feed screw machine made a rough cut towards the headstock first, then switched to the finisher bit on the rear of the saddle and cut away from the headstock, the spindle reversed directions at each cutter change. After each rougher/finisher cycle the depth control bar clocked forward to the next depth, there were 8 steps of cutter depth. After the 8'th depth cut (16 total cuts), you would let the clock do 5 or six spring cycles then break the edges of the threads and check your fit. The 4.5G fit was in the nut (the tap specs) that why I said it's probably best to single point it. there were three classes the screws were sorted in, Normal, which allows up to .003 backlash. Tool Room, and Government, which were the tightest spec. Later models also had a ground screw option.
If you run in to problems just give me shout.
Steve
 
There are taps that will yield a 3B class fit, but they are very expensive. For the cost alone, I would also single point the nut. But single pointing an original bronze nut is going to be difficult. First and foremost, you'll have to align the centerline of the nut to the centerline of your lathe. Then, since there are already threads there, you'll have to align your tool so that it hits the grooves near perfectly. Then, there is the need to grind an ACME profile on a cutter. But these difficulties can be overcome by a skilled operator. When you are cutting the threads, take 0.002 to 0.004" only each pass and check the fit AT EVERY PASS. Also take spring cuts after every pass.

I would just use a good press fit (hole in the original part, stub on the replacement). I wouldn't use Loctite - I have never trusted it. Anyway, the torque is very low on these parts, so a press fit should be sufficient. If I was still worried, I would drill and pin the parts together.
 
It may be easier to make a new, larger nut from scratch rather than upsize the one you have. On a worn lathe the centerline of the old nut may be off both side-to-side and up/down, because of the accumulated wear on the sliding elements. The wear has been taken up by the gibs but in doing so the leadscrew axis and the nut axis move apart.

One way to do this is, rough out the nut from bronze, and put the bushing in place in the carriage, without the leadscrew or dial in place. Then fab up a punch that's a sliding fit in the bore of the bushing. Mount up the crossfeed screw, slide it to the very front so your nut blank is as close to the front as possible. Lock down the gibs to hold the slide while you give the punch a tap.

The nut will now have a good guess about where the minor diameter bore for the screw should locate.
 
Nothing wrong with making but new lead screw nuts for the 1/2 inch rod are available on the internet, I was happy with the quality, bought one for the compound at the same time.
 
I suspect that single pointing the new thread will not be possible for me as the only machine I have is the one I am refurbishing. Any machining I do will have to be with the cross slide locked up and the compound swung round.
Has anyone had any experience with the screw and nut refurbishment kit available on eBay?
 
Has anyone had any experience with the screw and nut refurbishment kit available on eBay?

I had to replace both of mine on a 10L. They were both excellent products. The only problem I have is with the nut - it is made out of brass, not bronze, so it won't wear as well.

You CAN lock the cross-slide and only use the compound, if that is your only option. It works, but you have to just remember that that is what you are doing and adjust your technique.

You also might want to consider buying a tap for the nut.
 
Bob, contact me off list at: [email protected]
Send me your postal address so I can check shipping costs for you.
I will make you a 7/16 LH feed screw from 1144 stress-proof and I can make a bronze Nut Also.

Steve Wells
 
Machine porn for Jim...:)

932 bearing bronze nut.
Last photo is a new SBL screw on top and one of mine with the 3 parts bonded together.
 

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More...??
These are works in progress, testing different ways to build a handle/pinion gear blank, using 680 to bond them. Starting with a TGP 0.375 shaft and a 0.750 gear blank bored to fit. This is 1144 SP also.
 

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I am surprised that nobody has drawn a print of various cross slide screws and nuts and posted them for sale on eBay. Before one tears down a lathe, they could buy such a print and make the parts. Could just be made of CRS and last a good long time.
Turn the screw and buy the nut an option for guys not up to Id threading.
Here a youtube guy is doing so

 
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I am surprised that nobody has drawn a print of various cross slide screws and nuts and posted them for sale on eBay. Before one tears down a lathe, they could buy such a print and male the parts. could just be made of CRS and last a good long time.
Turn the screw and buy the nut an option for guys not up to Id threading.
I have a repair manual completed for the 9/10K that has the prints in it for turning the parts for a large dial conversion. These are factory specs, not guess-ta-mates. Have not sold the manual yet alone. I was hoping to be able to supply the parts a lower cost that a complete screw assembly.
I've been building a CNC machine for two years now to help lower the costs of the mount bushing, dial and spinner handle. It's been slow going, but I'm getting closer, it will be making parts before the leaves fall.
Steve
 








 
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